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Need help taking flats

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#1 Brians200

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Posted 23 May 2019 - 10:24 PM

I am unable to make my flats match my lights for some reason and am looking for some advice.

 

Here is an example of one my lights and flats side-by-side.

LTUYOV2h.png

 

After going through image integration, following the steps in Warren Keller's Pixinsight book, I can't seems to completely remove the ring.

 

4JeE55Lh.png

 

I experience this issue with all of my filters. Some of the other filters also create rings in the stacked image.

 

z0vFEPxh.png

 

The current method I am using to take flats is to point the telescope straight up and put a light panel on it. There is some printer paper between it to diffuse it / make the panel dimmer. I have the same issues if I place the scope horizontal with a white t-shirt and pointing it at a computer screen.

 

8fdgK7ol.jpg4e3euMSl.jpg

 

Does anybody have an idea on what I might be doing wrong? The flats seem to be partially working since it is successfully removing dust.



#2 44maurer

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Posted 23 May 2019 - 10:40 PM

Are you taking flats for each filter?

#3 Brians200

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Posted 23 May 2019 - 10:43 PM

Yes. I am taking sets of flats for each filter I use.

 

In the luminance example at the beginning, that was the only filter I used that whole night. So I know the filter is correct because I because I didn't rotate the wheel that night


Edited by Brians200, 23 May 2019 - 10:46 PM.


#4 pyrasanth

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Posted 24 May 2019 - 02:28 AM

I've seen this issue before. It could well be a reflection from one of the optical or internal surfaces of the adapters. Being black inside does not mean that they are not reflective. I had to remove most of my adaptors and spray them with anti reflective black paint. You could use some sticky flock paper inside the adaptors if that puts you off as well as flocking inside the dew shield. Try a range of flats at between 30-50% ccd saturation & you don't mention if your using dark and bias frames but if your following Warrens' book then that should be assumed.

 

Try not to have the panel to bright- longer exposures on a dimmer panel are preferable to being initially bright with shorter exposures. If you take flats in the day cover the camera assembly with a dark cloth or bag as well as the panel with a dark sheet- it is surprising how much stray light can creep into the optics ruining your flats. 

 

Good luck with resolving & let the forum know how you get on.


Edited by pyrasanth, 24 May 2019 - 08:55 AM.

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#5 happylimpet

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Posted 24 May 2019 - 02:49 AM

I've seen this issue before. It could well be a reflection from one of the optical surfaces of the adapters. Being black inside does not mean that they are not reflective. I had to remove most of my adaptors and spray them with anti reflective black paint. You could use some sticky flock paper inside the adaptors if that puts you off as well as flocking inside the dew shield. Try a range of flats at between 30-50% ccd saturation & you don't mention if your using dark and bias frames but if your following Warrens' book then that should be assumed.

 

Try not to have the panel to bright- longer exposures on a dimmer panel are preferable to being initially bright with shorter exposures. If you take flats in the day cover the camera assembly with a dark cloth or bag as well as the panel with a dark sheet- it is surprising how much stray light can creep into the optics ruining your flats. 

 

Good luck with resolving & let the forum know how you get on.

Yeah I'd agree with this explanation. On the positive side the flats are working quite well, and getting rid of dust is a win, even if theres still work to be done.

 

i'm not sure how much luck you'll have fixing it but looking up through the OTA with the camera removed will probably show some reflective surfaces which would benefit from some kind of flocking/painting. Bound to be informative anyway.



#6 terry59

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Posted 24 May 2019 - 07:05 AM

"It could well be a reflection from...."

 

Getting flats to match lights can be challenging, especially for luminance. Try lowering the ADU to 12k -15k and see if that helps

 

smile.gif


Edited by terry59, 24 May 2019 - 07:06 AM.


#7 MalVeauX

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Posted 24 May 2019 - 07:31 AM

Add bias frames. They calibrate the flat. waytogo.gif

 

Very best,


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#8 fewayne

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Posted 24 May 2019 - 07:37 AM

Focus position the same?



#9 AIP

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Posted 24 May 2019 - 09:16 AM

Remove the black parasol. Possibly the flats lamp is not perfectly parallel with the optics and the light enters inclined


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#10 BenKolt

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Posted 24 May 2019 - 12:26 PM

Another thing to do, and this is easy enough, is to mask out the portion of the light panel outside of the aperture of your dew shield.  You may have some light leaks, and that extra light outside of the scope system could be infiltrating.  I've had this happen before.  It should be easy to mask out the extra parts of the panel with thick paper or cardboard or something.

