Helios 150mm - with Corrector Lens
For many years I have been deep-sky observing using a conventional 9" reflecting telescope working at f6. This was Dobsonian mounted and used exclusively for visual observations. Using this telescope and a home made computer assisted finding system I could find and observe objects down to magnitude 12 with no trouble. I constructed an equatorial platform and drove it with a little plastic gearbox and a variable power supply borrowed from my son's model railway! Now I could watch my subject in peace and even make sketches at the telescope.
But, I have always been attracted by the possibility of doing some astrophotography and clearly this telescope was never going to be suitable as it stood. I was faced with the choice of buying a heavy duty mount to take the 9" scope or buying a new setup more friendly towards CCD imaging. I had a limited budget available, around £1200. The camera I decided on, and will review shortly, was the Starlight Express one-shot color MX5-C model, which cost £610. The 9" performed very well, but I always had found the tube slightly flexible and it was a very heavy scope. The case for a new scope was taking shape. I couldn't justify either a Celestron or Meade 8" SC. I reasoned that with a really compact fast Newtonian, which would be used almost exclusively for CCD work, providing I could get reasonable tracking accuracy, say unguided for a minute, then a 6" would give me decent images. After extensive web searching and phone calls to dealers I finally decided on a Helios Catadioptric Newtonian - Apollo 6".
The telescope, which I bought from Dark Star Telescopes in Wales, cost me £554, including RA and Dec axis drives. It was a 6" Newtonian with a large aperture correcting lens mounted in front of the secondary. This serves the dual purpose of flattening the field and maintaining a long focal length (1000mm) in a short (500mm) tube. I guessed it would probably introduce a fair amount of chromatic abberation as well. This was just within my budget, but would it do the job? It arrived the day after I ordered it in one large box! As I unpacked it and examined the various components I was impressed by the obvious quality of the machining and castings used. It felt comfortably solid. The telescope came fully equipped with an EQ3-2 mount, 20mm and 10mm Plossl eyepieces, a x2 short Barlow lens, 42mm threaded camera adaptor, 6x30 finder and a polarscope for polar alignment.
Assembling the mount was straightforward and the instructions were adequate. The mount felt and looked solid as it went together. Next the drive motors and here was a problem, no instructions. As it wasn't obvious to me how or even where the motors fitted, a call to Dark Star was made. Five minutes later I was confident I could fit the motors, and within 20 minutes or so the motors were on. After I placed four large D cells into the battery compartment and I was ready for action. The telescope tube felt extremely stiff and when mounted in the cradles the whole telescope felt quite sturdy. It was apparent that there were some vibration problems, typically taking 5 seconds or so to fully damp out. Provided I kept the tripod legs as short as possible and with such a compact tube assembly this was easy, then I felt the instrument would be rigid enough for my intended application. The finder looked inadequate and also looked as though it could easily be knocked out of alignment. The focuser was huge, and extremely smooth with very little backlash and no perceptible lateral movement during focusing. The spider assembly worried me because of it's size, the vanes were very thick which I though might produce diffraction problems, but it wasn't going to easily move out of collimation that was obvious. With a 2" obstruction on a 6" mirror I wondered whether contrast would be adversely affected. Adjustments to both primary and secondary looked very well engineered.
The first clear night duly arrived and the telescope was setup. Aligning the mount was and still is one of my pet hates. I have gradually worked out methods to increase my accuracy in finding the North celestial pole (NCP). The mount has a bubble level which works well. Latitude and azimuth adjustments are easy to use and work well and getting an alignment good enough for visual use is very easy, taking less that 1 minute. Getting the scope accurately aligned for CCD work is another matter - not helped by a complete lack of instructions on the use of the polarscope. Neither Dark Star or the Helios distributors in this country, Optical Vision, have been able to produce instructions or even provide much in the way of guidance in the use of the polarscope. This is ongoing, but is definitely a point to be considered by possible purchasers of this mount / telescope. The circles, alas are not quite large enough to be accurately read, which introduces another error when finding the true NCP. However, at the present I can get accurate alignment for a 20-30 second unguided ccd exposure, so things are not as bad as they may seem and will improve with experience.
The telescope itself is a dream. The mount rotates on it's axes with a silky smoothness and the motor drives themselves are inaudible (unlike my Dobsonian drive!) Once you are familiar with the quirks of equatorial mounts and the way the locking knobs keep moving to ever more inaccessible regions, the mount is a delight to use. The telescope produces beautiful tight images all the way to the edge of the field, well over 1½ degrees with the 20 mm eyepiece. Surprisingly the images are nearly as bright as my 9" Dob. though obviously it cannot see as deep. There is a little chromatic abberation visible, mainly around the edges of planets, but it is not objectionable and does not seem to be evident on CCD images, so maybe most of it comes from the eyepieces. Also, the expected diffraction from the thick spider arms was not excessive although I believe it did manifest itself as a slight smearing of planetary features, ie extended objects. The Barlow is effective at both increasing the image scale and darkening the sky background. M42 at x50 through this little scope is a beautiful sight, with much detail visible in the nebula. Surprisingly, using a UHC filter enhanced the view on both bright objects such as M42, and dim objects such as M76. This surprised me as I would not have thought the telescope could cope with the loss of light transmission which using this filter causes. On one exceptional night, detail in Jupiter's belts was so crisp and clear, I got the 9" out for comparison. The image from the 9" was clearly superior, in terms of image brightness, but little extra detail could be resolved by the larger mirror. When viewing Saturn, I could just make out Dionne, Tethys and Rhea along with Titan and Iapetus in both scopes, though they were clearly brighter in the 9". The finder proved to be easy to align and seems to hold alignment well, despite a rather poor mounting system. The finder itself has a plastic body but produces well corrected images of adequate brightness. To assist finding an EZ finder was fitted, which has proved to be a useful investment.
For the money this scope is great value. It is compact, easily handled, easy to use and gives first class images. The mount is just sturdy enough I feel for a telescope of this size. Just. Vibrations can be a problem, but they only take a few seconds to disperse and once this is appreciated there is no problem. Focusing at high magnification, say over 200x, is not easy and tries the patience, but it can be done. The focuser is smooth enough, it is just that there is too much shake on the tube at these mags. With the exception of the polarscope and possibly the finder, all the accessories that came with the telescope are of excellent quality. There was quite a lot of shiny hardware inside the tube, which I have since blacked out. Whether this has increased image contrast to any extent is difficult to say.
Whether the scope is able to fulfill its purpose in introducing me to the techniques needed for high resolution ccd imaging remains to be seen and will be the subject of a later review. The signs are hopeful at the moment. There surely cannot be many camera/mount/telescope combinations which can produce reasonable CCD images in a beginners hands for less than £1200 and I'm sure this one can.
Submitted by David Moth - firstname.lastname@example.org - UK