94 replies to this topic

[andythilo]

Triangulum.jpgIt's been a few months since anyone has posted on this thread, but I'm having the exact same problem with the 294 and the ZWO Duo-Band filter.  My images have a pretty dramatic color gradient and I'm wondering if anyone came up with a solution (increased gain?) to solve the problem.  The gradient appears in lights and flats pretty equally.  Interestingly, if you rotate the filter the gradient moves with it.

Interestingly I've been having this issue lately with my 294 and a L-Enhance filter. Posted about it on the PI forum and someone pointed out my flats could be the issue. Not sure if I'm allowed to post the link here or not.

[andythilo]

Has anyone determined an optimal flats exposure setting for the 294? I'm sure I read that it should be >2secs. However in my PI forum post it's been recommended to just run the my flat field generator at 12v and use the appropriate exposure setting. ADU should be about 27000?

[Stevemr2t]

I'm having almost this exact same issue on my 294MCPRO and Triad Quadband Filter (Very bad color gradient) and to some degree the L-Enhance. It's possible to work around the L-enhance, but the Quadband just isn't working well at all. 

 

Quadband Flat (Light Panel ~ 25000) - Almost identical with T-shirt etc.

 

[FreemanHK]

I recently purchased my 294MC Pro and I think I have the same issue. I tested this camera for a few nights with different filters. Here are some stretched master flats I have (stacked using SiriL, debayered). It seems to me that the more the lights been filtered out, the worse it'll be. 

L-Pro filter:

UHC filter: 

 

Dual Narrowband filter:

 

Ha filter:

Now I give up calibrating with flat frames though there are also red patterns in the light frame(dual narrowband filter, 130 x 120s, gain 120, -10C stacked):

Edited by FreemanHK, 10 February 2021 - 08:42 PM.

[Creedence]

I experienced the same thing with the 294MC, and because I was committed to using the Triad Ultra I ended up trading it in for the ASI071MC-Pro.  It does not share this issue with the Triad, and I like the fact that there seems to be less read noise and NO amp glow.

 

The 294MC was a great camera, but it does not play nicely with the Triad.

[StevenBellavia]

Hi,

 

I am late to the party, but could this be caused by the filter itself, or perhaps some type of Moire pattern or other diffraction pattern that develops between the filter and the sensor and/or the Bayer matrix? 

Or maybe this is just the PRNU or FPN for that sensor, and it is within spec? (though I don't know why certain filters would accentuate that.  It might be wavelength, bandwidth and/or exposure length dependent?).

 

I have the 294MM (not the MC, so no Bayer matrix).

 

Attached is a master flat from an astronomik 6nm Ha filter.  As you can see it is also a strange looking pattern.  So you can't blame the Bayer Matrix. There is none.

But I have always seen something like this with several Ha filters I have owned, on several cameras, and the light frames calibrate just fine. (Note, I always use Dark-Flats, not Bias).

 

Also attached are the OIII and Lum master flats (and red, green and blue all look reasonably "flat" as well).

 

So it seems to just be the Ha that gets the odd pattern.  "bad" filter?

Or is it a  "bad" sensor?  PRNU?  FPN?  Diffraction artifact ?  But that would imply that things are only "bad" at 656nm, 6nm bandwidth.  Maybe?

Note: the Ha flats were 0.33 seconds, so not exceptionally long.  I think that rules out an exposure dependence.

 

I have not had issues with calibration (though now I just jinxed myself).

 

I also own a 533MC and a 183MC.  I am going to do flats with that same exact filter, and compare.  I will also extract each color, as the 3 colors cannot be stretched the same due to the flat frame panel not being exactly uniform in all 3 colors.

I have never had an issue with those cameras either, and have used a ZWO duo-band with both.  But I have not tried a straight Ha filter with those cameras.

 

I will get back to you after the tests.

 

My early hypothesis:  This is PRNU or FPN of the sensor itself which, for some reason, is exaggerated in Ha narrow-band, and is "normal" (i.e., within spec and not a "defect", and can be calibrated out).