 

However, I agree that internal reflections, non-optimal brightness level or your processing or a combination of these are likely the larger issue.

 

Ben


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#11 Brians200

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Posted 24 May 2019 - 03:49 PM

I've seen this issue before. It could well be a reflection from one of the optical or internal surfaces of the adapters. Being black inside does not mean that they are not reflective. I had to remove most of my adaptors and spray them with anti reflective black paint. You could use some sticky flock paper inside the adaptors if that puts you off as well as flocking inside the dew shield. Try a range of flats at between 30-50% ccd saturation & you don't mention if your using dark and bias frames but if your following Warrens' book then that should be assumed.

 

Try not to have the panel to bright- longer exposures on a dimmer panel are preferable to being initially bright with shorter exposures. If you take flats in the day cover the camera assembly with a dark cloth or bag as well as the panel with a dark sheet- it is surprising how much stray light can creep into the optics ruining your flats. 

 

Good luck with resolving & let the forum know how you get on.

I was taking about 4 second exposures with the light panel. I will get some more printer paper for tonight to make it darker as it is on the minimum setting. If I have no luck, I will look into seeing if I can find what is reflecting.

 

"It could well be a reflection from...."

 

Getting flats to match lights can be challenging, especially for luminance. Try lowering the ADU to 12k -15k and see if that helps

 

smile.gif

I will try using a lower ADU tonight.

 

Add bias frames. They calibrate the flat. waytogo.gif

 

Very best,

I was use bias frames and dark frames per the instructions in Inside Pixinsight.

 

Focus position the same?

Yes. I did not adjust the focus.

 

Remove the black parasol. Possibly the flats lamp is not perfectly parallel with the optics and the light enters inclined

I will try this tonight. With and without, as well as a few different ADU levels.



#12 Brians200

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Posted 25 May 2019 - 07:15 PM

I took 6 sets of flats last night for testing. I added more paper to make the light panel dimmer so I could increase the exposure time (4 seconds to 13 seconds).

I only used one filter to keep things consistent and did not adjust the focus beyond the initial focusing at the beginning of the night.

Here are the results.

 

 

Parasol on

QqMKgftl.jpg

 

20000 ADU

A4xl9e5h.png

 

12000 ADU

tIBTjyth.png

 

 

Next, I removed the parasol

lkiWoDnl.jpg

 

20000 ADU

QmslC5Ch.png

 

12000 ADU

Sx2vsYQh.png

 

Finally, I collapsed the dew shield completely

BYkxyWYl.jpg

 

20000 ADU

E7lk67Oh.png

 

 

12000 ADU

lxSpkVph.png

 

 

I did not see much difference going from 20000 ADU to 12000 ADU. The position of the light panel definitely seems to matter.

Next on the list is to attempt to find reflections



#13 terry59

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Posted 26 May 2019 - 07:20 AM

The first set is the best but seems to show reflection. More diffusion may be a viable solution



#14 happylimpet

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Posted 26 May 2019 - 10:27 AM

Varying the ADU should never make any difference to anything (aside from noise levels in the flats depending on sub number). It seems to be one of those red herrings that people throw out when they dont have any real ideas, in much the same way as polar alignment gets blamed for everything.



#15 terry59

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Posted 26 May 2019 - 10:53 AM

Varying the ADU should never make any difference to anything (aside from noise levels in the flats depending on sub number). It seems to be one of those red herrings that people throw out when they dont have any real ideas, in much the same way as polar alignment gets blamed for everything.

Except I just went through this and it is partially documented in this thread. I've deleted all of the data from my failed attempts or I'd show what they looked like. Lowering the ADU value while ensuring a narrow spread between mean and median ADU solved my issue so I thought I'd share my experience FWIW

 

https://www.cloudyni...nging-fsq106ed/


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#16 jdupton

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Posted 26 May 2019 - 02:50 PM

happylimpet,

 

Varying the ADU should never make any difference to anything (aside from noise levels in the flats depending on sub number). It seems to be one of those red herrings that people throw out when they dont have any real ideas, in much the same way as polar alignment gets blamed for everything.

   In a perfect world, you are absolutely correct. However, the full answer is that ADU doesn't matter if the flat is exactly calibrated. It will not always be a big difference but is a difference.

 

   A flat frame consists of two main parts -- the illumination signal from the flat source and an offset made up of Bias signal and Dark signal. You could write this as FLAT + OFFSET. If the OFFSET is zero, then when you calibrate, there are no problems and the calibration will work regardless of the ADU level of the FLAT portion.

 

   In that ideal case where the FLAT has had the OFFSET removed in calibration, then when you calibrate your lights you would be doing:

 

CAL_LIGHT = LIGHT / (FLAT/mean(FLAT))

 

   If your calibration of the flat frame is off, then that calibration factor of (FLAT/mean(FLAT)) will also be off. This can be seen by trying a mathematical experiment.

 

  • Let's say your mean raw flat ADU is 15,000 and the Bias plus Dark for the flat has a mean of 2,000.
     
  • Let's also assume your flats include +/- 15% vignetting from the mean.
     
  • As a final assumption, let's say your light frame has a (dark calibrated) mean background level of 3,000 ADU.
     
  • When you calibrate the flat with your Flat-Dark or Bias, you end up with a mean flat at 13,000 ADU.
     
  • In the ideal case, the Light Frame calibration factor for the flat at a darker point where the actual value of the raw flat shows 12,750-2,000 ADU will be (10,750 / 13,000) or 0.827x. This will cause the image background to be lightened to 3,000 / 0.827 = 3628 to match the vignetting.

    Now, let's look at a case where your calibration of the flat was off by 500 ADU due to mismatched dark or other problems. We'll say the calibration took too much of the offset out of the flat.
     
  • In this case, the Light Frame calibration factor for the flat at the same darker point where the actual value of the raw flat shows 12,750-2,500 ADU will be (10,250 / 12,500) or 0.820x. This will cause the image background to be lightened to 3,000 / 0.820 = 3659.

   We can see that by removing a too large a portion of the offset in the flat when we calibrated caused the flat to over-correct and lighten the Light Frame a little too much (by almost 1% or ~30 ADU). A similar error can cause under-correction in our images when we don't remove all of the Bias-Dark offset.

 

   You can run through other examples and see how things are affected by incomplete (or over-) removal of the offset in the flat and how that will affect you images. If your mean raw flat ADU level is 32,000 instead of 15,000, you can run through the example above and see how a similar calibration error of the flat will affect its application to your lights. I had written it up as a simple spreadsheet a while back to get it clear in my own mind. Doing so will allow you to just plug in numbers and see how much your lights will be affected. As mentioned, it is a small number but when you get to post processing and severely stretch the image, you will usually be able to see the difference.

   You can often recover a poorly calibrated flat that is showing under or over-correction by experimentally adding or subtracting a fixed offset from it and then reapplying to a sample light. Checking the light for over or under-correction will then allow you to adjust the manual offset adjustment amount. Once you get something that looks good, you can then use that adjusted offset flat to calibrate all your lights.

 

   The net of all this is that varying the mean ADU level of the flat will not affect the correction so long as calibration of the flat is accurate. If the calibration of the flat is off due some reason, then the mean ADU level will have an effect on the calibration result. For a given calibration error of the flat, higher mean ADU levels cause less error that low ADU levels in the flat. This is yet another reason to shoot for a mid-range ADU level in a flat so long as there is not clipping or saturation going on.

 

 

John



#17 AIP

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Posted 26 May 2019 - 03:34 PM

With my Atik 16200 at now I'm taking flats with 45,000 - 50,000 ADUS. These flats calibrate better the lights than the 25.000 ADUS


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#18 happylimpet

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Posted 27 May 2019 - 05:21 PM

happylimpet,

 

   In a perfect world, you are absolutely correct. However, the full answer is that ADU doesn't matter if the flat is exactly calibrated. It will not always be a big difference but is a difference.

 

   A flat frame consists of two main parts -- the illumination signal from the flat source and an offset made up of Bias signal and Dark signal. You could write this as FLAT + OFFSET. If the OFFSET is zero, then when you calibrate, there are no problems and the calibration will work regardless of the ADU level of the FLAT portion.

 

   In that ideal case where the FLAT has had the OFFSET removed in calibration, then when you calibrate your lights you would be doing:

 

CAL_LIGHT = LIGHT / (FLAT/mean(FLAT))

 

   If your calibration of the flat frame is off, then that calibration factor of (FLAT/mean(FLAT)) will also be off. This can be seen by trying a mathematical experiment.

 

  • Let's say your mean raw flat ADU is 15,000 and the Bias plus Dark for the flat has a mean of 2,000.
     
  • Let's also assume your flats include +/- 15% vignetting from the mean.
     
  • As a final assumption, let's say your light frame has a (dark calibrated) mean background level of 3,000 ADU.
     
  • When you calibrate the flat with your Flat-Dark or Bias, you end up with a mean flat at 13,000 ADU.
     
  • In the ideal case, the Light Frame calibration factor for the flat at a darker point where the actual value of the raw flat shows 12,750-2,000 ADU will be (10,750 / 13,000) or 0.827x. This will cause the image background to be lightened to 3,000 / 0.827 = 3628 to match the vignetting.

