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Flat Frame Exposure Time?

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14 replies to this topic

#1 claws

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Posted 24 September 2021 - 07:45 PM

I was under the impression that flat frame exposure times should be around 2 or 3 seconds, but I've seen .2 to.3 second exposure times recommended. I was going to attempt to take flats with an ADU of 25000 with 2 or 3 second exposures by dimming my light panel sufficiently, but now I don't know if that's possible or desirable. What say you all?



#2 Dynan

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Posted 24 September 2021 - 07:56 PM

Use quality paper in between camera and panel. I use it successfully to raise the exposure time to 1-2 seconds for my flats using my flat panel at minimum setting.


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#3 Jim Waters

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Posted 24 September 2021 - 08:18 PM

I use one or two t-shirts.  "A Bad Flat Is Worst Than No Flat"....

 

I am moving into using Sky Flats.


Edited by Jim Waters, 24 September 2021 - 08:20 PM.

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#4 premk19

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Posted 24 September 2021 - 08:21 PM

I use flats down to 0.02s. It doesn't matter as long as the flats are correctly calibrated.


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#5 claws

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Posted 24 September 2021 - 09:01 PM

I use flats down to 0.02s. It doesn't matter as long as the flats are correctly calibrated.

I'm still confused. I can figure out how to dim my light source so I can take longer exposures but I'm not sure if longer exposures are necessary or beneficial. I'm not sure how to correctly calibrate my flats to achieve such short exposure times, premk19.

 

Thanks,

Kim



#6 OldManSky

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Posted 24 September 2021 - 09:08 PM

I'm still confused. I can figure out how to dim my light source so I can take longer exposures but I'm not sure if longer exposures are necessary or beneficial. I'm not sure how to correctly calibrate my flats to achieve such short exposure times, premk19.

 

Thanks,

Kim

All that matters is that the flats correctly model the optical system, and have ADU values from just under to just over half your total range.

Some cameras (notably the ASI1600 cameras) have problems with consistency or banding with very short (<1sec) exposures. For those cameras you need to increase exposure time. For everyone else, short ones are just fine.


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#7 jonnybravo0311

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Posted 25 September 2021 - 12:27 AM

All that matters is that the flats correctly model the optical system, and have ADU values from just under to just over half your total range.

Some cameras (notably the ASI1600 cameras) have problems with consistency or banding with very short (<1sec) exposures. For those cameras you need to increase exposure time. For everyone else, short ones are just fine.

The 294 series cameras also fall in the "behave badly with short exposures under 1 second" club.

 

Kim, while some cameras behave nicely, with predictable responses across exposure lengths, your 294MC does not (neither does my 294MM). The recommendation is to take flats that are around 2-3 seconds at a minimum. Also, you'll want to take corresponding dark flats and _not_ bias frames. To dim/diffuse your light source, you can use white tee shirts, paper, dimmable lighting, acrylic sheets, etc.


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#8 premk19

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Posted 25 September 2021 - 12:59 AM

The 294 series cameras also fall in the "behave badly with short exposures under 1 second" club.

 

Kim, while some cameras behave nicely, with predictable responses across exposure lengths, your 294MC does not (neither does my 294MM). The recommendation is to take flats that are around 2-3 seconds at a minimum. Also, you'll want to take corresponding dark flats and _not_ bias frames. To dim/diffuse your light source, you can use white tee shirts, paper, dimmable lighting, acrylic sheets, etc.

The 0.02s flats I shoot are with my 294MM and luminance filter. I calibrate with dark flats without any issues. There's a lot of inaccurate info floating around for the 294 mono.



#9 jonnybravo0311

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Posted 25 September 2021 - 01:19 AM

I ran quite a few tests, along with John Upton, showing quite conclusively that the 294 series cameras (both MC and MM) suffer from the same non-linear sensor response problems. Here's just one graph from those tests:

 

gallery_347158_15202_437.png

 

While I agree with you that there is quite a bit of inaccurate information, what I wrote about the 294 is documented and tested.


