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ASI2600MC-Pro: multiple, very faint light streaks from different directions

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#1 RazvanUnderStars

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Posted 19 September 2021 - 07:24 PM

I imaged the very faint Abell 85 for two nights and, to my surprise, I found several light streaks, from different directions showing in the integrated image (175 x 1 minute at -10C), that I cannot explain. Attached is a downsampled, stretched image, made from the best frames only, with StarNet applied just to make it obvious (there are many stars otherwise. I don't intend to use StarNet so ignore the artifacts). 

 

Things I know:

  • I was alone at a closed dark site with no local source of light
  • there are no bright stars just out of the frame in those directions (I loaded the image into SkyTools which matched the orientation so I was able to check with certainty in the atlas)
  • the moon had already set
  • the streaks are visible in the light frames (barely so, they are faint, but once I knew where they are in the integrated they are) so it wasn't a passing place or something. That would have been eliminated at integration time anyway. I used PixInsight's NormalizedScaleGradient script prior to integration, with Generalized Extreme Studentized Deviate for pixel rejection).
  • the streaks are not visible in the dark frames or the master dark frame (out of 36 1-minute exposures at -10C).
  • there is no way light can get into the frame. The OTA was a Tak TSA-120 with the dew shield extended. Everything was tight (I checked).
  • temperature outside was around 10C, so the cooler was working but not particularly hard. The camera's dew heater was on.

 

The master light was fully calibrated (with a master dark and a master flat + flat darks, everything at -10C as well), as I did in the past. 

 

Is this some kind of very faint amp glow that only becomes visible after hours of integration? Or is there any other explanation? I had another problem with the camera last week (the grease leak, documented elsewhere) but I'm not sure these are related. 

 

I sent an email to ZWO as well, but they are in the mid-autumn fall festival until the 21st.

 

Thanks for any suggestions.

 

light streaks.jpg

 



#2 ChrisWhite

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Posted 19 September 2021 - 07:55 PM

Bright stars just outside your field of view. Take a look at a larger view of this part of the sky and you will be able to tell exactly which stars are the offenders.

#3 RazvanUnderStars

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Posted 19 September 2021 - 08:25 PM

Hi Chris.

 

Thanks for the suggestion. Unfortunately, as mentioned that in the original message, I had already checked for that. I'm attaching the atlas view from SkyTools. The brightest streak is from below in this orientation and there's no bright star in that direction. The one at the very bottom of the screenshot, HD 224364, is mag 6.63, situated at about 42 arcmin from the frame border. (see EDIT note below)

 

There is no bright star beyond the up left corner either (in the atlas view) which is where the second brightest streak appears to come from.

 

I even drew lines along the edges of the streak until they converged but there was nothing bright there or nearby.

 

That's why I'm puzzled.

 

EDIT 5 minutes later: upon further inspection, it appears that SkyTools does not plot the image in the correct orientation, the image should be mirrored left-right. I noticed how the stars in the image were the mirror of the stars in the atlas. I cannot use the flip & mirror controls in SkyTools as it mirrors both the image and the atlas, but I will play with it manually. It's possible then that the lower streak comes from star HR9085 (mag 5.6). and the top streak comes from HR 9097 (mag 6.2). Hmmm.. I didn't expect stars that are not very bright to produce this, but at least the orientations of the streaks match their position, so I think you're right.

 

 

 

 

 

Bright stars just outside your field of view. Take a look at a larger view of this part of the sky and you will be able to tell exactly which stars are the offenders.

field.jpg


Edited by RazvanUnderStars, 19 September 2021 - 08:33 PM.


#4 andysea

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Posted 19 September 2021 - 08:26 PM

+1 what Chris said. See if there are any reflective surfaces in your imaging train. I would be surprised if this happened with Takahashi adapters tho as they are the best in darkening interior surfaces.

 

Incidentally, I just happened to be looking at that sensor earlier today and I was wondering what would happen if a bright star hit precisely one of the shiny electrical contacts around the sensor. Probably a far fetched hypothesis but it left me wondering.

Attached Thumbnails

  • 2600 sesnsor.jpg

Edited by andysea, 19 September 2021 - 08:31 PM.

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#5 ChrisWhite

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Posted 19 September 2021 - 08:33 PM

The bigger light streak coming in is Star Caph.  How long is your lens hood? If the star can see your glass it can streak. 


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#6 andysea

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Posted 19 September 2021 - 08:35 PM

My vote goes to Caph. Was there a meridian flip in the mix?


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#7 ChrisWhite

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Posted 19 September 2021 - 08:36 PM

Also the fainter one might be polaris as much of a stretch as that might sound.  Its in the right orientation is all. 


Edited by ChrisWhite, 19 September 2021 - 08:37 PM.

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#8 ChrisWhite

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Posted 19 September 2021 - 08:39 PM

My vote goes to Caph. Was there a meridian flip in the mix?

Meridian flip wouldnt matter.  The bright stars are still in the same relative position to the target.  I'm still betting on polaris for the fainter streak...


Edited by ChrisWhite, 19 September 2021 - 08:42 PM.


#9 RazvanUnderStars

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Posted 19 September 2021 - 09:16 PM

From the outer rim to the lens it's about 14cm and a bit. 

 

 

The bigger light streak coming in is Star Caph.  How long is your lens hood? If the star can see your glass it can streak. 



#10 ChrisWhite

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Posted 19 September 2021 - 09:23 PM

From the outer rim to the lens it's about 14cm and a bit. 

Imagine how far a star could be outside your FOV and still see your front element.  Even though its out of your camera FOV the light can still cause issues.  14cm is a pretty short hood.  You might solve your problem by extending the hood a ways.  You can test by wrapping cardstock or cardboard as an extension.  There is a reason that SCT hoods tend to be about 18 to 24 inches long. 



