SharpCap Forums

- recently found out about synthetic blue, works great for me during live stacking session (with low-budget refractor)
- what is not clear to me: when I save a .fits file of the stack (from live stacking mode), what is the blue channel in the .fits ? the original as produced by the camera or the computed one according to the selected algorithm ?

Hi,

the rule for raw frames saved during live stacking is that they are untouched - just as they come from the camera. This means that you can do a full reprocess later.

On the other hand, if you are not live stacking then all the effects in the preprocessing (dark, flat, background subtraction, satellite trail removal, synthetic blue, etc) are applied to the image before it is saved to file, so the saved files *do* contain the adjustments.

Hope this helps,

Robin

Hi Robin,
to make my question more clear:
it deals with the 4 save options during live stacking, as explained in this topic here on the forum :
viewtopic.php?t=7998

I can save a 16 or a 32 bit copy of the stacked image, so this is one file, not a directory of all subframes

As far as I can see, all preprocessing (righthand control boxes) is applied ( darks, flats, etc).
All stretching and color balancing is not applied.
For the new option 'synthetic blue' both cases could make sense: apply or not.

When I'm happy with the 'synth blue' image, I could save the 16 bit .fits with the synt blue channel and do more complex stretching etc operations (in my case: to show to the public in an outreach context)

In another context, I could save the 16 or 32 bit stacking result with the original blue from the camera. I could to further processing, including playing around with alternative algorithms for blue channel substitution. Without having the need to reprocess the whole bunch of subs that where used to feed the stack.

To come back to my initial question:
I have the impression that the 16 bit saved .fits does contain the computed synth blue,
can you confirm that ?

kind regards,
Ivo D.

Hi Ivo,

ah, OK, thanks for clarifying!

For those four options, all of the images have the synthetic blue channel. That's because the replacement of the blue channel happens to each frame from the camera *before* it is added into the stack (alongside other processing steps like flat and dark correction). So, the data in the stack is built from the synthetic blue data and the original blue channel data is no longer kept anywhere.

cheers,

Robin

Hi Robin,
- thanks for the clarfication !

- " replacement of the blue channel happens to each frame from the camera ",
this is interesting.

What would be the difference in outcome if the blue replacement is applied to the stack, and keeping the processing of the blue camera channel going on as usual ?
- computing load will not change I guess.
- no data lost (the original camera blue channel)
- outcome would be different in case the synthetic blue algorithm is changed during the stacking process, but to me that seems bad practice
- at any moment during stacking the user could have the options to 'stretch' and to save either with original blue either with synthetic.

Perhaps 'synthetic blue' is rather a first step in post-processing, not a last step in pre-processing ?
Playing around with stretching and color balancing the stack during live stacking, but first decide which blue channel to use.
Toggling between original camera blue stacked versus computed synthetic blue to my feeling could be a switch in the lefthand boxes while lifestacking.
From an educational point of view (outreach) that would be an interesting thing to show and explain to the public.
At the end of the live stacking session, I could save both versions with original blue and with synthetic blue: great for comparison and discussion !

But probably there is some issue that I'm not aware of yet !

kind regards,
Ivo

Hi Ivo,

the way it works (with the blue channel substitution happening in the pre-processing of the frame along with dark/flat/satellite trail removal, etc) is for a few reasons

1) It fits in with the way that the other pre-processing options work in SharpCap, so it's consistent

2) It can be used with multiple different tools in SharpCap rather than just in deep sky live stacking (for instance with planetary live stacking or just capturing and saving images to file).

cheers,

Robin