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questions/musings about cmos calibration

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#1 calypsob

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Posted 13 July 2018 - 09:54 AM

In order to remove ampglow from certain cmos bodies we have to do Darks, Flats, and Dark flats, bias are omitted. 

 

Im wondering though, why are bias frames skipped?  I read a thread a while back about sample variance issues with master bias frames in certain cmos models.  Could this be due to ampglow build up in the extraordinarily short bias exposure?  Would it make sense to attempt applying dark bias frames to a master bias?  I also wonder if these cameras are able to expose too fast to properly record a bias frame, could slowing the exposures slightly to .5 seconds improve bias results?  

 

Additionally, I found that after adding a pedestal to the output offset and applying large scale structure rejection to my flats, ampglow was severely attenuated if not completely removed.  Has anyone else experienced similar results?  Hopefully this will inspire some helpful discussion.  



#2 entilza

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Posted 13 July 2018 - 10:09 AM

I know QHY recommends a bias frame of .3 seconds, mainly to avoid a black bar at the top.

 

Honestly after getting my camera I've just used dark flats (I usually take 1" to 2" flats) and have never had any issue calibrating, it's actually such an enjoyable experience coming from DSLR that I just don't think about it anymore.

 

I have never needed to use a pedestal, but for me I've settled on two settings:  The default [ 174 gain, 77 offset for Narrow band] and [ 0 gain, 77 offset for LRGB ].  No issues calibrating.  I'm not sure how these gains are related to the ASI1600.  However just stating what I use.  I can see how easily setting an incorrect offset can mess up calibrations though, by not touching it I've kept myself from this danger.  I think a lot of people starting with these cameras have too many settings and I believe ZWO is now pre-locking the offset per gain which I think is a good idea to keep things simpler.  QHY hasn't locked the offset yet.

 

I have not tried to add bias (.3" bias frames ) to see if the resulting calibration would improve, perhaps it's worth an attempt.



#3 Jon Rista

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Posted 13 July 2018 - 10:37 AM

Bias on the Panasonic M sensor is unstable at short exposures and can have an uneven field (that changes with each exposure). It stabilizes around 0.2 seconds:

 

z3HkeXk.jpg

 

As long as your "bias" frames are 0.2 seconds or longer then you should be fine, however with camera-controlled exposures (usually the case <1s) amp glow can be higher. For flats, this is fine, I usually take one set of "dark flats" (biases, basically) and use them with all of my sub-second flat exposures. 



#4 bobzeq25

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Posted 13 July 2018 - 11:07 AM

In order to remove ampglow from certain cmos bodies we have to do Darks, Flats, and Dark flats, bias are omitted. 

 

Im wondering though, why are bias frames skipped?  I read a thread a while back about sample variance issues with master bias frames in certain cmos models.  Could this be due to ampglow build up in the extraordinarily short bias exposure?  Would it make sense to attempt applying dark bias frames to a master bias?  I also wonder if these cameras are able to expose too fast to properly record a bias frame, could slowing the exposures slightly to .5 seconds improve bias results?  

 

Additionally, I found that after adding a pedestal to the output offset and applying large scale structure rejection to my flats, ampglow was severely attenuated if not completely removed.  Has anyone else experienced similar results?  Hopefully this will inspire some helpful discussion.  

Dark flats and bias are redundant.  People can disagree about when to use one or the other, but doing both adds nothing.


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#5 Phil Hosey

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Posted 13 July 2018 - 01:53 PM

I only ever took bias so that I could use dark optimization in PI. That way I could use one calibrated master dark for my lights and flats. That really only works if you dont have to deal with amp glow in which case you would just use a matching master dark that hasn't been calibrated with a bias.
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#6 Ken Sturrock

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Posted 13 July 2018 - 02:30 PM

Because, as Jon pointed out, the glows aren't necessarily linear or stable.

 

Although a simplification, think about how this works: A bias frame contains the noise generated by the camera simply taking an exposure and reading out the data. A dark frame contains that same noise plus any additional noise generated by the longer term activation of the camera for the duration of the dark frame.

 

If, for example, you calibrate a four minute light frame with a four minute dark frame then it would remove all of the inherent, not time-related, noise in the camera and also the noise that built up through four minutes of use. Done.

 

Where a Bias frame comes into play is if you want to calibrate a light frame but don't have a matching dark frame. Suppose that you have a two minute light frame but a four minute dark frame. What happens is that the software will subtract out the noise in the Bias frame from both the two minute light frame and the four minute dark frame. It will then scale (aka optimize) the bias-subtracted four minute dark frame by dividing the remaining values of the pixels by two in order to simulate what a two minute dark frame might oughta look like if you had one. It will then subtract that "fake" two minute dark frame from your two minute light frame. In other words, the combination of the bias frame and the scaled four minute dark frame should have about the same noise as a genuine two minute dark frame.

