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  #21  
Old 27-01-2014, 11:06 PM
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I guess the only way I'll find out is to measure it to know.

When you say bias drift how did you measure it? Did you wait for 20mins for the camera to settle down after it reached its cooling temp?

Did it vary with the ambient temp falling as the evening progressed?

Richard also says bias will be different at the start and at the end of a read out process. So there is a variable there as well. I don't know how you measure that if you can at all.

Greg.
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  #22  
Old 28-01-2014, 09:43 AM
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Originally Posted by gregbradley View Post
I guess the only way I'll find out is to measure it to know.

When you say bias drift how did you measure it? Did you wait for 20mins for the camera to settle down after it reached its cooling temp?

Did it vary with the ambient temp falling as the evening progressed?

Richard also says bias will be different at the start and at the end of a read out process. So there is a variable there as well. I don't know how you measure that if you can at all.

Greg.
I measured the average bias in a selection of frames using the Statistics process in PixInsight. I spent several days collecting calibration frames interleaving bias frames and darks of various durations. I was indoors in a room that doesn't vary in temperature a lot (it is embedded in the side of a hill) and didn't start the calibration run until well after the camera temp had stabilized.

I didn't notice any significant variation in bias from the start to the end of the frames but I'll go back and have a closer look. In PixInsight it is easy to set up previews over different parts of the frame and grab stats for each preview.

Cheers,
Rick.
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  #23  
Old 28-01-2014, 12:13 PM
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I think he was referring to the readout process from the reading at the beginning of a single image to the end of that single image.

Also to falling temps as the evening progresses.

I'll do an experiment to find out next time I am using those cameras.

Greg.
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  #24  
Old 28-01-2014, 12:30 PM
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Originally Posted by gregbradley View Post
I think he was referring to the readout process from the reading at the beginning of a single image to the end of that single image.
Yes, that's what I thought he meant. He's expecting to see a variation in average bias between the first rows and the last rows due to the time delta between when they are read out, presumably due to accumulating a little extra dark current. I can check that by measuring different sections of the bias frames.

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Also to falling temps as the evening progresses.
He's suggesting that some of the bias drift may be due to thermal effects in the external support circuitry which is not kept at a constant temp like the sensor. I agree that's quite plausible but I haven't tried to test it. It would be interesting to know what causes the bias drift but my first priority was figuring out how to compensate for it.

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Rick.
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  #25  
Old 28-01-2014, 08:47 PM
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Originally Posted by gregbradley View Post
Richard also says bias will be different at the start and at the end of a read out process. So there is a variable there as well. I don't know how you measure that if you can at all.
Greg,

I checked my bias master. There is a slight difference in average value from the first rows to the last rows but it is tiny - about 0.015 ADU compared to a bias value of 1220 ADU. This is equivalent to about 1.5 seconds of dark current at -30C. That could be about how long it takes to download the data from the sensor into the RAM buffer on the camera.

Cheers,
Rick.
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  #26  
Old 29-01-2014, 12:09 PM
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very informative thread - thanks guys.

Presumably bias drift will vary with camera design. For interest, I looked up the recommended support circuitry for a couple of popular CCD chips - one manufacturer suggested what appears to be a thermally compensated output buffer, the other a straight emitter follower, with no compensation. One would likely drift only a little bit, the other probably more so.

It might be informative to have a more complete list of cameras that could benefit from overscan compensation. Will do a few tests on the H694 bias stability and post here - it doesn't produce overscan data as is, but I am not sure how much it drifts.

