Re: [APML]First M31 Prime Focus

Hi Ray,

Thanks! Yes, I do understand what you're saying. I'll try that next time out.

Gary

----- Original Message -----

From: Ray Butler

To: Discussion of Film Astrophotography

Sent: Wednesday, October 20, 2004 4:04 AM

Subject: Re: [APML]First M31 Prime Focus

Hi Gary,

That's excellent for a first attempt at that sort of focal length. The
guiding, which is the hard part, looks perfect.

Others have already given processing tips; what I noticed was the
off-axis defocus due to field curvature. This goes with the territory of
2-element refractors (I know, I try to shoot with an achromat too), but
you can reduce its effects by focusing your camera not on the very
centre of the field, but rather on stars which are about 1/3 of the way
from centre to edge. This gives a better average focus for the whole
frame - your flat film plane would be cutting through the middle of the
curved true focal plane, rather than being tangent to one extreme of it.

Good luck, & keep the pics coming!
Ray

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--
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788 FAX: +353-91-525700

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