 

If you have a light frame that has a longer exposure than the available dark frame, the process is similar. Suppose that you had a six minute light frame and a four minute dark frame with a bias. The software will subtract the noise in the bias frame from both the six minute light frame and the four minute dark. It will then multiple the remaining pixel values in the four minute dark by 50% to create a "fake" six minute dark frame. It will then remove the "enhanced" noise found in this fake six minute dark frame from the six minute light frame.

 

This scaling process generally works pretty well so long as the noise in the image is, in fact, linear. 

 

Aye. This is a simplification, but not too far off.

 

On my ASI-183 Mono Pro (which may behave differently than other cameras) I will use bias frames and a slightly longer mis-matched dark to calibrate my flats and it usually works fine. My actual light frames, however, are always calibrated with a matching dark and no bias. I don't have any experience with using the Pedestal, but the large-scale rejection doesn't seem to matter on my frames.


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#7 jdupton

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Posted 13 July 2018 - 03:43 PM

Ken,

 

   That's a good summary.

 

   I have seen a few folks comment that the glow growth for CMOS cameras is exponential rather than linear. I just tested that aspect on my ASI294MC camera a couple of days ago. I find that the glow area is quite linear with respect to exposure. However, it does appear slightly exponential with respect to temperature. While scaling Darks may not be optimum for all cameras, it isn't hard to check your camera. If the amp glows are linear as they are for me, then you can get by with scaling the Darks (not too far), so long as they are shot at the same temperature as the lights.

 

   My testing procedure was to shoot Bias and Dark frames at -10, -5, 0, +5, and +10 degrees C with exposure times of 0, 30, 60, 120, 240, 480, and 960 seconds. I compiled all the statistical data from those exposures at Full Frame, a 1000 by 1000 pixel Central Area Crop, and a 50 by 200 pixel Glow Area Crop centered on the brightest amp glow area at the edge of the frame. I then sliced and diced all the data (amounting to 475 frames) to come up with a wide variety of views of differing aspects of camera's Bias and Dark frame properties. I was surprised at first to see the linear growth of the amp glow brightness given the reports of others saying their cameras were decidedly non-linear.

 

Glow_Crop_vs_Exp.png

 

   My own conclusion for my camera is that so long as you are very careful to match temperatures, you can scale Darks to match your Lights within reason for at least some CMOS cameras.

 

 

John


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#8 Ken Sturrock

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Posted 13 July 2018 - 06:28 PM

Thanks for that insight, John. I haven't studied it as precisely as you have. I will also admit that when I started imaging, the regular use of Bias frames wasn't really "a thing" and I don't normally see a challenge in just maintaining a matched library for my two or three common light frame exposure durations. In that regard, I may be driven more by the habit than the absolute science.

 

At the end of the day, though, matching the dark (duration, temperature, gain, offset) is an obvious path but the scaled dark may work well also, so long as folks take the time to see for themselves. I appreciate your research on the matter. Very interesting to see such a clean growth of the glow.



#9 Michael Covington

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Posted 13 July 2018 - 07:07 PM

Here is why bias frames are not needed if you have lights, darks, flats, and dark flats, and are not scaling the darks:

http://www.covington...slr/#Arithmetic


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#10 Ken Sturrock

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Posted 13 July 2018 - 07:31 PM

That's a great article, Michael. Interesting that you mention bias frames as being "older". I never seemed to see people use bias frames until DSLR imaging started to really make the scene. Thanks for the link!


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#11 Michael Covington

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Posted 13 July 2018 - 08:38 PM

Well, I thought the use of bias frames was older practice, going back to early CCD days, but I could be mistaken.

 

I've written a revision of the PixInsight BatchPreprocessing script that makes things a little easier for people who don't take bias frames.  it is here:http://www.covington...PreprocessingFD

 

Actually, that's elsewhere on the same rather rambling page I referred you to.

 


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#12 freestar8n

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Posted 13 July 2018 - 10:07 PM

With my cmos cameras I do daytime flats that are very short exposures - so I just use a "bias" of similar short exposure but not exactly the same.  So nothing is very different for me from a normal ccd that has better behaved exposures in the 0 to 5 second range.

 

There is a slight change in behavior above one second exposures - but if you are just using darks with lights and "bias" with very short flat exposures - it all behaves normally.

