gradient dark

RE: Re: [SBIG] Re: STL dark frame comparison anyone?


"Anyone else see the vertical gradient in STL-11K dark frames?"

Yes, exactly what I get on my STL11K, and I've had two different chips - one of
the originals and a newer one replacing it when a
manufacturing defect was detected.

When you build a Darks, Flats & Bias Libraries and Calibrate your light
exposures the "readout" gradient will be removed from the
light image. The secret is taking 'proper', and proper number of Darks, Flats &
Bias', enough to remove what you're seeing. As
a minimum I'd recoment 8-10 Darks, Flats 'and' Bias for each filter and time you
expose at. If you are permenantly mounted,
create a Master Calibration frame whilc will be good until you change the camera
orientation.

It 'seems' what you're looking at are the actual Darks themselves. Remember
these Darsk will be 'subtracted' from your light
image to remove the readout noise in your light or image frane. This is what
you want, it's not a flaw...joe :)


"May You Go Among The Imperishable Stars"

Joe Mize
StarFields Observatory http://www.cav-sfo.com/
Chiefland, FL 29:24'33.4"N 82:51'37.7"W


------- Original Message -------
From : Philip Perkins[mailto:ppml2@...]
Sent : 8/14/2007 1:19:06 PM
To : SBIG@yahoogroups.com
Cc :
Subject : RE: Re: [SBIG] Re: STL dark frame comparison anyone?

Hi Steve,

Many thanks for your thoughts and condolences. I have long admired
your imaging work and excellent articles!

Very interested in your comments about the readout being from right
to left - that must be the same for all STL-11K cameras I would have
thought, which (possibly) makes presence of a vertical gradient even
more curious. But it seems normal according to Gary Ferdinand's
description which exactly fits the gradient in my images.

The gradient is also partially visible in the first image of SBIG's
App Note - this is clearly a crop but it looks as though the full
frame would show the same gradient as in my images:
http://www.sbig.com/pdffiles/column_defect.pdf

Anyone else see the vertical gradient in STL-11K dark frames? I've
often thought it would be useful to have a small library of dark
frames somewhere on the net, so that people can do comparisons - a
dark frame reveals so much about the CCD chip and imaging electronics.

I am not sure it is due to a light leak - the gradient is exactly the
same whether it's a 5 second or 20 minute exposure, whether indoors
or outdoors. The black nosecap is always firmly in place. The dark
frame I uploaded:
http://www.astrocruise.com/docs/dark.zip
... was taken outdoors in very dark conditions with the nosecap
firmly fitted (the camera was not attached to a telescope).

But according to accounts so far the gradient does seem to be normal
and thanks again for your great feedback!

Clear skies,
Philip
--

At 01:45 14/08/2007, you wrote:
>Hi Philip,
>
>Very sorry to hear about your brother. Hope things are getting back
>to normal for you. Your dark frame looks very good as far as defects
>go. In fact, it looks almost class 1 to my eye. The read out on my
>chip (also an STL11K class 2) is from right to left, not top to
>bottom, and it's seen most easily on the bias frames. I cannot detect
>a gradient from top to bottom on my darks. Nonetheless, the type of
>gradient that you are seeing is not outside of normal variation in my
>experience- there are some chips like the Sony interlines that
>commonly have this type of top to bottom gradient as well. The one
>thing that I would make certain of is that the dark frame was really
>taken in the dark- a very small light leak can cause all sorts of
>trouble- even from a distant streetlight, and can mimic an electronic
>gradient quite easily. In the end, I think that this will be normal
>variation, and that you have a great chip on your hands...
>
>Steve
>
>Steve Cannistra
>< http://www.starrywonders.com>http://www.starrywonders.com


Philip Perkins
<ppml2@...>
Wiltshire UK & Luberon France
www.astrocruise.com