Re: [TASS] Flat fielding

[Tom Droege <droege@WWA.COM>]

Andrew,

The flat fields were definitely not up to snuff.  I don't even have any
good idea about how to make a good flat field for the Mark IIIs.  You
cannot make twilight flats with a device that takes a 470 second long
exposure.  At twilight the conditions are changing much faster than that.
You can't point it at a "dome flat" spot because it does not point.  You
just have to look at the data and use that.  This is what Mike G's program
does.  I think that this is all one can do.  I think Mike went to a lot of
effort to think of the best possible way.

Tom Droege

At 12:22 AM 9/9/99 GMT, you wrote:
>In TN0019 (960903) Michael Richmond wrote:
>> I made a flatfield vector for each segment by
>>calculating the median value of pixels in each
>>column. I then divided each image by this 1-D
>>vector. After looking at some of the reduced
>>images, it appears that I'll need to find a
>>better way of calculating the flatfield vector;
>>this method can be screwed up by bright stars and
>>changing sky brightness.
>
>To which Herb Johnson in TN0057 (Aug 19 1999) added
>>[The image should be a twilight image of a
>>high and relatively uniform illumination]
>
>Ah! It should be. But is it? Are all the
>flatfield images up to standard? Or could this
>be a major contributor to the 0.05 magnitude
>noise floor?
>
>In other contexts, the importance is stressed
>of averaging as many flatfield determinations
>as possible. As far as I can see, our software
>does not allow for averaging. Should it?
>
>Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.
>
>