 

My new ASI183MC has very strong "amp glow" (I guess) that shows in darks - but it calibrates away cleanly.  It doesn't matter how strong or scary looking it is as long as it is repeatable - which it appears to be.

 

Flat exposures have such high photon counts that a small inaccuracy in matching the bias or "dark flat" won't matter much - unless it really shifts the "floor" significantly.

 

You do need to be more careful if you are scaling the darks to match the lights - but with my imaging I tend to use only a small set of exposure times - up to about 4 minutes.  So having exactly matching darks isn't a big deal.

 

So the lights are happy with a master dark of the same exposure time - and the flats are happy with a bias of similar, but not exactly the same, short exposure time.

 

Frank


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#13 Der_Pit

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Posted 14 July 2018 - 05:47 AM

Here is why bias frames are not needed if you have lights, darks, flats, and dark flats, and are not scaling the darks:

http://www.covington...slr/#Arithmetic

A very nice compendium, indeed :)

 

I stumbled across one thing there:

 

Another advantage of this method is that flats and lights do not have to be taken at the same ISO setting.

Have you actually verified this?  I agree it is correct if the camera response is strictly linear or at least does not change shape when changing ISO/gain.  But I've seen CMOS sensors failing in either (or even both) respects....

So I'd expect this conclusion to be somewhat camera dependent and one should confirm it for the actual configuration?


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#14 Michael Covington

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Posted 14 July 2018 - 01:03 PM

If the camera response were not linear, then calibration as a whole would not work.

 

Digital sensors are notoriously linear -- that is the best thing about them -- it's why we can dark-subtract digital images but not film images.  Film is highly nonlinear.



#15 Der_Pit

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Posted 15 July 2018 - 03:26 PM

Yes, compared to film even the bad digital sensors are very linear.  But as I wrote, especially CMOS sensors with small pixels (and thus small FWC) can have issues there.  Maybe I'm just too picky there (coming from the professional side, I work in a solar observatory).  We never touch gain for calibration frames....



#16 Lite2

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Posted 15 July 2018 - 04:19 PM

Well, I thought the use of bias frames was older practice, going back to early CCD days, but I could be mistaken.

 

I've written a revision of the PixInsight BatchPreprocessing script that makes things a little easier for people who don't take bias frames.  it is here:http://www.covington...PreprocessingFD

 

Actually, that's elsewhere on the same rather rambling page I referred you to.

Nice!! Thank you so much for this! Just installed and trying it out now.



#17 Jon Rista

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Posted 15 July 2018 - 04:44 PM

Ken,

 

   That's a good summary.

 

   I have seen a few folks comment that the glow growth for CMOS cameras is exponential rather than linear. I just tested that aspect on my ASI294MC camera a couple of days ago. I find that the glow area is quite linear with respect to exposure. However, it does appear slightly exponential with respect to temperature. While scaling Darks may not be optimum for all cameras, it isn't hard to check your camera. If the amp glows are linear as they are for me, then you can get by with scaling the Darks (not too far), so long as they are shot at the same temperature as the lights.

 

   My testing procedure was to shoot Bias and Dark frames at -10, -5, 0, +5, and +10 degrees C with exposure times of 0, 30, 60, 120, 240, 480, and 960 seconds. I compiled all the statistical data from those exposures at Full Frame, a 1000 by 1000 pixel Central Area Crop, and a 50 by 200 pixel Glow Area Crop centered on the brightest amp glow area at the edge of the frame. I then sliced and diced all the data (amounting to 475 frames) to come up with a wide variety of views of differing aspects of camera's Bias and Dark frame properties. I was surprised at first to see the linear growth of the amp glow brightness given the reports of others saying their cameras were decidedly non-linear.

 

attachicon.gif Glow_Crop_vs_Exp.png

 

   My own conclusion for my camera is that so long as you are very careful to match temperatures, you can scale Darks to match your Lights within reason for at least some CMOS cameras.

 

 

John

How are you measuring your median?

 

Consider this (IMX183):

 

1tfiBLp.jpg

 

The glows seemed to be quite non-linear vs. exposure time, considering that 1 hours worth of normalized short subs have significantly less glow than 1 hours worth of normalized long subs...

 

If you actually try scaling a long dark to a shorter sub...does it actually work? Or is there remnant glow left behind?


Edited by Jon Rista, 15 July 2018 - 04:48 PM.

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#18 Michael Covington

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Posted 15 July 2018 - 04:56 PM

What should rise linearly is the *intensity* of the glow at any given point -- not the *area* of glow above a particular level, because the glow itself is surely distributed nonlinearly over the area.

