[Aavso-photometry] RE: Rope Light Illumination for Flats

[Wolfgang Renz]

Hello Donn

> ~ If you blink the sky flat against the screen flat with these settings you will
> ~ notice a rotation of ~ 7degree in the "background" of the flats.
> ~ Did you rotate the camera between the two flat series ?
>  I am not sure where the possible rotation in part of the image might have
> come from. The images were taken about 24 hours apart.  If I had rotated the
> camera the first night, it should not have made any difference. The camera
> rotator homes back to the same spot after each imaging session and is
> accurate to a few arcsec according to the manufacturer [RC Optical Systems].
> The camera as well as the filter wheel is rotated as a unit.  I would not expect
> to see any dust-donuts from the primary or secondary.

At which f number did you take the flats ? 8.4 or shorter with an reducer ?
Does the reducer rotate with the camera and the filter wheel ?


> ~ > The new differential frame had the following values:
> ~ > Maximum      1.142
> ~ > Minimum      0.763
> ~ > Average      0.938
> ~ > Std Dev      0.005
> ~ Did you cut out the edges and the area that is affected by the guiding
> ~ chip mirror before you made your statistics ?
> After the two images were divided, the image was trimmed to eliminate
> the outer 50 rows and columns of the image before performing the statistics. 
> The values that I posted were copied and pasted from the screen data
> provided in Maxim DL's 'Information' window.  No chance for error in data
> transfer.

The "full shadow" of the guiding chip mirror is ~ 50 wide. The "half shadow"
of the guiding chip mirror is another ~ 50 pixel wide but is flatfielded well.
Are you satiesfied with the differences between the sky and screen flats ?


> ~ > You will also see easily two "donuts" in the upper left quadrant (the single,
> ~ > faint donut with a diameter of ~ 630 pixel and one of the seven ~ 150
> ~ > pixel donuts) and also slightly the center left ~ 150 pixel donut.
> ~ > The other five of the seven ~ 150 pixel and the numerious ~ 22 pixel
> ~ > donuts are not highlighted.
> ~ > Its a bit strange that two of the ~ 150 pixel donuts are highlighted and the
> ~ > other five are not.

Actual the donuts are not rotated by 7degree but seem to be moved just a few
pixel between the flats. Is this caused by a slightly changed illumination !?
Did you ever had any issues with scattered light with your short tube truss RC ?


> ~ > See image: http://starkey.ws/FTP%20Page/Picture%20035.jpg
Looks like the begin of the Rope Light overlap with the end by ~ 30 degree.
Might this be the cause for the gradient between the screen and the sky flat ?


Clear skies
  Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO