Determining "best" ADU level for ASI1600MM Pro Mono Flats

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STEVE333 United States of America
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Determining "best" ADU level for ASI1600MM Pro Mono Flats

#1

Post by STEVE333 »


I'm still in the learning stage with my new ASI1600MM-Pro camera. However, I've found an important fact about how to produce the best Flats. I'll just state the result here. If you are interested in an explanation I've included that below.

To produce the best flats, the mean ADU of the flats should equal the mean ADU of the Light frames. This result comes about because the camera (sensor + electronics) is not linear.

EXPLANATION:
I've seen a few recommendations for the proper ADU level (mean ADU) to use for the flats. However, when I tried the recommended values the flats didn't correct very well. I'm using a Gain of 140 and an Offset of 50, and, the Light frames used had a mean ADU of approximately 3600.

Because the flats didn't correct well I tried an experiment. I created Master Flats with mean ADU levels of 3000, 5000, 7000, 10000, 15000 and 25000. The maximum ADU value for this camera is 65535 (16 bits). If the camera (sensor + electronics) is linear, then, all of the Master Flats should produce equal corrections when used to calibrate the Light frames. The picture below shows the results of six individual calibrations. Each calibration used a different one of the Master Flats mentioned above. Their are 6 individual images in the picture below. Each individual image shows the same small portion of the original image. Counting from left to right, the 6th image (far right) shows the result of calibration with a Master Flat created using a mean ADU value of 25000. The 5th image shows the result of calibration with a Master Flat created using a mean ADU value of 15000 and so on. If you look at the extreme right edge and bottom of the 6th image you will see that it is slightly darker than the rest of the image, i.e., the Master Flat didn't fully correct the edge of the image. As you look at the 5th image then 4th image and so on you will see that the correction is slowly getting better. Finally the 1st image (the one on the left) looks best of all with the mean ADU of the Master Flat being 3000 which is close to the the Light frames mean ADU of 3600. A Master Flat with mean ADU of 3600 would be the best.

ImageASI1600 Flats by STEPHEN KING, on Flickr

Because the camera is not linear, taking the Flats with the same mean ADU as the Light frame ensures that any small change in brightness of the Light frame (caused by vignetting or a dust mote or both) will have the same change in brightness of the Flat, because, both are at the same nonlinear portion of the sensors response (same mean ADU). If the camera were truly linear this wouldn't be necessary.

I don't know if all of the ASI1600MM-Pro sensors have the same degree of non-linearity that mine does, but, this is now a requirement for me.

Hope this makes sense, and, is useful for you.

PS: I will now be adjusting my exposure times to achieve a desired "standard" ADU level so that my Master Flats (one for each filter) can be reused. I'll probably stick with around 3600 ADU for the RGB filters. I'm still experimenting with the Lum filter as well as the SHO filters. Taking flats requires quite a bit of setup time as well as processing time for me, and, adjusting the exposure time is a lot easier. I can use the Pixel Aid tool in APT to measure the mean (average) ADU level of the Light frames when setting the Exposure Time for each filter.

Steve
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by JayTee »


Steve,

It made sense and thanks for sharing that informative tidbit.

Cheers,
JT
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

#3

Post by ic_1101 »


Great info Steve. I've been having the occasional problem with my flats and will give this a try. Do you also use dark flats or bias frames?
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by STEVE333 »


ic_1101 wrote: Wed Apr 08, 2020 1:15 am Great info Steve. I've been having the occasional problem with my flats and will give this a try. Do you also use dark flats or bias frames?
I tried both Alex, and, the Bias frames worked best for me (fewer of the "black pixels" showing up). I took my Bias frames at an exposure of 0.0001 sec.

