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Flats over correcting


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Bottletopburly

Am i right in thinking that if dust bunnies etc are still showing after stacking and i have tried Dss and Siril that Flats are over correcting ?ūüėē ,though i need to give optics and sensor¬† a good clean ,currently have the 200pds setup though im still think the¬†¬†El panel¬† may be the cause (thought i had fixed that ) i need to get another light source rigged up big enough to fit the 200¬†pds to check .

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i was doing flats at around 35000 adu reading, about hal;f of the histogram and my dust bunnies were still there, blue channel was awful, started lowering the adu to around 14000 and found it was much better but still got some dust bunnies but they seemed easier to process out, also a lot depends on the settings used in deep sky stacker

 

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silentrunning

I have found that the most effective flats for my setup are about 21 ADU, so well below half the range of the camera and therefore well within its' linear response. I always get rubbish calibration if I start imaging before the sky is truly dark with flats that work perfectly once twilight has completely gone, this has thrown me before especially during the summer months. Not sure why this is a I would expect to see an image ruined by a bright sky but I have found that at those times I can also see various dust bunnies and other artefacts that are gone when the sky has darkened.

Edited by silentrunning
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1 hour ago, silentrunning said:

I have found that the most effective flats for my setup are about 21 ADU, so well below half the range of the camera and therefore well within its' linear response. I always get rubbish calibration if I start imaging before the sky is truly dark with flats that work perfectly once twilight has completely gone, this has thrown me before especially during the summer months. Not sure why this is a I would expect to see an image ruined by a bright sky but I have found that at those times I can also see various dust bunnies and other artefacts that are gone when the sky has darkened.

 

My simplistic explanation for this John is this.

Dust bunnies and artifacts display themselves as shadows on an image.

Once the background reaches a level that is equal and below to that shadow the Dust bunnies will disappear from the image. Only to reappear if they conflict with a bright part in an image, a Nebular ect.

Only at this point can the Flats do their job by removing that shadow from the final image.

Edited by Graham
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26 minutes ago, Clive said:

21 ADU?

 

Clive 

Another simplistic explanation. The in depth explanation go way over my head.

Every time a photon hits a pixel on the camera sensor it creates an electrical signal.

The ADU ( Analog Digital Unit ) is a measure of these signals.

The higher the number the more hits that pixel has received so the brighter is appears in an image.

By moving the cursor over an image in software ( Maxim , Photoshop ect ) that number is shown in the readout on the screen.

You can then set the high point for you flats to the desired number you want by adjusting the brightness or sub length until you get the desired ADU level.

Edited by Graham
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silentrunning

Just realised you may have been pointing out my error Clive. I should have put 21000 ADU not 21! My camera has a range of 65536 so you can see 21000 is somewhat below half way.

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Bottletopburly

interesting  when you search the net for flats info quite  a lot of people have issues with flats including experienced imagers  at times , I’m going to try the NINA flats wizard next time just to see how it compares with choosing Av mode on the camera ,on the 200pds took a bit messing with the flats panel to get it dimmed correctly to bright and the wizard throws an error , four layers of baking paper and a white cloth sheet seems to do the trick .

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one other method i have been looking at but yet to try is to roughly match your flats histogram to your lights subs histogram, not tried it yet myself but will take a few sets of flats next time to compare the difference if any.

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1 hour ago, red dwalf said:

one other method i have been looking at but yet to try is to roughly match your flats histogram to your lights subs histogram, not tried it yet myself but will take a few sets of flats next time to compare the difference if any.

 

Be interested to learn the results from that Rob.

Like many folks flats are the bane of my life. 

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Bottletopburly

 @GrahamReading various threads  some issues were down to simple things like changing usb leads or flushing the sensor prior to taking flats .

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33 minutes ago, Bottletopburly said:

 @GrahamReading various threads  some issues were down to simple things like changing usb leads or flushing the sensor prior to taking flats .

 

Interesting you mention flushing the sensor.

After I took a load of flats with the QSI the first proper sub on a target came out looking like an over exposed flat with no image detail at all.

I thought the camera had gone wrong or I had left the end cap on the scope.

The second sub was fine.

So obviously the sensor has some kind of memory or data retention. 

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Bottletopburly

Test flat using nina on the newt ,when i stretch it in Apt you can see the newt rings if it shows the rings off centre to the image can anyone tell me  does that mean your collimation is out ? https://drive.google.com/file/d/1Iqee9i0fFxQNkn7GlRv12ytrwKWUAbTE/view?usp=sharing @Graham

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That flat looks pretty good to me

The offset could be down to normal Newt coma , slight sag in the focus tube putting the camera chip slightly off line.

As long as you have not changed anything from your imaging session it will all line up with your lights so you should be good.

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