[Aavso-photometry] Fwd: Light box question]

[Ben Davies]

Hi Thomas,

The way it was explained to me was this; he drew a spiral galaxy on the 
blackboard and 4 rectangles surrounding it representing the fov of the 
flats.  I believe he thought that it was important to be in a close 
region of the sky.  The technique you describe would play out in 
parallel, but the results are maybe different. 

In both cases, the ghosts of the stars are a problem.  I suspect they 
are a consequence of seeing changes from frame to frame.  Its grin, like 
that of the Cheshire cat, remains behind.  This is probably one of the 
unadvertised downsides of oversampling.

Ben


> If I understood it correctly, one way of doing a flat from the night
> sky (specially if you have a lot of light pollution, as I do at home)
> is the following:
> combine all the (dark-corrected) frames you did during the night.
> To combine the frames, use a "median" or "sigma reject", after normalizing
> the frames, in orther to erase the stars. To use this method you need
> frames from many fields, or at least you have to dither between frames,
> so that, there is not a star always in the same pixel.
>
> Note that this is essentially the same as a twilight flat, but instead
> of twilight,
> you are using light pollution. Therefore you have to use frames taken where
> there is not much gradient. Thin clouds will reflect light pollution, and can
> spoil this method.
>
> I just did a quick experiment to check this method: I selected a night in
> which I had good weather and was able to take twilight flats both in the
> evening and in the morning. Then I took several frames made during that
> night, each one from a different field (all taken with the same V
> filter), already dark and flat corrected.
> Then I did a "sigma reject" combine (rejecting above 3-sigma) with Maxim DL.
> At this moment, what I get is basically a comparison of the twilight flat
> (which I had used to calibrate the frames used in the experiment) and
> the "night sky flat". In other words, if this method works, I should get a flat
> picture, and this is what I got, within a standard deviation of 0.5%, and
> no visible gradient. Then I did a quick estimate of the deviation I should
> have gotten (assuming only Poisson errors), and I got about 0.4%.
>
> The problem with this method is that it is very difficult to remove completely
> the stars. Even after the sigma reject combine, I can see the shadows of the
> bright stars. To avoid this from spoling the measurement, when making the flat
> for a frame, you should use only the other frames. Another source of problems is
> if there are nebulae or large galaxies in the frames.
>
> By the way, the system I used is specially easy to flat correct, because it is
> a newtonian to which I put an oversized secondary in order to have
> full illumination
> of the chip. In particular, this system has no vignetting.
>
> I hope this helps,
>
> -Tomas L. Gomez (GOT)
>
> On 6/15/07, BailyHill at aol.com <BailyHill at aol.com> wrote:
>   
>> Hello Ben
>>
>> Mike did an excellent job of explaining the dither technique.
>>
>> Like everything in photometry, it has its limitations.  It is very  hard to
>> collect enough images to get real high signal to noise ratio in the  flat.  As
>> Mike said, you can spend more time doing flats than  observing.
>>
>> The other trick, is to remove the stars from these flats.  For me, I  have
>> NEVER been able to make them really go away.  I can always see  residual stars
>> at a low level.  Perhaps there is a trick that I do not know  able.
>>
>> I guess the bottom line is that you must match the flat technique to the
>> problem you are working to get the optimum results.  What this probably  means is
>> to try several techniques, and see which answer you like.   This technique
>> will not always be the same.  I can tell you that I spend  some time almost
>> every day working on flat fielding techniques.  Its that  important.
>>
>>
>> Clear Skies
>> Gary
>>
>>
>>
>>
>>
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