One word of caution - If you use a mean ADU that is fairly low (like 3000) you will likely need to take a larger number of Bias frames to average than you normally would. This is because the lower mean ADU will produce individual flat frames with lower signal-to-noise, so, you will need to average a larger number of them to get the signal-to-noise back up. I used 100 of the 3000 ADU flat frames to create the Master Flat, and, it seemed to work fine. Of course you can delete the individual flat frames once you have the Master Flat. Just FYI.

Steve
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

#5

Post by DEnc »


Thanks for this, very interesting.

I've read that there's a difference in illumination with flats versus dark sky exposures--the light isn't collimated with the various methods of shooting flats. Could the flats with the longer exposures be picking up just a bit more internal reflection, and over-correcting your images?
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by STEVE333 »


DEnc wrote: Wed Apr 08, 2020 1:51 am Thanks for this, very interesting.

I've read that there's a difference in illumination with flats versus dark sky exposures--the light isn't collimated with the various methods of shooting flats. Could the flats with the longer exposures be picking up just a bit more internal reflection, and over-correcting your images?

Good question "DE" -
I don't believe that is the problem. The fraction of "internal reflection signal" relative to the "desired flats signal" would be constant independent of the length of the exposure. So, if there is significant internal reflection getting mixed into the flats it would corrupt the flat the same way no matter what the exposure time. At least, that's my understanding.

Steve
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by frajanssen »


Hello Steve,

Nice job.
Perhaps a stupid question: where can I find the mean ADU?

François
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

#8

Post by Graeme1858 »


frajanssen wrote: Wed Apr 08, 2020 8:29 am Hello Steve,

Nice job.
Perhaps a stupid question: where can I find the mean ADU?

François

Hello Francois

No questions are stupid here!

Steve captures his images using Astro Photography Tool. He uses the Pixel Aid tool in APT to measure the mean (average) ADU level of the Light frames when setting the Exposure Time for each filter.

Regards

Graeme
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by frajanssen »


Thanks Graeme, I found it.

Francois
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by TheButcher »


Great work Steve, I will try this out! I still use your chart you made for the gain and offset with ADU levels. :)
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by STEVE333 »


TheButcher wrote: Wed Apr 08, 2020 9:22 pm Great work Steve, I will try this out! I still use your chart you made for the gain and offset with ADU levels. :)
Thanks Butcher.

Just a thought on the suggested ADU levels in the graph. Those ADU levels are the minimum ADU that should be used. If you use higher ADU values (longer exposures) it won't degrade your image, but, it will reduce the dynamic range possibly leading to more stars being saturated. Of course longer exposures will also reduce the number of images collected. Always a tradeoff. Just FYI.

Steve
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by TheButcher »


Thanks again Steve I was also wondering about if I went higher. :)
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by STEVE333 »


Just a quick update. I've now taken new Flats for the LRGB channels. I'm away for the weekend and won't get a chance to work on a new image, but, I did calibrate the original data using the new Flats and have created the Integrated (stacked) images for all four channels. These Integrated images (LRGB) appear to be well corrected by the new flats. Processing/combining the channels is the next step.

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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by STEVE333 »


I started this post because I was not able to create Flats that would correct my images from center to edge. From the testing I concluded that my ASI1600MM-Pro camera is nonlinear at least in the lower ADU range. Further testing showed that the best mean ADU level for creating Flats is the mean ADU level of the Light frame to be calibrated.

Following the guidance from the testing mentioned above I created new Flats for the LRGB channels from recent data taken of NGC4236. The new MasterFlats corrected the RAW images all the way from center to edge for all four channels. It made the processing much simpler, and, reduced the amount of cropping needed for the final image.

This change from a DSLR camera to a cooled mono camera with filter wheel is more significant than I would ever have imagined. So many new things all at once (hardware, software, processing). I've had a few days when I was ready to "throw in the towel", but, fortunately, something would happen and a "light bulb would come on" and at least one small portion of the change would get conquered. I'm feeling more optimistic, but, not out of the woods yet.