Imagine a tiny sensor with just 4 pixels.  Their glow levels are respectively 1, 1, 4, and 20.  You consider glow detectable when it exceeds 10.  So 1/4 of the sensor has detectable glow.

So now you multiply your exposure by 5.  Now the glow levels are 5, 5, 20, and 100.  Now 1/2 of the sensor has detectable glow.  But you multiplied  your exposure by 5, not 2, in order to produce this. 

 

Nonlinearity?  Only in the distribution of the glow, not in the response of the pixels, which indeed increased by exactly a factor of 5 in this thought-experiment.

 



#19 Jon Rista

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Posted 15 July 2018 - 05:06 PM

What should rise linearly is the *intensity* of the glow at any given point -- not the *area* of glow above a particular level, because the glow itself is surely distributed nonlinearly over the area.

Imagine a tiny sensor with just 4 pixels.  Their glow levels are respectively 1, 1, 4, and 20.  You consider glow detectable when it exceeds 10.  So 1/4 of the sensor has detectable glow.

So now you multiply your exposure by 5.  Now the glow levels are 5, 5, 20, and 100.  Now 1/2 of the sensor has detectable glow.  But you multiplied  your exposure by 5, not 2, in order to produce this. 

 

Nonlinearity?  Only in the distribution of the glow, not in the response of the pixels, which indeed increased by exactly a factor of 5 in this thought-experiment.

Again...have you actually tried to scale long darks with glows when calibrating shorter lights? 

 

It usually does not work. You will usually end up with remnant glow, or inverted glow, something like that. So I am not convinced it is simply a change in the distribution. I don't know why, specifically...although I suspect that certain components heat differently with longer subs, and glows do indeed seem to be more sensitive to temperature changes than the normal dark current. 

 

Note that these are not always (or even ever) actual "amplifier" glows. These glows come from a variety of circuitry, and in some cases may even be NIR light emission. There are zero guarantees that they are consistent, with each other or the underlying dark current, over time. 

 

In any case...I would TEST, because simply measuring the median of dark frames of successive exposure length isn't going to tell you enough about the nature of the glows. I would bet that in testing, long scaled darks do not calibrate shorter lights as well as shorter darks. 


Edited by Jon Rista, 15 July 2018 - 05:07 PM.

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#20 jdupton

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Posted 15 July 2018 - 05:59 PM

Jon,

 

How are you measuring your median?

 

Consider this (IMX183):

 

 

The glows seemed to be quite non-linear vs. exposure time, considering that 1 hours worth of normalized short subs have significantly less glow than 1 hours worth of normalized long subs...

 

If you actually try scaling a long dark to a shorter sub...does it actually work? Or is there remnant glow left behind?

   To be precise, I did not measure the median; I measured and plotted the means. I have the data and could create a plot of median values instead when I get some more time.

 

   A more detailed explanation of my methodology follows:

  1. Use the same gain and offset for all tests -- 120 gain, 30 offset for my ASI294MC camera
  2. Take 50 Bias frames (not directly used for this analysis) (I used this same data to look at lots of other aspects of the camera.)
  3. Take 3 Dark frames at exposures of 60, 120, 240, 480, and 960 seconds
  4. Repeat Steps 2 and 3 set-point temperatures of -10, -5, 0, +5, and +10 degrees C.

   Use PixInsight to define areas to measure:

  1. Create a blank image of the same size as the camera frames.
  2. Define a preview of 1000x1000 pixels exactly centered in the frame.
  3. Define a preview of 50x200 pixels centered at the right edge of the frame centered on the brightest portion of the glow

 

   Using the Dark frame images gathered above along with the synthetic blank image, create new images of two cropped areas.

  1. Start the Blink Process
  2. Drag the center preview area to the Blink Image window
  3. Run the Blink Crop command to export the centered cropped preview of all images
  4. Reload the original images to Blink
  5. Drag the Glow area crop to the Blink Image window
  6. Run the Blink Crop command to export all the glow cropped preview images.

   Now get statistical data for all frames from PixInsight.

  1. Load images into the Blink Process.
  2. Export the statistical data with additional FITs Keyword fields to a text file
  3. Repeat step 2 for each of the data sets gathered including all Dark frames and all centered crops and glow crops. This was 475 sets of statistical frame data.

   Import the statistics files created by Blink into a spreadsheet. I did the analysis using pivot tables which allow you to create interactive graphs such as the one I originally posted.

