Flaws in Image

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Rocket_Guy
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Flaws in Image

#1

Post by Rocket_Guy »

I feel a little silly asking this, but lately I've been getting some flaws in my images. I have an 8" Newtonian and a ZWO 290 camera. I don't think the problem is with the camera since I don't see the flaws when I remove the camera from the scope. Also, when I look at the primary mirror down the scope tube I don't see anything obvious on the primary mirror which might be causing the flaws. Any ideas, anyone as to what might be the problem? Thanks.

Bill
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Menno555
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Re: Flaws in Image

#2

Post by Menno555 »

Hi Bill

What you are seeing are tiny dust particles close to the camera.
Not on the sensor because then they would be smaller and darker but maybe on the protection window of your camera or a filter that you used.
Sadly this is almost always on every capture because dus is everywhere.
But it can be removed.
First, get yourself a photo bellow like for example https://www.amazon.co.uk/KamKorda-Compr ... 717&sr=8-3. When blowing your filters and so on with air, a lot of dust can be removed. This is only for all the things behind your telescope, so not for your primary mirror. That can have a lot of dust before it becomes visible in captures.

If that doesn't help, then you have to make calibration files, so called flats, darkflats and for the best results also darks. The flats actually look a bit like your screenshot. These files are "subtracted" from your captures, in essence they get removed by software.
With SharpCap you can make and apply calibration files, see the manual on https://docs.sharpcap.co.uk/4.0/ under "Capturing and Using Dark /Flat Frames"
Making calibration files can be overwhelming for some but once learned, it becomes easier en most of the times, your captures really do look better.
There are a lot of manuals and videos around. Personally I like AstroBackyard where I learned a lot. See the tutorials on https://astrobackyard.com

Menno
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Rocket_Guy
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Re: Flaws in Image

#3

Post by Rocket_Guy »

Hi Menno,

Thanks for the reply. I am familiar with flats and I know that the defects can be removed through their use. This technique is still on my learning to do list, however. I thought about the dust explanation but there are some things which are puzzling to me as to whether that is really the problem. My questions arise because:

1) The camera does not show these defects when I remove it from the telescope. Wouldn't the defects remain when it is removed?

2) The only filter used with the camera was an IR cut filter. If I remove it, such that I have no filters in the optical path, I still get the defects when the camera is attached to the telescope.

By the way, I read your posts often and I always seem to learn something new from them. Thanks for sharing your expertise.

Bill
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Menno555
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Re: Flaws in Image

#4

Post by Menno555 »

Hi Bill

Thanks for the compliment :)
There is an easy, fast test to see where the flaws are. Attach everything on your scope as usual. Now make a capture.
Then only turn your camera 45 degrees around and make a new capture. If the flaws are now also around 45 degrees turned, the flaws are on the camera and/or filter (if that is attached to the camera).
If the flaws are still in the same place, then it's somewhere in your scope. In that case my guess would be on the secondary mirror. But that's a guess, I'm not that known with Newtonian scopes.

On the filter. There is also a protective window in front of your sensor, probably AR glass. Dust here also shows up as these flaws.

Menno
Jean-Francois
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Re: Flaws in Image

#5

Post by Jean-Francois »

Hello,

First to Menno ... if you rotate the camera and the dust is on the camera (detector protective glass, camera window or other fixed filter on the 1.25" adapter), it will rotate with the camera ... so it will be all the time on the same place relative to the camera detector ... so on the same place on the image too.
It is also the inverse what you explain. ... If the dust image is rotating when the camera rotates, then it is something in the telescope.

You can start some calculation ... you know the pixel size of your camera and the FD ratio of your telescope (telescope + any corrector/reducer).
From the image, you can calculate the size of the shadow (number of pixel in diameter * pixel size), here with the assumption that the diameter is 2 mm. Then you can calculate the distance between the detector surface and the particle ... 2 mm * FD ... if your telescope has a FD = 6, then the dust is at 12 mm from the detector. The calculation is only a coarse calculation, but you can estimate the surface in consideration with the particle.

Concerning that the camera does not show the defects when you remove the camera from the telescope ... do you use a different lens ? or do you use a collimated light source ? ... or simply the room illumination ?... in this case you will not see the defects because the light shadow coming from the particle is above a very large angle, maybe over the full detector.
You can repeat the test with the camera removed from the telescope ... but this time in the dark with only a point source (small lamp) at a long distance ... several meters.

Regards,
Jean-Francois
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Menno555
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Re: Flaws in Image

#6

Post by Menno555 »

Hi Jean-Francois

You are of course correct, I've had it mixed up.
Flaws stay at the same place with camera rotation: flaws are on the camera.
Flaws are rotated with camera rotation: flaws are in the scope.

And thanks for the point 2 explanation. It's what I had in mind but sometimes my English lacks the right way to describe it :P

Menno
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admin
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Re: Flaws in Image

#7

Post by admin »

Just to fill in one more detail (almost everything is covered by the answers above) :)

Dust spot shadows are more visible when the camera is on the telescope because the light getting to the sensor arrives along a much narrower path, meaning that the shadow is smaller and darker than if you have the camera off the telescope. If you switched to a telescope with a larger f-ratio then the spots would become even smaller and darker .

cheers,

Robin
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Rocket_Guy
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Re: Flaws in Image

#8

Post by Rocket_Guy »

Thank you all for your responses. The great thing about this hobby is that there is always something new to learn besides getting to look at all the amazing sights in the heavens. If I am not mistaken, I received help from people from 3 different countries. How fun to interact with people from all over the world!! Best to all!

Bill
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