FreeLists / az-observing / [AZ-Observing] Re: Gegenshein and a question

[Chris Schur <comets133@xxxxxxxxx>]

Dean, making a flat for your wide angle lens is easy. 
Drape a white t shirt over the lens and aim at the
sky, and take your flat.  Yes, it records the vignette
perfectly.

Chris
--- ketelsen@xxxxxxxxxxxxxx wrote:

> Hi All-
> 
> Steve Coe mentioned the other day how visible the
> Gegenshein was on the
> weekend - I was having some backlash issues with my
> mount, so went to wide
> field imaging.  Unfortunately, I forgot my adaptor
> to use my Nikon lenses,
> so I had my Canon zooms to use with the 20Da.
> 
> Anyway, one of my targets was the Gegenshein, and
> for it used the 10-22
> Canon zoom at 10mm wide open at F/3.5.  Cropped
> slightly, this is 4X3
> minutes at an ISO of 1600:
> 
>
http://alice.as.arizona.edu/~ketelsen/Gegenshein%204X3m.jpg
> 
> I'm really impressed with the sharpness of this lens
> - it is fun to cruise
> around the frame at full resolution.  There is a
> little vignetting and
> that brings me to my question - how do you shoot
> flats with a lens with a
> 107 degree diagonal FOV?  I usually shoot sky flats
> during twilight, but I
> don't think the sky is uniform enough that wide. 
> Any ideas?
> 
> BTW, here are a couple other shots - the winter
> Milky Way from south of
> Canis Major to Comet Holmes and a little Zodiacal
> Light, here with 18
> minutes total exposure:
> 
>
http://alice.as.arizona.edu/~ketelsen/WinterMilkyWay%206X3m.jpg
> 
> Here a quick shot of the western sky after moonset
> about 2200 local time
> with 9 minutes total exposure:
> 
>
http://alice.as.arizona.edu/~ketelsen/West%203X3m.jpg
> 
> Thanks in advance for thoughts on flat fielding...
> 
> -Dean
> 
> 
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> 



      
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