[Aavso-photometry] Flat Stuff

[Chuck Pullen]

Steve - I wish I had your problem with aperture size and light 
boxes.  Bummer.  My C-11 does light boxes fine - wanna trade?  :-)

Your half second exposure may be causing shutter effects, unless you have 
an Interline camera.  Maybe waiting until its a bit darker so you can take 
4 to 10 second exposure would get rid of the shutter effects as well as 
take any stars and smear them out so they better median combine?

Chuck Pullen

At 21:56 1/18/05 -0500, Steve Robinson wrote:

>  I have never had the option of using a light box.  The 18" mirror that I use
>makes getting a wide steady light source awful hard.  Twilight flats are 
>just about
>all I ever take.
>
>My procedure is to turn off the equatorial drive and point the scope a bit 
>to the
>east of the meridian when the sky is light enough to give me 60% full well 
>using
>the V filter using a shutter speed of up to 0.5 seconds.  I take several 
>of these
>until the illumination falls below 50 percent of full well.  Then I up the 
>exposure
>and continue until I get  9 or so exposures.  I then repeat using the R 
>filter,
>and finally the I.
>
>I use the Eastern direction because I believe the light gradient will be 
>less in
>that direction.   (what Arne said confirms what I have believed through 
>experience.)  It is hard to say if east is better than zenith, but I do 
>believe it is better than west.
>
>Finally, whatever the longest exposure was that was needed to take the 
>longest flat
>frame, I take 10 flat darks.  I then take bias frames, and subtract scaled 
>darks.
>
>I still get gradients, but I suspect that has more to do with local light 
>pollution
>than any problem with the flats.
>
>Steve Robinson
>
>
>
>
>_______________________________________________
>Aavso-photometry mailing list
>Aavso-photometry at mira.aavso.org
>http://www.aavso.org/mailman/listinfo/aavso-photometry