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Re: Good CCD Whats Needed????

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Posted by Roger Hamlett on November 16, 2002 14:16:00 UTC

This is a big subject.
The first question is what you want to image. In a sense there are three types of imaging involved. The first is imaging bright objects, where the scopes own guiding can be used. The second is doing things just a little fainter, where again the scopes own guiding can be used, and for short exposures, field rotation is not a problem. The final area is doing long exposure 'deep sky' work. Here there are a series of problems/difficulties:
1) Focussing. This is far harder than you'd probably credit!. The pixels on CCD's are small, and getting really good focus, requires a lot of work (or a 'software' solution). 'Full' solutions involve combinations like locking the main mirror, and adding a Crayford focusser, with computer control, combined with a software focussing package (like FocusAide).
2) Guiding. The problem here is that if your pixels are imaging (say) 2 arc seconds/pixel, then tiny movements of the scope are needed to keep alignment perfect.
3) Field rotation. Unless the scope is sitting on a mount, that is rotating on an axis, parallel with that of the Earth, as longer exposures are done, even if the centre of the image is kept aligned perfectly to the target, the edges of the field will be seen to rotate. There are three solutions to this. The first is to limit exposure time to below the point where this matters (the time involved depends on the FOV of the scope/CCD, and what part of the sky is being imaged). The second is to use a field 'de-rotator', which rotates the camera to keep the field stationary. The advantage of this is that it can be used with the scope running alt/az, and is quicker to set up. However the 'downsides', are extra weight, lower accuracy (three motors all have to run together to keep things still), and if a seperate guide scope is used, unless the guide star is 'dead centre' of the FOV, this will also need a derotator.. The final solution, is to mount the scope so that it's rotational axis, does align to the polar axis. This requires a wedge, and when moving, some time to achieve alignment.

Once wedge mounted, the cheapest way to achieve anything reasonable, is to go 'DIY', and modify a suitable webcam, and use multiple exposures with software 'stacking', with software image alignment to compensate for and guide errors. This can also be done with shorter exposures running Alt/Az. If this 'tempts you', then look at:

The second route 'up', is a camera that supports guide from the main CCD (the 'Starlight Xpress' MX models). This has the disadvantage of 'throwing away' half the cameras sensitivity when guiding, and introducing more thermal noise.

The third route is to use two cameras, with one guiding, and one taking the picture. This has advantages for colour, where the guide camera, can be left unfiltered.

The fourth route is to use an 'OAG', to pick a small amount of the main scopes light off the edge of the field, and either guide manually, or with a seperate dedicated guide camera.

The final route is to use a camera that has it's own guide CCD (the SBIG models are probably the most well known).

The best standalone guider on the market, is probably the Sbig STV. This can also image, but is not really aimed at this.
If you want to image colour, then you either have to use a 'one shot' camera (the Starlight MX7C for example), or seperate bandpass filters on a mono camera. The former has the disadvantage, that the effective 'colour' resolution, is half the CCD 'pitch', and the micro-lenses involved tend to reduce the overall image sharpness.
You will also need a software package to do the image processing - full packages like Maxim, or AstroArt, are expensive, but handle images in greater colour depth than things like PhotoShop, and include special features for image alignment etc.. However final processing is often done with packages like PhotoShop.

Have a look at "Ron Wodaski's" site (, which has some data about techniques, as well as data about his book, which is pretty much a 'bible' of the techniques needed.
The other thing to consider, is the focal length of your scope, and the pixel size of the CCD. In general, unless you are prepared to spend a lot of money, most CCD's are fairly small. To get a reasonable field of view, and to avoid 'throwing away' resolution, with lots of pixels all imaging the same star, the focal length of your scope, needs to be matched to the pixel size of the CCD. In general with scopes like the NS11, this will mean either using focal compressors, or considering the 'Faststar' accessory.

I have only 'brushed' the surface, and I am sure others will add more. A typical 'good' combination, would be something like:
Wedge, ST-8XE, RoboFocus on the main focusser, Maxim DL/CCD, and a F/6.3 focal compressor. Add the filter wheel, if you want to image in colour.

Best Wishes

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