Saturn 2019-08-10 21:00 UTC

[Ags]

The wind was blowing tonight and the seeing was atrocious, so I thought it was the perfect night to test what would happen with different exposure lengths on a night of proper bad seeing. I shot three 4-minute sequences of 10ms, 17.5ms and 30ms to see if very short frames really would give a benefit.

ASI 178 MM, Skymax 102, Wratten 29 filter

I am picking up very little detail on the planet. I suspect the strong red filter I am using suppresses the detail as the difference between the bands of the planet is mostly in the blue channel (I think).

@vlaiv I used a smaller ROI this time - 800x640. It is probably still too big but the AZ-GTI spontaneously slews at random moments, so I like to have a bit of time to countermand the slew before the planet slips out of view. I will try 640 by 480 on the next outing - and then I might give those 6ms frames you talk about a go 😃

30ms frames (best 37%)

17.5ms frames (best 15%)

10ms frames

 

Final image (with a little sharpening done in Gimp):

Edited August 10, 2019 by Ags

[oldfruit]

Considering the conditions I think you have done well.

[vlaiv]

I think that on this particular night, going for 10ms made a difference.

Btw at what focal length are you recording? Using a barlow or native?

I'm asking because you seem to drizzle images for some reason, but I think that SW Mak102 has F/12.75 - that is even a bit over critical sampling for ASI178, particularly in red part of spectrum. That means that you are getting all the detail that you possibly can with this pixel size - there is certainly no need to drizzle as it will lower your SNR and it won't improve things.

[Ags]

I don't use a barlow. In earlier experiments drizzling seemed to give some benefit. I will have a go without drizzle. 

And here it is, the 10 ms sequence reprocessed without drizzling, with the same sharpening in gimp.

@vlaiv you say that on this particular night 10ms made a difference... But is there ever any reason to use longer exposures? Why not just shoot 10ms or less every night?

 

 

Edited August 10, 2019 by Ags

[vlaiv]

10 minutes ago, Ags said:

@vlaiv you say that on this particular night 10ms made a difference... But is there ever any reason to use longer exposures? Why not just shoot 10ms or less every night?

Planetary imaging is a fine balancing act - on one hand you want to go as short exposure as you can, but on the other hand you want to achieve the best SNR that you can.

Short exposure brings you frozen seeing, but also increases number of good frames - seeing is changing moment to moment, and at those good moments you want to be at very high fps to record as many frames of good moment as you can. It also gives you more usable frames - if for example you shoot 30ms subs and seeing conditions change rapidly it can happen that out of those 30ms - 20ms was decent and 10ms was poor, with 10ms subs there is a chance that you have two good subs, but 30ms sub will be blurry.

On the other hand short sub means less SNR per sub. This is bad in two ways - first is read noise. Every frame has one "dose" of read noise - more frames, more total read noise there is in the end. In ideal world there is planetary camera with 0 read noise. With such camera it makes no difference if you stack 1000 x 5ms for total of 5 seconds, or 100 of 50ms subs. Result will be the same.

In our not so perfect world, planetary cameras have read noise and this read noise makes difference between a stack of 1000 x 5ms vs 100 x 50ms - first stack will have 1000 doses of read noise and second stack will have only 100 (total light and associated shot noise will be the same). This is the reason behind recommending very low read noise cameras for planetary imaging.

Second reason is related to actual shot noise - shorter sub, less signal you will capture in that sub, although in the end stacking enough frames will get you total needed signal, you need to stack them first and that means that software that is stacking subs needs to have enough signal per sub to figure out how to stack each sub. When you do planetary stacking - you assign so called "alignment points" over the planet. Software uses this points, or rather part of image under those points (and in vicinity) to try to figure out where to move pixels between different subs to align them properly. Due to seeing successive frames will "wobble" and planetary features will shift randomly. Software examines alignment points - finds features under each alignment point in all frames, calculates mean position and then shifts pixels in all subs to this mean position.

In order to be able to do that software needs each sub to have enough signal, or rather high enough SNR as noise in the image makes feature detection and alignment less precise. If software can't properly align alignment points (or features in that position) - it will produce blur due to stacking - which is a bad thing. You want your subs to have at least some signal for stacking software to work properly - that is why you can't go very short, unless you image the moon for example and there is plenty of light to produce enough signal with even very small exposures.

This is also reason why it is bad to over sample when doing planetary imaging - you will spread signal over too many pixels and SNR will be poor - there will be trouble with alignment (and if you over sample you won't have any benefit of additional detail because scope is not able to produce it - it is aperture limited after all).

All this means that you adjust your exposure to given seeing conditions - you need to lower your exposure until you effectively "freeze" the seeing, but no less than that as shorter subs will just have lower SNR and you want the best SNR that you can get.