This page last modified 2005 February 20

FAQ for uk.sci.astronomy


A. Introduction
B. Astronomical Societies
C. Astronomical Publications
D. Astronomical Software
E. Astronomical Equipment – Choosing and Purchasing
F. Astronomical Equipment – Maintenance
G. Astronomical Equipment – Mounts
H. Astronomical Equipment – Eyepieces
I. Astronomical Equipment – Accessories
J. Astronomical Equipment – Telescope Making
K. Astrophotography – Conventional Film
L. Astrophotography – Digital Cameras (DigiCams)
M. Astrophotography – CCD Cameras
N. Astrophotography – Other Electronic Cameras
O. Visual Observation
P. Light Pollution
Q.Famous UK Astronomers
W. Miscellaneous
X. Other Useful Internet Resources
Y. Contributors
Z. Requests for Other Sections

A. Introduction

  1. Where is this FAQ located?
    The primary location is .
    It is mirrored at
  2. What is uk.sci.astronomy?
    "uk.sci.astronomy is a forum for amateur and professional astronomers to discuss astronomy within the UK." (from the newsgroup charter.)
  3. What is the charter of uk.sci.astronomy?
    The charter is a definitive description of what is and is not permitted on the newsgroup. It is archived at The full text of the charter is:
    uk.sci.astronomy -- Astronomy in the UK
    uk.sci.astronomy is a forum for amateur and professional
    astronomers to discuss astronomy within the UK. Examples of
    suitable topics for uk.sci.astronomy include:
    Observing from the UK: Using the naked eye, binoculars,
    telescopes, The Moon, Planets, Stars, The Sun, Comets, Meteor
    Showers, Deep Sky Objects, Constellations, Aurorae, et. al.
    Equipment: Choosing Telescopes/Binoculars. Choosing Accessories
    (Finders, Drives, Eyepieces, Filters, Observatories) Building
    Telescopes, Backyard Observatories, etc.
    Astrophotography:  Traditional, CCD, Other detectors, Image
    Astronomy Software: Planetarium Programs, Deep Sky Programs,
    Observatory Programs, Educational Programs
    Relevant Social Events in the UK: Educational seminars, observing
    meetings etc.
    Astronomical events visible from the UK: Lunar and Solar Eclipses,
    Meteor activity, supernovae etc.
    Tips & Techniques for Amateur Astronomers
    Literature covering the topics listed above.
    Is forbidden except for short (less than 10 lines) private adverts
    for astronomy related items (telescopes, eyepieces, software,
    books etc.) and announcements of events relevant to astronomy
    in the UK.  Blatant off-topic and commercial advertising is
    not permitted.
    All posts must be made in plain text; HTML and other types of
    formatted text are forbidden. Posting URLs of relevant Internet
    sites, where appropriate, is encouraged.
    With the explicit exception of PGP signatures, all encoded
    binaries are forbidden. Forbidden binaries include,
    but are not limited to: pictures, sounds, word processor
    documents, spreadsheets, executable programs and "business
    cards". You are invited to read a guide on the World Wide Web
    at for further information
    on how to configure your newsreader to post to uk.* newsgroups.
    Anyone posting advertisements, binaries or other material contrary
    to this charter may be reported to their ISP or postmaster.

  4. Can the charter be changed?
    Yes, but it involves a non-trivial process. For information on the procedure for changing charters, please see
  5. What conventions are there for posting to uk.sci.astronomy?
    They are the same as those for any uk.* hierarchy newsgroup, and can be found at, where you will also find advice on how to configure your newsreader appropriately. In brief, these conventions/guidelines are:
  6. What is a troll?
    A troll is someone who posts something provocative to a newsgroup or other forum, not for the purposes of starting a productive discussion but of creating a "stir". They usually eventually go away if they are ignored.
  7. How do I deal with trolls and off-charter posting?
    If you must respond, please do so privately and not on the newsgroup..
    If you wish to complain, please do so to the "Complaints-To:" address in the header of the offending post.
    You can also report incidents of net abuse to the newsgroup, using the appropriate posting format.
  8. What other astronomy newsgroups and electronic discussion media exist?
    Graeme Skinner maintains an up-to-datelist of these at:

B. Astronomical Societies

  1. What national astronomical organisations are there in the UK?

  2. Where can I find out about local astronomical societies in the UK?

C. Astronomical Publications

  1. What UK astronomical periodicals are available?

  2. What other astronomical periodicals are available in the UK?

  3. What astronomy books are suitable for beginners?
    There are very many books suited to beginners. Those recommended on uk.sci.astronomy and elsewhere include:
  4. What astronomy books are suitable for intermediate and advanced amateurs?
    There are very many books suited to intermediate and advanced amateurs. Those recommended on uk.sci.astronomy and elsewhere include:

D. Astronomical Software

  1. What planetarium programs are available for the PC?
    There are numerous excellent planetarium programs available for PCs. Those recommended include:
    There are more comprehensive lists of astronomical software at:
  2. What planetarium programs are available for non-Windows platforms?

  3. What planetarium programs are available for hand-held computers?
    Of the myriad of astronomically related software available for the Palm there are three planetarium programs that stand out: 2Sky is commercial software. Planetarium and PleiadAtlas are offered as nagware. Like most Palm software, a small registration fee is required for continued, full-featured or nagfree use.

