[Aavso-photometry] GJ 436 update and upcoming transit times

[Michael Newberry]

I have some other possible explanations for the vignetting that remains 
uncorrected after applying the flat field. But let me first add a little 
description to the scattered light explanation that has been put forth.

The effective difference between scattered light and vignetting is that what 
we call "scattered light" is an additive effect which is not present in the 
shuttered darks and bias frames. Corrections applied by the flat field 
frame, such as for removing vignetting, are multiplicative. So the 
corrections for the two effects can't compensate for one another, no matter 
how hard we try. Scattered light is some source inside your system that a 
closed shutter does not allow the detector to see and that is independent of 
what light is coming in the front of the telescope. Look for things like an 
internal LED or microswitch light inside the filter wheel or some other 
equipment.

I hope you are applying dark frames to your flats (and bias frames too, if 
you use them in your processing). If not, then the dark count in the flat 
will contribute an additive component that mimics scattered light. This 
becomes most important when the dark count is relatively high and/or when 
the signal in the flat is relatively low. To avoid this problem, you always 
need to apply dark (and possibly bias) corrections to your flats, as well as 
t\o your science images.

Another potential cause of what you describe is shutter delay. If an image 
has a short exposure time, say a second or two, then the finite time it 
takes to travel from the edge to the center and back, creates a vignetting 
in the illumination toward the edge of the frame. Iris shutters vary in 
their open/close times, ranging from something like a few hundreds to 
perhaps a few tenths of a second. Typically, your science images have a 
sufficiently long exposure so the relative exposure difference between the 
edge and center is negligible. However, your flat field frames may have 
exposures of only a few seconds or less, and the shutter delay could be 
substantial. You can easily test for this effect by taking two successive 
flats of the exact same source, but using different exposure times, say 1 
sec and 10 sec. Apply the dark and bias corrections to the raw flats to 
create "calibrated flats". Then divide the calibrated flats. If the 
resulting image is not ruler-flat, then you are seeing vignetting caused by 
shutter delay. In that case, you need to increase the exposure time of all 
your flats to something more like 10 seconds (or whatever).

Michael Newberry


----- Original Message ----- 
From: "Tom Krajci" <tom_krajci at tularosa.net>
To: <aavso-photometry at mira.aavso.org>
Sent: Tuesday, June 19, 2007 9:25 AM
Subject: Re: [Aavso-photometry] GJ 436 update and upcoming transit times


> >From: "Geir Klingenberg" <geir.klingenberg at gmail.com>
>
>>I have checked the flatfield using NGC7790 and the method described by
>>Arne. And sure enough, I found systematics in the data. It looks like
>>there is a gradient from the center towards the edges. There is some
>>vignetting in this telescope, and it doesn't look like the flat
>>corrects it completely.
>
> You have identified a problem, but are you sure the cause is incorrect
> calibration of vignetting?  Could it be scattered light issues?
>
> Do you have any photos/diagrams that describe the baffling you use in your
> telescope?
>
> As for baffling (both telescope and CCD), I can send interested folks some
> diagrams/graphics that show what I do to try and reduce the scattered 
> light
> issues in my telescope.
>
> -------------------------------------------
> Tom Krajci
> Cloudcroft, New Mexico
> http://overton2.tamu.edu/aset/krajci/
>
> Center for Backyard Astrophysics (CBA)
> http://cba.phys.columbia.edu CBA New Mexico
>
> American Association of Variable Star
> Observers (AAVSO): KTC http://www.aavso.org/
> -------------------------------------------
>
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