Hi Richard,
Ah, interesting rig. I think I found that, here?
https://support.itelescope.net/support/ ... lescope-33
Well, big scope, big mount, and big old school CCD camera. With big pixels. But, not so big of an image circle?
I wonder where the vignette is happening. The farther from the sensor, the fuzzier I think the falloff should be. But if closer, then likely sharper. I imagine that could still be flat-fielded with the right calibration, but I wonder if it requires a little extra work. If there's too much falloff, perhaps the overall scaling isn't working out properly.
APP is known as a fine stacker, but I don't know what its defaults are. What did you feed in for the full calibration, if I may ask? Off the top of my head, CCD may require dark flats, whereas newer CMOS can use bias on the flats side of the equation.
In any event, though the edges can be cropped, query if a calibration problem with the flats is causing that strong blue cast in the center. There really was no shaking it no matter what I tried. Ultimately I even (after doing a dynamic crop in PI and re-star-aligning the files) ran Wipe on each filter individually at UnCal1, before recompositing, but I'm not sure that accomplished much - or even more than just UnCal1 after composition would have.
I also started inspecting each channel one by one. Siril turned out to be the easiest for that, with RGB composition, and I just left the L out. This was to try to figure out why the stars were multicolored, as if the channels weren't aligning right or had chromatic aberration (CA). Here's where things became intriguing. The green channel looks best. Very nice. But the blue and the red seem to be just out of focus, probably with red the worst. So, spatially, those stars were bloated in red and blue as compared to the green data. Or maybe seeing was worse during those filters, or something stacked wrong?
The stars in the red channel also looked "eggy," as if there were some tracking drift or guiding problems. The unfortunate result of all this is some bright red ringing around the stars, and even red wings to the sides of many. I wonder if blinking through the subs (autostretched) would provide any answers.
But, I tried to do the best I could with all that in mind. In order to not have especially the oval red stars influence the synthetic L, I composited as L, RGB, declining any synthetic contribution. Prior to that I tried using only green as the luminance data, but that didn't really pan out as I hoped. The flaws in the chrominance still showed up when the luminance data (green at that time) was stretched.
I used OptiDev with a few ticks of IFD and then drew a ROI, keeping the global stretch tame, and followed that up with a mild HDR. Have to keep some power in the core! Then SVD, Color, Denoise. That's it.
Color was of course the trickiest, and I played around with the saturations and the highlight repair (max) in order to try to achieve some nice coloration without revealing the aforementioned flaws
too much. Basically I tried to hide them. As such, the colors here are somewhat dull as you can see.
For the "true color" balance, I just used the sliders manually with frequent checks of MaxRGB, and aimed myself at some landmark stars I could find the B-V values for in Stellarium (listed right below magnitude when you click on a star).
How accurate are B-V values? Dunno. And it's not the only color index. I've seen somewhat different results, color wise, for the same star, if measured in B-V versus U-B or the others.
https://en.wikipedia.org/wiki/Color_index
This guy came up with a more complete chart, which I consulted for better gradation in between the rows of the Wikipedia chart.
http://www.vendian.org/mncharity/dir3/s ... tails.html
Of the handful of decent Tuc 47 images out there, I think this APOD seemed to have a pretty good balance that concurred with the B-V values, and didn't blow out the saturation or turn the white stars blue.
https://apod.nasa.gov/apod/ap110116.html
I suppose my lightly saturated rendition is kind of boring, since Tuc 47 is supposed to be a jewel box of the southern skies, right? Maybe one of our other regulars can figure this one out better.
-
- RKonrad Tuc47 test ST9 2A.jpg (666.94 KiB) Viewed 49312 times