    WindowsCE: An evaluation download is available for each except The Sky


    There is a comprehensive FAQ on astro software available for hand-held computers at
  4. What software is available for observation planning?
    Some good planetarium programs include this facility. Dedicated observation-planning software includes:
  5. What software is available for telescope making?
    See for a list of suitable ATM software.
  6. What software is available for astronomical image processing?
    Image processing software is used to manipulate the data held in a digital image. In astronomy this is typically applied to digital camera images, ccd-images and (less common) scanned photographs and slides. There are many different packages available. In addition to general image-processing software like Adobe PhotoShop® and Paint Shop Pro®, there is dedicated astronomical image-processing software. Most CCD cameras come with some image-processing software. Here is a reasonably extensive list of third-party offerings:

  7. What software is available for spectroscopy?

  8. Where can I obtain astronomical shareware?

E. Astronomical Equipment – Choosing and Purchasing

  1. What is the best telescope for a beginner?
    The best telescope is the one that will be used the most! A telescope that is easy to set up, easy to use, and which has good mechanics and optics will be most likely to meet this criterion. You should take portability into account if you will need to transport it to an observing site. Cost is frequently a limitation for beginners, and there is a cost below which it is advisable to save your money until a better telescope is available. There is advice on telescopes available in the UK, broken down into price ranges, at

    Beginners are frequently advised to get binoculars instead of a telescope. This is sound advice – most experienced observers continue to use binoculars as well as their telescope or telescopes. Binoculars also have a multitude of applications outside astronomy should the interest in observational astronomy wane. There is advice on choosing binoculars for astronomy at
  2. How do I choose a first telescope?
    Ideally, you should "try before you buy". The best way to do this is usually to make contact with your local astronomical society and attend an observing evening. Experienced members will be pleased to advise you on your prospective purchase and you will be able to arrange to try out equipment owned by the society or by members.

    If you are unable to attend an astronomical society meeting and want online advice, this is available at There is advice on evaluating a telescope prior to purchase at http://www.astunit.comtutorials/evaluating.htm. There is some excellent advice in this US-biased Telescope Buyers FAQ. Phil Harrington's excellent book, StarWare, contains information on a great deal of currently available astronomical equipment.
  3. What are the advantages of reflecting telescopes?

  4. What are the advantages of refracting telescopes?

  5. What are the advantages of catadioptric telescopes?

  6. How do I choose binoculars for astronomy?
    There is advice on choosing binoculars for astronomy at
  7. Where do I purchase new astronomical equipment in the UK?
    There are a number of reputable retailers in the UK. Most advertise in Astronomy Now. If you wish to get opinions on a retailer, ask on the newsgroup.
  8. Where do I purchase used astronomical equipment in the UK?
    There are a number of options.
  9. How safe is it to buy used astronomical equipment?
    As with any purchase, especially of used equipment, the best advice is caveat emptor. That said, the majority of vendors of used kit are honest amateur astronomers and reports on uk.sci.astronomy of someone being "fleeced" by a vendor of used kit are exceptionally rare. Steps you can take to protect yourself include:
  10. Can I buy directly from abroad?
    Yes, but you need to be aware of the following:
  11. What do all the numbers mean?
  12. Where may I obtain manuals for second-hand telescopes?

F. Astronomical Equipment – Maintenance

  1. How do I clean telescope optics?
    The short answer is: "As infrequently as possible and very carefully." The optical surfaces of mirrors and lenses are susceptible to damage if they are cleaned improperly, and they have to be truly filthy before cleaning becomes necessary. If you must clean the optics, first of all contact the manufacturer/supplier of the telescope to ensure that any proposed action will not be deleterious to the telescope or void the warranty, then:
  2. What is collimation?
    Collimation is the correct aligning of all the optical elements of a telescope.
  3. How do I collimate a Newtonian reflector?
    The following web sites have excellent advice:
  4. How do I collimate a refractor?
  5. How do I collimate a Schmidt-Cassegrain?
  6. How do I replace damaged optics?
    Contact the manufacturer/supplier of the telescope and solicit help and advice from there.

  7. How do I repair/replace mechanical items?
    Contact the manufacturer/supplier of the item and solicit help and advice from there. If you are unable to get help from there, Barrie Watts of Beacon Hill Telescopes is sometimes able to make parts to order. Small parts can sometimes be made to order by SRB – The Adaptor Factory (01582-572535).
  8. How do I repair electrical/electronic components?
    Contact the manufacturer/supplier of the device and solicit help and advice from there.

G. Astronomical Equipment – Mounts

  1. What is an altazimuth mount?
    An altazimuth mount has two axes of rotation. The altitude axis is horizontal, the azimuth axis is vertical.
  2. What is a Dobsonian mount?
    A dobsonian mount is a simple altazimuth mount that is usually made of wood, has bearings of Teflon® running on Formica® (or similar substances), and is held together by gravity. It is usually used for mounting Newtonian reflectors, and offers the most cost-effective way to own a telescope of reasonable aperture. It is named after the legendary John Dobson of the San Francisco Sidewalk Astronomers /, who popularised it.

  3. What is an equatorial mount?
    An equatorial mount has two axes of rotation. The polar (or right ascension) axis is parallel to the Earth's axis of rotation. The declination axis is orthogonal (at right angles) to the polar axis.

  4. What is a fork mount?
    The fork mount is an altazimuth or equatorial mount in which the telescope tube is held between the "prongs" of a two-pronged fork. The axis about which the telescope rotates is the altitude or declination axis. The fork itself rotates about the azimuth or polar axis. It is mostly used in its altazimuth form for refractors and in its equatorial form for Cassegrain-type catadioptrics. The Dobsonian mount can be considered to be a variation of an altazimuth fork. Fork mounts are usually the most compact way of mounting a telescope, but the base of the fork can limit the position of the observer's head.

