[Aavso-photometry] Light box question]

[Michael Newberry]

Ben,

Here's the short version of the answer:

The flat field correction is a complex assemblage of corrections: 3 of them 
to be sure. There is the overall illumination patter, which includes shadows 
and sips and bumps in the QE of the detector, the pixel-to-pixel variations 
of the detector, and the color-related variations in QE. To get the highest 
quality flat field correction, you want to correct the pixel values of your 
data frames according to the color of light they recorded, and the bulk of 
this is usually controlled by the color of the bright sky. If you take 
twilight flats, you get a correction frame that shows variations caused by 
blue light. Conversely, the dark night sky is rather red thanks to spectral 
emission from the natural airglow. If your object is observed under 
moonlight the sky is again blue. You get much closer to a "prefect" 
correction if the color of light in the flat and data are a good match. If 
you are doing spectroscopy, the color of the light starts to matter more 
than if you are doing broadband photometry.

The technique described by your professor is what I call a "Dithered Flat", 
This differentiates it from the other type of sky flat which is just a flat 
field frame taken by pointing at the sky. The term "dithered" tells exactly 
what is going on. The telescope is pointed at a slightly different, 
randomized, location and a flat taken repeatedly. The flats are then all 
combined into a single master flat using your favorite method. The result is 
a flat field correction that reduces the residual color differences between 
the flat field frame and the data frame, and also that does not include 
pixel-to-pixel scale noise---which you can take care of by using a separate 
pixel flat (also called a "chip flat").

This is a high precision technique that will give you the extra little 
"something" in your results. but it is more work and involves more observing 
time on calibrations rather than targets. So you have to weight its 
advantages and disadvantages before integrating it into your observing 
technique.

Michael Newberry


----- Original Message ----- 
From: "Ben Davies" <ben at davies.net>
To: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Sent: Friday, June 15, 2007 7:19 AM
Subject: Re: [Aavso-photometry] Light box question]


>
>
> While on the subject of flats:
>
> I was talking to an astronomy professor recently who told me that when
> doing astronomy on the big telescopes, he made his flats from the night
> sky surrounding the object he was looking at.  I tried to press him on
> how that could possibly work, but his English was not so good and I had
> trouble understanding exactly what he was saying.
>
> I assume it is doable, because he was insistent on the point.  Does
> anyone know anything about this?
>
> Ben Davies
>
>
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