Choosing a Telescope – Part 3
Catadioptric Telescopes Continued…
The [easyazon_link keywords=”Schmidt Cassegrain telescope” locale=”US” tag=”dublinnightskyob”]Schmidt Cassegrain[/easyazon_link] 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 planets 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 focusing.
[easyazon_link keywords=”apochromatic refractor” locale=”US” tag=”dublinnightskyob”] [/easyazon_link] |
| Typical 152mm (6″) [easyazon_link keywords=”apochromatic refractor” locale=”US” tag=”dublinnightskyob”]apochromatic refractor[/easyazon_link] |
[easyazon_link keywords=”Schmidt-Cassegrain telescope” locale=”US” tag=”dublinnightskyob”] [/easyazon_link] |
| 200mm (8″) [easyazon_link keywords=”Schmidt-Cassegrain telescope” locale=”US” tag=”dublinnightskyob”]Schmidt-Cassegrain reflector[/easyazon_link] |
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…computerized/GoTo compatible, simple and relatively cheap.
4-inch [easyazon_link keywords=”Vixen achromatic refractor” locale=”US” tag=”dublinnightskyob”]Vixen achromatic refractor[/easyazon_link] on an [easyazon_link identifier=”B00WGDELKE” locale=”US” tag=”dublinnightskyob”]Advanced Polaris equatorial mount[/easyazon_link]…nice optics and excellent mount.
[easyazon_link identifier=”B000GUHOYE” locale=”US” tag=”dublinnightskyob”]Celestron NexStar 5[/easyazon_link] or [easyazon_link identifier=”B01K7IJ1OA” locale=”US” tag=”dublinnightskyob”]Meade ETX 125[/easyazon_link] – GOTO capability and nice optics in a highly portable package.
[easyazon_link keywords=”8″ schmidt-cassegrain telescope” locale=”US” tag=”dublinnightskyob”]8-inch Meade LX / Celestron[/easyazon_link]. 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
- Analyse your lifestyle first. Minimise the set-up.
- Don’t buy in a camera shop / catalogue shop:- buy from a dedicated telescope shop or well-known online retailer.
- Spend as much as you can. Ignore accessory bundles. Note the resale value. Don’t overspend.
- Make sure you can find objects, consider a GOTO/compterized telescope if you can’t.
- Avoid ‘aperture fever’.
- Telescopes are heavy. Watch your back.
- Don’t buy a 60mm refractor or a 4 inch reflector….. Like I did!
- Forget astrophotography … for now.
Suggested Reading
Check out various types of telescope here
Generally speaking, the shape of the corrector plates and secondary mirrors, though both do have convex secondary mirrors but the exact shape differs.
Schmidt Cassegrains have thin, wavey shaped corrector plates and the appropriate shaped secondary mirror.
Maksutov Cassegrains have thick meniscus shaped corrector plates and the appropriate secondary mirror.
As others pointed out, Schmidt Cassegrains can be easily made in larger apertures, while Maksutov Cassegrains can't because the corrector plate is actually more akin to a lens and lenses get heavy quickly and are more difficult and expensive to produce.
Beyond that, the differences between the two get complex and typically deal with how much of a particular type of aberration which one suffers from. These aberrations are typically spherical aberrations, astigmatisms, and non-coma off axis distortions.
It is said that a Maksutov Cassegrain will outperform a Schmidt Cassegrain of the same size, but you don't see many Maksutov Cassegrains over 5" or many Schmidt Cassegrains under 6" so it's a bit of a nil point.