Choosing a Telescope Part 3

Catadloptric Telescopes Continued
The Schmidt Cassegrain is the most popular type. The Maksutov is a variation with a differently shaped corrector. The 'Mak' offers excellent image quality:- refractor like, although the thick corrector can cause thermal stability problems. In the past, these have been expensive but now Meade and Russian-built versions have lowered the price greatly.

The catadioptric's drawbacks? First, the cost. An 8" Schmidt-Cassegrain costs 50% to 300% more than an 8" reflector, although about the same as a good 4" achromatic refractor. Schmidt-Cassegrain catadioptrics do not have as wide a contrast range on the Moon and planets as a refractor or most f/6 to f/8 Newtonian reflectors, because of the extra light scattered by its larger secondary mirror and its multiple-element folded light path, nor will it be able to split dose binary stars as cleanly. However, a Schmidt-Cassegrain will usually outperform a fast (f/4.5) focal ratio reflector of similar aperture on the plan-ets and binary stars due to its lack of diffraction-causing secondary mirror spider vanes.

Although their large apertures allow detailed deep space observing, catadioptrics generally do not have as bright an image as other scope types of similar aperture at the same power. Also the focussing is done by moving the primary mirror which can cause annoying image shift during focussing.

Typical 152mm (6”) apochromatic refractor
200mm (8”) Schmidt-Cassegrain reflector
Finally, because of their eyepiece location at the bottom of the optical tube, catadioptrics on German Equatorial mounts are somewhat less comfortable to use than reflectors on similar mounts - when observing at the zenith, catadioptric eyepiece will typically be at waist level, moving to normal eye level only when the scopes are pointed towards the horizon. The eyepieces of fork-mounted catadioptrics are always at more or less eye level, no matter where the scopes are pointed, and are therefore more accommodating for visual observing.

Despite these drawbacks, if it fits your budget and you need a portable scope that does it all, you would be hard-pressed to find a better all-around investment than a good catadioptric.

Manufacturers: Meade, Celestron, Orion Optics, Vixen , Takahashi .

What would you recommend?
Below, please find some personal recommendations, reflecting my own deep seated and, quite possibly irrational, prejudices!

Portable 6-8 inch Meade/Celestron/Darkstar Dobsonian...Digital Setting Circle compatible, simple and relatively cheap.

4 inch Vixen achromatic refractor on GP equatorial mount...nice optics and excellent mount.

Celestron Nexstar 5 or Meade ETX 125 - GOTO capability and nice optics in a highly portable package.

8-inch Meade LX / Celestron. Catadioptric telescopes from the Industry giants. Great if you can set it up on your own. Try a 'dead lift' of the tube and fork onto the tripod at 2.30 am before committing to this scope.

Summary
 

 

      1. Analyse your lifestyle first. Minimise the set-up.

      2. Don't buy in a camera shop / cata¬logue shop:- buy from a dedicated telescope shop.

      3. Spend as much as you can. Ignore accessory bundles. Note the resale value. Don't overspend.

      4. Make sure you can find objects, consider Digital Setting Circles or GOTO if you can't.

      5. Avoid 'aperture fever'.

      6. Telescopes are heavy. Watch your back.

      7. Don't buy a 60mm refractor or a 4 inch reflector..... Like I did!

      8. Forget astrophotography... for now.


Suggested Reading

Star Ware: The Amateur Astronomer's Guide to Choosing, Buying, and Using Telescopes and Accessories, 3rd Edition
Star Ware: The Amateur Astronomer's Guide to Choosing, Buying, and Using Telescopes and Accessories, Third Edition (eBook)
Choosing and Using a Schmidt-Cassegrain Telescope: A Guide to Commercial Scts and Maksutovs
The 20-Cm Schmidt-Cassegrain Telescope: A Practical Observing Guide
Star Watch: The Amateur Astronomer's Guide to Finding, Observing, and Learning About over 125 Celestial Objects
Turn Left at Orion: A Hundred Night Sky Objects to See in a Small Telescope - and How to Find Them


< Part 2


Choosing A Telescope 3 News



Creation was Godless says Hawking

2 Sep 2010 at 1:18am  There is no place for God in theories on the creation of the Universe, Professor Stephen Hawking concludes in a new book.
Read more...



Old star wallows in 'steam bath'

1 Sep 2010 at 10:46am  Europe's Herschel space telescope spies an aging star that has surrounded itself in hot water vapour.
Read more...



Facial cancer hits Tasmanian devil Cedric

1 Sep 2010 at 3:41am  A Tasmanian Devil who experts hoped was immune to a facial cancer that threatens the marsupial species is euthanised after developing tumours.
Read more...



The Superwind Galaxy NGC 4666

1 Sep 2010 at 3:00am  The galaxy NGC 4666 takes pride of place at the centre of this new image, made in visible light with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. NGC 4666 is a remarkable galaxy with very vigorous star formation and an unusual ?superwind? of out-flowing gas. It had previously been observed in X-rays by the ESA XMM-Newton space telescope, and the image presented here was taken to allow further study of other objects detected in the earlier X-ray observations.
Read more...



Nasa booster rocket passes test

31 Aug 2010 at 1:52pm  One of the giant booster rockets intended to power the first stage of flight on Nasa's next rocket has been tested in the US.
Read more...



Big rocket booster in second test

31 Aug 2010 at 11:09am  A further test ignition takes place on the giant booster intended to power Nasa's next big rocket.
Read more...



Killer space blast 'off the hook'

31 Aug 2010 at 7:08am  The theory that the great beasts living in North America 13,000 years ago were killed off by a space impact can now be discounted, a new study claims.
Read more...