Would a super telephoto or a telescope work better?

Started Nov 6, 2012 | Questions thread
RustierOne
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Re: Would a super telephoto or a telescope work better?
In reply to Astrophotographer 10, Nov 8, 2012

Astrophotographer 10 wrote:

For lunar and planetary work neither of those scopes. Ritchie Chretien is not the best design for planetary. Nor is F3.9 Newtonian. You need F40 for planetary.

Usually Schmidt Cassegrains are used. They work better if cooled. You also would need a Televue Powermate to extend the focal length for planets, not the moon though.

They have long focal length and large aperture for the cost. Some planetary imagers have specially made 12 inch Newtonians.

Most planetary work is done with smaller mp fast frames per second cameras like Point Grey Research Flea 3 or Lumenera.

Greg, I agree in that neither of these telescopes would be my first pick for planetary imaging. I too think the Schmidt-Cassegrains are a good way for that. Another issue with the Newtonian is the f/4 focal ratio. An f/6 or higher would be much easier to collimate and more tolerant of collimation errors. There is one very successful Moon imager named Bob Pilz, who has used f/6 reflectors. I believe he is one of the contributors to the Registax project. Here's some of his outstanding work:

http://www.pbase.com/bob_p/moon_8_telescope

His use of Imaging Source video cameras (with their small sensor size) is a good match to the f/6 "sweet spot" of acceptable image quality. An f/4 of course has a much smaller such area.

It looks like the Orion f/3.9 was meant for wide field imaging, rather than lunar/planetary work. It would probably do OK on full disc Moon imaging, especially with a coma corrector. But with a larger sensor (i.e. APS-C), even then it might be pushing the limits.

I have owned a number of reflectors (still have an 8-inch and a 10-inch), but have issues with the open tube. They certainly benefit from ventilation with a fan and being concerned about cool-down of the mirror. They also seem to be dust collectors, like all optics. But to clean a reflector mirror many times will require recollimation as well. Not that the Schmidt-Cassegrains are immune to dust or cool-down issues. But at least for dust, its just cleaning the corrector lens with no recollimation necessary. Of course dust should be minimized to preserve low contrast planetary details.

These are just some of my opinions and observations. There's no intention here of sparking a spirited debate of the merits of Newtonian versus Schmidt-Cassegrain reflectors (let alone vs refractors). Each design has its merits and weaknesses. Once that is recognized and dealt with the results can be good whichever way you choose to go.

Best Regards,
Russ

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