    Now, let's look at a case where your calibration of the flat was off by 500 ADU due to mismatched dark or other problems. We'll say the calibration took too much of the offset out of the flat.
     
  • In this case, the Light Frame calibration factor for the flat at the same darker point where the actual value of the raw flat shows 12,750-2,500 ADU will be (10,250 / 12,500) or 0.820x. This will cause the image background to be lightened to 3,000 / 0.820 = 3659.

   We can see that by removing a too large a portion of the offset in the flat when we calibrated caused the flat to over-correct and lighten the Light Frame a little too much (by almost 1% or ~30 ADU). A similar error can cause under-correction in our images when we don't remove all of the Bias-Dark offset.

 

   You can run through other examples and see how things are affected by incomplete (or over-) removal of the offset in the flat and how that will affect you images. If your mean raw flat ADU level is 32,000 instead of 15,000, you can run through the example above and see how a similar calibration error of the flat will affect its application to your lights. I had written it up as a simple spreadsheet a while back to get it clear in my own mind. Doing so will allow you to just plug in numbers and see how much your lights will be affected. As mentioned, it is a small number but when you get to post processing and severely stretch the image, you will usually be able to see the difference.

   You can often recover a poorly calibrated flat that is showing under or over-correction by experimentally adding or subtracting a fixed offset from it and then reapplying to a sample light. Checking the light for over or under-correction will then allow you to adjust the manual offset adjustment amount. Once you get something that looks good, you can then use that adjusted offset flat to calibrate all your lights.

 

   The net of all this is that varying the mean ADU level of the flat will not affect the correction so long as calibration of the flat is accurate. If the calibration of the flat is off due some reason, then the mean ADU level will have an effect on the calibration result. For a given calibration error of the flat, higher mean ADU levels cause less error that low ADU levels in the flat. This is yet another reason to shoot for a mid-range ADU level in a flat so long as there is not clipping or saturation going on.

 

 

John

Hiya - yes, my assumption was that the calibration was being done properly! If people are taking shortcuts which are only masked by having a specific subset of ADU values in their flats, then, well, Im not counting that as they should be doing it properly! Which isnt hard to do.....

 

Cheers


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#19 RossW

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Posted 27 May 2019 - 06:12 PM

Brians200, have you tried reversing (flipping) the filter in the filter wheel? If your filter has an AR coating, flipping it may help reduce the reflection. This helped solve a reflection problem I was having.

 

Cheers,

 

Ross



#20 Brians200

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Posted 27 May 2019 - 06:56 PM

Hi Ross,

 

I was under the impression that Astrodon filters were directionless. Is there a way to tell which way the filters should be facing?

 

I will flip them around when I look for reflections in a day or two. I was wanting to collect as much data as I can before I take everything apart as I won't be able to match the same rotation.



#21 Brians200

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Posted 29 May 2019 - 08:45 PM

I have taken a video of the inside of my telescope while pointing it outside during the day.

The view point is in front of the filter wheel as the camera (Atik 16200) is screwed onto the filter wheel (Atik EFW 3).

 

https://www.youtube....h?v=JuowvnbAGAk

 

The only obvious thing I notice is that the Moonlite focuser tube is really bright.


Edited by Brians200, 29 May 2019 - 08:47 PM.


#22 happylimpet

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Posted 30 May 2019 - 04:20 AM

I have taken a video of the inside of my telescope while pointing it outside during the day.

The view point is in front of the filter wheel as the camera (Atik 16200) is screwed onto the filter wheel (Atik EFW 3).

 

https://www.youtube....h?v=JuowvnbAGAk

 

The only obvious thing I notice is that the Moonlite focuser tube is really bright.

Gosh yeah, could that be flocked or painted matt? Ive put flockng inside my focuser tube.....


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#23 RossW

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Posted 31 May 2019 - 01:47 AM

Hi Ross,

 

I was under the impression that Astrodon filters were directionless. Is there a way to tell which way the filters should be facing?

 

I will flip them around when I look for reflections in a day or two. I was wanting to collect as much data as I can before I take everything apart as I won't be able to match the same rotation.

Hello Brians200

 

I haven't used Astrodon filters, but here is what happened when I inserted my Chroma narrowband filters the wrong way (for my system at least, with the AR coatings toward the camera sensor):

 

Chroma%20H-inserted-with-filter-closest-

 

All is well after flipping them.

 

Cheers,

 

Ross


Edited by RossW, 01 June 2019 - 01:05 AM.



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