 

Steve

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[StevenBellavia]

These are the master flats for that same 6nm Ha filters on the ZWO ASI 183MC camera.

 

Note: I had to run different panel brightness for each color, to keep the extracted histograms near the middle.

 

As you can see, completely different patterns, but patterns are there.  (not as pronounced as the 294MM).

 

So I believe this confirms it is not the filter.

 

So, for the ASI 294 (MM or MC) it seems to be the sensor response to 656nm @6nm bandwidth.

 

For me this is good news, as I had intentions of getting the 294MC, and why I landed on this thread.

I've been very happy with the 294MM, so I think I will be okay with the 294MC.  I do not plan to use it for narrow or multi-band.  I will only be using the 294MC for obtaining color data, as I will continue to use the 294MM for luminance and narrow-band.

 

I am not even sure how to obtain and use flat frames with a narrow-band filter on a OSC camera, as it must be nearly impossible to pick a single exposure that has a mid-histogram for all three colors. In some of my attempts, either red was totally saturated, or if I got red to move left, blue was completely gone.   With the duo-band filter, it is better but not ideal.

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Edited by StevenBellavia, 26 October 2021 - 12:58 PM.

[StevenBellavia]

Attached / below  is an Ha Flat from the 183MM (left), next to the Ha Flat from the 294MM (right).
 

The two patterns are similar. I have never had any issues with either of my 183's (I have both the MC and MM).

(And I have no issues with the 294.  Just very interesting Ha flat frames.  This pattern is not visible on my raw light frames, regardless of how hard I stretch them)

 

This leads me to my next hypothesis: 

 

Silicon is partially transparent, and becomes more transparent as the wavelength increases.  The frame and amplifiers behind the sensor affect some of the light passing through and reflecting back.  This is more evident in narrowband Ha, as it is a long wavelength.  A flat panel is a lot of light, so reflections, ghosting etc., could be possible.  I think we are looking at the structure of the sensor, via light passing through the silicon and reflecting back and forth between it and the very reflective filter. (My narrowband filters look like mirrors when you hold them in your hand).

 

Conclusion:  I think it's a selfie.  The sensor is taking a picture of itself, using the narrowband filter as the mirror. 

 

The original poster's post #12 confirms this.  An image of the sensor is identical to the flat frame (but flipped left-to-right, due to the sensor taking the picture versus being taken of the sensor).

 

****************************************************************************

 

This is an edit to this post (Oct-28-2021):

 

I just did a test with the narrowband Ha filter in front of the camera lens, (not close to the sensor, as before) and I got an identical pattern.

I don't think reflections could make it past 6 AR-coated elements.

And I don't think a reflected image would be the same with the filter so far forward.

I also tried putting another, well coated Luminance filter directly in front of the camera with the Ha filter still out in front of the lens, with no effect.

 

So now I am unsure what is causing it.

 

It must be something to do with how the pixels themselves transmit and reflect narrowband Ha light.  Or perhaps doping or coating of the silicon, etc.

The frame and structure might still have something to do with how the light is absorbed and reflected, but it is not simply the image of the frame, as I had thought. And it is not temperature, otherwise the image of the sensor, without anything connected to it, would not look just like the flat frame (post #12)

 

It is interesting how the 183 and several (all?) the 294 Ha flats have a "frame-like" structure to them.

 

So back to the drawing board...

 

Steve

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Edited by StevenBellavia, 28 October 2021 - 01:30 PM.

[StevenBellavia]

A friend and co-worker on a large telescope camera project (he asked to remain anonymous), had the following to say:

 

If it's only seen when 656nm light shines on the sensor, and if there's no cover glass, there must be an etalon effect within the sensor itself. At wavelengths where the silicon is semi-transparent (more precisely, when the absorption length is longer than the thickness) the light reflected from the front and back silicon-air interfaces interferes to create standing wave intensity patterns depending on the local thickness -- i.e. constructive interference when the optical thickness is an integer multiple of the wavelength. For polychromatic , broadband light these patterns wash out. It's similar to the Newton's ring patterns on oil films.

