[Aavso-photometry] Banding in flats

[robertjmodic at att.net]

Wolfgang,

Thanks for the reply.

> 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 ?

This is a twilight flat with no diffuser.  The shadow at the top and the
reason for the larger gradient top-to-bottom vs. side-to-side is the shadow
of the autoguider pick-off mirror from the ST-7XME.

> 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).

The flats where properly bias and dark subtracted (see the .fits header).
All frames were made at the same temperature.  

> 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 ?

Yes, it has a 2.14" minor axis secondary.  I get 100% illumination over
most of the CCD in the ST-7.

> 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 due to the bonding wires, then why are they only
seen on the left side?  The bands are tilted a bit too, not exactly horizontal
as I would expect if they were due to the bonding wires.

> 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) ?

No.  This faint grid pattern can only be seen in master flats that
are stretched quite a bit.  Probably just structure on the CCD.

> 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.

The subtracted dark frame is actually a thermal frame (already bias 
subtracted).  I use many different exposures for both flats and light frames
and it is not practical to take matching darks for everything.

Darks and bias frames are not the source of the banding since they are
visible on raw flat frames that have not been bias or dark subtracted.

I've posted a .jpg of a V master twilight flat here that shows the banding
a little better:

http://chagrinvalleyastronomy.org/images/mflatsky2s-1-10V%20MinMaxClip.jpg

I'm still thinking that the banding is caused by the cover slip.  Unfortunately,
the cover slip is epoxied to the chip and I cannot remove it to test my theory.

Bob


>> -----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