It is possible to get perfect stars right across the diagonal of an APS-size chip such as the Starlight Xpress M25C or M26C – but only if your Hyperstar imaging system is in very good collimation. Getting good collimation is not a trivial exercise, but once you have it set up then it only becomes a matter of very small tweaks if any mirror movement requires re-collimation.
The very first thing you must do is make sure that your imaging chip is perfectly normal to the optical axis. For the Starlight Xpress cameras mentioned above there are adjustment screws to move the camera chip with respect to the front plate which connects to your OTA. Starlight give a method here http://www.sxccd.com/maintenance_info/Aligning_CCD.pdf for getting the chip flat using a laser pen. I have found that this method works extremely well and you can get precise chip flatness very quickly and easily once you have gone through the process a couple of times. The only change I made to the Starlight setup was that I work in a vertical plane with the laser pen pointing upwards from the ground as using this geometry you don’t have the camera mounted sideways against gravity. Being vertically mounted makes it a lot easier to maintain the camera flat against whatever rotation holder you use.
With the camera chip perfectly flat to the front plate, fix the camera to the Hyperstar and get a good focus. I recommend the electric MicroFocuser and MicroTouch software for all your focusing. You can do it manually, but it takes much longer using that route. Get a good focus and take a 20-second image.
Unless you are extremely lucky you will find that stars towards the centre of the image look pretty good, but stars out towards the edge are “tailed” like comets – and this is the collimation needing adjusting. You will need the program CCDInspector to get really good collimation, and to get a numerical feedback that your collimation is good – but the initial “rough” collimation is done by eye by referring to the image on the monitor.
Pick any one of the Hyperstar collimation adjusters and move it either in or out a touch, where a touch is no more than a quarter of a turn. Retake the image and note the result on the outermost stars. It will soon become obvious which adjustments “push the tail” of the stars up into the main star until you end up with a decent looking round star. With the collimation adjustments made so that you can see you have decent round stars in all 4 corners, you are good to go, and you will get extremely good results from your Hyperstar – but you can fine tweak things a little better than your eye can pick up just looking at the monitor – and this is where CCDInspector comes in.
Take a 20-second image of a star field preferably without galaxies or nebulae in the field as these can “throw” the CCDInspector values out. Put CCDInspector into arc seconds mode rather than giving “pixels” output, as this gives you more accuracy in the next part of the setup. You will see values for the tilt in the X and Y directions, and the aim now is to get the smallest value you can for these. The next adjustments of the collimation screws on the Hyperstar are TINY, no big movements or you will throw away all your hard work so far. You now need to make tiny adjustments of the collimation screws on the Hyperstar and note their effect on the tilt X and Y values as you adjust them. You will need to write these figures down as relying on memory alone, especially if you are working out in the freezing cold, simply doesn’t work. You will also need to re-focus after each adjustment and this is why it is a very good idea to have computer-controlled focusing, otherwise this step will simply take far too long. Make adjustments to the collimation screws to get the smallest values you can for the tilt in X and Y in CCDInspector, and don’t forget to refocus after each adjustment. You will see that a natural by-product of small X and Y tilt values is a small value for the collimation and for the field curvature. Proceed iteratively until you get the smallest X and Y values possible.
Finally, when you have managed to get the Hyperstar collimated as best you can, it is a good idea to then re-run all your V-curves for FocusMax so that they apply to your new, nicely collimated system.
That’s all there is to it – Happy Imaging J