Telescope wish-list – 8″

Beyond the starter sizes of astronomical telescopes, keen amateur astronomers usually yearn for something bigger.
The ‘something bigger’ often takes the form of the popular 200mm (8″) size.
At this size there is some shake-out of the options available, as the refractors are long and heavy, and price themselves out of the running except for enthusiasts with deep pockets.

On the other end of the scale, you can get a Newtonian 200mm, on a Dubsonian mount, for under £300, cheap enough to tempt the unwary beginner. The Dobsonian mount was designed to be cheap, light and portable, for mounting large Newtonians and taking them out to astromeets and dark-sky sites, and designed to be easy to make as a DIY project. It is not really intended for starter-scopes. No single-axis tracking, no slow motions, no powered drives and no Goto.

For a Newtonian mounted on a solid equatorial mounting with electric drives and Goto, you can triple the above price.
The advantages of a Newtonian are cheapness and (usually) a short focal ratio good for astro-photography. That’s it.
Disadvantages: needs frequent collimation checks, awkward eyepiece position, requires eyepieces designed for short-focus telescopes.

Further up the scale are the catadioptric telescopes, usually a SCT (Schmidt-Cassegrain Telescope) in this size range. These have a folded optical train and a corrector plate at the front, giving a compact tube and a convenient eyepiece position, with a long focal ratio. They do not require frequent checking or adjustment. With a Goto mounting, they cost from around £1300. Good for astro-photography and general viewing (except wide-field). If you want the very best, you can spend £10,000 on a superb and fully equipped catadioptric 8″ telescope and mounting. Not that many people do, other than university astronomers.

A Goto mount will allow you to rapidly find objects by their RA and declination, something that setting circles are supposed to do but actually don’t, except at an observatory. Also they can be hooked up to a laptop for sundry purposes including accessing observing lists from the Internet.
Norton’s Star Atlas and Goto firmware databases contain small lists, but download a full list of what double stars your telescope should be able to resolve, and you’ll be gobsmacked.

Astronomy bits

I recently acquired some more astronomy hardware. First, a Helios 200p OTA (or in plain English, a large Newtonian reflector without mounting). I hoped that this would provide some extra power for looking at planets in particular, as well as making use of my under-employed EQ-5 mount with electric RA drive. The OTA was very cheap, a fraction of the cost of a new one.
I also got two premium quality eyepieces, a Baader Classic Ortho of 10mm and a Celestron X-Cel of 8mm focal length. I got the X-Cel cheap on Ebay- they retail for around £70.
Last night I managed to test the new eyepieces in my 127 Maksutov telescope. Both are an obvious improvement on the 9mm stock eyepiece that came with the Celestron Mak. The 9mm struggled to split Epsilon Lyrae 1 or2 (not a severe test for this scope) while the other two split this double-double easily, showing round dots with diffraction rings.

I am still working up the Helios, having so far tried it on the EQ-5 mount three times. I have adjusted the collimation, given it a spare red-dot finder (only good for finding very bright objects, unfortunately) and checked that it works with various eyepieces. I fitted a grab bar across the tube rings so that I could handle the scope safely. With the tripod legs fully extended, the eyepiece proved to be too high to reach when the telescope was aimed at the zenith. At 9Kg this is the heaviest scope one can put on an EQ-5 without exceeding the maximum recommended loading. It seems stable enough for visual use.
The Helios has a f5 focal ratio, like almost all the larger Newtonians for sale today. This f-ratio is not really well suited for visual work, but makes the scope shorter, lighter, and easier to mount. In this case, making the tube any longer would bump up the mounting requirements to a stronger, heavier and more expensive mounting.

The Helios seems much less user-friendly than my 127mm Mak go-to outfit, and I wonder what a beginner would make of it.