 

Our sensors show complicated fringing patterns when illuminated with narrowband light at wavelengths >930nm. For a long time we attributed this to reflections between the silicon front and back interfaces, but recent analysis showed it is actually comprised of two patterns overlaid. One is from the silicon front-back side, but the more prominent one seems to be coming from the layer of epoxy where the silicon is glued to a support structure. In your sensor it's possible that there is a similar non-silicon layer whose thickness, and thickness variation, conspire to produce an interference having the structured pattern you see. In that case one would need to know the optical properties (refractive index and absorption length at 656nm) as well as the thickness to get a sense of the thickness variation that could produce that pattern. Since there's no fringe-like pattern, it would have to mean that the thickness variation is never more than one wavelength.

 

Here's the absorption length vs. wavelength and temperature for silicon for reference. If you know the QE at H-a (and reflection losses) you can estimate whether the silicon is transparent enough for it to be a silicon etalon effect.

 

So I was close, in that I attributed it to underlying structure (which could be the adhesive, as he points out).

I believe this, or at least something very similar, is the cause of these patterns in Ha and Sii wavelengths.

 

Note:  I just tried sodium (589nm @10nm bandwidth) and a long-pass, 685nm filter, and neither show the pattern. So, apparently 589nm is too short, and It will not show up in broad bandwidths, respectively. The thickness of the silicon and/or adhesive must be around 650 to 680nm, or an integer multiple of that, as it only shows up in Ha and SII light.

 

So this is just a plausible answer to why it happens.  I cannot give much guidance on how to work around it for calibration.

 

Steve

 

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[StevenBellavia]

I am going to go out on a limb, and offer a possible mitigation strategy for this odd flat-frame pattern issue. 

 

But I could be wrong about this, and I hope some other CN heavy hitters set me straight if I am.

 

I think "regular" reflections are a threshold phenomenon.  That is, if you have a bright star in the field, it will show a reflection, but the other dimmer stars won't.  I think that is because the reflections from the dimmer stars don't register, or are below the read noise, so they get lost when stacking (which is a good thing).

And I also think that this etalon effect is similar.  It has to be, if it doesn't show up in the light frames, right?

And if something doesn't show up in the light frames, but does in the flat frames, then the flat frames are going to wreck the light frames (and hence the reason for this thread).

 

So, if we can gather flat frames at the dimmest possible level, that is, close to the light frame intensities, while still being within the linear regime of the sensor, then perhaps this will reduce, or even eliminate the problem?

 

Attached are 2 of the final stacks of 11 sets of flat frames, (5X of each) with exposures ranging from 0.2 seconds to 2.2 seconds.

The mean histogram varies from about 1/16 to 1/2 histogram.  And from a plot of mean ADU's versus the single sub exposure time, it all seems to be linear.

It also seems that shorter exposures have less noise (lower StdDev), but I think that is misleading.  I think (?) we will need many more flats at the lower exposures so that we don't dominate the calibrated and stacked image with flat frame noise.

Note:  I am also unsure if the visual aesthetics of the auto-stretched master flat is of any use.  But the shorter does "look" better (?)

So I think short exposure flat frames  (and then bias or corresponding dark-flats, which will be essentially the same) is one way to reduce this issue.

 

But to repeat:  I COULD BE WRONG.

 

Steve

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Edited by StevenBellavia, 31 October 2021 - 07:15 PM.

[sbharrat]

I am going to go out on a limb, and offer a possible mitigation strategy for this odd flat-frame pattern issue. 

 

But I could be wrong about this, and I hope some other CN heavy hitters set me straight if I am.

 

I think "regular" reflections are a threshold phenomenon.  That is, if you have a bright star in the field, it will show a reflection, but the other dimmer stars won't.  I think that is because the reflections from the dimmer stars don't register, or are below the read noise, so they get lost when stacking (which is a good thing).