  5. What is a German equatorial mount?
    The German (so-called because it was designed by the Bavarian astronomer, Josef von Fraunhofer) equatorial mount carries the telescope tube at one end of the declination axis. Its weight is balanced by counterweights on a shaft on the other end of the declination axis. It is a very versatile mount, but the telescope has to be "flipped" if it crosses the meridian.

  6. What are setting circles?
    Setting circles are graduated scales on the right ascension and declination axes of the telescope. They are an aid to locating objects in the sky.

  7. What are digital setting circles?
    Digital setting circles employ optical encoders on the shafts of a mount. These encoders are read either by a dedicated computer or, via an interface, by a desktop or laptop computer. They serve the same purpose as ordinary analogue setting circles, but can be configured to compensate for poor polar alignment (or for no polar alignment at all) and usually come with catalogues of objects in the computer so that these objects may be easily located.
  8. How do I add drives to my mount?
    Contact the supplier or manufacturer to see if there are dedicated drives available. If not, you may be able to obtain them from JMI in the USA.
  9. What is GOTO?
    GOTO is a computerised system for driving a telescope mount. A target object is selected, usually on a computerised handpad, and the computer slews the telescope to the object.
  10. Can I computerise my telescope?
    If the telescope mount can be driven, it can be computerised. The ease with which this can be achieved depends upon the mount.
  11. Can I use a planetarium program with my telescope?
    It depends upon the mount and the planetarium program, but the better planetarium programs will work with the more common mount and drive systems. Contact the publisher of the planetarium program to ascertain whether it can be used with your mount and drive system.

H. Astronomical Equipment – Eyepieces

  1. Why are there so many different eyepieces?
    There are eyepieces to suit almost every purpose and budget.
  2. What do the letters and numbers mean?
    The number on an eyepiece is its focal length in millimetres.
    The most common letters and their meanings are:
    Er – Erfle. An extremely good relatively wide-angle eyepiece with a flat field. Also used in good quality wide-angle binoculars. Use at f/6 and greater.
    H – Huyghenian. Most commonly provided with budget telescopes. Use at f/10 and greater.
    K – Kellner. A 3-element eyepiece usually provided with budget telescopes. It is often used in binoculars. Use at f/7 and greater.
    MA – Modified Achromat. A variation on the Kellner.
    Or – Orthoscopic. An excellent eyepiece, with a relatively small field of view by today's standards. Use at f/6 and greater.
    Pl – Plössl. A good mid-range eyepiece. Currently probably the most common eyepiece. Use at f/5 and greater.
  3. Which eyepiece is best for me?
    The eyepiece is an essential part of the optical system, so get the best that you can afford. In order, acquire a medium, low and high power eyepiece:
  4. Are zoom eyepieces any good?
    Modern astronomical zoom eyepieces are of good optical quality and can replace two or more eyepieces from a set. However, they tend to have narrow fields of view at the low magnification end.
  5. What is a Barlow lens?
    A barlow is a negative lens that increases the effective focal ratio of the telescope.

I. Astronomical Equipment – Accessories

  1. What are LPR filters?
    For more information on applicability of LPR filters, see Filter Performance Comparisons for Some Common Nebulae.
    For a graph of filter transmission characteristics, see
  2. What colour filters are useful?
    There is a list of colour filters and their uses at
  3. What is a "minus violet" filter?
    A minus violet filter removes violet from the light, thus reducing the effects of chromatic aberration in achromatic refractors.
  4. What is a star diagonal?
    A star diagonal is a prism or mirror that is placed between the focuser and the eyepiece of a refractor or Cassegrain-type telescope, and which reflects the light through 90 degrees. Without a star diagonal, observing near the zenith is extremely uncomfortable unless the observer is reclining.
  5. What is a finder?
    A finder is a small telescope with low magnification (typically x8) and wide field (typically 7º) that is piggy-backed on the main telescope tube, with which it is aligned. It usually has a cross-web or other reticle in the eyepiece, thus facilitating the centring of an object in the field of view, and thereby aligning the main telescope to that object.
  6. What is a unit-power finder?
    A unit-power finder is a no-magnification reflex device that projects the image of a red dot or concentric circles of known angular diameter onto the night sky and which can be used as a substitute for, or in addition to, a normal finder. It is valuable as a finder if the main telescope has a wide-ish field of view. The concentric circles are an aid in star-hopping.
  7. Why do I need a red light?
    Dim red light has the least effect on night vision and is therefore preferred by astronomers.

J. Astronomical Equipment – Telescope Making

  1. Can I make my own telescope?
    Yes. It is very common for amateurs to make their own telescopes.
  2. Will it be cheaper than buying one?
    This depends to what extent you improvise and on how creative you are. You should not necessarily expect that a first self-made telescope (usually 6" or 8" aperture reflector, if you make your own optics) will be significantly cheaper than a commercially made one; the benefits of a first telescope are usually the sheer pride and satisfaction of having made one! However, the skills that you will learn can be applied to more ambitious projects, where there are definite savings to be made.
  3. Where can I get more information?
    There is a page that points newcomers to telescope making to the required informational and material resources at:

K. Astrophotography – Conventional Film

  1. What is Astrophotography?
    Astrophotography is the photographing of astronomical objects. For anything other than photographing bright solar-system objects, long exposures are required.
  2. What sort of camera do I need?
    The minimum specification for long-exposure astrophotography is: Additional desirable specifications include:
  3. Where can I get a camera for astrophotography?
    New cameras may be acquired from some suppliers of astronomical equipment.

    Some of the favourite cameras for astrophotography are no longer made and are therefore only available used. Used cameras may be acquired from:

  4. What common film is suitable for astrophotography?
    The specifications of film emulsions regularly change. A recent (November 2002) article in Astronomy Now recommended the following:
  5. How can I prevent star-trailing with a fixed camera?
    Trailing with a fixed camera can be prevented by ending the exposure before the trailing exceeds the resolution of the film. The formula for calculating this is T = 1000/(F × cos d), where T is the maximum exposure time in seconds, F is the focal length of the lens in millimetres and d is the declination of the centre of the star-field being photographed. For a 50mm lens photographing stars on the celestial equator, this corresponds to a maximum exposure of 20 seconds.
  6. How can I take long exposures without getting star-trailing?
    This can be achieved in several ways:
  7. What is piggy-back photography?
    Piggyback photography is the mounting of a camera onto a telescope or onto a mount that has a telescope attached. The telescope may be used for guiding, but is not part of the photographic optical system. The term is also loosely applied to a camera that is mounted alone on an equatorial mount.
  8. What is prime-focus photography?
    Strictly speaking it is photography with the film plane directly at the focus of the primary mirror or objective lens of the telescope. The telescope then replaces the camera lens. The term is also used for photography with the camera at the focuser of a Newtonian (strictly Newtonian focus) or Cassegrain (strictly Cassegrainian focus) type telescope. The camera is directly attached to the telescope, usually with a suitable adaptor.
  9. What is afocal photography?
    In afocal photography, the camera (with lens) replaces the eye at the eyepiece of a telescope. The camera is best supported by a tripod. It potentially gives a high magnification and is suitable for bright objects such as the Moon.
  10. What is eyepiece-projection photography?
    In eyepiece-projection photography, the telescope eyepiece projects the image onto the film plane. The camera is directly attached to the telescope, usually with a suitable adaptor. It gives a higher magnification than prime-focus photography.
  11. What other accessories do I need?
    For prime-focus and eyepiece-projection photography you need a camera adaptor that mates the camera to the telescope tube or focuser, as appropriate. If you cannot get what you require from your usual supplier, it is likely that SRB will either have it or be able to make it for you. For long exposure photography, some form of guiding is appropriate.
  12. What is a guidescope?
    A guidescope is a smaller-aperture telescope attached to the main telescope or camera. It is fitted with an eyepiece with a reticle. Its focal length should be at least that of the photographic optical system.
  13. What is an off-axis guider?
    An off-axis guider attaches between the telescope tube and the camera. A small prism "picks off" a small amount of the light and directs it to a reticle eyepiece. The eyepiece image is generally less bright than that in a guidescope, but errors due to flexure between the main scope and the guidescope and due to mirror-shift in the main telescope are eliminated.
  14. What is a cold camera?
    A phenomenon called reciprocity failure that reduces the efficiency of the photographic emulsion is reduced or eliminated at low temperatures. A cold camera uses dry ice (solid carbon dioxide) to chill the film. They are technically very complicated to use.
  15. What is hypering?
    Hypering is an abbreviation for hypersensitising. It is a process by which the film is bathed in a gas containing hydrogen (usually "forming gas", a mixture of hydrogen and nitrogen). This nearly eliminates reciprocity failure.
  16. What is field rotation?
    Picture a constellation or asterism rising in the northeast, passing near the zenith, then setting in the northwest. When it sets, it will appear "upside down" as compared to when it rose – it has rotated. This rotation quickly becomes apparent in any long-exposure astrophoto taken with a camera on a mount that is not properly polar-aligned. Remedies include proper polar alignment or a computerised "field derotator" that rotates the camera appropriately.
  17. Where can I get more information?
    The acknowledged "bible" for amateur astrophotography is Michael Covington's excellent Astrophotography for the Amateur.

L. Astrophotography – Digital Cameras (DigiCams)

  1. How is a Digicam different from an ordinary camera?
    A digital camera ('DigiCam') uses a CCD chip in place of film as the image sensor. It is 'digital' because all the image information is recorded as digital data and stored electronically. The more expensive examples have larger CCD chips offering better resolution. The size of the chip is quoted in Mega-Pixels (millions of pixels), the largest commonly available is approx. 6MPixel.
  2. Is a Digicam better than a webcam for astro imaging?
    A webcam uses similar image sensor technology to the digital camera, though with much lower resolution. The webcam does not possess any built-in recording capability, nor sophisticated controls of exposure time or colour balance. The lens is usually very simple, but a big advantage is that it can be removed for Prime Focus work. Most digicam lenses cannot be removed.
  3. What features do Digicams offer?
    To appeal to the general public the vast majority of consumer digicams look very like compact 35mm film cameras. Only a few high-end digicams replicate some of the functions of the traditional SLR (e.g. interchangeable lenses). Most digicams have a retractable optical zoom lens of short range with a built-in lens cover. All digicams offer a 'digital zoom' which simply magnifies the central portion of the image. Virtually all have a small TFT liquid crystal display which acts as a finder and also shows the camera settings, and there is usually a small optical finder too. Image data is stored on a removable memory card of varying capacity, and there are several different formats in common use. All digicams are designed to allow the transfer of image data to a personal computer from where the final image can be printed.
  4. What are their limitations for Astronomical use?
    Digicams were not designed with astronomical imaging in mind! However, amateur astronomers can take advantage of certain special features they offer. Exposures are limited to a maximum of few tens of seconds, and often less than 5 seconds, which means that faint objects cannot be recorded. With long (i.e. greater than a few seconds) exposures the major limitation is digital 'noise' in the image. Some of this noise (but not all) can be removed during image processing. CCD cameras specifically designed for Astronomy are electronically cooled to reduce this noise. Additionally, apart from a very few models, digicams have non-removable lenses which means they cannot be used at the Prime Focus of a telescope. Eyepiece-projection (Afocal Projection) is therefore the only option for recording an image through an astronomical telescope.
  5. What features should I look for when purchasing a digital camera?
    Ideally, the camera which generates the lowest noise in the image during long exposures is the best option. The ease with which the digicam can be mounted (attached to a telescope) is another feature to look for; adapters can be obtained from SRB. Cameras with retracting zoom lenses are difficult to attach solidly to an eyepiece and are limited to some sort of bracket suspending it in close proximity to the eyepiece. A great benefit is the option to remove the camera lens entirely, this not only enables a firm connection to be made but also enables it to be used at the telescope's prime focus (thus allowing wide-field imaging at lower magnification). It also avoids vignetting which can be a problem with afocal projection. Digicams possessing a threaded lens cell for accepting filters can more easily be connected securely to a telescope.
  6. Why is the maximum exposure time important?
    The brightest astronomical targets e.g. the moon, major planets, (and the sun using a suitable filter) easily fall within the normal exposure range of a digicam. The equivalent ISO rating (speed) of a digicam may be adjustable up to 800 or even 1600ASA if the camera offers control over its' electronic gain. For astronomy, most deep-sky targets are so faint that they cannot be recorded within the maximum exposure of a digicam. However, new and ever more sensitive models are being produced, and as the technology advances then so more targets come within range. Recent top-of-the-range models (e.g. Canon D60) extend the maximum exposure time to a minute or more whilst still retaining low image noise, this extends their target list to the brighter deep-sky objects (e.g. Nebulae such as M42, bright globular clusters).
  7. Why is cooling important for best image quality?
    The amount of noise in the image is directly related to the temperature of the CCD sensor, which is why astro CCD cameras are cooled. A digicam can be artificially cooled (with a fan) which offers a moderate improvement. The electronics in a digicam generate considerable heat when it's turned on, so a further improvement will be seen if it is switched off and allowed to cool for a minute or so between exposures.
  8. Is the type of lens important for a digicam?
    Because the only option with most models is to use afocal projection through an eyepiece it is worth investing in a digicam with a good-quality lens. Cheaper Digicams have poor lenses which limit the achievable resolution.
  9. Should I use the Digital Zoom on my Digicam?
    Digital zoom is a feature of little use to ordinary photography because all it does is expand the central portion of the image – which is easily be accomplished using a computer anyway. However, it is of some use for astronomical imaging because it makes focusing a little easier. Use the fully zoomed-in view to focus the object, then zoom out again to properly frame the object.
  10. Is the type of memory chip my Digicam uses important?
    The type of storage medium or memory chip (e.g. Compact Flash, SmartMedia) is unimportant, but it is very important that images be saved in 'uncompressed' or 'raw' format. This takes up more memory, and this is why most cameras have the option to reduce file size by using JPG compression within the camera's firmware. However, the effect of using compression is to introduce artefacts which severely limit subsequent image processing. Because raw images are preferred you need fairly large memory capacity in the camera.
  11. How do I get the images into my PC to process them?
    The most common PC interface protocol at time of writing is USB, older cameras used serial (RS232) communications which is relatively slow. For serial interface cameras it's usually possible to buy a device which enables the image data to be read directly from the digicam's memory chip into the computer greatly speeding up the process.
  12. Should I buy the digicam offering the highest resolution?
    The latest multi-megapixel digicams with the largest chips compare well (but are still inferior) to 35mm film in terms of raw pixel density (which equates to image resolution). The difference however is of little concern for the prime targets for which the digicam can be used. Even a digicam with a 1 Mpixel chip – which is quite small these days - can produce very detailed images of, for example, the moon and planets. There are two major types of CCD chip used in digicams, standard silicon sensors and CMOS sensors. Once thought to be inferior (lower sensitivity, higher noise), CMOS is the type of chip used in the new Canon D60. Great advances in the implementation of CMOS technology and on- camera image processing have resulted in less expensive and higher resolution chips.
  13. Will my Digicam produce 'True-Colour' images?
    Digicams offer the great advantage of one-shot colour imaging which works well for photographing brighter objects (Jupiter, Saturn etc.). It begins to fail with longer exposures where colour balance changes with increasing length of exposure. Post-processing can restore the colour balance to some extent though much of the correction is guesswork. It is better to use tricolour imaging for long exposures (that is, taking 3 separate monochrome exposures through red, green and blue filters). For this method to be successful the colour sensitivity of the chip has to be calibrated, a process which defines the exposure times for each colour filter (the colour 'ratio'). Whilst most digicams do offer a 'black and white' mode in fact all that happens is that the chrominance data (colour) is discarded from a colour image. Digicams have tricolour filters incorporated into the CCD chips themselves, there is no way of taking a true monochrome image.
  14. What is the best way of supporting my Digicam for astro imaging?
    There are three ways you can make use of it: You can use a fixed tripod to record nice images of the constellations, and also events like Aurora displays and sunsets. At night you can set the exposure to it's maximum length as star trailing will probably not be a problem unless it's longer than about 30 seconds. Secondly, it is possible to 'Piggy-back' your digicam on a tracking platform or driven equatorial telescope mount. However, most digicams do not offer a sufficiently long exposure to make piggy-back mounting worthwhile. A few do though – those digicams offering a minute exposure or longer would benefit from being able to track the movement of the stars. Thirdly, most amateur astronomers want to photograph what they can see through a telescope, hopefully in greater detail and in colour. While an acceptable image can sometimes be obtained holding the camera to the eyepiece, best results are obtained with the camera firmly attached to the telescope. Two methods are in common use: a special bracket attached to the Digicam's tripod socket holding the camera close to the eyepiece, or (better) a threaded adapter connecting the camera's lens directly to the eyepiece. For afocal projection it's best to use a low-power, wide-field eyepiece (about 35mm), and then adjust the optical zoom on the camera to avoid vignetting. Set the camera's focus to infinity and fine adjust using the telescope's focuser. Although reasonable results can sometimes be obtained using the digicam's autofocus in my experience this method is not reliable for objects other than the moon or other high-contrast object.
  15. What is the best way to power my Digicam for astro imaging?
    The digicam will come with a battery pack as standard but this will probably not last long enough for an imaging session using a telescope. Quite often a long time is spent fine tuning the focus, which rapidly drains the battery. Further, cold conditions significantly reduce battery capacity. A better option is to use an AC supply or a larger lead-acid battery.
  16. Can I control my Digicam from my computer?
    If the digicam can be firmly mounted to a telescope then remote control software (available for newer Nikon, Olympus, Canon and some other cameras) allows virtually all functions of the camera to be operated remotely using a computer. Older cameras may be restricted to viewing the image and taking an exposure. If full remote control is available the great advantage is that operating the often tiny manual controls is avoided (very frustrating on a cold night!).
  17. What is the best way of focusing my Digicam for use with a telescope?
    Accurate focusing is the most critical stage of astronomical imaging and it's importance cannot be over-stressed. It is difficult to judge correct focus using the small built-in display so either use an external monitor (a small TV or PC display helps) or another focusing aid such as a Hartmann or hole-mask. Use maximum digital zoom whilst focusing so that errors are more easily seen, then back off the zoom to frame the object normally.
  18. How do I avoid vibrations spoiling the exposure?
    Use the digicam's delayed shutter feature to allow vibrations to stop before the image is taken. Take several exposures of each object (at least 3, and more if the 'seeing' is less than excellent).
  19. I took some images, how can I improve the result?
    Almost all images will benefit from some degree of post-processing using a computer program designed for the job. This involves reducing spontaneous noise by subtracting 'dark' (or blank) frames, and averaging multiple frames. Most image processing programs, and all of the astronomy oriented ones, allow multiple images of the same object to be 'stacked' or averaged. After sorting out and discarding obviously poor quality examples the remaining images can be compiled in this way to provide an image possessing lower background noise with more detail. After that, further enhancement can be achieved by use of sharpening filters and also correcting the colour balance. The best tool for the job is a specific astro image processing package (e.g. MaximDL or AstroArt), but if you have a good working knowledge of Paintshop Pro or Adobe Photoshop most jobs can be done using these. Astro image processing programs also offer special procedures called deconvolution to recover blurred detail from images slightly out of focus or trailed. These programs are also specifically designed to best deal with low contrast and poor signal-to-noise ratios typical of astronomical images.
  20. Should I buy an expensive Digicam now or wait a while?
    Digicams are currently on the threshold of being a viable tool for the budding astro-imager. It is unlikely however, that for long exposure work it will be able to replace the functionality of a dedicated astro CCD camera any time soon. Nevertheless, amazing results have been achieved by amateur astronomers using both digital cameras and also webcams, extending their range to capturing stunning images of the brighter deep-sky objects. These results have been achieved through both improved CCD technology in the cameras, DIY techniques for cooling the chip and electronics, and development of image processing techniques to improve the final image. Progress is a continuous process so digicams definitely have a promising future.
  21. Where can I find more information on using a Digicam for astro imaging?
    [URLs of Various websites devoted to using digital cameras in astronomy] Digital camera astro imaging group (Yahoo).

