Flats trouble?

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Stuart United States of America
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Re: Flats trouble?

#21

Post by Stuart »


What James said. You should go much longer--I recommend greater than 0.5 seconds. If it's too bright, then just put some white paper or more white t-shirts over the objective.
Personal equipment: TEC 140 F7 on Astro-Physics Mach 1 mount. Camera QSI 683ws7. Guide with Vario guiding scope
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Re: Flats trouble?

#22

Post by bobharmony »


Thanks for the input, guys. I read up on the effects of DSLR shutter shadows and was surprised to learn that there are actually two curtains involved here. Most of the articles I read concerned syncing flash with the shutter opening and closing, but I expect the effect would be the same for any exposure of a constant light source. I know that the shadow is an issue with dedicated astrocams, which have a single shutter/curtain, but the readings on DSLRs indicate this may not be such an issue for these "old school" cameras.

If I understand correctly, when the exposure begins the sensor is covered by the first curtain, which moves out of the way from one side to the other, then after some time (the length of the exposure, I'm guessing) the second curtain covers the sensor, moving in the same direction as the first curtain. If this is the case then each part of the sensor is exposed for the same of amount of time regardless of exposure length, and there is no need for longer exposures to mitigate the effect of shutter shadow.

If this is indeed the case, then what I need to concentrate on is getting the proper ADU saturation without allowing the dominant color (in my case it is the blue) to become oversaturated. I suspect for me this will be 1/40, as 1/50 was still a bit under the mean, and 1/30 is very close to filling the blue pixels. That is backed up by my processing experience. The 1/50th flats made for easier processing and a better result than the original flats I used. Processing the stack with 1/30 flats was very difficult and did not provide proper flat correction in post-processing. I don't see any indication of a gradient due to shutter shadow in any of the flats I took, even the original ones I took the night I collected the lights.

I have no doubt that dimming the light source more would allow for longer exposures, I am not clear that it would add anything to my process. I think I would be better served by looking for a better balanced light source that would allow me to get closer to the median ADU that seems to be the Holy Grail of obtaining the best flats.

Bob
Hardware: Celestron C6-N w/ Advanced GTmount, Baader MK iii CC, Orion ST-80, Canon 60D (unmodded), Nikon D5300 (modded), Orion SSAG
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Re: Flats trouble?

#23

Post by Baskevo »


bobharmony wrote: Sun Nov 03, 2019 2:56 pm Thanks for the input, guys. I read up on the effects of DSLR shutter shadows and was surprised to learn that there are actually two curtains involved here. Most of the articles I read concerned syncing flash with the shutter opening and closing, but I expect the effect would be the same for any exposure of a constant light source. I know that the shadow is an issue with dedicated astrocams, which have a single shutter/curtain, but the readings on DSLRs indicate this may not be such an issue for these "old school" cameras.

If I understand correctly, when the exposure begins the sensor is covered by the first curtain, which moves out of the way from one side to the other, then after some time (the length of the exposure, I'm guessing) the second curtain covers the sensor, moving in the same direction as the first curtain. If this is the case then each part of the sensor is exposed for the same of amount of time regardless of exposure length, and there is no need for longer exposures to mitigate the effect of shutter shadow.

If this is indeed the case, then what I need to concentrate on is getting the proper ADU saturation without allowing the dominant color (in my case it is the blue) to become oversaturated. I suspect for me this will be 1/40, as 1/50 was still a bit under the mean, and 1/30 is very close to filling the blue pixels. That is backed up by my processing experience. The 1/50th flats made for easier processing and a better result than the original flats I used. Processing the stack with 1/30 flats was very difficult and did not provide proper flat correction in post-processing. I don't see any indication of a gradient due to shutter shadow in any of the flats I took, even the original ones I took the night I collected the lights.

I have no doubt that dimming the light source more would allow for longer exposures, I am not clear that it would add anything to my process. I think I would be better served by looking for a better balanced light source that would allow me to get closer to the median ADU that seems to be the Holy Grail of obtaining the best flats.

