LiveStacking on-screen display issue

[mjanzou]

Greetings,

I have tried 2 times unsuccessfully to 'see' any on-screen dso target using live stack.
The screen appears linear and when I do an auto-stretch, I get a more detailed star field but no nebula data. The histogram levels also appear flat in nature with no noticeable 'usual' hump.
I'm using an asi533mc-p (zwo duo filter) with a askar fra300-p mounted on a cem26.
What confuses me more is that I saved the fits subs from the livestack session and when I point SC to them with the folder watch option, the dso appears and stacks without issue. Histogram/image looks normal and all is well.
Im sure its a setting or 'user' oversight and would appreciate your feedback.

Thx...

I have a onedrive share that I saved the stacked images to for your review, the 10-1-22 is the session livestack fits save and the 10-2-22 is the watched folder stack from the subs I saved during the previous nights same livestack session. Here:

https://1drv.ms/u/s!AoVoVwu4yv-HiZR4uqF ... A?e=hnLxFk

Thx again,,,,, DOH!

[admin]

Hi,

thanks for sharing the images. It's odd, since both of the FITS stacks contain nice nebulosity data when I download them and view them in FITSLiberator.

I think that the key is when you look at the image statistics for the two - the *mean* pixel value of the one from the actual live stack is negative! That means that most of the image will remain black, since while SharpCap allows for negative pixel values they are really there to keep track of random variations and after a few stacked frames the number of them should be negligble.

Almost certainly, the negative pixel values have been caused by a mismatch between the dark and light frames - probably in the value of the 'brightness' camera setting, which is what ZWO call 'offset' or 'black level'.

It looks like you didn't use the dark frame when you re-stacked, so you sidestepped the issue.

Hope this helps,

Robin

[mjanzou]

I did notice that I used the wrong dark but when I selected the dark file, it defaulted to the correct 120s directory, but you can clearly see it used the default startup profiles 5s dark

Just to be clear, is there any condition that will apply cal frames to you your raw subs during a livestack session

And 2, do I have to use cal frames created in SC when stacking outside of SC folder watch? For example if i stacked the rawfiles in siril?

I also created master cal frames in asistudio for asilive but understand they are a bit different then your wizard cal frames.

Thx Robin…

[admin]

Hi,

SharpCap will not automatically select a dark frame for you, although you may get a dark selected if you have previously saved a capture profile with a dark frame set (or you have turned on the option to auto-restore camera settings).

It is possible to use master calibration frames from sharpcap in other software, but you may find that it gives imperfect results or is hard to set up correctly. Some people do this and get excellent results, others have problems, it is difficult to work out the exact reason as there are so many different software applications for processing and so many different ways that each can be configured.

cheers,

Robin

[mjanzou]

Thanks for your reply and clarification Robin.

I also meant to ask is there any condition in livestack 'settings' (that I manage to screw up) that would compromise the rawfile subs? or will they always be clean (no cal frames/adjustments) applied.

Update: I just reran the folder monitor with the 200g/120s dark cal frame with the same (bad) on-screen results.

However when I switched to hot pixel removal only, still applying the DUO band flat, the screen lit up in all its splendid glory and all is well....

mj

[admin]

Hi,

the raw frames are saved before any processing is applied when you are live stacking, so they will always be pristine, allowing you to do the sort of re-run you have been doing with the folder monitor camera or stacking with other software.

If you are still having issues with black screen output when applying the dark, I'd suggest checking out the master dark frame with FITsliberator - I suspect you will find that the average value of the dark frame is higher than that for the light frames, which you will want to track back to the camrera settings when you took the two different sets of data.

You might be able to work around this by turning on background subtraction - that will usually bring the background level up to a standard value (which isn't zero). I have never tried it with a negative background, but it could work.

cheers,

Robin

[mjanzou]

Thanks Robin,

I did redo my 200g/120s dark frame with a new 20 offset (brightness) and ran it without issue. As you described, the 'live stack' 20 offset camera setting used with the zwo default 70 offset dark frame was indeed the culprit.

I read all over the forums that the 20-30 offset range is optimal for the 533 but wonder why zwo defaults it to 70? Is there any benefit running a significantly lower offset?

Thanks,
mj

[admin]

Hi,

a lower offset gets you just a little bit of extra dynamic range in each frame. The effect is pretty small though. The higher offset makes you safe against black level clipping if you capture using higher gains. As we have discussed, the real problem comes when different values get mixed up

cheers,

Robin

[mjanzou]

Hi Robin,

I have one additional cal frame question.
I was under the impression from our conversation that no SC 'preprocessing' dark/flat frames were applied to raw saved stack images for use in later postprocessing.
However, it clearly states in the 'flat frame' docs section that if you apply a 'preprocessing' flat frame it will be applied to the on-screen AND saved stack frames.
Has there been a change or am I mistaken???

Thx,
mj

[admin]

Hi,

looks like the documentation isn't consistent on that - in the flat frame section it says that the flat correction is applied to any saved capture file. In the live stacking section it says that the saved raw frames in stacking do not have any corrections applied.

The live stacking section is correct - the bit in flat frames is true as far as *normal* saved images go - ie captured using 'Start Capture', 'Snapshot' etc, but the raw frames saved in live stacking are an exception to this.

Hope this helps,

Robin