visual scopes vs photo scopes

Started by Tom Bourg, 10/30/2005 05:58PM
Posted 10/30/2005 05:58PM Opening Post
Hello,

I've both read and heard from people who know a lot more than I do that certain scopes are optimized for visual use and others are designed for photography. I've especially heard that R-C scopes are great for imaging but just so-so for visual use.

Can someone explain how this is done? Is it the scope design, such as R-Cs, that is better for photos, or is it the figure on the mirror, etc. In other words, can a Newtonian or Cat be optimized for photography and not visual use or vice versa?

Last night I was using a 20" R-C on Mars and although I have heard repeatedly that it is better for imaging that visual photography the views of Mars were about the best I have seen, so this really got me thinking about this topic.

Thanks, any explanation will be an improvement on what I know now. smile

Good luck and clear skies.

Tom B.
Posted 10/30/2005 06:41PM #1
The RC scope have a larger central obstruction than other types of scopes - and this reduces contrast, and makes it a little tougher to get your eye in the right spot.

Posted 10/31/2005 04:28PM #2

The requirements for a telescope design are different for visual vs. photographic. For example, a telescope designed for photography must be able to achieve good focus across the entire field of view in a flat image plane. This is to ensure the stars are sharp and in focus across the entire film frame (I talk film because all the photographic designs were of course designed for use with large photographic plates or film). For a visual scope, this is not as stringent a requirement because the eye tends to accomodate for slight changes in focus. Another requirement is that off-axis, a photographic scope should provide nice, round images of stars (whether or not they are in perfect focus). This is partly because odd-shaped stars are ugly looking, but also for professional fields like astrometry it is easier to pinpoint the exact location of a star in the image if they are round blobs (the actual position is in the center of the blob) versus streaks (astigmatism) or shaped like a comet (coma). In a visual scope, this isn't as important because you can either refocus for best shape or it's just not as noticeable. These are just a couple of examples of requirements driving the design.

This is why there are many different telescope designs. Some will be optimized for visual and some are optimized for photography. Some are a compromise of the two, meaning they are not best at either but are good at both. For example, the RC design and the Schmidt Camera are examples of scopes designed for photography. The Newtonian and the Schiefspiegler are good examples of visual scopes. Finally, the Schmidt-Cassegrain (as Celestron and Meade have designed theirs) would be an example of a scope designed as a compromise, giving generally good performance either as a visual or as a photographic telescope. The SCT is interesting because it is robust enough that you could design one that would be a killer visual scope, or you could design one that would be a killer photographic scope.

Hopefully I haven't confused the issue too much. I'd recommend you getting a copy of "Telescope Optics" by Rutten and van Venrooij. This is a very good reference that talks about just these issues.

-Jason