M. Astrophotography – CCD Cameras

This section is currently being written. Until it is available, visit:

N. Astrophotography – Other Electronic Cameras

Until this section is written (offers gratefully received!), you are invited to visit the QuickCam and Unconventional Imaging Astronomy Group. If you require an adapter to mate the camera to teh telescope, try SRB.

O. Visual Observation

  1. Where can I find out what is happening in the sky?
  2. How do I record observations?
    Notes and drawings can be made with soft pencil and paper; mini-tape recorders can be used for verbal notes. In both cases, they can be written up in neat afterwards. Essential information includes
  3. How can I see fainter objects?
    The following have all been known to work:
  4. Why can't I see Saturn's rings?
    Reasons have included:
  5. How do I know if what I am seeing are Jupiter's moons?
  6. How can I estimate the magnitude of an object?
    Compare it to a 'standard star' of known magnitude. The magnitude scale is calibrated by arbitrarily assigning a magnitude to a star – the magnitudes of other stars follow by comparison to this standard star. In practice, there are many standard stars, so distributed that standards of different spectral types are likely to be reasonably close to any star being observed. One should choose a star of similar spectral type when estimating a magnitude of another star – the eye and photometers are differently sensitive to different wavelengths. (See also Q2)
  7. What is limiting magnitude?
    This can have different meanings according to the context.