Bob
Bob, I do not believe that you can tell if you are experiencing the shutter shadow affect... I believe it just makes your flats darker. So if I'm not mistaken and unless your camera does not have a shutter, if your flats are below a certain exposure length, you will not be able to reach above a certain brightness. When I was testing my flats, I noticed when I decreased the brightness and increased exposure time, my flats got brighter.

good luck
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Re: Flats trouble?

#24

Post by pmwolsley »


Bob,
I took a look at the two FLATs you provided and have some comments. The brighter your FLAT frames are the less noise they are going to introduce into your calibrated frames. This is because SNR increases with signal. You also want to avoid saturation. I think both of your FLATs should work but your dimmer FLAT will introduce more noise into your calibrated frames that the brighter FLAT. It's also imperative that you shoot FLATs immediately or very soon after your imaging session is over. The optical path cannot be disturbed or disassembled. Camera rotation and focus are important. I have written some programs that let me interrogate my data and I used my program to interrogate yours:

Darker FLAT Data.jpg
This is what a single horizontal row of pixels looks like at the centre of your FLAT frame. This is RAW data before debayering so there are four colour channels here (Red, Green1, Green2 and Blue). You can clearly see that the blue and green channels are yielding values that are pretty much equal. Your red channel data is quite low. For most Canon cameras the black point in this data is around 2048. Be sure to note the noise in this data. This noise reduces when you combine multiple FLATs to create a master FLAT.

Brighter FLAT Data.jpg
This is the data for the same location in your brighter FLAT. The values are all higher but the red channel is still quite low. The noise is actually quite similar to your darker FLAT but because there is more signal, the SNR is much higher which is desirable. You might want to consider using a computer screen displaying a full screen from a paint program. You can adjust the colors using the paint program so that either there is more red or the green and blue are less. You want all channels recorded by your camera as bright as possible while staying away from saturation. Try to use the same ISO as your LIGHT frames as DSS likes this arrangement. I use 1/30th sec exposures which works well for me. Hopefully you can use between 1/10th to 1/2 second. You can always adjust the monitor brightness as well.

Darker FLAT dust motes.jpg
This is a special viewing mode I invented which allows me to see dust motes. This is the entire FLAT frame but I have fitted the data to a 4th order polynomial and subtracted the polynomial from the FLAT to create this image. What struck me is that there are no dust motes which is an excellent result.

Brighter FLAT dust motes.jpg
This is what your brighter FLAT dust motes look like. The size of the dust motes tell me that you have very little dust directly on your sensor. Sensor dust motes appear as very small dark donuts. The dust motes on this FLAT tell me that there is dust on a lens in your optical train or on a filter. I have never seen a dust mote that ended up being on the aperture of my telescope.

Darker FLAT vignetting.jpg
This is the darker flat once again only here I am not using the polynomial. This is the RAW data and I have greatly increased the contrast to highlight the vignetting of your optics. What strikes me right away is that the brightest spot, or centreline point for your optical train is significantly offset to the right of where your camera is located. I don't know what could be causing this but you might want to think about this issue.

Brighter FLAT vignetting.jpg
Just for completeness here is the vignetting present in your brighter FLAT.


Hopefully in all this there is something that can help you resolve your issue with your FLATs

Peter
Scopes:Celestron 8" EdgeHD + 0.7focal reducer, Skywatcher BK80ED +0.85 focal reducer
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Re: Flats trouble?

#25

Post by bobharmony »


pmwolsley wrote: Sun Nov 03, 2019 9:29 pm


Hopefully in all this there is something that can help you resolve your issue with your FLATs

Peter
Thanks, Peter. This is very interesting data and very useful in helping me visualize the impact of longer exposures on the average ADU value. In addition to the improvement in ADU and SNR with the longer exposure, I find the image showing the asymmetry of my flats interesting. This has been noticeable to me in my flats for a long time, I assumed it had to do with the drawtube protruding into the lightpath of my reflector. I suppose it would be easy enough to test that by rotating the camera in the focuser and shooting another flat to see whether the center of light appears to change it's orientation in the image.

The graphs showing the noise indicate to me that when I get to a properly lit flat, I will have very little rejection of anything except extremely hot or cold pixels, as the majority of the results are withing a narrow band of ranges.

I understand the need to take flats as close in time to the lights as possible. I try to get them before I break down for the night, and before I have powered any of the gear off. Worst case, I will bring in the scope without removing the camera from the drawtube and take the flats that morning.


James - I don't believe the shutter shadow is an issue for a DSLR, as there is a first curtain and a last curtain which open and close the lightpath to the sensor in an even way. I believe that is an issue for dedicated astrocams, which have a single curtain. I do have to consider whether I am getting any flicker effect from the light panel, and can see where longer exposures could help with that possibility.


Finally, I now have gotten two different opinions about setting the ISO value for flats the same as my lights, one saying I should, and one saying it doesn't matter. That is always an interesting thing to see.

I will keep mucking about with what you all have shared, and see about making further improvements to my process.

Bob
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Re: Flats trouble?

#26

Post by Stuart »


Bob,
The ISO doesn't matter (really). What's most important is getting your ADU value correct, having an even field, and avoiding flicker in your panel.
Stu
Personal equipment: TEC 140 F7 on Astro-Physics Mach 1 mount. Camera QSI 683ws7. Guide with Vario guiding scope
Shared equipment through Star Shadows Remote Observatory through PROMPT/ CTIO/Chile 16" RCOS 16803 chip
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Re: Flats trouble?

#27

Post by bobharmony »


Stuart wrote: Mon Nov 04, 2019 3:53 am Bob,
The ISO doesn't matter (really). What's most important is getting your ADU value correct, having an even field, and avoiding flicker in your panel.
Stu
Thanks, Stu. I didn't have any trouble with DSS stacking with flats at ISO 100 to go with my ISO 800 lights. I am running the 4.2.2 version. I recall that DSS used to be fussy about matching values back in the 3.3.4 days, which is where I started with it. Perhaps they have adjusted the programming in the past couple of releases. DSS still puts out a warning that the ISO values don't match, but it doesn't have trouble stacking them.

I run the light panel on house power which cycles 60 times per second, which makes me think I am good at exposures of 1/30 or longer. Based on the ADU values you, James, and Peter have reported, it appears I can still go a bit longer to get to the ADU levels I need to be at. Perhaps the the clipping I start to see at 1/20 is just hot pixels finally reaching full-well values, which would be nothing to be concerned about.

I have a 1/30 and 1/20 flat I could upload. I will do that later and ask if any of you are willing to take a measurement of them. There may be more room still to grow the flats level.

I am processing a stack using auto0adaptive weighted average for the lights with the 1/30 flats right now, I'll see what kind of result I can get out of that combination.

Bob
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Re: Flats trouble?

#28

Post by Stuart »


Hi Bob,
I wouldn't worry about the clipping. What you're interested is your mean ADU value for the flat.
Stu
Personal equipment: TEC 140 F7 on Astro-Physics Mach 1 mount. Camera QSI 683ws7. Guide with Vario guiding scope
Shared equipment through Star Shadows Remote Observatory through PROMPT/ CTIO/Chile 16" RCOS 16803 chip
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Re: Flats trouble?

#29

Post by bobharmony »


I took a 1/20 second flat when I was setting up the 1/30 flats for processing. Could someone take a look at the linked file and post their measurements on it?

Thanks,
Bob

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Re: Flats trouble?

#30

Post by Baskevo »


bobharmony wrote: Tue Nov 05, 2019 3:55 am I took a 1/20 second flat when I was setting up the 1/30 flats for processing. Could someone take a look at the linked file and post their measurements on it?

Thanks,
Bob

Here is your ADU value:
Screen Shot 2019-11-04 at 10.12.42 PM.png
Did you find out what your maximum ADU value is, though? Because yours could be 30,000 in which it is still too dark, or it could be 15,000, in which it is too bright, from my understanding
-James W.

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Re: Flats trouble?

#31

Post by SkyHiker »


Can't you just use the camera's LCD screen to judge if the lighting was sufficient? This is what I do, regardless of whether I take them at night or the morning after and it seems to work fine. If it looks medium grayish with the sides a bit darker, it's good.

At night I use a white stucco wall that I aim my headlamp at. Since the scope does not have the wall in focus it looks evenly white to the camera. That way flicker is als no problem. In the morning I use a flattened paper towel.

It sounds crude but I have no reason to believe it's wrong, from experience. I have a feeling that there's a good deal of tolerance or else it would not work.

But thanks for raising the problem, and I would like to know if you get noticeably better results eventually.
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Re: Flats trouble?

#32

Post by pmwolsley »


Here is some info regarding your latest FLAT

Latest FLAT.jpg
This is the RAW data as I have previously displayed but now showing the data from your latest submitted FLAT. The RED channel is still quite low but I expect you are in the ballpark for creating a quality FLAT.

Histogram.jpg
This is the resulting RAW histogram for your latest FLAT. Please ignore the Std Dev values shown along the R.H.S. of this image but the Mean values are valid. You probably have been looking at your histogram in another program and the red channel is much closer to the other channels. This typically happens when the program you are using displays the white balanced data. For your camera the white balanced multiplier for the red channel is roughly 2.2. The multipliers for the green and blue channels are closer to 1.0 This causes the red channel to look as if the data is right in the same ballpark as blue and green. In reality, the RAW data for the red channel is much lower. Once again, I think you are ok with your current set-up for taking FLATs. Just keep in mind how the histogram you are looking at may be displaying the values.

I used DCRAW to determine that, for your camera, the black point corresponds to 2048 counts and the max ADU value is 12279 counts.

Stu is right in not worrying too much about the ISO of your FLATs matching the LIGHTs. DSS gets concerned about this issue because DSS is using your BIAS frames for your LIGHTs to compensate your FLAT frames so that it can calculate what the exact amount of vignetting and dust motes darkening is. This calculation is then used to perform the "divide by your FLATs" calibration step. In reality, DSS would really like to know what the DARK FLATs look like but this is typically asking for too much. If the exposure time of your FLATs is less than one second then there is virtually no difference between a DARK FLAT and a BIAS frame for the same ISO as your FLAT frame. The other point that's closer to Stu's comment is that the difference in a BIAS frame at different ISOs is pretty small...especially when multiple FLATs are combined together to create a master flat. DSS shares this belief and will allow you to calibrate your LIGHT frames using FLAT frames taken at a different ISO. DSS generates the warning message just so that you are aware that this assumption is being used.

Peter
Scopes:Celestron 8" EdgeHD + 0.7focal reducer, Skywatcher BK80ED +0.85 focal reducer
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Re: Flats trouble?

#33

Post by Stuart »


Responding to James and Bob above
The max ADU value is based upon the scale you measure it in. James, you measured it in a 16 bit scale so the max ADU value will be 65,536.
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Re: Flats trouble?

#34

Post by Baskevo »


SkyHiker wrote: Tue Nov 05, 2019 3:31 pm Can't you just use the camera's LCD screen to judge if the lighting was sufficient? This is what I do, regardless of whether I take them at night or the morning after and it seems to work fine. If it looks medium grayish with the sides a bit darker, it's good.

At night I use a white stucco wall that I aim my headlamp at. Since the scope does not have the wall in focus it looks evenly white to the camera. That way flicker is als no problem. In the morning I use a flattened paper towel.

It sounds crude but I have no reason to believe it's wrong, from experience. I have a feeling that there's a good deal of tolerance or else it would not work.

But thanks for raising the problem, and I would like to know if you get noticeably better results eventually.
This is what I tried to do when I started taking flats, and they were way too dark. As Stuart has been schooling us, I found out that the brightness you see on the LCD screen is in a non-linear state, so it is not an accurate representation and can often be way off. That is why my flats originally were not working. I had to increase the brightness of my flats by 2x, and now the histogram on my LCD screen is all the way to the right.
Stuart wrote: Tue Nov 05, 2019 5:01 pm Responding to James and Bob above
The max ADU value is based upon the scale you measure it in. James, you measured it in a 16 bit scale so the max ADU value will be 65,536.
So my MAX ADU value was not 65,536, when I measured it by taking an over-exposed super long flat... I thought it is different for different cameras? For example, I cannot expose above 16,131 ADU, which is where my mean and median ADU values are equal. So then wouldn't half of MY max ADU value be around 8,000?
-James W.

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Re: Flats trouble?

