Re: [APML]First M31 Prime Focus

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

Vicent Peris wrote:

> 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.

Yes, I noticed that too, but thought it might have been due to some 
cropping.

Wait a minute...misaligned optics?? In a _Stellarvue_ telescope??!! 
There's shurely shome mishtake??!!

Ray "with apologies to the most defensively proud astro group on the 
net: Stellarvue fans" Butler


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