Egg shaped stars when guiding ?

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Egg shaped stars when guiding ?

Postby Aardvark » Sat Jul 13, 2013 1:51 pm

This is a little checklist that I have thrown together that may help some avoid egg shaped stars. There are of course many possibilities, these are probably the most common when guiding.

1) Inaccurate guiding due to backlash in the gears driving the mount, this happens when the gear teeth are not fully meshed to each other. One way of avoiding this is to have the balance 'slightly' offset towards the weights in the RA balance, and usually biased towards the camera in the Declination axis.

2) Possibly field rotation, this could be caused if the chosen guide star were too far away from the intended target. For example if your target were low in the sky and your guide scope was using a star 10 degrees above this then the mount will guide to the guide star which has a different linear movement to the stars at the horizon. 1 degree movement from East to West along the horizon is a much greater apparent distance than a 1 degree movement of Polaris for example. The example given would be extreme but easier to understand. In practice the guide star should be as close as possible to the target.

3) Any cables snagging or trailing ?

4) (See 1) ), it is also a good idea to ensure that backlash in the gears is reduced, by moving onto the target and then manually slew scope to the East and slowly slew back to the West onto target. And similarly when on target slew scope slowly North and then South back onto target. I have assume balance is biased to weights and camera as above.

5) It could also be due to the settings in PHD not quite set correctly. There's a great guide HERE by Craig Stark author of PHD.

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