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Thermal Venus Nightside

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#1 rehling

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Posted 21 May 2020 - 01:25 PM

This was a long and winding road: I bought my 1nm filter back in 2018 but had no good opportunities to use it ~19 months ago, due to weather. This time, I had several chances, but found myself struggling to find the right parameters for success, battling unknown sources of internal reflections and double images, variable transparency, and wasted opportunities while I iterated and problem-solved.

 

Last night, I finally got a result, despite poor seeing. I used an C 9.25" SCT, ASI-1600mm with the cooling on, gain jacked up to the max, and 100ms frames. There is no sharpening being applied. I did not use a Barlow, in order to eliminate one possible locus of internal reflections. Maybe I just need a new, factory-clean Barlow rather than the one that I've been using for 4 years now.

 

That dark spot should be Beta Regio, so I'm delighted to have captured any surface detail. Endless thanks to the people on here who have posted their successes as an inspiration.

 

Venus 20200521 1nm.png


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#2 John Boudreau

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Posted 22 May 2020 - 08:53 AM

Any capture of the dark side thermal emission is a special feat! I'm sure the poor seeing made focus difficult--- you certainly captured the dark side with enough contrast to catch features, but the strong blurring makes it tough to tell with certainty--- although I must say the darker areas seem to be in the right places. wink.gif

 

I had a session last night where after getting good focus on the thin crescent while the sky was still very bright, I decided to try to tweak it a bit better just before starting the thermal emission exposures when within moments the seeing degraded and never recovered! I should have left well enough alone! crazy.gif


Edited by John Boudreau, 22 May 2020 - 02:05 PM.

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#3 rehling

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Posted 22 May 2020 - 10:33 AM

Thanks, John. I am hoping for clear mornings next month, because my local geography reliably offers much better seeing before sunrise than after sunset. Venus' declination will not be quite as favorable then, but the seeing can only improve. The individual frames at 10ms during my evenings make Venus look as crooked as a lightning bolt so I know that longer exposures have to be terribly blurred.

 

It is difficult when the opportunities are so fleeting. I hope you get more chances soon. It is remarkable how many distinct adverse factors conspire to make this kind of imaging difficult. On the other hand, it is also wonderful how many different kinds of imaging Venus allows.



#4 ch-viladrich

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Posted 22 May 2020 - 12:22 PM

Well done !

This is interesting to know that the ASI-1600 seems to be free of artefact in spite of having a rolling shutter.


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#5 rehling

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Posted 22 May 2020 - 06:23 PM

Well done !

This is interesting to know that the ASI-1600 seems to be free of artefact in spite of having a rolling shutter.

I don't think my ASI-1600mm has any sort of shutter, but maybe there's something I'm missing!



#6 sunnyday

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Posted 22 May 2020 - 07:34 PM

realy nice work .

thanks 


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#7 ch-viladrich

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Posted 23 May 2020 - 06:17 AM

I don't think my ASI-1600mm has any sort of shutter, but maybe there's something I'm missing!

This is not a mechanical shutter but an "electronic shutter". There are two possible types of electronic shutters in CMOS sensor : global shutter and rolling shutter.

The word "shutter" should be understood as the way the photosites of sensor are read.

Each mode has its pro and cons.

Most sensors with rolling shutter (like IMX290) have very bad artefacts in imagery of Venus.


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#8 Tom Glenn

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Posted 24 May 2020 - 01:44 AM

I don't think my ASI-1600mm has any sort of shutter, but maybe there's something I'm missing!

Christian summarizes it above.  Global shutters expose the entire sensor at the same time, whereas rolling shutters scan the sensor line by line, and although the exposure for each photosite is the same, the time it takes to expose the entire frame is longer.  This can lead to artifacts, especially with fast moving objects.  How this would impact imaging of Venus is entirely unclear, although empirical evidence shows that it does.  This was discussed in another thread, in which John Boudreau explained some of these findings.  

 

https://www.cloudyni...cron/?p=9938991



#9 rehling

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Posted 24 May 2020 - 02:30 PM

Christian, Tom, and John, thank you very much. I may be seeing the same thing – and may be in the market for a new camera! I definitely have seen multiple overlapping images of the crescent, but I have also had other possible sources to check for this. In fact, I have made a point of rotating my camera in the eyepiece for this Venus thermal work because there is a persistent reflection that is offset in one particular direction and I am making sure that it is aimed away from the nightside. I was also seeing more reflection when I used a Barlow and eliminated it after a few failures.

 

In the past, with a different telescope, I have also had issues with reflections in still other places and inserted flocking paper to address that.

 

Most observations don't suffer as a result of the reflections, because the brightest thing in the image is, usually, not so much brighter than the dimmest details that I hope to capture; I have really only noticed these before when trying another observation where one thing is much brighter than the other thing: Trying to capture Sirius B through the glare from Sirius A.

 

All told, I don't doubt that the rolling shutter is a big issue here, but I'm afraid I may be chasing more than one "ghost" with not many opportunities to experiment and eliminate them all.




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