Re: [APML]First M31 Prime Focus

[Ray Butler <ray.butler@nuigalway.ie>]

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.

The other benefit with this method is that if your focus judgement is 
slightly off, in either direction of focus travel, then either the 
centre or the edges will actually get better focus - because your film 
plane will still intersect the curved focal plane, only at a different 
radius off-axis. Whereas, if you try to focus right at the centre of the 
field, where the film plane is a tangent to the curved plane, and if you 
err in the wrong direction, the tangent lifts away from the curved plane 
altogether, and so_everything_ will be more out of focus. I hope that 
you can visualise the geometry I'm talking about - if not I can email 
you a digi-sketch.

Obviously, if the target is very small and demands the sharpest focus, 
like maybe a globular cluster, you might want to discard that idea and 
just keep it centred in the field, where the aberrations are minimised 
and the composition looks better. Then focus _very_ carefully.

As always, there's also a solution if you're prepared to pay some money: 
you could buy a field-flattener for 500mm-600mm focal length refractors, 
such as the ones TeleVue and Borg sell for their scopes. I've seen 
images from a Synta 100mm f5 achromat (500mm f.l., about the same as 
your Stellarvue) equipped with a TeleVue 0.8x flattener/reducer designed 
for the TV85, and they were excellent - pinpoint stars, corner to corner 
(but it can't fix the chromatic halos, of course!).

Good luck, & keep the pics coming!
Ray

GARY HATFIELD wrote:
> I want to post 3 new shots.  The M31 prime focus is my first attempt at 
> a galaxy.  I don't really know how to process it but this is my first 
> draft.  I'm going to work on it some more and hopefully make it better.  
> Comments and processing tips welcome!
>  
> http://www.starryestateobservatory.com/My_Homepage_Files/Page1.html
>  
> Gary
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo

-- 
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

_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo