This page last modified 2001 October 30

Tonkin's C5++ Page

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OTA Specifications
First Impressions
Other C5+ Pages


This sobriquet was suggested by Pietro Carboni, as an indication of the amount of accessories I have acquired for this scope.


The Celestron C5 optical tube assembly -OTA has been in production in production since 1971. Since then, the telescope has come with various mounts and accessory options as standard. These include:

At the time of writing, only the Spotting Scope, G5 and NexStar are in production.

OTA Specifications

Aperture 125mm
Focal Length 1250mm*
Focal ratio f/10* at Cassegrain focus
Focusing Movable primary
Length 280mm
Weight 6lbs -2.7kg
Rear Cell 48mm male thread
Theoretical Resolution 0.9 arcsec

* The effective focal length and, therefore, the focal ratio depends upon the separation of the primary and secondary mirrors. In a Cassegrain telescope in which focusing is achieved by moving the primary mirror, stated focal lengths and ratios are thus nominal values whose precise values will depend upon the position of the focal plane. Used with a star diagonal, mine has an effective focal length of about 1290 mm -f/10.3.

The 48mm thread on the rear cell accepts a variety of accessories including a visual back -standard equipment, a T-threaded photographic back, an off-axis guider, and a f/6.3 focal reducer/coma corrector. The C5 OTA has a deserved reputation for optical and mechanical excellence. One of the pitfalls of focusing by moving the primary mirror in Schmidt-Cassegrains is that, unless the mechanism is very precisely machined, rocking of the mirror during focusing introduces image shift. This is not detectable in my C5; reports from other users suggest that this is the norm for this instrument. In addition, the focusing is exceptionally smooth, another quality which reports suggest is the norm. The optical is excellent. A star-test suggests that there is a tiny amount of undercorrection, but this is sufficiently small that it does not contravene the Rayleigh Criterion and prevent the telescope performing to its theoretical limit on a night of steady seeing.


My C5 is the C5+ incarnation, that I obtained second-hand. The OTA fixes by a dovetail joint to the tine of a single-arm fork mount. The fork is itself mounted on the drive base, and has a male ¼-20 threaded attachment for counterweights. The fork/drive assembly has manual slow motions in RA and Dec and an electric spur-gear drive in RA. The electric drive can be powered by a PP3 -9V alkaline battery that resides in a recess in the drive base, or by an external 12V supply. I have found the drive to be extremely good – when it is properly polar-aligned, the mount maintains sidereal rate with less than an arcminute of periodic error with no abrupt jumps. -I have never bothered to precisely quantify this, but it is in the region of 30 arcsec. It is also very quiet, unlike the drives of some other commercial Schmidt-Cassegrains – a definite advantage if one does not wish to disturb the neighbours on warm "open window" nights!

The drive base attaches to an equatorial wedge which itself suffices as a tabletop mount. It has an integral circular bubble level to assist in levelling. Like the fork mount, the wedge is of extremely sturdy metal construction.

The weakest link in the mount is the tripod. I have the Celestron "field tripod" that is dedicated to this telescope. It is adequate for visual work, but by itself it is inadequate for long exposure photography or CCD imaging. For these purposes I use it with the legs fully retracted and standing on vibration suppression pads. The accessory tray that attaches to the tripod braces adds more rigidity. Like this it is not ideal, but it is usable with care.


Reflex finder Replaces the original 6x30 finder and is sufficiently precise for me to be able to locate my target object in a low-power field.
Declination motor Essential for guided long-exposure photography and imaging.
Hand paddle This is required to operate the declination motor and RA drive rat. Useful for centring an object as well as for guiding.
Solar filter

A home-made filter, using Baader Planetarium Astro-Solar film.
Dew shield A dew shield is therefore essential for Schmidt and Maksutov type telescopes. This one packs flat, a quality that makes transport easy.

I made these from a length of 1" diameter mild steel rod and some collars that were designed to secure barbell weights to a bar. This combination is very versatile.
Piggyback attachments

These allow a 35mm camera or a CCD camera to be attached to the accessory bar, either directly or via a ball-and-socket joint.
Focal Reducer/Coma Corrector This dedicated attachment reduces the focal ratio to a nominal f/6.3 -actually f/6.9 when used with the flip mirror and CCD.

I made this compact one from a 300mm camera lens and a x2 tele-extender to which I cemented a star diagonal. This has an aperture of 56mm and its 600mm focal length gives a magnification of x67 with my 9mm guiding eyepiece. The Barlow lens doubles the effective focal length to 1200mm. I made the mounting rings from old metal drainpipe brackets
Off-Axis Guider It eliminates any flexure between the main telescope and a guidescope, and automatically compensates for any primary mirror shift. However, a very small amount of light is deflected – this quality makes it an interesting exercise to find a suitable guidestar!
Flip-Mirror Finder This is an essential accessory for CCD imaging.
Digital Setting Circles These are JMI MicroMAX computerised circles. They can compensate for the difficulty of polar-aligning this mount.
Power Supplies The telescope has a 9V alkaline battery that will provide sidereal tracking for a nominal 50 hours. Frequent replacement of alkaline batteries is an expensive exercise, so I have two alternatives. One is a simple home-made device which contains ten RX6 -AA NiCad cells in series, giving a nominal voltage of 12 volts. When I wish to power more than just the telescope, I use a Draper PP12V portable power pack, which contains a 12-volt 12Ah rechargeable lead-acid battery, sufficient for a full night's power needs.
Field Tripod