As I understand it, once you have reliable set of darks (for which you might need overscan compensation), the issue is dealt with - all you need overscan for is to get the right dark current to scale and subtract. Bias drift in the lights will only show up as a variable offset, which can be easily dealt with - is this a reasonable statement? EDIT: of course this only applies to pretty pictures, not science data - overscan compensation would be essential for some forms of photometry.

regards Ray

Last edited by Shiraz; 29-01-2014 at 10:28 PM.
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  #27  
Old 29-01-2014, 10:57 PM
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As I understand it, once you have reliable set of darks (for which you might need overscan compensation), the issue is dealt with - all you need overscan for is to get the right dark current to scale and subtract. Bias drift in the lights will only show up as a variable offset, which can be easily dealt with - is this a reasonable statement? EDIT: of course this only applies to pretty pictures, not science data - overscan compensation would be essential for some forms of photometry.
Ray,

I do overscan calibration of all my data - calibration frames and light frames. PixInsight subtracts the median of the overscan region from all of the data pixels and then everything else happens as usual. This is intended to remove any bias drift from the data before it is processed.

Cheers,
Rick.
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  #28  
Old 30-01-2014, 05:59 PM
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How do you measure bias drift? Do you check background ADU of a series of bias images taken at the same time and same temp?

I get the idea from Richard that it may also tend to shift when ambient temps change.

Here is a copy of a post by Richard today where he measured his Microline. Bias seems extremely stable.
So this overscan callibration may only be of benefit if your camera has bias drift.



Richard Crisp
Today at 3:14 AM
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I did a little bit of temperature testing in an older ML4022 to measure the bias value under different ambient temperature conditions and different sensor temperatures


I operated the ML4022 in three different ambient temperature conditions: Room, Refrigerator, Freezer


I didn't have a quality thermometer handy but would estimate the ambient temperature was about 70F in the room. I guess the Refrigerator temp was around 38F and the Freezer was probably below 20F maybe as cold as zero F.


here are the results for -20, -25, -30 and -35C sensor operating temperatures


Bias Values (ADU)
op temp-20-25-30-35freezer977977977979refrigerator987988989991room991991992993

Since my night temps usually fall within the range of 70-40F the room temp and refrigerator temps cover my normal range. It is rare for me to encounter really cold freezing temperatures.


looks like the spread for my expected conditions is in the range of 2-4 ADU.

Richard Crisp




Greg.

Last edited by gregbradley; 30-01-2014 at 07:12 PM.
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  #29  
Old 30-01-2014, 09:37 PM
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Originally Posted by gregbradley View Post
How do you measure bias drift? Do you check background ADU of a series of bias images taken at the same time and same temp?

I get the idea from Richard that it may also tend to shift when ambient temps change.
I measure drift by looking at the mean value of each bias frame (or a smaller section of each bias frame if you prefer - Dave and I did some tests on a 100x100 pixel area at the centre of each frame). The time and ambient temp (did you mean ambient or sensor temp?) shouldn't matter. If your bias is not stable regardless of these then overscan calibration is likely to be of benefit.

Ambient temperature is definitely a possible cause of bias drift by affecting the electronics external to the temperature regulated sensor. I didn't ever see an obvious correlation in my results but that's not conclusive.

Both Dave and I did tests that demonstrated a significant benefit to overscan calibration with our cameras - a SBIG STF-8300M in his case and an Apogee U16M in mine. With overscan calibration we got a much more linear looking growth of dark current over time and could also measure a reduction in effective read noise (bias drift can't be distinguished from read noise). YMMV of course, and as I said before: if you don't have bias drift then there's no value in overscan calibration.

I'm pretty busy with work stuff at present but once I dig my way out from under that I'd be happy to post some numbers and graphs. Dave might weigh in at some stage too but he's in the middle of a significant lifestyle change and may be too busy to spend time on IIS for a while.

Cheers,
Rick.
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  #30  
Old 10-03-2014, 09:26 AM
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Quick question re meridian flip when using the over scan feature.

I'm guessing that when it comes to stacking images that have overscan visible I need to stack each side of the meridian separately or the overscan regions will not align? This also brings up another point what if I dither then the overscan regions wont line up either? I'm missing something arnt i?? I need to go back an re read the entire thread.. don't I. Ok I'll go back a re read the entire thread.

