Derby and District Astronomical Society
A Review of the Celestron C6-N Newtonian Reflector
by Chris Newsome
I have been keen on astronomy ever since I was at school (many years ago!!). Back then, I had always dreamed of owning a 6” Newtonian reflector but my finances at the time would only stretch to a cheap 2” refractor telescope purchased from Dixons, with no finder and mounted on an altazimuth mount that allowed movement in declination of about 30-45deg (my grandfather was a wizard in the workshop and he built me an adaptor to put the telescope on a camera tripod which would allow me to point the telescope at any point of the sky). At the time, though, with three set magnifications of 15x, 30x and 45x I could see the rings of Saturn, the moons of Jupiter and also the craters on the surface of our nearest neighbour. That was back in the late 1970’s.
Now, after having been at work for some years, buying houses, cars, living etc, I finally found myself in a position, in December 2003, to buy what I had always wanted – my own 6” Newtonian. I didn’t want a modified one with an internal corrector lens, I didn’t want an SCT or anything complicated – just a good old Newtonian on a German Equatorial mount. But where to buy one? Scanning the internet, I found the UK distributor of Celestron (and knowing the name from reading Sky and Telescope in the 1970’s), I reckoned that they would be a safe bet for a quality telescope. So after a few clicks on the mouse, I ordered my C6-N from David Hinds Ltd. It arrived the following day. I eagerly opened the box and set it up according to the instructions. But would it live up to my expectations?
The C6-N is a 6” diameter, 750mm focal length Newtonian (f/5). It is a very simple design, the light being collected by a parabolic mirror and brought to focus just outside the side of the optical tube by means of a 45deg flat secondary mirror. The resulting image is then magnified by whatever eyepiece you want to use. The mount, a CG/4 (German Equatorial), is very well made, solid and heavy! The basic mount comes with hand wheels to rotate the telescope to manually track an object as it moves across the sky. Also included in the basic “package” is a 6x30mm finder and a 20mm eyepiece (providing a magnification of 37.5x). As soon as you have assembled the bits, you can take the telescope outside and, assuming it is a clear night (!), use the telescope. It is that easy.
The C6-N on it’s first night – note the empty boxes on the left!
When I purchased the telescope, I also bought a 7.5mm Plossl eyepiece (magnification 100x) and a 2x Barlow to allow a greater range of magnification. Is there a downside to this telescope and mount in it’s basic format? Yes, there is! It’s not mechanically driven so photography was not possible. This is easily rectified, though, with the spending of a little more money (what a surprise!). To improve the telescope, I purchased a motor drive and polar finder for the CG/4 mount, again from David Hinds Ltd. The addition of these two pieces of kit to the C6-N, changes the telescope from a very good one to an excellent one. The motor drive is incredible in it’s accuracy. It is a dual drive, allowing electronic movement in both RA and Dec (you can get a single drive one that just drives in RA). It is a silent drive, run from 4x SP2 batteries (user supplied). It has 4 speeds to drive, 2x, 4x and 8x sidereal speed, and an “N” and “S” facility to drive the RA motor in either direction depending on whether you are using the telescope in the Northern or Southern Hemisphere. Having a dual drive motor is beneficial, in my view, over a single drive version in as much that it allows slow adjustments in the alignment of the telescope in both directions to smoothly centre an object in the field of view.
The hand controller for the dual axis CG/4 drive.
The RA drive on the CG/4 mount.
The Dec drive on the CG/4 mount.
To make full use of the accuracy of the drives, accurate alignment is a must. The CG/4 mount is fitted with RA and Dec setting circles for manual movement. But these can only be used if the mount is aligned correctly with the Celestial Pole. If you know your latitude accurately then you can set this manually, and ensure the polar axis is pointing due north. However, a lot of the manual adjustments can be removed with the addition of a polar axis finder. This is a small finder telescope that sits within the polar axis void in the CG/4 mount. In the viewfinder, there are a set of cross hairs and a circle wire set at a predetermined distance from the centre. Situated on this circle wire is a smaller circle that is to be used as a target to get Polaris in. Assuming that, on set up, the polar finder is aligned correctly, then when you come to align the mount with the celestial pole all you need do is get Polaris in the small circle and you are very accurately aligned for tracking. Even if you don’t get Polaris in the target circle, as long as you get it close to the main circle in the finder eyepiece, tracking will be accurate enough for long exposure photography. Small adjustments to the alignment with Polaris can be made by two sets of bolts that move the mount to the East and West of due North and also allows minute adjustments in the altitude of the North Celestial Pole.
The eyepiece of the polar axis finder.
Once the telescope has been set up in your observing position for the night and aligned with the Celestial pole, you are ready for a night of fascinating sights. Moving between objects is quick and easy. Simply release the clutches on the CG/4, move the telescope to your chosen object, locate in the finder and lock the clutches. As soon as you lock the clutch on the axes, the telescope is driven again. The first observations with the kit were awesome and I was hooked again on sitting out in a back garden, looking like the proverbial brass monkey, and looking at the sky! The images were crisp, sharp and very clear and bright. There was no noticeable degradation of the image at the edge of the field, and overall the optics were of astounding quality. Simple but astounding. The quality of your eyepiece is very important and the Celestron eyepieces I purchased are very good. The 20mm has very good eye relief (useful for those who wear glasses) and with the addition of the 2x Barlow, the magnification is doubled yet still retaining the good eye relief. The 7.5mm is very good on magnification but the eye relief isn’t as good, hence I don’t use it that often. The eyepiece mount on the telescope is a standard 1.25” rack and pinion setup. The mount itself can be stripped down and re-setup with adapters to allow the fixing of a camera for prime focus photography.
Around the optical tube assembly (OTA) the holding brackets on the mount, have a fixing that allows you to attach a camera using a standard camera thread. This is ideal as it allows you to drive a camera for long exposure wide field photography. As I said, the whole setup is solid but heavy. Because of the design of a German Equatorial mount, the telescope needs to be balanced. The mount comes with a set of counterweights to achieve this and these are moveable along the counterbalance arm to achieve a setup. Indeed, when the telescope is balanced accurately, you could drive the telescope without the need for locking the clutches on the mount itself – but I wouldn’t recommend it!! With the addition of other pieces of equipment (e.g. a camera), the counterweights need to be moved to re-balance the setup. With practice, and if needs be markers can be notched onto the counterweight arm, this can be done in the dark. A word of warning, though! If you intend to attach a camera to the rack and pinion eyepiece mount be very careful as the weight of it could put too much strain on the mount/OTA fixing. Modern digital cameras are light and should not present as much a problem as my old Pentax K1000!
The Celestron C6-N is, in my opinion, an excellent telescope. It is ideal if you want to learn the basics of telescopes, astrophotgraphy, and starwatching. It doesn’t have modern computerised GoTo functions which means that you get to learn your way around the sky and how to star hop. The quality of the optics will not disappoint, what you can see will astound the learner astronomer. The operation of the telescope is very easy to master, it is reasonably portable and takes about 10mins to setup at an observing site. In total, I spent around £600 on the telescope and the extras (drive, eyepieces etc) – an excellent purchase and highly recommended.
The C6-N setup for observing with my Meade ETX-105 and laptop used for CCD imaging.
DDAS member Chris Newsome is a regular contributor of astrophotographs to this website. Many examples of his work may be found in our gallery.Related Links: