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Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Wed Oct 01, 2025 3:07 pm
by admin
Hi Mike
clear skies here last night, so I ran an hour on M13 and then compared that to similar lengths of time on M31 and IC5068. The colour calibration on the globular cluster definitely gave higher blue values (about 1.9x) than we calculated for the other two targets (~1.6x), so something is going on with globular clusters.
My current favourite theory for what might be causing it is that perhaps the measurement of star colours from the image is being affected by the high density of stars in and around the core of the image. That may account for the fact that the measured B-G values for stars is higher (near to +1) than for other targets, which in turn leads to the calibration thinking more blue is required. I will have to spend some time trying to work out if this is the true reason or not (I have 2 or 3 other possibilities too) and then see what might be done to address the issue.
cheers,
Robin
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Wed Oct 01, 2025 8:27 pm
by MarMax
admin wrote: Tue Sep 30, 2025 2:20 pm
Hi Mike,
thanks for sharing - I see two distinct things going on with the colour calibration in that image
1) the background has a red tinge in the center of the image and blue towards the edge/corners. I think that this is caused by uncorrected vignetting of the image, meaning that there isn't really a consistent background level - this in turn makes it hard to get the background subtraction just right when using the calibration. Cutting the image down to the central region (keeping 50% of width & height) cuts out enough of the vignetting on the edge/corners that the calibration then does a good job of getting a black background.
2) The calibration seems to be boosting the blue channel too much, giving a blue globular cluster. I need to dig into this one a bit more - is it the result of relatively noisy data in the B-G calibration graph? Or could it be due to the bulk of the calibration stars in that area being the brighter (and hence perhaps bluer) stars. If I can find some previously saved globular cluster images then I will test them out to see if I see similar things on other globs - if not then I can see if I can capture some on a clear night (if we get any soon...)
cheers,
Robin
Robin, thank you for both replies and doing some GC's yourself. It seems like the extra blue is more associated with GC's and OC's than other targets. As for the red center and blue perimeter . . . I'm having a devil of a time with flats. I'm going to change my workflow and only use an offset of 60 for the 2600 and 2400 sensors. I'm using the "capture and subtract bias" and "monochrome" choices and plan to start using your "auto gain/exp" instead of the same gain as the lights.
Here is my recent saga on CN.
https://www.cloudynights.com/topic/9775 ... s-for-eaa/
I'm all ears if you have any words of wisdom.
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Thu Oct 02, 2025 1:42 pm
by admin
Hi Mike,
I can give some 'scattergun' ideas on why flat correction might not work out properly based on my experience - not sure if any will apply to you, but worth checking...
* Mismatch of the flat and dark flat/bias flat frames (different brightness/offset settings or different colour balance) - that would mean the bias level in the flats not being subtracted out properly, leading to flats that either over or under correct (depending on whether it is over or under subtracted).
* Similarly mismatch between the light frames and dark frames - any change in offset/brightness or colour balance or exposure/gain - this leads to the bias/thermal component of the lights not being removed correctly and again over/under correction by flats
* Any obstructions *beyond* the top of the telescope tube (skyward of where the flat panel goes) - this one caught me as I have multiple scopes mounted side-by-side, and the longer telescope I use for solar was cutting out some of the light that should have got to my deep sky quad... Because the reduction is not part of the optical path inside the telescope, flats will not correct this sort of thing.
cheers,
Robin
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Thu Oct 02, 2025 6:23 pm
by MarMax
I think I'm OK on the first two but the third suggestion is interesting. With the Edge 11 I can use either a 315mm Gerd Neumann Aurora panel or a 24"x24" square LED panel. I typically use the Aurora when running the scope and just set it on top of the dew shield. The 315mm diameter is just barely more than the OD of the dew shield.
Thinking about your third suggestion, it seems like the 24"x24" panel might be better since it goes much farther beyond the OD of the dew shield. When using this panel I place the scope front of it but not touching it. I'll usually have a gap of 3-4" from the dew shield. With the SCT I've wondered if placing a flat panel on the OTA or dew shield is not ideal. Thinking about how light enters from the sky it seems like a larger light source spaced some distance away from the dew shield would be better.
I also have another way to take flats and this is with a projected light source and a 4'x4' white board. This seemed to work best when I was using a Hyperstar with a C11. I can also try the projected light flat and see how things look.
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Thu Oct 02, 2025 7:09 pm
by Gfischer
Max,
I was having issues with my flats awhile ago. I have a 12” Newtonian with a dew shield. I was putting my led screen on the dew shield at first and flat would over correct. I changed to putting led screen directly on ota and helped a lot. Just fyi
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Fri Oct 03, 2025 12:41 pm
by admin
You could always try taking flats of a blue sky on a clear day - the exposures tend to be a bit long, but it provides light getting to the front of the telescope in exactly the way that light from the sky does.
cheers,
Robin
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Fri Oct 03, 2025 3:33 pm
by MarMax
Gfischer> Good tip and I'd agree the flats are over correcting.
I'm using a 3D printed dew shield that incorporates two 50mm fans for dew control. This is also a "short" shield and it's not opaque. It does pass some light so using it during the day is not possible. I have a standard Celestron aluminum dew shield but it's really heavy compared to the 3D shield and it's like a sail because of the length.
- EdgeHD_11_OTA_wAccys_08122025s.jpg (240.7 KiB) Viewed 1474 times
I'm also using a Celestron dew heater ring so removing the dew shield involves a bit more than just pulling it off. It does seem worth trying flats w/o the shield.
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Mon Oct 06, 2025 12:37 am
by MarMax
As a follow up to the flats with dew shield and flats without dew shield discussion I've completed one EAA session with flats without the dew shield. IMO the flats are correcting better. They can still be improved but I'm reasonably happy with the results.
And I'm not sure if this is related or not but my impression of the color balance using SCC is less blue. Perhaps the 5/10 flats are doing something that changes SCC and the 7/10 (improved) flats are not impacting SCC as much. This is only one night of testing but there seemed to be a lot less blue with the brighter stars.
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Mon Oct 06, 2025 1:04 pm
by admin
Hi,
yes, it would make sense that you might get better colour correction if the flats work better... The colour calibration does the calculation for the R/G/B colour balance sliders from the star colours, and that's not going to be significantly impacted, but... the background subtraction estimation isn't going to be so robust and that could be affected by poor flats.
In particular, the background subtraction cuts off the edges/corners of the image (to limit the effect of any vignetting) and then calculates the background from the inner region remaining. Some of your earlier flats seemed to be over-correcting the edges/corners, which would be ignored from the background subtraction point of view, but the background in those edge/corner areas is still being affected by the adjustments and is therefore prone to come out with odd colours.
cheers,
Robin
Re: New Feature : Stellar Colour Calibration for Live Stacking
Posted: Wed Oct 08, 2025 10:09 pm
by roelb
Also, when the object is small in comparison with the FOV, the Stellar Color Calibration results in higher blue values.
Perhaps, this is due to a big stellar field as with the globular clusters?
I reviewed 13 objects from 08/21 using the latest SC version.
The fits for a series of objects can be found here:
https://mega.nz/folder/EbsjhBBB#r-fiAEAzWrKXokTCD64ntQ