...
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  #31  
Old 10-03-2014, 09:45 AM
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Quick question re meridian flip when using the over scan feature.

I'm guessing that when it comes to stacking images that have overscan visible I need to stack each side of the meridian separately or the overscan regions will not align? This also brings up another point what if I dither then the overscan regions wont line up either? I'm missing something arnt i?? I need to go back an re read the entire thread.. don't I. Ok I'll go back a re read the entire thread.

...
The overscan is used during calibration of individual subs then cropped off before they are registered and stacked. It doesn't have any impact on meridian flips or dithering.
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  #32  
Old 10-03-2014, 10:03 AM
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Ann, perfect thanks. One more piece to the puzzle added.
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  #33  
Old 02-06-2014, 08:57 AM
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Dave

this is extremely helpful - I've been playing with calibration with PI last weekend as I was encountering readout issues.

I have a SBIG STX16803 - is the 60 pixel overscan setting appropriate for that camera?

Pete
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  #34  
Old 02-06-2014, 11:07 AM
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Dave

this is extremely helpful - I've been playing with calibration with PI last weekend as I was encountering readout issues.

I have a SBIG STX16803 - is the 60 pixel overscan setting appropriate for that camera?

Pete
Hi Pete,

Dave has been pretty busy lately so I suspect you won't hear back from him soon. I also suspect he won't know the answer to your question. You should ask SBIG and/or poke around the extra data in the frame to figure out where the overscan data lies. On my 16803 camera (Apogee U16M) there is a set of 10 columns at the far right of the frame that do the trick but the SBIG driver will almost certainly map things differently.

Cheers,
Rick.
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  #35  
Old 02-06-2014, 01:17 PM
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My QSI583 (Kodak KAF 8300 sensor) has 279 columns on the right. Spoke to QSI's Kevin Nelson about it, and he recommended just using the right-most 100-150 columns. So it varies from sensor to sensor, and possibly(?) camera manufacturer to manufacturer.
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  #36  
Old 02-06-2014, 01:38 PM
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Hi Pete,

Dave has been pretty busy lately so I suspect you won't hear back from him soon. I also suspect he won't know the answer to your question. You should ask SBIG and/or poke around the extra data in the frame to figure out where the overscan data lies. On my 16803 camera (Apogee U16M) there is a set of 10 columns at the far right of the frame that do the trick but the SBIG driver will almost certainly map things differently.

Cheers,
Rick.
Thanks Rick

I think it was the discussion about real vs dummy overscan that threw me. I can access my obs PC from work so I'm playing with this as a I type.

Looking at a 1200 second dark, I have a dark border to the right and bottom of the frame. However at the right side, there is a brighter line (several pixels wide) between what appears to be the edge of the image and the dark border. I wonder if this is the readout channel for the chip. I've posted a pic.

Interestingly, I have no dark border at the base of a bias frame, only to the right. Also attach a pic.

How do I distinguish the real overscan from the dummy overscan?


Quote:
Originally Posted by troypiggo View Post
My QSI583 (Kodak KAF 8300 sensor) has 279 columns on the right. Spoke to QSI's Kevin Nelson about it, and he recommended just using the right-most 100-150 columns. So it varies from sensor to sensor, and possibly(?) camera manufacturer to manufacturer.
Sadly, SBIG are less forthcoming - though there may be something deep in the bowels of the SBIG site - off to do some digging now

Pete
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  #37  
Old 02-06-2014, 01:39 PM
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oops - that is the bias - here is the dark

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  #38  
Old 02-06-2014, 02:05 PM
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Did some more digging - the KAF16803 fact sheet suggest that its only a 20 pixel border to the left and above/below the chip that are useful for overscan and 9 pixels to the right.

Here is the link

http://www.ccd.com/pdf/ccd_16m.pdf

My head is spinning - so I guess its time to use the time-honoured trial and error method

Pete
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