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weird artifact--what is this?

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#1 stuart827

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Posted 05 December 2020 - 01:00 PM

I took a picture of orion nebula---weird artifact. it is slashes in an arc formation. they are in 4 quadrants of the image. any idea??

 

i was using asiair pro, 0.8 reducer flattener on williams optic scope 81 zenithstar and asi533 pro one shot color camera

 

I am SUPER new to this. 

 

let me know if you have any ideas?!

 

thanks

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#2 AZ Maverick

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Posted 05 December 2020 - 01:06 PM

Looks like dust motes to me.

Did you use flats in your calibration files?


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#3 stuart827

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Posted 05 December 2020 - 01:23 PM

I did not use flats. I just used bias and darks. Also just prior to this I imaged pleiades and these artifacts were not there. In between though I placed the tele cap back on to do the darks though. whats weird is that the three slashes are exact duplicates in the top and bottom right sided ones. 

 

Would dust motes do that? the same ones show up in different places on a single photo?

 

thanks for responding AZ



#4 stuart827

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Posted 05 December 2020 - 01:25 PM

ALSO..what light source do you use for flats. My worry is that If i use a light source that is not entirely uniform I am at risk of creating vignetting. What do you think?



#5 AZ Maverick

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Posted 05 December 2020 - 01:27 PM

I did not use flats. I just used bias and darks. Also just prior to this I imaged pleiades and these artifacts were not there. In between though I placed the tele cap back on to do the darks though. whats weird is that the three slashes are exact duplicates in the top and bottom right sided ones. 

 

Would dust motes do that? the same ones show up in different places on a single photo?

 

thanks for responding AZ

It's quite possible.

Dust motes can be anywhere in the image train, on the objective lens, on the a flattener/reducer or filter or on the sensor itself.

Calibration flats are used to remove those issues as well others such as vignetting.



#6 JEPott

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Posted 05 December 2020 - 01:28 PM

I make flats by putting a white t-shirt stretched over the lens and then point at a white wall with a light pointed at the wall - works well.



#7 AZ Maverick

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Posted 05 December 2020 - 01:30 PM

ALSO..what light source do you use for flats. My worry is that If i use a light source that is not entirely uniform I am at risk of creating vignetting. What do you think?

The old standby is an early clear sky using the tee shirt method.

Although I used this method for quite a while, I now use an inexpensive LED tracing panel for taking my flats.

Flats can sometimes be a tricky/finicky calibration to acquire and use if not done just right - but they are really needed in a lot of cases.



#8 brian_a_paden

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Posted 05 December 2020 - 02:23 PM

Dust in the imaging train. Flats will fix it. I used the t shirt method for a long time and finally got a 15$ led tracing pad and the flat calibration improved quite a bit. Anyhow, t shirt flats will fix that artifact.

#9 brian_a_paden

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Posted 05 December 2020 - 02:25 PM

IMHO flats are more important than bias or darks.

#10 idclimber

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Posted 05 December 2020 - 02:52 PM

The huge reason flats are more important than darks or bias is as follows. Flats must be taken with the camera undisturbed from the lights. The same focus, gain, offset, temperature, etc. If the dust motes move before you capture them you will have a lot of work touching up the images.

 

Sometimes at 6am I am lazy and just throw a cover over the scope and wait to take my flats the following night. That is fine as long as nothing moves. I have even carefully moved my scope inside and take them indoors. That also works as long as you are careful. But remove the camera or simply rotate it by bumping it and you are toast. 

 

Darks and bias and even dark flats with todays cooled astro cams are trivial to add later. Months later if you have too. 

 

Beginners often underestimate the importance of all these calibration frames and skip them. You sometimes see post here claiming good results doing just that. Their images may look good, but it despite the calibration techniques not because of it. 

 

If you value your sleep and imaging time and have the cash, a good flat generator is priceless. 



#11 Alex McConahay

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Posted 05 December 2020 - 05:26 PM

I did not use flats. I just used bias and darks. Also just prior to this I imaged pleiades and these artifacts were not there. In between though I placed the tele cap back on to do the darks though. whats weird is that the three slashes are exact duplicates in the top and bottom right sided ones. 

 

Would dust motes do that? the same ones show up in different places on a single photo?

 

thanks for responding AZ

First step is to use flats. I gotta agree with that. 

 

As for the Pleiades.....it could well be that these artifacts were there, but not visible. Remember, M42 has a lot of nebulosity. Pleiades has some, but mostly blank sky, which may not be affected as much. 

 

The duplicates are very similar....but they are not identical. (I selected an area around one, and tried to paste it directly over the other spots. Close match, but not perfect even after translation, rotation, and scaling.  

 

Remember, when we say "dust" we mean dust, pollen, ash, sand particles, Covid-infected sneeze particles, flecks of paint, and anything else that gets on our optics. Could it be an insect left footprints of approximately the same shape as he (or she) walked across your optics?

 

I'm still with the flats thing. 

 

Alex



#12 stuart827

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Posted 05 December 2020 - 06:06 PM

Sounds like flats are the way to go. I appreciate everyone’s feedback. I noticed on my flattened reducer there is a filament inside the lenses that is not accessible. It is long and narrow similar to the images. Also if you reflect the light onto the flattener you get reflection in 4 quadrants similar to the dust spots. Additionally the image of the filament gets multiplied as I think it reflects off the 4 lenses. It’s a new field flattener so perhaps I should exchange it. I bet this is the issue. 
 

again though, it looks like flats are pretty helpful so I think I will give that a try. 
 

thanks everyone...super helpful info. 
 

stuart


Edited by stuart827, 05 December 2020 - 06:07 PM.


#13 Der_Pit

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Posted 06 December 2020 - 08:02 AM

Just a question - you referred to 'slashes'.  I do see three(?) lines like from satellites crossing the FOV.  Did you talk about those, or about the dark smudges?  If the former, do you use sigma clipping for stacking?



#14 Kevin_A

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Posted 06 December 2020 - 08:58 AM

Definitely not dust motes as it is the same pattern repeating.



#15 stuart827

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Posted 06 December 2020 - 10:03 AM

I think it is from the small filament in the field flattener. I did see the satellite lines but I was referring to the dark smudges. I have no idea what sigma clipping is. What is it?

If it is not dust motes then what could it be?

#16 acommonsoul

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Posted 06 December 2020 - 12:21 PM

Elon Musk strikes again? Looks like satellites to me.


Edited by acommonsoul, 06 December 2020 - 12:24 PM.



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