Catadiopteric telescopes

img_2832My 9mm eyepiece in place.img_2834

I forgot to mention one of the best designs of telescopes. These scopes are composed of a series of lenses and mirrors that permit a long focal length in a short tube. A correcting lens on the front cleans us spherical aberration too. That is all I know about this design.

Some months ago I thought I would like to have a scope with a long focal length for viewing planets. At the time Mars, Jupiter, and Saturn were in the sky at night. I wanted a unit that was light and smallish so it would be easy to set up. And not expensive!

I purchased an Orion StarMax 90 mm Maksutov-Cassegrain scope from Amazon on sale for $209 Prime. It came on a well constructed altazimuth base (refer to previous comments about scope mounts) and will fit on my equatorial mount if I rotate the scope 90 degrees to the right so its mounting base is in line with the tripod. This puts the targeting scope in an odd position, but it works. The focal length is 1250 mm and the tube length is 9 inches (23 cm). The overall length including the star diagonal is 13 inches (33 cm). The “maximum useful magnification” under perfect viewing conditions is reported to be 180X using a 7 mm eyepiece. That is pretty tight and I would not buy such a lens until you are sure your situation will permit useful views with it. The good quality eyepieces included provide 40X ad 125X.

I have used it to see the planets and the view is very sharp. The images are small, but I clearly see the ring of Saturn (but not the separation of the rings) and a little of the bands of Jupiter. I have not looked at deep sky objects yet. The field of view is narrow because of the long focal length so the scope is optimized for planet and moon viewing, but should work for DSO also especially at low power.

This scope can be used for terrestrial viewing if you buy an non inverting star diagonal. (The dedicated astro scopes present an inverted image.)

If I had only one  telescope I would go with this design, but with a larger diameter like 6 to 8 inches(15.25-20.3 cm) and perhaps a shorter focal length too unless one wants to concentrate on moon and planets. Prices go up with increasing diameter.


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