And I also think that this etalon effect is similar.  It has to be, if it doesn't show up in the light frames, right?

And if something doesn't show up in the light frames, but does in the flat frames, then the flat frames are going to wreck the light frames (and hence the reason for this thread).

 

So, if we can gather flat frames at the dimmest possible level, that is, close to the light frame intensities, while still being within the linear regime of the sensor, then perhaps this will reduce, or even eliminate the problem?

 

...

 

Steve

This seems like a detailed analysis leading to the same conclusion I have seen posited in several threads about flats on a 294: you need to use a very dim flat panel (to stay within the linear portion of the sensor) and ensure that your flats are over 3s long (to get sufficient total ADU). 

 

See for example:

https://www.cloudyni...-flats-for-294/

https://www.cloudyni...and-flats-help/

https://www.cloudyni...-compare-notes/

 

Or am I misunderstanding the outcome of your analysis? 

[OrionSword]

Yannick,

If you haven't already I invite you to read this thread and you will see this appears to be a sensor specific problem but with variances from camera to camera in the 294M or C series.  I have the mono and these odd patterns do indeed calibrate out,  In my case I use a Tracing panel for flats, exposures are kept in the 2-5 sec range and I seek a 5000 ADU values in that exposure range.  You go much higher and the flat over  compensates and spoils the lights.  Johnbravo in the same thread has seen the same anomaly with the 294 cameras.  Mine is QHY but I am sure ZWO sources similar sensors.  This was distubing at first but I deal with this camera in my inventory as a special use case and it works well thus far.  We have examples posted in the thread below:

 

https://www.cloudyni...-color-pattern/

[StevenBellavia]

This seems like a detailed analysis leading to the same conclusion I have seen posited in several threads about flats on a 294: you need to use a very dim flat panel (to stay within the linear portion of the sensor) and ensure that your flats are over 3s long (to get sufficient total ADU). 

 

See for example:

https://www.cloudyni...-flats-for-294/

https://www.cloudyni...and-flats-help/

https://www.cloudyni...-compare-notes/

 

Or am I misunderstanding the outcome of your analysis? 

I have quickly read those threads, and it seems the "blotchy" issue is for all filters, whereas this thread specifically addresses narrowband Ha and SII.  My 294MM only has the odd flats in those, with no issues in broadband or shorter wavelength narrowband.

The other threads also show blotchiness in light frames, which I do no believe is what this thread is about (?)

 

I think the advice to go 3 seconds and as dim as possible is true for all cameras, and is a standard "recipe" for flat frames.

I believe the etalon effect is strictly unique for Ha and SII narrowband and needs a non-conventional recipe as I have proposed.

 

Another thought I had:  Take much longer light frames, until the pattern appears.  Then it will be "matching" the pattern of the flat frames.

 

And when I said shorter exposure, what I really mean is lower intensity, regardless of the exposure or gain that gets you there. I think around 1/16th (4000 ADU's out of 65535) seems to still be within the linear range (though I did not try shorter or less intensity).  And I did see a 5000 ADU recommendation on one of the links you provided, so if that does indeed work, it confirms the theory somewhat.

Edited by StevenBellavia, 01 November 2021 - 05:38 PM.

[OrionSword]

In my case the only filter I used since getting this camera was Ha.

[StevenBellavia]

In my case the only filter I used since getting this camera was Ha.

Well, that certainly fits the current thinking about wavelength-specific effects, and is very useful information.  Thanks!

[rnshagam]

Has there been any resolution on the issue?  I'm using an ASI294mc pro and an L-extreme filter in two different OTAs and using a tablet as the light source with each OTA setup.  I also tried generating flats from skylightt-shirt. My flats look wonky in the red Ha channel, but more or less ok in green and blue OIII. I'm playing around in SiriL with constructing a synthetic flat, in which I take the green channel flat and copy it over to the red and blue channels.  So far, not great results, but I'll keep trying to fiddle with the levels in the file, which would correspond to changing ADU values.

 

Any other suggestions are appreciated.

[terry59]

A friend and co-worker on a large telescope camera project (he asked to remain anonymous), had the following to say:

 

If it's only seen when 656nm light shines on the sensor, and if there's no cover glass, there must be an etalon effect within the sensor itself. At wavelengths where the silicon is semi-transparent (more precisely, when the absorption length is longer than the thickness) the light reflected from the front and back silicon-air interfaces interferes to create standing wave intensity patterns depending on the local thickness -- i.e. constructive interference when the optical thickness is an integer multiple of the wavelength. For polychromatic , broadband light these patterns wash out. It's similar to the Newton's ring patterns on oil films.

 

Our sensors show complicated fringing patterns when illuminated with narrowband light at wavelengths >930nm. For a long time we attributed this to reflections between the silicon front and back interfaces, but recent analysis showed it is actually comprised of two patterns overlaid. One is from the silicon front-back side, but the more prominent one seems to be coming from the layer of epoxy where the silicon is glued to a support structure. In your sensor it's possible that there is a similar non-silicon layer whose thickness, and thickness variation, conspire to produce an interference having the structured pattern you see. In that case one would need to know the optical properties (refractive index and absorption length at 656nm) as well as the thickness to get a sense of the thickness variation that could produce that pattern. Since there's no fringe-like pattern, it would have to mean that the thickness variation is never more than one wavelength.

 

Here's the absorption length vs. wavelength and temperature for silicon for reference. If you know the QE at H-a (and reflection losses) you can estimate whether the silicon is transparent enough for it to be a silicon etalon effect.

 

So I was close, in that I attributed it to underlying structure (which could be the adhesive, as he points out).

I believe this, or at least something very similar, is the cause of these patterns in Ha and Sii wavelengths.

 

Note:  I just tried sodium (589nm @10nm bandwidth) and a long-pass, 685nm filter, and neither show the pattern. So, apparently 589nm is too short, and It will not show up in broad bandwidths, respectively. The thickness of the silicon and/or adhesive must be around 650 to 680nm, or an integer multiple of that, as it only shows up in Ha and SII light.

 

So this is just a plausible answer to why it happens.  I cannot give much guidance on how to work around it for calibration.

 

Steve

 

 

I am going to go out on a limb, and offer a possible mitigation strategy for this odd flat-frame pattern issue. 

 

But I could be wrong about this, and I hope some other CN heavy hitters set me straight if I am.

 

I think "regular" reflections are a threshold phenomenon.  That is, if you have a bright star in the field, it will show a reflection, but the other dimmer stars won't.  I think that is because the reflections from the dimmer stars don't register, or are below the read noise, so they get lost when stacking (which is a good thing).

And I also think that this etalon effect is similar.  It has to be, if it doesn't show up in the light frames, right?

And if something doesn't show up in the light frames, but does in the flat frames, then the flat frames are going to wreck the light frames (and hence the reason for this thread).

 

So, if we can gather flat frames at the dimmest possible level, that is, close to the light frame intensities, while still being within the linear regime of the sensor, then perhaps this will reduce, or even eliminate the problem?

 

Attached are 2 of the final stacks of 11 sets of flat frames, (5X of each) with exposures ranging from 0.2 seconds to 2.2 seconds.

The mean histogram varies from about 1/16 to 1/2 histogram.  And from a plot of mean ADU's versus the single sub exposure time, it all seems to be linear.

It also seems that shorter exposures have less noise (lower StdDev), but I think that is misleading.  I think (?) we will need many more flats at the lower exposures so that we don't dominate the calibrated and stacked image with flat frame noise.

Note:  I am also unsure if the visual aesthetics of the auto-stretched master flat is of any use.  But the shorter does "look" better (?)

So I think short exposure flat frames  (and then bias or corresponding dark-flats, which will be essentially the same) is one way to reduce this issue.

 

But to repeat:  I COULD BE WRONG.

