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Issue with flats and dust donuts


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Hi

I have this issue where flats correct some of the background but make dust donuts look much worse. The problem is exaggerated by applying the DBE (and ABE) process in Pixinsight.

Here's the autostretched, calibrated Lum before and after DBE:

LF.thumb.jpg.1f54ac87a44f7269dcf93de48045f9a4.jpg

stretchedLF.thumb.jpg.5d9a390cfb95d929e926550217f93a5a.jpg

and here's the stacked Lum without flats, also before and after DBE:

LnoFlat.thumb.jpg.b4fca9853be52f65ef8b3b80c078bb9f.jpg

stretchedLumNoFl.thumb.jpg.3f708ec6957c868ba3d1876bfc0a4948.jpg

I tried preprocessing the images in both Nebulosity and Pixinsight with the same results. I usually take flats at the end of each imaging session so I don't think it's a focusing issue. I believe that they were properly exposed (1.17s, 21000 ADU).

The master flat looks like this (second image is autostretched):

masterflat.thumb.jpg.ba54b18243e69616552b6106f8f9b9f6.jpg

5ab4c344e8c90_stretchedflat.thumb.jpg.f036571ad0a176e1b7b7281e01f9563f.jpg

What do you think the problem is? 

* camera is ASI1600MM-Cool.

 

Edited by Shahd
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How are you taking your flats?

I ask because stretching your flat frame shows a weave pattern as well as a number of very regular, circular artefacts that don't resemble traditional dust bunnies.  These patterns are also present in the 'no flat' image.  

This raises two questions, what is causing them? and why aren't the flats helping?

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4 minutes ago, almcl said:

How are you taking your flats?

I ask because stretching your flat frame shows a weave pattern as well as a number of very regular, circular artefacts that don't resemble traditional dust bunnies.  These patterns are also present in the 'no flat' image.  

This raises two questions, what is causing them? and why aren't the flats helping?

I take them using SGPro flat wizard, t-shirt and an iPad. 

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Do you calibrate your flats with biases or dark-flats to remove offset?

I actually was not able to find a way to proper calibrate images with Pix. I use Maxim for this - its Set calibration library is fast and excellent tool. 

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Just now, drjolo said:

Do you calibrate your flats with biases or dark-flats to remove offset?

I actually was not able to find a way to proper calibrate images with Pix. I use Maxim for this - its Set calibration library is fast and excellent tool. 

Yes, I calibrate them with biases. 

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Whenever you see ‘Bas Relief’ effects (3D) in subtracted or divided images this is almost always due to mechanical movement between the two source images, in this case the images are the flat master, or flat stack and the target image stack.

DBE has detected and enhanced the image artefacts resulting from mechanical differences beween the master flat and image stack. The dark areas around the galaxies is most likely due to poor placement of the sample points in DBE/ABE

Most likely there was some physical movement of the L filter, camera, flattener, focuser etc, either during the flats acquisition or the lights acquisition and this may have affected the entire series of flats, or lights, or just a few individual subs in either stack.

Since you report that RGB were ok but only L was affected I would suspect the filter wheel maybe was knocked during the sequence, possibly the L filter is loose in its holder or the wheel detent is a bit loose in the L postion allowing some movement as the telescope assumes different positions for flats and target imaging, or possibly the flattener to filter wheel coupling is a little loose, even slight focuser movement can cause these effects.

The artefacts are quite large making me think these are dust particles further up the image chain than the filters. You can measure the donuts in the image and determine how far from the sensor the dust particles are and that will give you a better idea of what moved, filter or coupling between filter wheel and flattener, focuser, OTA etc using the formula:

D=Pdf

Where D= the distance from the sensor to the dust casting the shadow, P=The width of the dust donut in pixels, d=The width of a single pixel and f=The focal ratio of the telescope. If you use the pixel size in millimetres then the distance will be in millimetres too, if you use pixel size in microns the distance will be in microns so you just need to be able to convert accordingly. Once you have the distance calculated you can work out what moved.

HTH.

 

Edited by Oddsocks
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54 minutes ago, Oddsocks said:

Whenever you see ‘Bas Relief’ effects (3D) in subtracted or divided images this is almost always due to mechanical movement between the two source images, in this case the images are the flat master, or flat stack and the target image stack.

DBE has detected and enhanced the image artefacts resulting from mechanical differences beween the master flat and image stack. The dark areas around the galaxies is most likely due to poor placement of the sample points in DBE/ABE

Most likely there was some physical movement of the L filter, camera, flattener, focuser etc, either during the flats acquisition or the lights acquisition and this may have affected the entire series of flats, or lights, or just a few individual subs in either stack.

Since you report that RGB were ok but only L was affected I would suspect the filter wheel maybe was knocked during the sequence, possibly the L filter is loose in its holder or the wheel detent is a bit loose in the L postion allowing some movement as the telescope assumes different positions for flats and target imaging, or possibly the flattener to filter wheel coupling is a little loose, even slight focuser movement can cause these effects.

The artefacts are quite large making me think these are dust particles further up the image chain than the filters. You can measure the donuts in the image and determine how far from the sensor the dust particles are and that will give you a better idea of what moved, filter or coupling between filter wheel and flattener, focuser, OTA etc using the formula:

D=Pdf

Where D= the distance from the sensor to the dust casting the shadow, P=The width of the dust donut in pixels, d=The width of a single pixel and f=The focal ratio of the telescope. If you use the pixel size in millimetres then the distance will be in millimetres too, if you use pixel size in microns the distance will be in microns so you just need to be able to convert accordingly. Once you have the distance calculated you can work out what moved.

HTH.

 

The distance is roughly 1.5 cm, which means it's either the L filter or the wheel that moved. Anyway, I tightened and changed the L filter position and will take new images and flats tonight. Hope this resolves the issue

Thanks a lot!

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On 3/23/2018 at 11:23, Oddsocks said:

Whenever you see ‘Bas Relief’ effects (3D) in subtracted or divided images this is almost always due to mechanical movement between the two source images, in this case the images are the flat master, or flat stack and the target image stack.

DBE has detected and enhanced the image artefacts resulting from mechanical differences beween the master flat and image stack. The dark areas around the galaxies is most likely due to poor placement of the sample points in DBE/ABE

Most likely there was some physical movement of the L filter, camera, flattener, focuser etc, either during the flats acquisition or the lights acquisition and this may have affected the entire series of flats, or lights, or just a few individual subs in either stack.

Since you report that RGB were ok but only L was affected I would suspect the filter wheel maybe was knocked during the sequence, possibly the L filter is loose in its holder or the wheel detent is a bit loose in the L postion allowing some movement as the telescope assumes different positions for flats and target imaging, or possibly the flattener to filter wheel coupling is a little loose, even slight focuser movement can cause these effects.

The artefacts are quite large making me think these are dust particles further up the image chain than the filters. You can measure the donuts in the image and determine how far from the sensor the dust particles are and that will give you a better idea of what moved, filter or coupling between filter wheel and flattener, focuser, OTA etc using the formula:

D=Pdf

Where D= the distance from the sensor to the dust casting the shadow, P=The width of the dust donut in pixels, d=The width of a single pixel and f=The focal ratio of the telescope. If you use the pixel size in millimetres then the distance will be in millimetres too, if you use pixel size in microns the distance will be in microns so you just need to be able to convert accordingly. Once you have the distance calculated you can work out what moved.

HTH.

 

This must be one of the most informative posts ever seen on SGL. Every day's a school day when you read an Oddsocks post! Much appreciated here.

Olly

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4 hours ago, ollypenrice said:

This must be one of the most informative posts ever seen on SGL. Every day's a school day when you read an Oddsocks post! Much appreciated here.

Olly

So true. If there was such a thing as a best post challenge, this would be the winner.

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22 hours ago, Shahd said:

I processed the data from last night, and it worked! thanks a lot

You are welcome Shahd, I'm happy I was able to suggest a possible answer to the problem and that you did't waste time retaking the luminance and calibration frames unnecessarily. :smile:

17 hours ago, ollypenrice said:

Every day's a school day when you read an Oddsocks post! Much appreciated here.

Thank you for the kind comments Olly.

When diagnostic imaging (medical) was my career I found the professionals I used to support appreciated an explanation for the problems they might be seeing with their equipment rather than just a blanket response that told them nothing as some of my colleagues tended to do.

Some of my bosses over the years used to complain that I never sent e-mails, only novella! :huh2:

I do try to include as much as I can in as few words as possible but as you know it isn't easy!

13 hours ago, wimvb said:

If there was such a thing as a best post challenge, this would be the winner.

You are too kind Wim, I do appreciate your comments and trying not to blush. :blush:

The post was written in rather a hurry and looking back now I would have changed the sentence structure and shortened it a bit to make it easier to read, there's always room for improvement!

William.

Edited by Oddsocks
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  • 2 years later...

Came across this thread while investigating exactly the issue the OP had, after ABE. Turns out the rear element on my flattener has come loose and had clearly shifted between taking the lights and flats! 

The flattener is now off for fixing, but wanted to give a shout to @Oddsocks and for a 3 year old thread!!

 

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