Re: [APML]First M31 Prime Focus

[Vicent Peris <vicent@cepheusastrophoto.com>]

Hi,

I think thare can be a bit of optics misalignment too, because the 
aberrations are more noticeable in the bottom
part of the image than in the upper one.

Regards,
Vicent.



At 09:04 20/10/2004 +0100, you wrote:

>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

Vicent Peris
http://www.cepheusastrophoto.com/Index.html
http://www.pleiades-astrophoto.com/pixinsight


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