 

   While I only used three Dark frames for the statistical data, I doubt that the plots would become non-linear with more samples being gathered. I did not judge any of the Dark frames or their glows by eye. It was all done by the statistics numbers that PixInsight extracts for each Dark Frame or cropped portion thereof. In addition, I did not integrate the frames. I used the spreadsheet pivot table to automatically take the means of the means of each triad of frames for a given Set-Point and exposure.

 

   The graph I posted is the output of the pivot table analysis in graphical form.

 

   I have not yet played with scaling a Dark to see how well it will calibrate.

 

 

John


Edited by jdupton, 15 July 2018 - 06:00 PM.


#21 jdupton

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Posted 15 July 2018 - 07:08 PM

Jon,

 

   As a follow up to the above, I tried a quick experiment.

 

If you actually try scaling a long dark to a shorter sub...does it actually work? Or is there remnant glow left behind?

 

   Rather than calibrate a light, I took a single 240 second Dark Frame and a single 120 second Dark Frame. I first subtracted a Master Bias from each then scaled down the 240 second Dark by a factor of 2. I then subtracted the scaled 240 sec Dark from the 120 sec Dark with a pedestal of 0.1 (normalized).

 

   What I got was a quite flat frame that had a mean of 0.1 (the pedestal). I did a FlatContourPlot of the difference frame and it looks quite flat but with slightly increased noise where the amp glow was in both frames. I expected that because the larger standard deviation of the longer frame. It looks a lot like a Bias frame but without any sort of offset other than the 0.1 I purposely added.

 

   I can see no evidence of a residual glow after scaling the Dark by a factor of two and using it in place of a 120 second Dark. You just need to be sure to have a clean Master Bias Frame if you are going to do this.

 

 

John


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#22 calypsob

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Posted 16 July 2018 - 12:05 AM

My 183 ships monday so Im taking alot of notes here. 



#23 Ken Sturrock

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Posted 16 July 2018 - 04:16 AM

John:

 

Thanks for doing this experiment. It's a clever design. I just tried it with some of my ASI-183 frames and got a solid black frame (almost entirely zero ADU) without the pedestal. Although only a one-tailed experiment it is, I guess, what I'd expect. With the pedestal, PI complains about the match, so trying to see if it over-corrected past the image floor didn't work for me.

 

I'm also not firing on all my mental cylinders right now - trouble sleeping.

 

I'll give it a try with some real data next time I do a project.

 

Calypsob:

 

Which IMX183 did you buy again? So long as you've thought through the tiny pixels, I suspect that you'll be happy with it.

 

-Ken


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#24 freestar8n

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Posted 16 July 2018 - 04:33 AM

If you are working with a large patch of pixels you need to be sure the overall values aren't skewed by a number of hot and cold pixels that really are badly behaved.  Ideally you should detect them and exclude them from any statistics.

 

The only nonlinearity I have seen with cmos cameras is slightly odd behavior below around 1 second exposures.  I haven't checked for nonlinearity of amp glow - or whatever that star pattern is - but I usually use darks matched in time to lights anyway.

 

Just selecting a number of pixels in those regions and plotting them vs. (dark) exposure time would be a pretty direct way to see any nonlinearity.  The slopes will be different and there will be shot noise - but it's nice to see the raw behavior of some pixels - and if you see what looks like nonlinear behavior in time with raw data from a number of pixels - that would be a good indication something weird is going on.

 

But I haven't looked for it myself.

 

There's no guarantee all these effects have a mean current rate for a given pixel - but there's also no reason they couldn't behave pretty well - and linearly.

 

Frank



#25 calypsob

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Posted 16 July 2018 - 06:57 AM

John:

 

Thanks for doing this experiment. It's a clever design. I just tried it with some of my ASI-183 frames and got a solid black frame (almost entirely zero ADU) without the pedestal. Although only a one-tailed experiment it is, I guess, what I'd expect. With the pedestal, PI complains about the match, so trying to see if it over-corrected past the image floor didn't work for me.

 

I'm also not firing on all my mental cylinders right now - trouble sleeping.

 

I'll give it a try with some real data next time I do a project.

 

Calypsob:

 

Which IMX183 did you buy again? So long as you've thought through the tiny pixels, I suspect that you'll be happy with it.

 

-Ken

I ordered a qhy183 osc.

I plan to pair it with either my 600mm f4 newt, 480mm f5.6 refractor or soon to be 400mm f4.2 refractor and of course the 135mm f2 and 50mm lens for wide fields. The small pixels should play nicely with these optics and if calibration works the way I want it to, a second 183 will be paired with the osc, probably a mono.

 

how many darks are people using after lets say a 4hr lrgb session with 30-60s subs?




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