P. Light Pollution

  1. What is light pollution?
    Light pollution is artificial light that shines where it is not required or where it is detrimental in any way to the environment. For astronomers the main problems are skyglow (brightening of the sky by light reflecting off water droplets and particulate matter) and light trespass.
  2. What is light trespass?
    Light trespass is artificial light that shines on your property from sources that you do not control outside your property. This may be from streetlights, a neighbour's poorly directed security light, light from a local recreational facility, or any other source.
  3. How can I get light pollution and light trespass reduced?
    You can get up-to-date information about what you can do from The Campaign for Dark Skies
  4. How can I observe in light-polluted areas?
    Aids to observing in light-polluted areas include:
  5. Which areas of Britain have the least light- pollution?
    For a guide to how your county compares to other areas, visit

Q.Famous UK Astronomers

  1. Who are the UK astronomers of note?
    Those marked with an asterisk (*) have held the post of Astronomer Royal
    Adams, John Couch - Born 5th June 1819, died 21st January 1892
    Airy, George Biddell* - Born 27th July 1801, died 2nd January 1892
    Bell-Burnell, Jocelyn - Born 1943
    Bliss, Nathaniel* - Born 28th November 1700, died 2nd September 1764
    Bradley, James* - Born 1693, died 1762
    Burbidge, Eleanor Margaret Peachey - Born 12th August 1919
    Burbridge, Geoffrey Ronald - Born 24th September 1925
    Christie, William H.M.* - Born 1845, died 1922
    Dawes, William Rutter - Born 1799, died 1868
    Dyson, Frank W.* - Born 8th January 1868, died 25th May 1939
    Eddington, Arthur Stanley - Born 28th December 1882, died 22nd November 1944
    Fallows, Fearon - Born 4th July 1789, died 25th July 1831
    Flamsteed, John* - Born 19th August 1646, died 31st December 1719
    Graham- Smith, Francis* - Born 1923
    Halley, Edmond* - Born 1656, died 1742
    Hawking, Stephen William - Born 8th January 1942
    Henderson, Thomas - Born 28th December 1798, died 23rd November 1844
    Herschel, Caroline - Born 16th March 1750, died 9th January 1848
    Herschel, John - Born 7th March 1792, died 11th May 1871
    Herschel, (Frederick) William - Born 1738, died 1822 Hewish, Antony - Born 11th May 1924
    Hoyle, Fred - Born 24th June 1915, died 20th August 2001
    Huggins, William - Born 7th February 1824, died 12th May 1910
    Jeans, James Hopwood - Born 11th September 1877, died 16th September 1946
    Jones, Harold Spencer* - Born 29th March 1890, died 3rd November 1960
    Lassell, William - Born 18th June 1799, died 1880
    Lockyer, (Joseph) Norman - Born 17th May 1836, died 16th August 1920
    Lovell, Bernard - Born 1913
    Maskelyne, Nevil* - Born 5th October 1732, died 9th February 1811
    Moore, Patrick - Born 4th March 1923
    Newton, Isaac - Born 1642, died 1727
    Payne-Gaposchkin, Cecilia - Born 1900, died 1979
    Penrose, Roger - Born 8th August 1931
    Pogson, Norman Robert - Born 1829, died 1891
    Pond, John* - Born 1767, died 1836
    Rees, Martin John* - Born 23rd June 1942 Ryle, Martin* - Born 1918, died 1984
    Thom, Alexander - Born 1894, died 1985
    Wolfendale, Arnold Whittaker* - Born 1927
    Woolley, Richard* - 24th April 1906, died 24th December 1986
  2. Who are the Astronomers Royal?
    Flamsteed, John 1675-1719
    Halley, Edmond 1720-1742
    Bradley, James 1742-1762
    Bliss, Nathaniel 1762-1764
    Maskelyne, Nevil 1765-1811
    Pond, John 1811-1835
    Airy, George Biddell 1835-1881
    Christie, William H.M. 1881-1910
    Dyson, Frank W. 1910-1933
    Jones, Harold Spencer 1933-1955
    Woolley, Richard 1956-1971
    Ryle, Martin 1972-1982
    Graham-Smith, Francis 1982-1990
    Wolfendale, Arnold Whittaker 1991-1995
    Rees, Martin John 1995-

W. Miscellaneous

  1. Why is the image inverted/reversed?
    This is a function of the optics. Assuming there are no "image erector" lenses or similar in the optical system of an astronomical telescope, the orientation of the image will depend upon the number of reflections in the system. It is irrelevant to astronomy – a star has no "right way up".
  2. What is magnitude?
    The magnitude is the brightness of an astronomical object. As observers, we are primarily in the apparent magnitude, which is the magnitude we perceive. There is also absolute magnitude, which is the magnitude that a star would appear at a distance of 10 parsecs (except for asteroids it is at 1 AU with zero phase angle).

    The magnitude scale that we use was derived from Hipparchus, who informally classified stars into different magnitudes. The first to appear at dusk were "1st magnitude" and the last to appear were "6th magnitude". The scale was formalised by Pogson, who formalised it into a logarithmic scale in which a 1st magnitude object is 100 times as bright as a 6th magnitude object.

    The magnitudes assigned to standard stars are such that they are consistent with each other, and the calibration is based on a star of magnitude 1.0 having an energy of 9.87 × 10-9 W m-2 at the top of Earth's atmosphere.

    There are other types of magnitude relating to specific parts of the electromagnetic spectrum.

    Here are the visual magnitudes of some common objects:
    Magnitude Object(s)
    -26.8 The Sun
    -12.6 Full Moon
    -4.7 Venus (at its brightest)
    -2.9 Jupiter (at its brightest)
    -2.8 Mars (at its brightest)
    -1.42 Sirius (brightest naked eye star)
    0 The approximate magnitude of Alpha Centauri, Arcturus, Vega, Rigel
    +0.7 Saturn (at opposition)
    +1 The approximate magnitude of Antares, Spica, Pollux
    +2 The approximate magnitude of Polaris and the stars in Orion's Belt and the Big Dipper
    +4 Approximately the faintest stars visible in brightly-lit urban skies
    +5.5 Uranus
    +6.5 Approximately the faintest stars visible in dark, transparent skies
    +8 Neptune
    +14 Pluto
    [Source: Tonkin, AstroFAQs]
  3. What is gravity?
    In general terms, gravity is a force of mutual attraction amongst all objects. Its importance in astronomy includes the understanding of: See John Stockton's Gravity FAQ for more information.
  4. What is relativity?
    This is best answered by Ned Wright's Relativity Tutorial.
  5. Why do astronomers believe that the Universe is expanding?
    Distant galaxies have redshifted spectra, indicating that they are receding. The greater the distance the greater the redshift and apparent rate of recession. This is consistent with an expanding universe.
  6. What is the Big Bang Theory?
    If the Universe is expanding, common sense suggests that it was smaller in the past. Taken to its extreme, this suggests that the universe was once a point; prior to that it did not exist. The theory in which the universe was suddenly created from a "singularity" is the Big Bang Theory, which is currently the dominant theory of cosmology. The name was coined in derision by the late Sir Fred Hoyle, who supported a different theory of the universe.
  7. What happened before the Big Bang?
    If the theory is correct, time came into existence with the Big Bang, so the conditions that gave rise to the Big Bang are probably unknowable.
  8. Will Earth be hit by a "doomsday asteroid"?
    Yes. It is not a matter of "if", but of "when". It may be millions of years in the future, but we currently have no way of knowing as we only know the orbits of a small fraction of the NEOs (Near Earth Objects).
  9. Can I do anything about "doomsday asteroids"?
    Jay Tate, Director of Spaceguard UK, writes:

    There are a number of very efficient NEO detection programmes in the USA that are out performing amateur searchers by a significant margin, though with larger telescopes (12" to 16" bracket) and high end CCDs discovery is still possible. The most valuable contribution that the amateur can make is performing follow-up observations of NEOs discovered by the professional programmes. There is an excellent guide to Minor Planet Astrometry available at
    Another valuable contribution that amateurs can give is to pursue those who control the purse strings, in other words, write to your locally elected officials. The government began to take the NEO hazard seriously in 2000, but, since then, there has been an almost total lack of activity.

    More details are available at: .
  10. Why does the Moon appear larger on the horizon?
    This is the "Moon Illusion". It is an optical illusion, related to the Ponzo illusion, resulting from a combination of the phenomenon of size constancy and our perception that the celestial sphere is flattened (i.e. that the zenith is nearer than the horizon). See the Moon Stereogram Demonstration.
  11. How can I combat dew formation on my telescope?
    Dew forms when the temperature of a surface falls below "dew point". Telescopes cool by radiative cooling, so dew can be counteracted by reducing radiative cooling of optical surfaces and by reducing the amount of warm moist air (e.g. breath) that comes into contact with them.
  12. How can I keep warm when observing on cold nights?
  13. Where can I get current weather information?
  14. How are eclipses caused?
    An object is eclipsed when it passes into the shadow of another. Therefore an eclipse of the Moon occurs when the Moon passes into Earth's shadow, i.e. when the Moon and Sun are on diametrically opposite sides of Earth. An eclipse of the Sun is, strictly speaking, an occultation of the Sun, i.e. the Sun's disc is occulted (hidden) by the Moon for an observer on Earth.
  15. What are the causes and effects of tides?
    Sea-tides are caused largely by the gradient, at the Earth, of the Moon's gravitational field; the Sun is the only other body which makes a significant contribution. Tides tend to cause the orbits of planets and moons to circularise and synchronise. For more information see:
  16. How is time related to astronomy?
  17. How is the calendar related to astronomy?
  18. What units are used in astronomy?
    Astronomy is an international undertaking and therefore uses Systeme International (SI) units. In addition, the following units are also used:
  19. What co-ordinate systems are used in astronomy?
    The most commonly used are horizontal co-ordinates (altitude and azimuth – referenced to the plane of the horizon and due north) and equatorial co-ordinates (right ascension and declination – referenced to the plane of the equator and the vernal equinox). For more information on these and the conversion between them, see: Also encountered are ecliptic co-ordinates (ecliptic latitude and ecliptic longitude – referenced to the plane of the ecliptic and the vernal equinox) and galactic co-ordinates (galactic latitude and galactic longitude – referenced to the galactic plane and the galactic centre).
  20. Where can I find satellite predictions?
    The most popular (to the extent that it is sometimes inaccessible owing to business) seems to be: Heavens-Above; have your latitude and longitude to hand. Other useful sites include:
  21. Where can I get information about solar system objects?
  22. What do all the strange new words mean?
    There is a hyperlinked Glossary of Astronomical Terms at

X. Other Useful Internet Resources

These are useful links that don't quite fit anyhere else. They are in alphabetical order.

Y. Contributors

  1. Who maintains this FAQ?
    This FAQ is maintained, on behalf of the subscribers to uk.sci.astronomy, by , to whom corrections, updates and suggestions should be emailed (i.e. don't assume that he will automatically see them on uk.sci.astronomy).

    The text version of the FAQ is maintained by David Clapham. Copyright remains as assigned in the HTML original, which should be regarded as definitive if any errors, omissions or contradictions between the two documents are noted.
  2. Who contributed to this FAQ?
    David Clapham (text version; proof-reading and HTML checking of original; numerous suggestions and additions)
    Chris Heapy (K)
    Pete Lawrence (D3, D6)
    Graeme Skinner (A8, Q)
    John Stockton (W15, W20, W21, numerous links, proof-reading and HTML checking)
    Jonathan Tate (W9)
    Stephen Tonkin
  3. Where did this FAQ originate?
    The original FAQ for uk.sci.astronomy was initiated and written by Dave G Smith. We, the current authors of the FAQ, acknowledge, with thanks, Dave's initiative.

Z. Requests for Other Sections

There have been requests for sections or subsections, or expansions on the following topics

If you are able to assist by writing one or more of these, please email the maintainer of this FAQ with your offer.

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