 

Steve

Hi Steve,

 

  I have this issue with my QHY 183M and it is visible only with a Ha or SII filter so I'd already reached the same conclusion as your friend. After months of testing different methods of collecting flats I found a method that has proven to be 100% effective.

 

  Flats are taken on a bright sunny day, pointed at blue sky away from the sun (I use my park position), exposure time is set so the brightness is enough to reach ~8k ADU. 

 

  Here is my Ha flat. You can see the outline of the problem but it is dim enough to calibrate out

 

  Gain 11/offset 30

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Edited by terry59, 14 December 2021 - 08:15 AM.

[rnshagam]

I posted this message on the ZWO forum:  https://bbs.astronom...ield-question/8

 

"I have had an ASI294MC Pro for about a year now, and recently started using an Optolong L-eXtreme filter. I am getting light frames and flat frames at 656nm similar to what Vittorio and bellavia have been describing. I have been unable to properly calibrate my images with these flat frames. What should I be doing to get successful results, if what Sam@zwo claims is possible? This is really getting frustrating.

By the way, I'm convinced that, as as described by some, this is a thin film etalon interference effect due to the particular transparency/reflectivity combination of Silicon at the longer wavelengths--the etalon possibly being formed by front and back surfaces of the silicon wafer I would consider this to be a defect of this camera, unless someone can show me how to mitigate it with proper flat frame calibrations. (My background is in optical interferometry, by the way).
Thank you"

 

I hope I get a reasonable  answer.  I've been able to ignore flat frames on most of my recent work with the L-eXtreme filter by  moving the black point a relatively atrocious amount to the right.  But it's not a satisfying way of solving the issue.  

[AdamJ]

I posted this message on the ZWO forum:  https://bbs.astronom...ield-question/8

 

"I have had an ASI294MC Pro for about a year now, and recently started using an Optolong L-eXtreme filter. I am getting light frames and flat frames at 656nm similar to what Vittorio and bellavia have been describing. I have been unable to properly calibrate my images with these flat frames. What should I be doing to get successful results, if what Sam@zwo claims is possible? This is really getting frustrating.

By the way, I'm convinced that, as as described by some, this is a thin film etalon interference effect due to the particular transparency/reflectivity combination of Silicon at the longer wavelengths--the etalon possibly being formed by front and back surfaces of the silicon wafer I would consider this to be a defect of this camera, unless someone can show me how to mitigate it with proper flat frame calibrations. (My background is in optical interferometry, by the way).
Thank you"

 

I hope I get a reasonable  answer.  I've been able to ignore flat frames on most of my recent work with the L-eXtreme filter by  moving the black point a relatively atrocious amount to the right.  But it's not a satisfying way of solving the issue.  

Dont know what gain you have all been using, but there are some suggestions that the IMX294 becomes non-linear at gains between 120 and 190, hence it wont calibrate. Try using gain 200 for all lights and calibration frames and see if it helps with calibration. 

 

Adam

Edited by AdamJ, 07 February 2022 - 11:52 AM.

[StevenBellavia]

I posted this message on the ZWO forum:  https://bbs.astronom...ield-question/8

 

"I have had an ASI294MC Pro for about a year now, and recently started using an Optolong L-eXtreme filter. I am getting light frames and flat frames at 656nm similar to what Vittorio and bellavia have been describing. I have been unable to properly calibrate my images with these flat frames. What should I be doing to get successful results, if what Sam@zwo claims is possible? This is really getting frustrating.

By the way, I'm convinced that, as as described by some, this is a thin film etalon interference effect due to the particular transparency/reflectivity combination of Silicon at the longer wavelengths--the etalon possibly being formed by front and back surfaces of the silicon wafer I would consider this to be a defect of this camera, unless someone can show me how to mitigate it with proper flat frame calibrations. (My background is in optical interferometry, by the way).
Thank you"

 

I hope I get a reasonable  answer.  I've been able to ignore flat frames on most of my recent work with the L-eXtreme filter by  moving the black point a relatively atrocious amount to the right.  But it's not a satisfying way of solving the issue.  