Steve
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by Baskevo »


I hope this works for me too! Clouds are supposed to break tomorrow, so I can report back soon :) Thanks for sharing all of this Steve!
-James W.

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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

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Post by STEVE333 »


The purpose of this post is to show graphically how a camera with a nonlinear response (camera chip and/or A/D converter) can affect how Flat frames should be taken.

The graph below shows a greatly exaggerated nonlinear response curve to help illustrate what is going on. The horizontal axis is the number of photons captured by one pixel, and, the vertical axis is the corresponding output ADU value from the camera. If the camera were perfectly linear the response would be represented by the solid black curve from the lower left corner up to the upper right corner. The dashed black line shows an exaggerated nonlinear response to help illustrate the problem of creating Flats for a nonlinear sensor.

Image

The Red dot represents the mean ADU for a Light frame. The Blue dot represents an arbitrarily higher mean ADU value that might typically be used to create Flat frames. From the graph it is seen that the slope (ADU counts/photon) is higher at the Red spot than at the Blue spot (for a linear sensor the slopes would be the same). This will cause flats created with mean ADU values at the Blue spot to undercorrect images taken with mean ADU values at the Red spot.

However, if Flat frames are created with the same mean ADU as the Light frames, then, any small changes in the Flat frame illumination (vignetting, dust donuts, etc.) would produce the exact same "nonlinear" change in ADU output as would any equivalent small change in the Light frame because they are at the same place on the nonlinear curve.

I hope this makes sense. Because the Flats I've created using this approach have produced images corrected corner to corner I believe it is correct.

Comments welcomed.

Steve
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

#17

Post by Baskevo »


Steve,

I gave it a shot. I measured my mean ADU value at around 3250, so I rounded up to 3500. It failed to correct :/

I shot flats of 3.5k and 20k, here's the result (single subs corrected):
Screen Shot 2020-04-25 at 9.40.30 PM.png
I'll stack em both and see how the gradients comes out.
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Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
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Software: APT, SharpCap Pro, PHD2, CPWI | PixInsight, DeepSkyStacker, Photoshop

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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

#18

Post by STEVE333 »


Your 20,000 ADU Flats worked perfectly. Good outcome for you. I wish mine did!!!

Thanks for checking this.

Steve
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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

#19

Post by Baskevo »


STEVE333 wrote: Sun Apr 26, 2020 5:28 am Your 20,000 ADU Flats worked perfectly. Good outcome for you. I wish mine did!!!

Thanks for checking this.

Steve
I'll probably see some gnarly color stuff after I combine :/ I wish your same ADU method worked!! Flats drive me crazy... But thank you for putting in the work to give this a go, hopefully it continues to work for you, Steve!
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
Mount: EQ6-R Pro
Cameras: ZWO ASI1600mm Pro (Cooled) | Canon DSLR EOS T7i
Auto-guiding: ZWO ASI120mm-Mini + Astromania 50mm Guidescope

Filters: ZWO 31mm Ha/Oiii/Sii 7nm + LRGB | Orion 2" Skyglow Filter
Accessories: Explore Scientific 2" Field Flattener, ZWO EFW 8 Position
Software: APT, SharpCap Pro, PHD2, CPWI | PixInsight, DeepSkyStacker, Photoshop

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Re: Determining "best" ADU level for ASI1600MM Pro Mono Flats

#20

Post by STEVE333 »


Baskevo wrote: Sun Apr 26, 2020 5:33 am
I'll probably see some gnarly color stuff after I combine :/ I wish your same ADU method worked!! Flats drive me crazy... But thank you for putting in the work to give this a go, hopefully it continues to work for you, Steve!
You're welcome James.

I just tried processing M16 data provided by yobbo89 in a thread in the Image Processing sub-forum ("Anyone interested in ...." is the Subject). It's my first real attempt as SHO imaging and has been very instructive. Gives me confidence that I really may be able to do this after all!!

Cheers and good luck with your Flats.

Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
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