Flats Calibration Wizard ADU Value


#1

Hi, I am using an ASI1600 MM camera and I am confused about what value to use as a target ADU for the Flats Calibration Wizard. The camera itself has a full well capacity of about 20000 electrons so as I understand it, I want to take flats that will have about 1/3 to 1/2 of the electron sites occupied - say 8000 electrons. My confusion is how to translate 8000 electrons into ADU for the FCW. I believe the Ascom driver for the camera always converts the output signal into a full 16 bit range from 0 to 65535. Is this correct and should I use a Flats target value around 20000?


#2

I have the QHY163M. I normally aim for 22,000ADU for flats, or perhaps a little less than that. So yes, 20,000 ADU in SGP is a good target value.


#3

Hi guys, I am new to SGP and have the same question regarding the ASI071MC? What should I target for an ADU value for my flats?

I have been doing astrophotography for about 9 years , but I am just making the transition from DSLR to CMOS for deep sky imaging. For DSLR, I used BackYardEOS and just took flats using the "sky with a T-shirt " method. (I have also used the iPad screen method occasionally.) ISO was always set low at 100. I am not too much of a purist, I normally shot flats once and just used those over and over again until they stopped correcting dust donuts. Typically that would be at least a year. Normally, if I shot the flats with a clean sensor, I could just clean the sensor and reuse the same flats. I never had any issues with matching orientation, but I was always careful to have the camera at “deep sky” focus for flats. This approach worked great.

I will try to upload some photos of my first project with SGP and the ASI071MC. I shot flats at a target of 30000 ADU. I had 3 shirts on my 80ED refractor and the exposure time was 0.14 sec at a gain of 69. That was the same gain I used for the light frames. Dust donuts seem to have been corrected, but there are two larger circles that were not. I am really not sure what these are - have never seen them before. Wondering if they are some type of artifact caused by the sun being too bright? I shot the flats at 4pm yesterday, while pointing the scope south east, away from the brightest part of the sky.

BTW, SGPro is a great product! Easy to get started with, powerful, and I really appreciate the detailed help file.

Appreciate anyone’s thoughts.

Thanks and clear skies!
Pat

Here is my flat - with enhanced contrast to show the dust donuts and mysterious circles.

Here is the stack (stretched in PI) of my Leo Triplet image. All dust donuts were corrected, but two of the mysterious circles remain.


#4

One of the spots seems to be undercorrected while the other one is overcorrected which seems very weird.

Are the donuts there in the uncalibrated frame?
Do you use bias frames?


#5

Yes, I made a stack without flats and the dust donuts (and the larger, mysterious circles) were all there - in exactly the same places as on the flat. I trashed that stack, so I don’t have it to upload. I also use bias frames, just like I always have.

I agree, it’s really weird. The flats I shot yesterday were good enough to correct the dust donuts and get rid of vignetting, but for some reason those two circles persisted.

When I shot sky+tshirt flats for the DSLR, the exposure time ended up being 1/50 sec (0.05 sec) using Av mode on the camera, ISO 100, and using BackYardEOS to program the shots. For the ASI071MC, I am at .14 sec (3x the exposure time). I am wondering if there is a better combination of exposure time and gain for flats on the ASI071MC. Figured that knowing a good target ADU would give me a place to start. I am assuming also that since low ISO was desirable for the DSLR, I should try reducing the gain for the flats - not really sure, though.

Thanks,

Pat

Pat Darmody’s Astrobin Site


#6

There seems to be a large contrast difference between the center and the edges. In the flats I get there is not this large contrast gradient. When I look at my flats I see a low contrast vignette, not a big bright spot like this. Also, it also seem like there is some indication of the grain of the tee shirt fabric. With the large bright spot in the middle, I’m wondering if the spots you are seeing are internal reflections or other aberrations in the scope cause by the bright center.


#7

Your flats definitely look different then mine when I used an ASI071 with a small refractor. I agree with DesertSky that the spot in the middle seems overexposed. If it is still possible for you to retake the flats, I suggest you do, perhaps with an LED panel instead.

Also, you didn’t mention how you do your pre-processing. If you are using the batch pre-processing script in PixInsight, beware that it can do strange things with flat frames. I have never gotten a satisfactory result out of it and always go back to the manual technique.

Glenn


#8

A dark circle is the out of focus image of a small obstruction somewhere in the light cone of the scope and it’s possible to work out where from the size of the circle and the focal length of the scope. The large circles are further away, maybe on a filter or even on a focal reducer if you have one.

As for determining the ADU setting try checking the histogram of an image that has areas that are fully saturated. If the histogram stops before the full 16 bit ADC reading of 65535 then that’s the full well limit in ADU.


#9

Thanks guys for all of the input. I would have replied sooner, but did not get notifications via my email.

  • The reason for the bright center is that I applied a contrast curve to the flat, just to bring out the corrections to show you in this email chain. The unmodified flat looks normal.

  • I use PixInisight and the BatchPreProcessing script (BPP). It has always worked well for me on DSLR images. This is my first go at CMOS / CCD deep sky imaging.

  • I shot some new flats last night and waited for the sky to get darker - so as to be able to use longer exposures. I ended up with flats at about 0.35 sec and 2.3 sec. I kept the ADU target at 30,000.
    Last night’s flats seemed to do a better job of correcting things. I still see the larger circles though.

  • I understand that small/dark spots are typically dust specks close to the sensor and that larger, fuzzier things are typically imperfections that are farther away from the sensor. I checked my LPS-D1 filter and my Skywatcher field flattener. There was not much dust on anything, but I cleaned it anyway with my hand-bulb blower. There has always been a slight speck (bubble?) on the flattener, but it has never been an issue before.

Anyway, I’ll keep plugging away and see if this persists.

Thanks!

Pat


#10

Something to consider about your examples. The circle above and to the left of the bright center is also showing up in your corrected image. If this artifact was from the optical train I would not expect to see it in the final image. If that artifact was both in the flats and the lights, the stacking should have eliminated it. This suggests that artifact is in the flats but not the lights. I don’t know what this means but it may be a reflection in the scope which shows up in the brighter flats and not the lights.


#11

I wondered about this too. I will need to do a stack that has nothing but the lights in it to test. Thanks!


#12

I had a similar issue with my sbig stf-8300c when trying to process with Pixinsight, error message unable to find a valid set of star pair matches. Seems the flats taken by SGP with flats calibration wizard did not input the correct value for Ha. The ADU on my camera is 25,500 for 1x1 it set the time at 3.03 seconds which is what the unfiltered images uses. I ran a series of flats with different ADU till the correct flats turned out good at about 36 seconds for Ha 1x1. SGP normally inputs the values correctly for the different filters but somehow it messed up. All good now.


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