Double Telescope Setups

[Nath2099]

I've seen a few photos of people's rigs where they have something like a 12"+ reflector, with an expensive 4"+ refractor attached to it. Do you think they'd be using the refractor as a tracking scope and just have too much money, or do people actually collect two sets of data?

If two sets of data, what's the benefit of each set?

[gaseous]

From Barry Armstead's article in the "projects and articles" section of the site:

"Typically, you would have one telescope on the equatorial mount, taking a long exposure while the mount is tracking. Piggybacking on top of that, would be a second, less powerful telescope, with a guiding camera in it. This guiding camera sends corrections to the mount, based on a star that you have designated it to lock on to. All mounts have some degree of mechanical error in them, but a guiding camera and guiding program like PHD (http://www.stark-labs.com/phdguiding.html), takes the pain out of it and keeps it all on track."

[Nath2099]

They seriously spend 4 grand plus on a guide scope? Too much money it is then.

[gaseous]

I'm a purely visual person myself Nath, so can't really comment on the cost specifics, but from what I've seen as a casual observer you can certainly drop a fair chunk of coin on a decent AP setup.

[tonez]

in that case they are both imaging scopes, just of different aperture and focal length. you image through the scope that suits whatever you are going for.

[Nath2099]

Ah ok. that makes sense. They were mostly in fancy home observatories. I guess this saves time unmounting, mounting and aligning scopes.

[el_draco]

There are various ways of guiding but most guiding can be done with a small, low cost refractor or even a modified finder scope. You don't use a guidescope on SCT's because the optics often move slightly as these scopes track (called mirror flop). A separate guidescope wont pick up that movement so we use an "off axis" guider which takes a tiny amount of the light from the main scope.

As to why else you would chuck a 4k scope on top of another telescope, there are several reasons:

1/ If the mount has a lot of capacity and is very high quality, it provides multiple imaging options.
2/ You can attach other equipment to this scope, like a photometer or spectroscope and do several sets of observations.
3/ You can do visual obs through two, or more, different scopes.
4/ You can use the second scope, if its a wide field scope, to track down fainter targets, or image a wide and narrow filed at the same time.
5/ You can show off. Some people are, for example, TAK addicts :rolleyes: (and I'll leave it at that.:lol:)

[poider]

So if the mount has a mechanical error and the guide scope corrects that error.... wouldn't that cause movement whilst the scope catches up with itself, and therefore blur the image?
Peter

[Atmos]

The guiding is occurring at scales of a fraction of a pixel in size so as long as the mechanical error is small (not a big jitter) it can be guided out without any noticeable blurring. On a good mount the guide errors are considerably lower than the seeing conditions so the sky covers up the guiding :)