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#10 michael8554

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Posted 25 September 2021 - 03:20 AM

Some astro cameras have a shutter, so a shutter shadow can appear in Flats taken at too-fast exposures.



#11 premk19

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Posted 25 September 2021 - 07:22 AM

I ran quite a few tests, along with John Upton, showing quite conclusively that the 294 series cameras (both MC and MM) suffer from the same non-linear sensor response problems. Here's just one graph from those tests:

gallery_347158_15202_437.png

While I agree with you that there is quite a bit of inaccurate information, what I wrote about the 294 is documented and tested.

This shows that as long as you calibrate flats with matched dark flats it should be fine, irrespective of exposure length. You just cannot use bias for calibration due to the inconsistency in ADU values.

Edited by premk19, 25 September 2021 - 07:27 AM.


#12 jonnybravo0311

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Posted 25 September 2021 - 10:02 AM

This shows that as long as you calibrate flats with matched dark flats it should be fine, irrespective of exposure length. You just cannot use bias for calibration due to the inconsistency in ADU values.

 

If the short exposures are working for you, that's good. I will continue to take my multi-second flats and matching dark flats because that process works for me smile.gif


Edited by jonnybravo0311, 25 September 2021 - 10:37 AM.

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#13 jdupton

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Posted 25 September 2021 - 10:29 AM

premk19,

 

This shows that as long as you calibrate flats with matched dark flats it should be fine, irrespective of exposure length. You just cannot use bias for calibration due to the inconsistency in ADU values.

   What you say is true. You can take short matching Flat-Darks even with a '294 series camera if you are very careful. However, I (and many other) recommend against that approach -- especially for those new to the camera or new to astrophotography in general.

 

   The issue is one of degree of matching. To get a really clean calibration of your Lights, the Flat must be itself calibrated extremely carefully. The reason is the slope of the curves involved in the short exposures. You will note that the Dark frame plot @jonnybravo0311 shows for his camera has a very smooth and low slope for exposures beyond 2 seconds. In the very short exposures note that the slope of the response curve is actually much steeper in comparison.

 

   That steep slope is aggravated by the temperature and the Gain settings you use. For higher than Unity Gain, it gets much steeper still. What this translates to is that for such short exposures, a small difference in temperature or exposure begins to matter much more compared to longer exposures. If you look at the FITs header for a set of Dark frames, you will see that the actual sensor temperature often varies +/- 0.5°+ between frames and can be more between different imaging runs. The sensor temperature and stability can also be sensitive to TEC cooling power (%) and ambient camera temperature. At high TEC power (%) consumption, the sensor temperature control suffers.

 

   All too often, a new imager may not be paying strict attention to the details of the conditions when gathering Flat and Flat-Dark frames. That leads to issues with Flats not properly doing their job. The solution is rather simple -- shoot Flat and exactly matching Flat-Dark frames at the same time and use longer exposures to get to the more linear low slope region of the curve.

 

   So, while you can shoot very short Flats and Flat-Darks if you are scrupulously careful with technique and they will work great. I just don't recommend doing that for folks who may be struggling to learn the process and may not understand all of the underlying implications of such simple decisions.

 

 

John


Edited by jdupton, 25 September 2021 - 10:47 AM.

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#14 premk19

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Posted 25 September 2021 - 10:50 AM

John, thanks for the extra details! I shoot dark flats every session right after I do flats, so good to see my approach validated.

#15 premk19

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Posted 25 September 2021 - 10:53 AM

If the short exposures are working for you, that's good. I will continue to take my multi-second flats and matching dark flats because that process works for me smile.gif


I'm pretty sure I'll run into the same issues OP does. Even at the lowest setting my flat box is still very bright, and I'd rather not carry layers of diffusing sheets with me everywhere I go!

Edited by premk19, 25 September 2021 - 10:54 AM.

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