Field Rotation Blues

Hi All,

Last Friday I took about 80 pics of Saturn through a sony cybershot digital camera connected to my 16" f4.5 dob telescope driven by a DDII system. Used a 9mm Nagler and 3x zoom no the camera. The shots spanned over about 15-18 minutes in time. I can't take 'em any faster due to blazingly slow download time to memory stick. Anyway, I later noticed while progressing thru the stack of images that field rotation had noticably rotated the images! When I stacked them in Registax, this rotation apparently ruined the result. Sigh and bummer. I guess I had forgotton this would occur in alt-az type of tracking system. Does anyone have a formula or tool to calculate how long of a time span one can image over (using still cams or webcams) before field rotation will become a problem? I guess I can trial and error this if I need to.

Thanks.

John

Hi John.

The actual rotation depends a lot on when your photography is taken. that is to say, the change in rotation is not constant. It changes relative to your orientation to Saturn as well as it's position in the sky. You may have been taking your photos during a time when the rotation may have been more accelerated.

One way to predict the amount of rotation is by using an astronomy software package like Starry Night. There are many out there which would allow you to predict, with pretty reasonable accuracy, what amount of rotation to expect, and in what time frame to expect it.

In addition, although 80 photos is a lot to be dealing with, it seems to me one method would be to manually rotate the individual frames using convntional photo processing software such as Adobe Photoshop or Paint shop pro prior to aligning in Registax. Place a straight piece of tape over your monitor along an identifiable axis. This would serve as a guide to help rotate all of the images to a common axis.

It does sound like a lot of work, but I can't think of any other alternatives for the photos already taken.

I use an LX200GPS in Alt-Az config in conjunction with a de-rotator to compensate. You may want to consider this kind of option, although I'm not sure what types of de-rotators are available for your type of mount.

Best of luck,

Derrick.