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DLSR flats not working


AstroRookie
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Hello,

when applying the flats taken in my last session (to find out what is causing the strange diffraction spikes) with Siril, the final stacked result still shows the vignetting and the dust spots. I also did the whole preprocessing with Nebulosity, same result.

I took the flats as follows:

  • same iso as my subs
  • camera and focus not touched
  • I use a homemade flatbox combined with the a white t-shirt
  • with Ekos took test shots till the histogram was half-way to the left
  • checked all my flats, they all show vignetting and the same dust spots as in my subs

I tried using them with and without using a bias frame, same result, the final result looks as if no flats were used.

Anybody any idea what is going on? An other question I have, will the vignetting and dust spots also show in the master flat (flats stacked)?

Thanks for your help,

AstroRookie

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Flats will fail if you don't use proper darks.

Whether you will see that - depends on how much dark current your sensor produces.

You also need to use flat darks.

There are other reasons why flats can fail and general rule is - if there is some signal not removed (or removed when it should not have been removed) flat correction will fail. This is because flat correction works only on signal that came thru the telescope opening and was subject to vignetting dust.

Any other signal that is there and not removed - will be corrected although it was not subject to vignetting / dust shadows and that will leave dust/shadows and vignetting mark on the image.

Possible causes:

- dark current not removed

- dark flats not applied

- light leak when shooting lights, darks or very rarely flats - flats tend to overpower any small leak

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1 hour ago, AstroRookie said:

Siril

Create a master bias frame: cover telescope, take 30 frames at minimum shutter speed. Stack using median without normalisation.

Set your camera on Av

Take a few flat frames with your light box.

Pre-process the flat frames with the master bias.

Stack the flat frames using multiplicative normalisation.

Pre-process your light frames with the master bias and the pre-processed  stacked flat.

Register.

Stack.

Result: a stack ready to process.

That's it.

Cheers

 

 

Edited by alacant
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Hi Rookie

Although I agree with the complete application of corrections that vlaiv and alacant have posted, just Lights and Flats should give a pretty good correction.

Since you have tried various stacking softwares, I suggest your Flats are wrong.

Sounds like you are using a DSLR, so shoot on the AV setting to get the correct exposure, and adjust the flatbox and T-shirt to make that about 1 second.

Post a link to your subs ?

Michael

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On 21/12/2020 at 11:34, michael8554 said:

Hi Rookie

Although I agree with the complete application of corrections that vlaiv and alacant have posted, just Lights and Flats should give a pretty good correction.

Since you have tried various stacking softwares, I suggest your Flats are wrong.

Sounds like you are using a DSLR, so shoot on the AV setting to get the correct exposure, and adjust the flatbox and T-shirt to make that about 1 second.

Post a link to your subs ?

Michael

Hello Michael,

when taking my flats, the camera was in manual mode. I took some "test" flats to have the histogram +- in the left half. I took two sets one at 1/500s - histogram in the middle, an other set at 1/250s - histogram more to the left. You mention an exposure time of 1s, tried that as well, but my test flat was overexposed. Do you want me to post a link to one of my subs or to one of my flats?

AstroRookie

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Hello,

having read this I presume my diy flatbox is a bit too bright, and maybe I should lower the iso for my flats; I used the same iso as my subs, so maybe next time I lower the iso and calibrate then with flat darks at the same iso as my flats.

And this more or less confirms it; exposure should be around 0.5s to 2.5s, and one should first try to adjust the brightness rather then the exposure.

But unfortunately according to yr.no the next imaging session will not be for this year. One must be insane to try astrophotography as a hobby living in a windy shithole by the Northsea 😉

AstroRookie

 

Edited by AstroRookie
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I've always used & read that your flats need to be at the same ISO as your lights (subs).

If your flatbox is too bright then add another layer of t-shirt to darken it down a bit. Some use a sheet of white paper.

All I do when I take flats is open a blank MS Word document on my PC screen, put two layers of t-shirt over the scope, change the camera mode from Manual (Bulb) to AV and let the camera choose the exposure. This normally works out at 1/13 with my Canon EOS 1300D.

For Dark Flats; I note the exposure time for the flats, change the camera mode from AV to TV, set the same exposure time as the flats, put the cover on the end of the scope and fire off my dark flats.

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7 hours ago, michael8554 said:

Never mind the weather, use it as a chance to investigate.

Make some new Flats with settings and T-shirts as sugested, and apply to your previous Lights, to get an approximate idea if you're on the right track.

Michael

Hello Michael,

I wish I could not care about the weather, but since I dropped my C8, I leave the new  scope permanently outside under a cover. So even during the day I depend on the weather to thinker with it. I'm working on a solution for this, but will not be before mid 2021. An other thing is that I have already removed the camera from the focuser to check the coma corrector, so would not be the same camera orientation.

But what puzzles me now is that some sources say flats must be the same iso as the subs, others say you must have an exposure between 0.5s and 2s and one can change the iso to obtain that (see this).

Many thanks for the advice!

AstroRookie

 

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3 minutes ago, AstroRookie said:

But what puzzles me now is that some sources say flats must be the same iso as the subs, others say you must have an exposure between 0.5s and 2s and one can change the iso to obtain that (see this).

ISO (gain and other settings) at which flat is taken has nothing to do with settings of other subs except for flat darks. These two need to match in everything (only darks taken with scope covered).

Duration of flat exposures will depend on several factors. Most important being to get good exposure that is in linear range of sensor's response and with strong enough signal. Here good rule to follow is to get histogram peak at around 2/3 up to 3/4 of histogram. With color cameras - make sure all three peaks are in histogram and that right most peak is hitting 3/4.

Other thing to worry about is presence of mechanical shutter. If you have mechanical shutter on your camera - you want longer flat exposures (dimmer light panel). This is to avoid gradient caused by moving shutter when exposure time is not long enough in comparison to shutter opening/closing time.

Other than that - you can use exposure length that suits you. I use exposures in 2-5ms range for color filters as I have very strong flat panel and cmos sensor with rolling electronic shutter (not mechanical one).

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