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

Tue Jun 19 20:18:51 EDT 2007

For scattered light, your recipe should detect it (provided they didn;t 
stick an LED inside the camera head), and provided you are using the 
telescope in precisely the same way you observe. For example, pointed 
upward, using the same filters, the same lights on inside the building, etc. 
That means, if you observed with any LED's glowing in the dome, or with 
someone's porch light hitting the west wide of the camera---things like 
that---then you need to do this dark frame test inth those factors present. 
To get the answer you'll need to first subtract the bias from both darks, 
then divide them. The resulting image should be constant +/- random noise, 
and have a mean/median value equal to the ratio of dark times.

However.... usually, such "unexplained" vignetting is a shutter delay 
problem rather than one of scattered light. What shutter type does your 
camera have: Iris or chopper (rotating blade)? And what were your flat field 
exposure times?

Here are some other questions: Are you doing filtered photometry or going 
filterless? Is your sky heavily light polluted?

Michael Newberry

----- Original Message ----- 
From: "Geir Klingenberg" <geir.klingenberg at gmail.com>
To: <aavso-photometry at aavso.org>
Sent: Tuesday, June 19, 2007 1:43 PM
Subject: Re: [Aavso-photometry] GJ 436 update and upcoming transit times

> Hi Michael and Tom,
>
> Thanks for the info. I am not sure what is done to avoid scattered
> light at GRAS, but I will share this information with them (and I am
> sure Ed Wiley is reading this too).
>
> Michael: can one detect scattered light by comparing darks taken with
> the shutter closed, and darks taken with the shutter open, but with
> the cap on the telescope (and with all lights off, of course)?
>
> The telescope I used is called gras001 and is shown here:
>
> http://www.global-rent-a-scope.com/
>
> Move your mouse over the text on the right to see details and a picture.
>
> Geir Klingenberg
>
> On 6/19/07, Michael Newberry <mnewberry at mirametrics.com> wrote:
>> 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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>> > Aavso-photometry at mira.aavso.org
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>> >
>> >
>>
>>
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