M52

[JR]

Mark commented on his friends' recent amazing image of Mars.  It was taken using several thousand very short exposures, totalling only about a minute in total.  He said it was alchemy.

Sharpcap has a utility for setting up such image runs.  It involves a one off calibration of the camera.  The camera is then set on the night to measure sky brightness, by taking images of a dark portion.  This takes a few seconds.

A subfunction named The Brain (what else!) then calculates optimum settings for resolving faint objects.

More details are here

http://docs.sharpcap.co.uk/3.2/#Smart%20Histogram

I tried it out last night with the open cluster M52, around 200 stars in Cassiopeia, at an uncertainly measured 5000 LYs from earth.

The Brain advised 8.3 second exposures at gain of 0 giving max resolution.

[Technical note which I hope isn't too far off the mark:  Gain of zero means that one electron displaced in the camera sensor by incoming photons ends up as a 'count' of 1 on the brightness scale.  Zero applies because gain is measured in decibels, ie a measure of change between input and output, in this case none due to the 1:1 ratio.  A gain of 0 gives the best possible dynamic range ie shades of grey.  As gain is increased, more and more overall brightness levels are amplified to a count value which exceeds the maximum value built into the hardware.  Saturation occurs and detail is therefore lost]

I set a total imaging time of 30 minutes (but take your pick).  The Brain set up an imaging run for me of 217 x 8.3 second exposures.  This monochrome image was taken with a ZWO 178MM that I bought for solar imaging, but which is versatile enough to do DSO as well.  Esprit 80ed f5/400.   I fear focus could have been bettered.   The sky clouded and cleared a lot which didn't help.  I set DSS to stack the best half of the 217 frames using 50 darks.  A curves adjustment in Photoshop was applied.  

Maybe not lead into gold here, but nevertheless some degree of alchemy from ~9 second exposures.

Sent from my iPad

[trevsie7]

Hi James, That is very interesting, thanks for posting it.  This is one function in Sharpcap that I have always skipped over as I didn't really understand it.

I assume that if imaging in narrowband and wideband filters then the analysis would need to be done for each filter, since the perceived background level could be different.

In your image the stars appear to me to be over exposed especially around the cluster and the background too dark, though this might be a result of the processing and not the image capture.

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[JR]

Thanks very much Trevor.  I'm glad you found the post interesting.  

I don't think I have got to the bottom of the theory, not least how ’noise' decreases in all of the ZWO graphs as gain increases.  It's obvious it doesn't as anyone who's tried a dslr at iso 6400 knows.

I think it would be good to try some more experiments and maybe you would be able to do it colour or narrowband as I don't have the filters.  I'd really like to see how a narrowband image comes out.  

It makes sense as you say that sky calibration would need to be done for each filter, but assume that the camera noise calibration is constant enough across all wavelengths, even if quantum efficiency in terms of electrons per photon isn't.  I may use my ZWO 120 colour to try to dodge the issue!

I will have a go again with better weather.  I couldn't get a proper lock using the FWHM focus tool in Sharpcap with figures all over the place without touching the focus knob.   I discovered at the end of the run that the telescope lens was starting to mist up but I was not going to go through 217 frames to see which were any good.  I lazily relied on the scores in DSS using the best 50%. 

Even so, I was mightily surprised at what came out of Deep Sky Stacker.  All I did for processing was a curve to bring out the smaller faint stars.   It did seem a bit like magic as each individual frame showed no sign of the cluster at all and only three or four stars.  The image is very high contrast, which could be adjusted down.   Maybe more rather than less processing was needed.  

The upshot seems to be support for many short exposures using the newer CMOS cameras, not least as guiding or a high end mount is less relevant for 9 second exposures.   I took nearly 5gb of images, so a fast large drive is definitely needed.  

James

Sent from my iPad

On 20 Nov 2020, at 20:34, trevsie7 <trevs...@gmail.com> wrote:



Hi James, That is very interesting, thanks for posting it.  This is one function in Sharpcap that I have always skipped over as I didn't really understand it.

I assume that if imaging in narrowband and wideband filters then the analysis would need to be done for each filter, since the perceived background level could be different.

In your image the stars appear to me to be over exposed especially around the cluster and the background too dark, though this might be a result of the processing and not the image capture.

On Fri, 20 Nov 2020 at 12:39, 'JR' via croydonastro <croydo...@googlegroups.com> wrote:

Mark commented on his friends' recent amazing image of Mars.  It was taken using several thousand very short exposures, totalling only about a minute in total.  He said it was alchemy.

Sharpcap has a utility for setting up such image runs.  It involves a one off calibration of the camera.  The camera is then set on the night to measure sky brightness, by taking images of a dark portion.  This takes a few seconds.

A subfunction named The Brain (what else!) then calculates optimum settings for resolving faint objects.

More details are here

http://docs.sharpcap.co.uk/3.2/#Smart%20Histogram

I tried it out last night with the open cluster M52, around 200 stars in Cassiopeia, at an uncertainly measured 5000 LYs from earth.

The Brain advised 8.3 second exposures at gain of 0 giving max resolution.

[Technical note which I hope isn't too far off the mark:  Gain of zero means that one electron displaced in the camera sensor by incoming photons ends up as a 'count' of 1 on the brightness scale.  Zero applies because gain is measured in decibels, ie a measure of change between input and output, in this case none due to the 1:1 ratio.  A gain of 0 gives the best possible dynamic range ie shades of grey.  As gain is increased, more and more overall brightness levels are amplified to a count value which exceeds the maximum value built into the hardware.  Saturation occurs and detail is therefore lost]

I set a total imaging time of 30 minutes (but take your pick).  The Brain set up an imaging run for me of 217 x 8.3 second exposures.  This monochrome image was taken with a ZWO 178MM that I bought for solar imaging, but which is versatile enough to do DSO as well.  Esprit 80ed f5/400.   I fear focus could have been bettered.   The sky clouded and cleared a lot which didn't help.  I set DSS to stack the best half of the 217 frames using 50 darks.  A curves adjustment in Photoshop was applied.  

Maybe not lead into gold here, but nevertheless some degree of alchemy from ~9 second exposures.

<image0.jpeg>

Sent from my iPad

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[Graham Cluer (CAS)]

Trevor/James

Dear Both

Although it may appear that you are having a private conversation please
be aware that at least one other person (me) and probably others are
reading your exchanges with great interest.  It shows the benefit of
having these discussions on a forum rather than just privately.

Thanks

Graham Cluer

[drja...@aol.com]

Graham

My messages are for anyone interested and invite any views!

James

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