Color aberration at star center(s)
I'm very new to APP, I've just tested my first integration (HA, S2, O3) and I see the following artifact on some of my stars (zoomed image):
Could anyone point me out on the right direction ?
I was not sure, but I went back and redo the integration.
Integration settings: quality, average, 1st deg LNC (3 iterations), MBB 5%, winsor clip k3 i1 d0, lanczos-3 no overshoot, topHatKernel size 1.0, no drizzle scale 1.0.
- L100: St-avg-5760.0s-WC_1_3.0_none-x_1.0_LZ3-NS-full-qua-add-sc_BWMV_nor-AAD-RL-MBB5-Lum_1stLNC_it2.fits
- R100: St-avg-4800.0s-WC_1_3.0_none-x_1.0_LZ3-NS-full-qua-add-sc_BWMV_nor-AAD-RL-MBB5-Ha_1stLNC_it2.fits
- G100: St-avg-4800.0s-WC_1_3.0_none-x_1.0_LZ3-NS-full-qua-add-sc_BWMV_nor-AAD-RL-MBB5-OIII_1stLNC_it1.fits
- B100: St-avg-4800.0s-WC_1_3.0_none-x_1.0_LZ3-NS-full-qua-add-sc_BWMV_nor-AAD-RL-MBB5-SII_1stLNC_it2.fits
Result (zoom ~175%):
I'm integration in total 83 images, so I set the diffraction protection to 8px.. it had no effect on the star centers.
Playing around I found that it has somehow to do with the Lum channel, when combining RGBL if I set the Lum channel over 40% I get the effect at bright star centers, increasing up to 100% makes the star center hue shift from yellow to the ones you see on the previous pictures. Does this help identify the problem ?
I may post the link to the full batch if required (2.6GB lights, 4.2GB flats and 6.0GB darks+bias).
So, I followed up a bit by mail. The result I got I attached here (I think I mixed the colors wrong, but that's not the issue 😉 ) and while I see the effect very clearly after combining the data (which is looking great seperately) I can reduce it substantially with a lower addition of the Luminance overall. Also my steps may have caused it to be less pronounced, so here they are;
- Loaded all the data using "multi filter processing" enabled.
- Registered with "dynamic distortion correction" enabled.
- Integrated using the "Quality" weights, stacking 95% of lights, 1st degree LNC/3 iterations, MBB enabled at 5%, MAD winsor clipping at standard values, diffraction protection at 10.
I then combined with 75% Luminance added to the rest at 100% R, G, B (with Ha to red etc). In this case reducing the luminance had a big effect in reducing the star center anomalies (which I guess are due to clipping, although I thought a Luminance addition wouldn't cause that).
To answer a few of your other questions;
What exactly does the “x” and “bg” sliders do ?
The "x" is the factor at which the data is mixed, kind of the weight of that channel overall. The bg is like a stretch I believe upping or lowering the background level which in cases of nebulae in a certain channel can be nice to do, although it didn't have a huge effect here.
All the data seems reasonable and like it should look, in terms of flats, bpm and darks ?
The flats are not great I think, you do see the dust bunnies and it doesn't seem they are of great, bad, influence, but it might be an idea to reprocess without flats and see how that looks after light pollution correction etc. If flats are not ok, it's better not to use them. I think you might need to have different exposures for the flats and also a more even light-source as I'm seeing some weird cut-offs and such. We can talk about the flats in a new topic if you want.
BPM looks ok, bit on the bright side maybe, master dark seems ok but you do see light bleeding at the edges, same for the master bias. It might be that your setup isn't completely dark when you take these fames.
Thanks for taking time to look at this.
I’m far from being an expert on this but I also do believe it has to do with clipping when integrating the luminance channel.
1) The defect scales with the amount of integration of Lum (I was able to integrate 40%, you went to 75%).
2) The defect appears first on the brighter, more saturated, stars.
Why aren’t more people having this issue ? Are my individual channels overexposed ? Is the Lum channel overexposed ?
I really hope @mabula-admin has time to have a look at this, if required I send him the same data I sent you.
Without going to much off topic while we wait for Mabula to kick in..
I do believe my darks were taken during the night, I thought what I see on the four corners are what people call amp glow, is it not ?
For the flat it’s clear I need to clean the whole imaging train and I even need to check if my filter wheel is working properly as the illumination is rather even I don’t know why so many differences between the shadows on each channel.
I created the flats using SGP 30000 ADU tarde value, when you say the BPM is in the bright side it means I have to lower the ADU ?