#35

Post by Stuart »


Hi James,
Yes, it's different for different cameras. DSLRs are 14 bit and some CMOS cameras are 12 bit. BUT (and it's a big "but") when you open it up in the statistics process in Pixinsight, it will default to 16 bit, so whatever value you have will be rescaled to 16 bit. So, for example, if you have a DSLR and you expose it to 50% ADU, your raw ADU value will be about 8000. If you use the Statistics process and you change the scaling to 14 bit, you will see that your average ADU will be 8000. But if you rescale it to 16 bit, your value will change to about 30,000. Does that make sense? If it's still confusing, I'll switch computers and do some screen shots and it will explain it.

Stuart
Personal equipment: TEC 140 F7 on Astro-Physics Mach 1 mount. Camera QSI 683ws7. Guide with Vario guiding scope
Shared equipment through Star Shadows Remote Observatory through PROMPT/ CTIO/Chile 16" RCOS 16803 chip
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Re: Flats trouble?

#36

Post by Baskevo »


I think that makes sense... I just measured it in 16-bit and I try to take flats that have a mean value of around 8,000, and that seems to work. Is that okay? Or is that still too low?
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
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Re: Flats trouble?

#37

Post by Juno16 »


Man, I had no idea how complicated the flats process was!

I just keep things simple like Henk commented. Histogram peak in the middle. Seems to work for me, but what do I know.

Has anyone used the flats wizard in APT? I would try it, but my D5300 is off to Ha mod wonderland. Ill check it out when it comes back.

Thanks,
Jim
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
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Re: Flats trouble?

#38

Post by Stuart »


Baskevo wrote: Tue Nov 05, 2019 9:14 pm I think that makes sense... I just measured it in 16-bit and I try to take flats that have a mean value of around 8,000, and that seems to work. Is that okay? Or is that still too low?
Still too low. I'll switch computers and post again with examples.
Personal equipment: TEC 140 F7 on Astro-Physics Mach 1 mount. Camera QSI 683ws7. Guide with Vario guiding scope
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Re: Flats trouble?

#39

Post by Stuart »


Here is a link to a random flat from a scope that I am a partner in.


Here is the flat when measured on a 16 bit scale. Mean 24,774 (lower than I'd do but whatever)
capture 1.JPG
Here is the same file when I just flip the drop down menu to 14 bit scale. It's the same file. Mean is now 6193
capture 1.JPG
My preference is to make it a tad brighter than that but these work well enough.

So your DSLR outputs a number, and the number can be measured at whatever scale you want. If you are measuring at the 16 bit scale, then your goal is 25,000-30,000 ADU. If you are measuring at a 14 bit scale, your goal is about 7000-8000

Does that help?
Stu
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capture 2.JPG
Personal equipment: TEC 140 F7 on Astro-Physics Mach 1 mount. Camera QSI 683ws7. Guide with Vario guiding scope
Shared equipment through Star Shadows Remote Observatory through PROMPT/ CTIO/Chile 16" RCOS 16803 chip
Shared equipment through San Diego Astronomy Society 14" RC with 16803 chip on a paramount
Software (for my stuff) PemPRO, SGP, PHD, Focus Boss, ASCOM, and Pixinsight on the other end.
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Re: Flats trouble?

#40

Post by Baskevo »


Stuart wrote: Tue Nov 05, 2019 9:40 pm Here is a link to a random flat from a scope that I am a partner in.


Here is the flat when measured on a 16 bit scale. Mean 24,774 (lower than I'd do but whatever)
capture 1.JPG

Here is the same file when I just flip the drop down menu to 14 bit scale. It's the same file. Mean is now 6193

capture 1.JPG

My preference is to make it a tad brighter than that but these work well enough.

So your DSLR outputs a number, and the number can be measured at whatever scale you want. If you are measuring at the 16 bit scale, then your goal is 25,000-30,000 ADU. If you are measuring at a 14 bit scale, your goal is about 7000-8000

Does that help?
Stu
But I've been measuring at 16-bit, and the maximum mean ADU value I can possibly do is a little over 16,000... That was with a 15 second exposure at 1600 ISO with the brightest setting on my iPad. That was where my mean and median were equal. So I thought that I am supposed to aim my flats at 51% of that..? Because I cannot do 25,000 in 16-bit. When I get home I will post my example (sorry for hijacking this bob)
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
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