I wish I had an answer for you.  With the MM, the calibration works.  But with the MC, I do not know.  But I hope ZWO responds with a reasonable course of action.

One thought is to do two sets of flats: (1) to have the blue and green channels about mid-histogram; (2) to have the red channel at a low histogram (4K to 8K).

Then, in processing, extract the blue, green and red channels, and apply flats to each, separately.  Essentially treating the MC as an MM camera. I often do this anyway with my MC cameras, as it allows better control of the final colors, and avoids any "noise" from de-mosaic algorithms. (though some will claim VNG de-Bayering is better than individual R-G-B data, a topic for another thread).   But it is time-consuming

Keep us posted.

Steve

[JayScope]

Just to add another data point ... I'm getting the same sensor artifact using the new Antlia ALP-T duo 5nm filter. Newly acquired so still doing tests - but so far flats at an ADU 8000 average correct better than my flats at ADU 24,000 - still noticeable but definitely better.

Also, the artifact is more noticeable on subs with my f5.9 refractor than on my SCT at f6.3

[mrkhagol]

Dont know what gain you have all been using, but there are some suggestions that the IMX294 becomes non-linear at gains between 120 and 190, hence it wont calibrate. Try using gain 200 for all lights and calibration frames and see if it helps with calibration. 

 

Adam

What about at 100 gain? Does it not calibrate as well either?

[rnshagam]

Followup to my question:  I found a painfully obvious solution to my problem.  Turns out I hadn't been paying attention to matching gains and offsets in my calibration frames to my light frames--my calibration library was faulty.  One big facepalm for me!!!  Now everything is working just fine, despite the funky looking flats in my 294.  Furthermore, I've found that as long as I keep the flats exposures averaging to about 20,000ADU and reasonably spectrally balanced, the exposure time doesn't matter much.  I have discarded my LED tracing tablet in exchange for a white t-shirt in a needlepoint hoop and skylight when making my flats.  

 

Complex problems do sometimes have simple solutions.

[mrkhagol]

Followup to my question:  I found a painfully obvious solution to my problem.  Turns out I hadn't been paying attention to matching gains and offsets in my calibration frames to my light frames--my calibration library was faulty.  One big facepalm for me!!!  Now everything is working just fine, despite the funky looking flats in my 294.  Furthermore, I've found that as long as I keep the flats exposures averaging to about 20,000ADU and reasonably spectrally balanced, the exposure time doesn't matter much.  I have discarded my LED tracing tablet in exchange for a white t-shirt in a needlepoint hoop and skylight when making my flats.  

 

Complex problems do sometimes have simple solutions.

so what you're saying is that if you took lights at 3min OFS 14 Gain 200 then you took your flats at Gain 200 and OFS 14 but then adjusting your exposure so that you can achieve 20K ADU?

[timmymac]

I too have been frustrated with calibration frames with the 294mc pro. With the Lenhance filter it was easily manageable with the longer (5-10sec) flats/dflats with adu averages around 28k. My recently acquired antlia  Alp dual band was a whole different story. First go with it on the Jellyfish left that distinct blotchy sensor pattern real bad on my light frames that never appeared with the Lenhance (240 sec exposures at gain 122 through an f 5.5 triplet ).I tried so many different flat frame times between 5 -15 sec and nothing worked to calibrate out that pattern. Tonight I got back in to experimenting with the flats using the same lights. I upped the gain to 300 and shot 5 sec flats (many layers of tshirts) ADU around 28K.....meh maybe slightly better but pattern still pretty extreme. Next tried a gain of 225 1sec exposures and ADU around 24k. To my surprise a drastic improvement ! Not perfect yet but getting there. If anything it's looking like shooting your flats at a higher gain and more normal shorter flats/dark flats may be the key to calibrate out that mottling with these dual narrow band filters. Will keep experimenting to find the right balance.