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Flats issue


Andyk93

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Hi, I am getting better and better at imaging I think and was even successful for the first time plate solving. I am starting to give up on my modified canon 1000d and I feel like I have tried everything apart from cleaning the sensor.

I use a round LED light against my 130pds and set the canon to aperture priority mode. the flats were 1/750s and there were 20 of them. the mean ADU in PI is 23k. when they are stacked with DSS the master flat comes out at 19.3k.

once combined with my image it completely over corrects the image.

I have tried EV compensation +1 and +2 and the +2 gives the best ADU range. I have attached a stacked image and the masterflat

Autosave.jpg

MasterFlat_ISO800.jpg

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52 minutes ago, Pompey Monkey said:

You mention both PI and DSS in your post: How do you calibrate the subs? What is the exact method that you used to calibrate your subs?

How does a single calibrated sub look?

Hi, I use DSS to stack because I don't know properly how to do it in PI. I have followed tutorials before and the problem still persists so its definitely not a software based thing. I only use PI for post processing at the minute. a single frame shows signs of the dust bunnies.

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9 hours ago, Andyk93 said:

I use a round LED light against my 130pds and set the canon to aperture priority mode.

Can you explain the physical setup used to take flats e.g. illumination source, illumination evenness, distance, what covers scope aperture? I wonder if the round features in the flat are the LED light. 

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Hi. Anything which gives a perfectly even source of light. Distance doesn't matter.

Focus on a star. Now make another flat with 4 layers of white t-shirt type cloth stretched over the telescope aperture. Even better, lose the led and use the same covered telescope pointed at evenly illuminated sky. Have the artefacts gone? If so, now add more layers of cloth so that the exposure is around 1 second. HTH.

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10 hours ago, bobro said:

Can you explain the physical setup used to take flats e.g. illumination source, illumination evenness, distance, what covers scope aperture? I wonder if the round features in the flat are the LED light. 

Okay so I have a 20cm white round led panel that I sit on top of my 130pds with about 5 sheets of plain white paper in between the light covers the whole aperture of the scope. I do this before I start imaging when it's completely dark. The round dots are definitely dust spots because they are visible on single subs. I have also tried without paper in between the light and telescope.

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3 hours ago, alacant said:

Hi. Anything which gives a perfectly even source of light. Distance doesn't matter.

Focus on a star. Now make another flat with 4 layers of white t-shirt type cloth stretched over the telescope aperture. Even better, lose the led and use the same covered telescope pointed at evenly illuminated sky. Have the artefacts gone? If so, now add more layers of cloth so that the exposure is around 1 second. HTH.

Thanks, I have tried with a white t shirt but it was still a problem. Will give it a go with an evenly illuminated sky. I tried yesterday adding so much paper and got the flat to 1 second. It took so much paper I couldn't actually see the light shining through

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18 minutes ago, Andyk93 said:

Okay so I have a 20cm white round led panel that I sit on top of my 130pds

That could be the problem. As the panel sits directly on the scope it can only provide even illumination if the panel itself is evenly illuminated. Layers of paper won't help that much - only attenuated the light. If some distance can be put between the panel and the scope, with the front of the scope covered, that would provide more even illumination. Or use the sky as alacant has said.

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20 hours ago, Andyk93 said:

I use a round LED light against my 130pds and set the canon to aperture priority mode.

MasterFlat_ISO800.jpg

Are you sure the led light has an even illumination? I would at least have a double folded t-shirt between the light and the scope. For flats to work, the light source that is used to create them, needs to ba as "flat" as possible.

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1 hour ago, bobro said:

That could be the problem. As the panel sits directly on the scope it can only provide even illumination if the panel itself is evenly illuminated. Layers of paper won't help that much - only attenuated the light. If some distance can be put between the panel and the scope, with the front of the scope covered, that would provide more even illumination. Or use the sky as alacant has said.

I will try it with some distance between them. It most likely doesn't have a proper flat field but wouldn't that only help the vignetting and not the dust spots? 

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1 hour ago, wimvb said:

Are you sure the led light has an even illumination? I would at least have a double folded t-shirt between the light and the scope. For flats to work, the light source that is used to create them, needs to ba as "flat" as possible.

I have tried t shirt flats with the t shirt folded in half and quarters. Although it cured the vignetting it never gets rid of the spots

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3 hours ago, swag72 said:

Here's an interesting link about flats - Sadly though I thought it left you a bit hanging as it didn't offer any solutions..... But interesting nonetheless http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

Thanks, it looks interesting. I will definitely be checking for reflections off components. The telescope is flocked though so it should be a little better

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33 minutes ago, bobro said:

And can you post a single unaltered sub (jpeg version fine)? That will allow comparison with the masterflat.

The DNG file near the top is a single unaltered sub. If you would prefer a jpeg I'll get one for you. Thanks

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14 minutes ago, alacant said:

Here's a .jpg of the .dng.

Thanks for this. Same issue I had - the jpg file is only 1/4 resolution and heavily compressed. I wanted to look for the spots. Difficult with the small image, but couldn't see them. They should be obvious if the master flat is correct.

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