Hi Ralph

Thanks for posting your images. I think you're right - you're probably
operating at something more like f2 ! Making flats with the f3.3 reducer is
tricky. I use one (actually my ratio is about f3.6) on an f10 10" LX200 with
a CB245 camera. There is noticeable vignetting at the corners even with the
small TI245 chip. I routinely see pixel variations of 1000 PV (on a scale
of 0-4095) from the center to the edge of the field at f3.6. The good news
is that if you make the flats carefully (I use a lightbox) the raw images
can be easily corrected. A lot of the image processing programs divide the
flat into the raw image then multiply by the median value of the flat. This
can cause some interesting "effects" when the range of pixels in the flat is
quite uneven.

Gregg L. Ruppel

Visit my astronomy site at:
http://www.biz1.net/~ruppelgl

----- Original Message -----
Sent: August 24, 2000 9:56 PM
Subject: [M]: Color image with a 0.33 focal reducer


> I have just posted a couple of images to my web site taken with the Meade
0.33x focal reducer. I used a Pictor 1616 and a 616 color filter wheel. I
have not estimated the focal reduction but it looks like more reduction than
a factor of three.
>
> My problem has been getting the flat field correct. I am starting to
suspect that the dew shield shifts and the field is wide enough to get some
of the glare or vignetting from this. This is particularly noticible on the
M31 image where the right corners are dark and the left corners are not. I
needed to bilinear adjust the pixel values to get to this point.
>
> The image are of M13 and M31. The M31 image shows both M32 and M110 along
with M31. Not bad for a 10" F/6.3 focal reduced. You can find the images
under deepsky at:
>
> http://rppass.com
>
>
> Ralph Pass
>
>

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