[Aavso-photometry] Banding in flats

[Wolfgang Renz]

Hi Bob

Your master flat shows a top to buttom gradient of ~ +550 ADU and
a left to right gradient of ~ +150 ADU. This is probably due to an
uneven illumination of the flats.
Did you take sky flats ?
With a diffusor, or without one ?

A top to buttom gradient might be explained with not dark subtracted
images also by the accumulated dark current during read-out time.
But such a gradient should disappear when applying appropriate darks.
Darks might not be appropriate e.g. when the CCD temperature was not
the same as with the flats or lights, or when the read-out times dif-
ferered (e.g. due to different CPU or I/O loads). This might be true
too when taking exposures of less than a few seconds and using scaled
darks due to not considering a shutter correction (= true shutter
opening time doesn't equal stated exposure time).

Aside of this, the master flat is really pretty evenly illuminated.
So I guess that your 8" f/5 Newton is optimized for photography and
has a larger secondary mirror than if optimized for visual observ-
ing, right ?

It has a dark band of ~ 12 pixel at the top that is probably caused
by the shadow of the guiding chip mirror of your ST-7XME when using
it at a fast scope as you do. This usually leaves no dark band in
a flat calibrated images and should therefore not affect photometry
much.

The master flat gets brighter towards all 4 edges within a range of
~ 20 to 35 pixel. I have not seen this for quite some time. I don't
know the reason for this for sure, but if I would have to guess, I
would say its due to a reflection off the bright golden package areas
sourrounding the CCD chip. So these areas are effectivly lost for
photometry due to inappropriate flat fielding.
The master flat shows 5 a bit darker bands in the top half, a band
free area in the middle, and 4 a bit darker bands in the buttom half
on the very left. If the above is the true cause for the bright ed-
ges,  the bands are probably due to the shading of the bonding wires
to the CCD chip. The are 6 of them on both sides in the top half and
5 of them in the buttom half. If one doesn't count the ones going to
the very corners of the CCD chip and therefore can show at most half
a band, the expected number of full shadows is 5 and 4. So this would
fit the observed shadows.
If this is the true cause, you might get rid of the bright edges
and the banding by placing a mask directly over the CCD chip. The
mask would have to be a bit larger than the active area of the
CCD chip to consider the f# of the used scope (chip width/height
+ distance of chip to mask / f#).

If the banding is not caused by the above, it might be due to an
electronic issue. Most common cause for such things are undervol-
tage issues and clocking issues due to undervoltage issues (bad
power supply, corroded pins or contacts, false jumpering, ...).

The calibrated master flat shows also some kind of 1 pixel wide,
a bit darker grid with a period of ~ 22.5 pixel in y. Something
like this could easily be explained for an interline chip as a
possibly present block structure. But I have not seen something
like this with a full frame chip yet. So it might be due to an
incorrect calibration of the flat.
Is this pattern also present in your master bias, master dark
(without bias subtraction), and mast flat frames (without bias
and dark subtraction) ?

If I look at the FITS header, it says:
   EXPTIME 1.0000000000000000 Exposure time in seconds
   HISTORY Changed data type to 32 bit real 
   PROCESS '1001' Processing flags
   HISTORY Subtracted bias frame mbias6-1-20 MinMaxClip.fts 
   HISTORY Subtracted dark, fdkth20s-1-10MMC-20C031409.fts, scaled by 0.05 
   HISTORY Combine Images: Min/max clipped mean of 10 images 
So it looks like as you used a scaled dark which rises the question if
the subtracted scaled dark frame was bias subtracted too. If not, you
might have ended up subracting the bias twice (once with the bias sub-
traction and once with the dark subtraction) which would introduce the
basic bias structures into the dark calibrated flats again.

Some amateurs have shown that the dark current of the warmest pixel
and hot pixel population of their camera don't behave linear with
time. Therefore I would not recommend to use scaled darks for photo-
metry (just as a fall-back if one has missed to taken equal exposure
time darks for an usued exposure time). Using equal exposure time
darks also avoids issues due to a required but not applied shutter
correction.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany



----- Original Message ----- 
From: "Tom Richards"
To: "Robert Modic"<aavso-photometry at mira.aavso.org>
Sent: Tuesday, April 14, 2009 7:09 AM
Subject: Re: [Aavso-photometry] Banding in flats

> Bob,
> I had a similar problem with my Apogee U13, but it did not exactly repeat
> image to image. It was rather random and some images lacked it. Apogee
> concluded it was some sort of electronics issue that could not be resolved,
> and did the decent thing by me. In your case I'd suspect electronics, but
> that can mean anything. Try shielding your download cable, try keeping mains
> power away from it (or very close to it to see if it gets worse), try
> imaging with the fan motor turned off, try different on-chip resolutions -
> anything that might show a difference for better or worse. I've even found
> on my replacement camera I can get banding when running under certain ACP
> scripts but not otherwise!
> Good luck!
> Tom Richards
> 
> 
> 
>> -----Original Message-----
>> From: aavso-photometry-bounces at aavso.org On Behalf Of "Robert Modic"
>> Sent: Tuesday, 14 April 2009 10:54 AM
>> To: aavso-photometry at mira.aavso.org
>> Subject: [Aavso-photometry] Banding in flats
>> 
>> Does anyone know what causes the horizontal light and dark bands seen
>> on the left side of this image?
>> http://chagrinvalleyastronomy.org/images/mflatsky1s-1-10Rc%20MinMaxClip.fts
>> 
>> This is a master twilight flat made with a Rc filter.
>> 
>> The banding is present all the time with all my filters and with light box
>> flats as well.
>> 
>> I read somewhere that the banding may be an interference pattern caused by
>> the coverslip not being parallel to the CCD.
>> 
>> The reason I ask about the banding is that it does not always flat out properly.
>> I have already checked the scope for and eliminated any scattered light in
>> the system.
>> 
>> Bob