The āusualā stack Iāve seen described at DPRās astro forum numbers in the dozens to hundreds of captures, each with ~30sec exposure. So, in terms of ālight on sensorā, I think 19000 1sec images would be similar to about 600 30sec imagesā¦ ???
The advantage I see with regard to short exposures is less reliance on precise tracking, which Iād imagine is more challenging with longer focal lengths.
Am I hunting in the right preserve here? I live within an hourās drive to some of the best dark sky in the US; what better social-distancing endeavor is this??
That really depends of the read noise. With the newer Sony sensors it is really close yes. With the benefits of better resolution.
But if you take a Canon for example it will be better to shoot 1min/2min each frame.
Hello, thanks for your messages.
The advantages for this technique are:
a basic mount can be used : no guiding is required and even a non equatorial mount can be used like the dobson altaz mount that I am using. The mount setup can be done quickly (alignment,ā¦)
higher resolution can be achieved than with regular long exposures: thousands of pictures with bad seeing or tracking issue can be eliminated easily with siril stacking based on fwhm, I will show this improvment on another target (M64) which is more suited
Please find below the improvments obtained by using 18000 images compared to using respectively 500,1500,7000 images (check in fullscreen the sky background and galaxy extensions with the 7000 version compared to 18000)
@fab4space sorry if I go completely off-topic but is your avatar, by chance, a result from some kind of stratospheric balloon launch that you did or something?
@fab4space, It is a totally unexpected and counter-intuitive result. I understand the point about guiding (and tracking, I guess) and mount. Although I guess youād get some image rotation over the course of the night, Siril would probably correct for that.
But noise. There are a number of sources of noise, Read noise, Shot noise and light pollution and they donāt all pull in the same direction. But if you are using an untracked mount, Iām guessing 1 or 2 secs is probably about right.
Youāve managed to get a fantastic amount of detail, and I imagine thatās your lovely 400 mm F/4.5 Dobsonian at work. I canāt get anything like that on my 200 mm F/5 Newtonian.
In my own images, Iāve gone up to 300 secs, and down to 30 secs. I find 180 s a happy medium, but tracked and guided. If I was unguided, Iād probably drop to 45 secs.
Iāve also experimented with ISO from 400 to 6400, and again, I find that 1600 is better.
My main battle at the moment is with fixed pattern noise (FPN) on my DSLR. Iām experimenting with dithering to remove that, but itās a slow and painful process. Does your method help with FPN?
Itās always nice to have our preconceptions challenged and youāve certainly done that.
Thanks @rbarbera@heckflosse@darix@Steve_Cookson for your comments @Steve_Cookson , with this altaz mount Siril handle the field rotation via global star registration algorithm and in order for this technique to succeed , yes the read noise of the sensor is the key factor and the read noise of the A7s sensor is very low compared to other DSLRs.
What DLSR /sensor are you using?
I agree that the FPN is also an issue and this not a problem with this method because with this kind of dobson mount almost all the frames are not alignedā¦ (I can get more than 200 pixels shift relative to the reference image during a session even with precise mount alignment).
Feel free to ask if you have more questions.
out of curiosity tried to do some selective denoising on itā¦dont think much critical detail was lostā¦but then I donāt know this genre of photography. Perhaps de-noising is a faux pas !
PSā¦ if its rude or rule breaking to post this please let me know and ill remove itā¦Obviously this is the OPās image and just wanted to see how much detail could be retained in relation to noise removed.
I wonāt speak for the poster, but I tend to think it is bad form to edit someoneās image when they havenāt asked for it; that is my personal feeling.
Thanks for having improved the image with this technique, can you please provide more details about the tool/settings that you have used to achieve this result?
I donāt see any problem with posting new versions of the image as long as the original reference is keep
Many thanks,
Fabrice
No problemā¦the technique is essentially two steps.
I denoise the image. I used Dfine in the NIK collection which was previously free while google was developing it (or rather not developing ). Have never seen denoising as good as this. I used the manual mode to adjust the result, push it up a bit.
Use some form of masking to determine where the denoised bit should be used and where the original image should show through.
The masking here is to avoid the loss if detail in focal points, such as the galaxy at the centre which has no masking at all. Lens flares around the starts, which would turn to mush, and smaller starts which might disappear altogether given they are essentially indistinguishable from noise.
I used a few forms of masking hereā¦
luminosity mask so that portions of the image which are essentially black reveal the full denoised effect/image. This leaves the brighter portions of the image mostly unmasked and so more detailed. Critical setting here is the mask feathering which must be tweaked to blends smoothly (or not).
manually painted mask in the centre focal point, which i feathered out along the cosmic dust halo with a soft edged brush.
The settings parameters are not really informative as they would be different for every imageā¦and also need fine adjustment at 100% scale to see exactly whats happeningā¦no secrets here. Very simple, just context dependent.
I spent 5 min maxā¦a less rushed job could produce better results no doubt.
Thanks @PhilipB for your detailed explanation.
used the same kind of noise reduction tools with Pixinsight with mask too with bright areas but the tradeoff between noise reduction, keeping details and not having a artificial result was difficult to achieve so I used the noise reduction very cautiously.
It seems that you managed to overcome this issue with the tool/settings that you have used, many thanks!
Fabrice
hello @fab4space just for the sake of curiosity: did you made these 19000 shoots blocking the shutter opened? is it possible on the sony A7? Otherwise, 19000 shoots would impact the shutter-life of your cameraā¦
Considering a middle way of 400k shoots, this single awesome picture cost 5% of the total shutter-life of his camera. I mean, one of the goal in my astrophotography hobby is to shoot one stunning picture like this, because I do really think it is perfect, but, does it really pay you for this 5% of camera life?
Btw: I canāt find the information I need about the electronic shutter, does it impact the total shutter-life too?
What do you mean by ādo not pull in the same directionā? Signal-to-noise-ratio will always be improved by adding up images (if properly weighted). The more the better. @fab4space
Thanks a lot for the series showing how image quality is improving with the number of stacks! May I use these in my astronomy courses to demonstrate noise treatment by stacking? Berry & Burnell (2005) in their book on astronomical image processing show a similar series in black& white. Yours is much nicer!
Proper flat fielding should get rid of that, if the pattern is constant with time (otherwise it is not an FPN).
