Refractor or Reflector – Which is “Best” for Astrophotography?

Started Nov 30, 2012 | Discussions thread
wfektar
Contributing MemberPosts: 688
Like?
Re: Refractor or Reflector – Which is “Best” for Astrophotography?
In reply to DuncanDovovan, Dec 4, 2012

DuncanDovovan wrote:

Thx for the explanation RustierOne!

I think I understand what you are saying. Basically the refractor is the better design, but not the most efficient if you take money into account.

Nothing is ever quite that simple. But it's probably fair to say that a good refractor always beats a good reflector in $$/primary size (not a good thing), and that the difference increases rapidly with primary size.

There's no such thing as a zero-dispersion refractive element (broadband, anyway), and there's no way to cancel completely chromatic aberration over the visible spectrum. Extra-low dispersion glasses are expensive, and bigger pieces are a lot more expensive than littler (goes as the volume, not the diameter). Pyrex for mirror blanks is dirt cheap in comparison. Also, high end apochromats reduce CA to less than the dimension of the Airy disc. That dimension goes as 1/primary, so the bigger the primary, the more stringent the CA correction needed. All of this gets very expensive very quickly.

You can relax the CA requirement if you're willing to put up with purple halos around bright objects, like Jupiter or Sirius. That gets you the much cheaper achromat. In theory the achromat will have less resolution than a similarly sized apochromat, though in practice you probably won't notice (until you compare your achromat to a high end apochromat), and for dimmer objects (less than, say, mag 3 or so) you probably won't notice the CA. In a good achro the purple halos around bright objects might be noticeable but not objectionable, depending on your tolerance for such things. Achromats are more common in smaller sizes, in part because if you're after a big refractor, you're picky and have money to burn. And in part because the Airy disc is larger in small scopes, relaxing the required level of CA correction.

Reflectors, of course, don't have CA issues.

If money does not play a role, a refractor is the way to go, although I think you may run into a design limit where reflectors can still continue. Something the size of the James Webb telescope could hardly be made as a refractor I guess?

Keep in mind usability. The best scope is the one you use. A long refractor is great optically because it's easier to correct for things like CA at large f-numbers. But operationally it is ... non-trivial. A C14 is f/10, 31" long, and 45 lbs. A beast, but manageable. The fastest large refractor I could find was 12" f/7.5, which is 2.3 m long (7.5 feet) and 186 lbs. Most large refractors are even slower (ie, longer). Because the balance point is somewhere near the midpoint of the tube, the mount has to be quite tall. The weight, height, and moment arm of the OTA means pretty much a permanent installation, unless you enjoy schlepping a quarter ton of equipment (or more) every time you use it.

You don't even need to go to such a large scope: a 6" f/8 is long and spindly, the mount has to handle a much longer moment arm than, say, for a 8" or 9" SCT. Small forces on the end of a long tube, like focusing, exert significant torque on the system, and you'll need a bigger mount. Large f-number Newts also suffer from this. All of which is to say that everything is a compromise. And that for anything over, say, 5-6", reflectors are much easier to handle and much easier on the wallet. But you do have that central obstruction to deal with ...

The recommendation (which cannot be repeated enough) concerning a good mount is key for AP. It's more important than the scope.

Reply   Reply with quote   Complain
Post (hide subjects)Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow