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Old 02-05-2022, 08:47 AM
Todo43 (Lachlan)
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Very Short Exposures for Lum in Galaxies

Hi All,

I have been browsing through Astrobin and came across this image. It is an amazing image but what is interesting is the exposure time. Romain uses lucky imaging to capture heaps of detail in the galaxy using 1-second exposures.

Why would only 1-second images be used? I understand that the camera is probably highly sensitive, and has very low noise, but how would he get away with this much detail in such short exposures with very low noise? Do the 1-second exposures somehow negate a lot of atmospheric distortion? Again I'm astounded by the amount of detail that he has got out of it and how bright it is.

Thanks!
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Old 02-05-2022, 09:10 AM
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Camelopardalis (Dunk)
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A couple of points…

1) fast newt - lots of photons concentrated on every pixel
2) very low noise camera - from what I’ve seen, there appears to be almost no CMOS pattern noise with that sensor…unlike every other sensor
3) light pollution? He’s taken a bucket load of subs, so maybe he’s exposing the background “enough”?

The short exposures would mean it would be able to reject any bad ones pretty easily. With conventional “long” exposure imaging, each sub is an average of the seeing throughout the sub length. (This is still true for 1 second subs).

The brightness is just a factor of stretching in post processing.

The days of “live viewing” will soon be upon us with the way the sensor tech is going
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Old 02-05-2022, 09:49 AM
Dave882 (David)
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Very interesting find Lachlan but take a look at the image specs… Having 560mm aperture would help! I think for the vast majority of normal sized telescopes 1sec is simply not practical for the amount of data you would have to work with (50mb/sub x 60subs per min x enough hours to expose the faint areas of a galaxy).
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Old 02-05-2022, 11:01 AM
Todo43 (Lachlan)
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Hi David,

I did notice that and was astounded. It would let in heaps of light! My dream! I'm actually about to get access to a 16" scope. Would be very interesting to try an experiment and see how it performs!


Dunk,

It is certainly very fast. Would you consider that sensor one of its kind? Any idea how it does that? That would revolutionize the Astro world if they were mass-produced. Are you, therefore, saying that the 1-second exposures are getting better clarity because between the start and the end of the exposure, the atmosphere moves less than between the start and end of say a 5-minute exposure?
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Old 03-05-2022, 06:19 PM
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Camelopardalis (Dunk)
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That IMX571 certainly gives cleaner output than any other camera I've been exposed to. A mate has one and I've seen darks and lights, so I'm not speculating entirely.

Regarding the atmospheric turbulence...it can really change on a dime. If you take a 10 minute shot, you get the averaged seeing over the whole 10 minutes. If you take 10x 1m shots over the same period, you still get dished the same seeing, but if it's particularly bad over a couple of shots, you can chuck them out and keep only the better ones.
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Old 03-05-2022, 06:59 PM
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When I got back into astro photography I was moving my rig between here and Sydney and could never get decent polar alignment..before pole master, Sharpcap Nina etc...so my polar alignment forced me to go for short exposures and I guess I got into the habit as the longest I select now is one minute ..this may change as I am going to stop using insane high gain and follow the rules..however what I see as an advantage is being able to throw out any sub that even hints at not being perfect ..the times I have done long exposures ( only 2 minutes) I found I really had to keep poor subs because ofcthectime investment and really they were all generally poor...but keeping only good ones really pays off...I have started on one occasion with 850 subs that I kept cutting down until I only used less than half but the image was far superior to a stack with everything.
And with planes going by where I am you need to be able to drop a few just for that reason..but in reviewing my images it surprises me how things can change from one sub to another..the mount, the wind whatever...

The main reason I purchased the RASA was in the hope that I can take very short subs..basically along the lucky imaging lines..Simply to get better detail..early days and I have yet to have one night that it was reasonable...it is just too wet..even if clear the water is up there...

Anyways I do think there is hope and for me personally I liked the big advantage of having less attachment to a sub..once I look for reasons not to throw them out now I ask are they really good enough not to upset the good "croud" that I have assembled.

I don't know how noise will affect my plans but I feel it should be manageable and see the advantage as having in effect better seeing.

Alex
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