Re: [APML] OT: 240 Galaxies in 3 square degrees of Virgo

["Don Westergren" <westergren@nethere.com>]

Hi Jerry,

If your camera was set with the upper left corner of the FOV either up or 
down, the adapter could let the camera tilt and cause local distortion.  It 
all depends on your local hour angle, but that would confirm your 
hypothesis.

I wondered about it because I'm about to start using a DSLR, and they are 
more sensitive to star distortions than film.  I'm also using an AP 
refractor (180f9) and don't know if I will need to use a flat field lens as 
well.

Don

> Hi Don,
>
> I don't know for sure what's causing the elongated stars in the upper 
> left, but I have a guess...  It's  not field rotation for sure, I spent an 
> hour drift aligning and the exposures were only 7.5 minutes.  It's not 
> atmospheric differential refraction.
>
> I think it's astigmatism because the camera's focal plane was not 
> perfectly coincident with the focal plane of the scope.  The camera is 
> very heavy and I have an Astro-Physics 2 inch adapter with EOS T-mount. 
> There is a LOT of slop and play in the EOS T-mount adapter the AP is 
> using, and I just think the weight of the camera pulled that corner down.
>
> At least that's my guess right now.
>
> The AP adapter also vignettes, even with the smaller size chip than a 
> piece of 35mm film, so I'm looking for other solutions to this problem 
> right now.
>
> Jerry
>


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