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Unlocated Dirt In Optics

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14 replies to this topic

#1 Mugrelo

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Posted 10 August 2022 - 10:39 AM

Hi Guys

 

From one day to the next my images started to show a lot of dirt. As you can see in the attached picture there are small circles everywhere. I own an 8" Hedge HD with Hyperstar and a cooled ZWO camera. I have thoroughly cleaned all the hyperstar and camera lenses, but I am still experiencing the same problem. The dirt didn’t even change position after cleaning so this dirt must be in other part of the telescope. I am using a microfibre cloth, compressed air (with hand bulb) and a small brush tool . What else I have to clean to get rid of these circles? In the past I could get rid of them easily but not now. Any advice would be greatly appreciated. Thanks

Attached Thumbnails

  • masterLight_BIN_1_4144x2822_EXPOSURE_600_00s_FILTER_NoFilter_RGB_integration_ABE.jpg


#2 SgrB2

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Posted 10 August 2022 - 10:47 AM

The large dust motes that you see on the image are on the ZWO camera

sensor window and not the sensor itself.  You need to carefully clean that

window.


Edited by SgrB2, 10 August 2022 - 10:48 AM.

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#3 rgsalinger

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Posted 10 August 2022 - 11:03 AM

Actually, I'd argue that you need to take flats. It's a hopeless task to keep the senor/cover glass/filters perfectly clean. Much better to use flats when processing your images. If that shot was done with flats, then the flats are bad which is another matter. 

Rgrds-Ross


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#4 David Boulanger

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Posted 10 August 2022 - 11:28 AM

+1 on the flats. That sensor window does look to be extremely dirty though. For cleaning I use Photosol Sensor Swabs and Photosol Eclipse cleaner. The swabs come in different sizes for different window dimensions.
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#5 Der_Pit

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Posted 10 August 2022 - 01:56 PM

I'm a bit irritated that the circles/donuts are bright.  To me that would indicate he is using flats, but has badly calibrated either them or the lights (i.e., wrong/missing darks/bias), resulting in an over-correction....

 

(or using an old flat that "corrects" dust that is no longer there because of proper cleaning grin.gif )


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#6 Tommyent

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Posted 10 August 2022 - 02:50 PM

At that size I'd agree that the culprit is pretty darn close to the sensor but not on the sensor itself. If it was me I'd take a hard look at the sensor window. If it didn't go away the first time you cleaned it, it might be on the inside of the glass (which would suck)


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#7 Phil Sherman

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Posted 10 August 2022 - 07:58 PM

There's actually a mathematical formula that will tell you where the dust is located relative to the imaging sensor.

      D = pdf

 

D = Distance from the sensor

p = pixel size

d = donut outer diameter in pixels

f = effective focal ratio of the scope. At prime focus, this will be the focal ratio of the scope.

 

The distance will be in the same distance units as the pixel size, usually microns. 1000 microns = 1mm.


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#8 F.Meiresonne

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Posted 11 August 2022 - 02:38 AM

+1 for flats


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#9 Mugrelo

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Posted 11 August 2022 - 09:42 AM

Awesome. thank you very much for all the good advice. I agree with Tommyent, this must be an inside of the glass becuase the dirt didnt move. Flats are a good solution too but will try  to clean it first. 

 

I will let you know how it goes and thank you very much for all your comments. Truly appreciated



#10 Mugrelo

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Posted 11 August 2022 - 09:49 AM

By the way, I have another question if I may. What do you guys think about my picture of Andromeda? I am using autoguiding with 10m exposures but I am still still getting elongated stars . Is it because my telescope is out of focus or maybe poor tracking? I guess is poor tracking but would like to know your opinion. Thanks again 



#11 Seiten

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Posted 11 August 2022 - 11:00 AM

It might help to provide a profile of the gear you are using.  To me, the stars dont look crisp indicating a general out-of-focus condition.  At ten minute exposures, you could see drift associated with bad polar alignment, differential flexure and a few other sources.  I suggest much shorter exposures for such a bright object as Andromeda.


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#12 David Boulanger

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Posted 11 August 2022 - 11:10 AM

Edge + Hyperstar = short exposures. Maybe even one minute or less. You are just going to blowout the core with long exposures. Can't revive saturated pixels.
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#13 unimatrix0

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Posted 11 August 2022 - 11:46 AM

I keep advising people to stop trying their lens and filters sparkling clean by various chemicals and rubbing it around. You're destroying it and mess up the coating on them and then it's useless. 

 

I recently changed parts in my imaging rig and I had A LOT of dust spots all over when I took the flats. I took it apart, blew air on it, I even cleaned the sensor cover glass. 

No effect.  Dust particles all over. 

It turns out it's on my secondary mirror inside my RC. 

 

Here is what I'm gonna do : 
NOTHING. 

 

It's fine. I took flats and the spots were gone. I'm not ready to take apart my RC telescope only to spend the next 2 weeks collimating it as well as scratching up the mirror surface. 

It's not worth it and pointless. 

Dust spots will not degrade your image. 

 

Again, stop trying to keep your camera sensors and telescope objective lens sparkling clean, they will never stay clean , especially with pollen and dust flying around all the time and your telescope spends hours outside at night. 


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#14 rgsalinger

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Posted 11 August 2022 - 12:27 PM

Mike's right. If the donuts are white then the flats are over correcting. If there were no flats then the donuts are dark. 

 

Rgrds-Ross


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#15 Mugrelo

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Posted 12 August 2022 - 03:17 AM

Super helpful, thank you all for your replies. I am using an Alt Az Mount with wedge (Celestron evolution mount). I feel that I reached the limits of my equipment and maybe is time to move to an equatorial mount.

You guys are right, I am not getting meaningfully better result with 10min exposures. I will try shorter exposures and to improve the focus using a Bathinov mask.

Noted all your comments on flats and the damage to optics. I hate cleaning because it causes me a lot of distress so I try to fix this issue with flat images.

And again thanks for sharing all your thoughts.


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