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CCD And What's Needed?????

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Posted by Roger Hamlett on February 28, 2003 20:18:28 UTC

Realistically, this is a _very_ 'open ended' question....
You can take images, quite competently, by just attaching a standard digital camera, 'piggybacked' on the scope, and taking an exposure of a few seconds. Multiple images, can then be 'stacked'. This can give very acceptable results for the brighter objects. The same technique can be used with eyepiece projection, for smaller objects, and planets. Going beyond this, you need a method of keeping the scope pointing accurately at the stars (guiding), and avoiding field rotation (wedge or a derotator). Now pointing a CCD system accurately, takes a different meaning than for normal work. If (for instance), you are imaging at a normal scale (perhaps 2 arc seconds/pixel), then you want the scope to be kept pointing with an error below perhaps 0.5 arc seconds. This requires some form of auto-guider. Now there are a number of different methods of getting the light needed for guiding. The first is a seperate guide scope - advantage is it can point either at the same object, or another nearby. Disadvantage is the weight, and actually aiming the device. Then you move to using an OAG (this takes part of the main scopes light, and feeds it to the guide device). Devices offering this, are external 'guide' pick off devices, like the Taurus tracker, or the Van Slyke easy guider, and the SBIG cameras, which have such a device effectively 'built in'. Advantage is the stability (it can compensate for movement from mirror shift), and light gathering power. Downsides are finding a suitable guide star in the field of the device, and it can become impractical when imaging through narrow band filters. The third method is using a camera that allows part of the imaging device to be used for guiding (Starlight Express MX series).
Now even the best of these approaches have problems compensating for the irregularity in the scopes drive, and more rapid atmospheric movements. The only system than offers a solution for this, is the SBIG AO7.
All the devices will need some optical changes to make good use of the pixel size, given the limits imposed by the atmosphere (probably a focal compressor).
So solutions that are 'complete', would comprise (as examples):
One of the SBIG 'self guide' cameras and a wedge and a F/6.3 focal compressor.
Upgrade for colour by adding a CFW8.
Guide with the supplied cable directly into the Nexstar guide port.
Upgrade to give really fast guiding with the AO7.
With care this can all be spaced to give a good compression ratio, and a reasonable field of view.

Alternatively using a seperate guide scope, with something like the Starlight SVX. Same focal compressor. You will have to wait a few weeks for the guide camera (just starting to ship), and use something like a small refractor as the guide scope, on a Losmandy dovetail mount.
Again upgrade with a colour wheel, and filters as needed.

In either case, you will also need a two axis counterweight system.
You will ideally need some improved software for image processing (the SBIG software is pretty competent 'out of the box'), but neither package offers the flexibility of something like Maxim DL for processing.

The actual permutations are pretty 'endless', with other options, such as alternative 'unguided' cameras, and a seperate guider (like the STV). However you should also try to minimise the cables required (the two alternatives suggested, are probably the 'lowest cable' solutions available).

To run each, you will need a suitable laptop computer.

You should also consider something to help with focussing - RoboFocus, combined with Maxim DL/CCD, or a 'third party' package like FocusAide, will save hours of wasted time when focussing, which is not easy...

Best Wishes

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