The tripod is made much more rigid if the optional accessory tray is fitted!
Vibration Suppression Pads Essential for photography or imaging at the Cassegrain focus.
Hartmann Mask This is a simple focusing aid, that consists of a full-aperture mask with two widely spaced holes in it. When the telescope is out of focus, a bright star will form two images. These will merge as focus is attained.
35mm SLR Camera Ancient and sturdy Canon FTb, with excellent mechanical shutter and mirror lock-up facility. With the Reducer/Corrector in place, a 35mm film frame -24mm x 36mm covers an area of sky of 1.5º x 2.4º.
CCD Camera

I have recently acquired a Starlight Express MX5 CCD camera. With the focal reducer in place, the focal length of the C5 is of the order of 860mm, giving just under 3mm per pixel and a field of 15 arcminutes x 19.5 arcminutes.
Infra Red Filter Because CCDs are sensitive to infra-red, an infra-red blocking filter is an essential accessory for CCD imaging.
Oxygen III Filter Among my favourite visual targets are planetary nebulae. The O-III filter enhances the visibility of these objects.

The proprietary cases available for the C5 seem to me to be over-priced. I have found that a ZAG Mobile Tool Chest is, without modification, an neat fit for the drive base and optical tube assembly, with accessories packing neatly into the tool tray. Unfortunately, with digital setting circles fitted, the cover for the declination encoder prevents the previous neat fit and modification – the removal of the central section of the accessory tray – is necessary. The integral wheels and pull-out handle enhance the transportability of the telescope.

First Impressions

This extract from my "first night" report sums it up:

Just after sunset I went back outside to a ready-cooled telescope, put a 32mm Plössl -x39 in the diagonal, and found Jupiter – the telescope snapped to focus and both equatorial belts were clearly visible. I tried the 6.3mm Plössl -x198 and again achieved a well-defined focus. The equatorial belts showed ragged edges, that of the southern edge of the SEB being particularly clear. Temperate belts -I haven't a clue which – I'm not a planetary observer were also visible. Diffraction rings showed around the Galilean Moons.Saturn was similarly impressive at x198. The Cassini division was immediately clearly visible, and I was surprised to see the planet's shadow on the E side of the ring – its width must be close to the limit of resolution of the telescope-*. Markings were visible on the planet's surface, with the polar regions being distinctly darker than the equatorial region.

The diffraction rings around the Jovian moons suggested that a star-test was in order. Aldebaran was conveniently placed and, at x198 showed a bright Airy disc surrounded by four diffraction rings, the inner of which was quite bright. Defocused images revealed some undercorrection. Focusing is extremely smooth and I could not detect any image-shift – I'll try it at another time with a reticle in a high power e/p.

At this point I was called indoors and, when I returned some 20 minutes later, was pleasantly surprised to find that Aldebaran had hardly shifted in the FOV, suggesting that the tracking was good and that my daylight polar aligning was -fortuitously? accurate.

After an evening meal, I returned to the telescope under darker but light-polluted skies. M42 was emerging from the branches of the neighbour's birch and at x39 showed some pleasing structure. The four stars of the Trapezium were clearly resolved. I tried an OIII filter and almost gasped at how much more structure I could see.

Chasing objects nearer the zenith, particularly between the zenith and the pole, proved to be an interesting task and reminded me why I like GEMs – the fork tine and base always seemed to be where I wanted to put my face. A red-dot finder would be a boon here, allowing the head to be further from the telescope than does the 6x30 finder.

At x39, M31 was about what I expected with a 3.2mm exit pupil, suggesting that the f/6.3 focal reducer might be an early purchase – it would give a 5mm exit pupil with the 32mm Plössl. The open clusters in Auriga were beautiful as ever and cruising through Perseus and Cassiopeia revealed several more small clusters which I enjoyed but didn't bother to identify.

By this time the Moon was rising over the neighbouring bungalows and the sky was brightening significantly. Some dew was beginning to form -must make a dew cap or this corrector plate is going to be almost permanently wet! and it seemed a good time to call a break.

* This is not unusual for linear features.


I acquired the MX5 CCD with the intention of doing some serious imaging. A combination of atrocious weather and an injury which prevents me from standing -e.g. at the guidescope for any length of time has conspired to put this intention on hold, but I have managed a few unguided shots. I make no pretence that these are of high quality, but they do indicate what is obtainable very simply, and give some indication as to the accuracy of the drive.

M13, The Great GlobularCluster in Hercules
2000 Jun 09; 30s unguided
M57, The Ring Nebula in Lyra
2000 Jun 09; 60s unguided
M92, The Other Hercules Globular that is ignored because of its proximity to its better-known neighbour.
2000 Jun 09; 60s unguided

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