[Aavso-photometry] Light box question]

[Michael Newberry]

Wow, some bad typo's! Sorry about that. The second sentence mentions 3 types 
of flat field correction (call them "cases"). The first case should read: 
"There is the overall illumination /pattern/, which includes /dips/ and 
bumps in the QE of the detector, ..."  The corrections are shown between / 
/.

 In addition, I would like to clarify that this first correction refers to 
the *spatial* pattern of sensitivity, not the CCD's QE as a function of 
wavelength. I list the wavelength dependency in the 3rd case.

Michael Newberry

----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: "Ben Davies" <ben at davies.net>; <aavso-photometry at mira.aavso.org>
Sent: Friday, June 15, 2007 7:48 AM
Subject: Re: [Aavso-photometry] Light box question]


> 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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>
>
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