#11 RazvanUnderStars

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Posted 19 September 2021 - 09:23 PM

I uploaded the image (the version with stars) to astrometry.net, plate-solved it there, exported their FITS file and then imported it into SkyTools instead of the image plate-solved in PixInsight. This time the orientation matched (why they differ is an interesting question but I'll leave it for a different time).

 

I drew lines along the edges of the streak, as best I could, to see where they intersect. They do way before Caph, though, which in the screenshot is near the bottom-right corner, at about 2deg49min away from the image's border. The closest star to the intersection is HR 9085 (mag 5.6), so it's possible if my lines were not accurate enough. 

 

field2.jpg



#12 ChrisWhite

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Posted 19 September 2021 - 09:25 PM

I dont think that drawing lines is effective here at locating the star in your FOV.  The light is scattering after it hits your lens not starting from the source star.  So your intersecting lines are probably somewhere on your front element where the light is refracting from, which is just outside your sensor FOV. 


Edited by ChrisWhite, 19 September 2021 - 09:26 PM.

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#13 RazvanUnderStars

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Posted 19 September 2021 - 09:26 PM

Will do, that's an easy test to make. 

 

Imagine how far a star could be outside your FOV and still see your front element.  Even though its out of your camera FOV the light can still cause issues.  14cm is a pretty short hood.  You might solve your problem by extending the hood a ways.  You can test by wrapping cardstock or cardboard as an extension.  There is a reason that SCT hoods tend to be about 18 to 24 inches long. 



#14 jdupton

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Posted 19 September 2021 - 09:30 PM

RazvanUnderStars,

 

   It might also be possible that the light from Caph is reflecting off of something just outside the sensor in the area you have marked at the crossing point. The faint radiating lines in your image may be a glint off something close even though Caph is the source of light.

 

   It could be a reflective area in or near the camera nose-piece, or one of several adapters in the imaging train.

 

 

John



#15 andysea

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Posted 19 September 2021 - 09:40 PM

Meridian flip wouldnt matter. The bright stars are still in the same relative position to the target. I'm still betting on polaris for the fainter streak...

duh! Thank you for bringing me back to sanity haha

#16 RazvanUnderStars

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Posted 19 September 2021 - 09:42 PM

Maybe something in the sensor, as Andy suggested in message earlier as well? I have recent pictures of it (posted in the thread about the oil leak). The protective glass extends well beyond the sensor itself and the image circle will cover those areas as well. I had cleaned the sensor with 99% alcohol and one thing I noticed it was that while there were no marks on the glass, the areas where I had made a cleaning pass shone differently in the light than those that I had not touched. I don't know if this can change the reflectivity. Eventually I had cleaned the entire sensor. 

 

 

RazvanUnderStars,

 

   It might also be possible that the light from Caph is reflecting off of something just outside the sensor in the area you have marked at the crossing point. The faint radiating lines in your image may be a glint off something close even though Caph is the source of light.

 

   It could be a reflective area in or near the camera nose-piece, or one of several adapters in the imaging train.

 

 

John



#17 ChrisWhite

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Posted 19 September 2021 - 09:48 PM

RazvanUnderStars,

It might also be possible that the light from Caph is reflecting off of something just outside the sensor in the area you have marked at the crossing point. The faint radiating lines in your image may be a glint off something close even though Caph is the source of light.

It could be a reflective area in or near the camera nose-piece, or one of several adapters in the imaging train.


John


Good points John.

#18 ChrisWhite

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Posted 19 September 2021 - 09:51 PM

duh! Thank you for bringing me back to sanity haha


If you keep imaging at f2.8 you'll never be sane again! Lol...

Edited by ChrisWhite, 19 September 2021 - 09:51 PM.


#19 andysea

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Posted 19 September 2021 - 10:09 PM

If you keep imaging at f2.8 you'll never be sane again! Lol...

oooh I got that sorted now! I aged ten years in two months but it was worth it…..maybe

#20 RazvanUnderStars

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Posted 20 September 2021 - 12:05 AM

I found a more convenient (and free) way to overlay the image with an atlas - posting this for the benefit of others who stumble on this thread.

 

Upload the image to nova.astrometry.net to plate-solve it there. Once you get the solution and you're on the results page, click on the "View in WorldWide Telescope" link. It will show your image overlaid on a sky survey with a handy transparency control. 

 

So I did the above with the image I was asking about and it's clear now that the streaks come from the stars. Each streak has a star in its direction but interestingly, not every close star has a streak. The streaks tend to enter the frame from the shorter sides, which I think makes sense given that the sensor is longer on the other side, thus reaching further into the image circle, exposing it to sources outside of the sensor area.

 

Thanks everyone for the help, the orientation mismatch in SkyTools sent me on searching for other explanations !

 

 

 

wwt.jpg


Edited by RazvanUnderStars, 20 September 2021 - 12:08 AM.

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#21 RazvanUnderStars

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Posted 20 September 2021 - 10:04 AM

At a third look, I realized that an equally bright star (HR9100, mag 5.9) along the left long edge in the previous message has not produced a streak, even though it's closer to the center of the frames than the other stars that did. I think this indicates that the streak is related to the rectangular shape of the sensor rather than any light that enters the scope, i.e. the streak happens on the surface of the sensor, probably the UV/IR filter.

 

Even if the light enters the scope, if it doesn't hit the rectangular sensor cell, it does not produce a streak, so in yet other words, it's not the optics of the scope, but the camera that creates them.



#22 ChrisWhite

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Posted 20 September 2021 - 03:00 PM

It's possible that that other star was focused on messing up someone else's imaging session...
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