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

Water in moon rocks likely came from comets

11 Mar 2010 at 1:15pm 

Genuine moon water has been found for the first time in rocks that were brought back to Earth during NASA's historic Apollo missions 40 years ago.



Read more...

February 2010 Extra

19 Feb 2010 at 10:00am  In this show we find out about space weather from Jim Wild [00:43-09:01] and talk to Marek Kukula about all things Solar [09:01-14:19], Tim answers your astronomical questions [17:21-37:31], and we round-up your feedback.


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February 2010

1 Feb 2010 at 3:59pm  ING direct. We have a couple of interview with scientists at the Isaac Newton Group of telescopes. We find out about the observatory and compact binaries from Dr Pablo Rodríguez-Gil [09:39-32:45] and planetary nebulae from Dr Miguel Santander-García [32:45-48:40]. As always, Megan brings us the latest news [02:00-09:24] and Ian Morison tells us what we can see in the February night sky [52:19-64:39]. Our cover art shows the Isaac Newton Group of telescopes from the air.


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January 2010

2 Jan 2010 at 3:23pm  Looking back, looking forward. It's the end of the International Year of Astronomy 2009. We bring you some of our highlights of the year which celebrated 400 years of the astronomical telescope. We get updates on the 365 Days of Astronomy podcast [16:48-23:37], Universe Awareness for Young Children [46:19-51:42] and Jen - our roving reporter - visits the AstroBunker [23:37-46:19] in Newbury. As always, Megan brings us the latest news [01:21-10:21] and Ian Morison tells us what we can see in the January night sky [52:06-67:37]. Our cover art shows the logo of the International Year of Astronomy.


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December 2009 Extra

22 Dec 2009 at 3:50pm  Live Part 2. In this show we talk to Sir Francis Graham-Smith about his life, being Astronomer Royal and his work on pulsars [01:00-23:44]. We put your astronomical questions to the Jodcast panel [24:19-46:03], and round-up the feedback we've received during the Live shows.


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December 2009

1 Dec 2009 at 3:30pm  Live! After months of talking about it we finally recorded an episode (1 of 2) with a live studio audience. In this bumper show we have an interview with Dr Chris Lintott where we find out about the latest results from Galaxy Zoo [18:36-50:08]. As always, Megan brings us the latest news [08:12-17:56], we discuss recent events in space [67:54-83:00] and Ian Morison tells us what we can see in the December night sky [50:29-67:32]. Our cover art shows some of the presenters and audience who braved the rain at Jodrell Bank during Jodcast Live.


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November 2009

3 Nov 2009 at 8:00am  Moon Calling. In this show we have an interview with John Sarkissian about the history of the Parkes Radio Telescope in Australia [12:50-45:54]. As always, Megan brings us the latest news [01:50-10:35] and Ian Morison tells us what we can see in the November night sky [43:53-57:24]. Our cover art shows Fox Mason at the control desk of the Parkes Telescope in 1970.


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