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Luminance eccentricity issues [Deep Sky] Processing techniques · Chase Davidson · ... · 7 · 213 · 2

Black_Dog_Astrophotography 0.90
Chase Davidson
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Hi everyone, I'm having a particular issue that I can't seem to get around. I've been taking subs of the fireworks galaxy for the last week and after each night I usually do a quick subframe selector to see what I need to gather for the next night. I generally use this as my approval and weighting:

Approval:  FWHM < 6 && Eccentricity <= 0.6 && Stars > 500
Weighting:  SNRWeight


I've generally gotten 90+% of acceptance with this when I have a clear night...except with my Lum data. It's very frustrating. I understand Lum data needs to have a much lower exposure so I do RGB at 120 second and Lum at 60 seconds...but they are still coming out terrible in eccentricity. Anyone else know what's going on? My tracking has been between .2-.3, autofocus ever hour, so I don't think it's the mounts fault. In fact I even take Hydrogen data before Lum the last two nights and every hydrodgen sub has been accepted. Whats going on with my lum?

You can see the Lum data vs the RGB (right side) acceptance in the screenshot below. Should I increase the acceptance to .65 eccentricity (it'll add another 3 hours of acceptable lum data) and just live with it? 

Lum Issues.PNG
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ONikkinen 4.79
Oskari Nikkinen
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You should investigate the source of your eccentricity problems as a start. I am not convinced your tracking accuracy is as good as your stats indicate, so how are you guiding and with what scope? There could be differential flexure problems if a reflector without an OAG, or just an unstable guide scope mount if thats what youre using. Eccentricity of 0.6 is really not very good so i am quite sure you have some issues to work out.

Another possible reason is dropping altitude. I see that you started with the target decently placed at 58 degrees, but towards the end of the night this means you are quite low in the sky and RGB separation due to atmospheric effects will play some role in this. There is also a suspicious drop on quality towards the end, which could be local seeing effects for example from thermal currents rising from buildings down range or something like that.

* edit: ignore the last comment above, i read your graph wrong as the last data is the best. Still some issues to work out.
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Black_Dog_Astrophotography 0.90
Chase Davidson
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Oskari Nikkinen:
You should investigate the source of your eccentricity problems as a start. I am not convinced your tracking accuracy is as good as your stats indicate, so how are you guiding and with what scope? There could be differential flexure problems if a reflector without an OAG, or just an unstable guide scope mount if thats what youre using. Eccentricity of 0.6 is really not very good so i am quite sure you have some issues to work out.

Another possible reason is dropping altitude. I see that you started with the target decently placed at 58 degrees, but towards the end of the night this means you are quite low in the sky and RGB separation due to atmospheric effects will play some role in this. There is also a suspicious drop on quality towards the end, which could be local seeing effects for example from thermal currents rising from buildings down range or something like that.

* edit: ignore the last comment above, i read your graph wrong as the last data is the best. Still some issues to work out.

The seeing conditions very well could be the issue. I use an OAG on the astro-tech at130 edt but I've been taking Hydrogen shots for the first 3 hours of the night and then taking Lum shots when it's low in the sky. I did capture 3 hours of good Lum data a few nights ago (middle of the graph above) but I think it was when the target was high in the sky.
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Gondola 8.11
Tony Gondola
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As you get very short on integration time, variations and seeing and guiding will be more apparent and will be more likely to have non-round stars. All those things have a tendency to average out over 2 or 3 minutes so it's easier to get round stars.
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andreatax 9.89
andrea tasselli
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At your image scale you shouldn't take luminance frames below 40 deg or better still below 45 deg.
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ONikkinen 4.79
Oskari Nikkinen
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Take a look at this article: https://britastro.org/2017/atmospheric-dispersion-and-its-effect-on-high-resolution-imaging

The article refers to planetary imaging but the dispersion due to altitude also of course applies to long exposure DSO imaging. Scroll down a bit and you will find a nice graph showing how much the red and blue ends of your luminance are dispersed by altitude. At 50 degrees its already 1 arc second, and it only gets (rapidly) worse with lower altitudes. Try to shoot L at high altitudes and do RGB otherwise is the take away here.
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Black_Dog_Astrophotography 0.90
Chase Davidson
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Another night, another bit of odd lum data. I started before the meridian flip last night and it seems everything before the flip had good tracking. I'm starting to wonder if I have a weight balance issue or a wire is pulling once the flip happens. I do notice a strange loud popping sound when the AM5 tracks to the right every now and then.

I stop taking subs before the target got below 40 degrees last night but as the data shows, the tracking issues started at ~47 degrees right after the flip.

Lum Issues v2.PNG
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Gondola 8.11
Tony Gondola
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Oh gosh, a big clue there. I wouldn't think the AM% would be too bothered by slight balance issues. Something changed after the flip though, a mystery to track down for sure. I have no idea what that pooping sound means for a harmonic drive but it doesn't sound good.
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