Light Ring When Processing

T

TCMars

New member
Ladies and Gentlemen, this is frustrating me…

Running DBE division, then subtraction on this image, without any prior processing, is revealing this ring that I just can’t seem to get rid of, no mater what I sample or how I play with tolerance/shadows relaxation. This image was stacked with flats, darks, bias; ASI2600MC (OSC). I don't believe this is normal vignetting, it doesn't match the pattern from my flats. I haven’t had this issue before, the only difference between this and my other pictures is this is the first time I used a filter wheel and the first time I shot without my L-eNhance filter (I got the filter wheel so I could cycle from nebula to galaxies). I’m in Bortle 6/7 skies, C8 OTA.

Any idea how to get rid of the ring? What do you think is the cause of the ring in the first place? The gradient looks fine in STF, unlinked looks fantastic. Running DBE makes it look like crap. What am I missing here?

Original File: https://drive.google.com/drive/folders/1LJLJXqktwfxyWOq7HuzjhLmlvR0y9TFx?usp=share_link

NGC3344_DBE_Div_Sub.jpg
 
i used to get this all the time with my TMB92. i think it must be due to internal reflections inside the telescope, that is, off-axis light bouncing around off of surfaces that are not perfectly black in the focuser, the filter wheel or any reducers. it seemed to happen more with L exposures, but was definitely present in all filters. panel/sky flats didn't seem to illuminate the telescope/sensor exactly the same as the sky, which meant flats would not fix the problem. however i did have some success doing "night flats" where you take a bunch of heavily dithered images in a blank-ish part of the sky near your target, so you can reject all the stars during integration. but this is a huge hassle and kind of an extreme measure.

i know some C6s had some shiny parts inside the main baffle, but it would result in a different kind of artifact.

the suggestion here is usually to remove the camera only, and during the day point the telescope to the sky (but far away from the sun!!) and look up thru the telescope and try to identify any bright parts inside the optical train, then flock them. i'd start there.

rob
 
I was thinking that might be the case, time to break out the electrical tape. My only pause is the fact that I haven't seen it with my l-eNhance filter, only when shooting without a filter. Maybe the light leakage is being filtered out?
 
well it is possible the existing flocking in your telescope is more reflective at IR or UV wavelengths than in visual wavelengths. assuming the L-Enhance cuts UV and IR, that could explain it.

i'm not strictly talking about light leaks though, i'm talking about off-axis light coming in thru the objective of the telescope and then reflecting off of shiny surfaces inside the telescope and into the sensor. so you might need to "blacken" various parts of the optical train that are too shiny.
 
a few companies make different flocking products. some are like tape with black felt on one side, others are like stiff plastic that you roll up and insert into the OTA or the focuser drawtube and they hold on by friction as the plastic tries to flatten out but can't.

try googling "telescope flocking material" and you'll see a lot of stuff.

some people use the "black 2.0" or "black 3.0" paint (which is similar to VantaBlack but a lot cheaper). i've heard that the black 3.0 and black 2.0 paints are kind of thick though.
 
Any suggestions on how to remove such artifacts by using Pixinsight? DBE and ABE do not prove to be useful, at least so far I have not found a set of parameters to do it sucesfully, neither on stary nor on starless frames.
Such artifacts are clearly seen also in flat frames, but in spite of that, WBPP callibration process does not 'clean' them from light frames.
 
with light leaks and reflections it's really hard to match flats and lights given the different light sources and exposure times.

i've sucessfully (but partially) removed these artifacts by placing a zillion DBE samples along and just to the sides of each arc. it is a very time consuming process and difficult when there are a lot of stars, which must be avoided. setting the sample tolerance really high will help but if there are any stars and star halos under samples this will give artifacts. also reducing the smoothing factor can help, but again you need to be careful and check the results carefully for introduced artifacts. examining the background model carefully as well is useful as you can see if there are any light "blobs" introduced by improperly placed samples.
 
hello @mvrecko
ideally, this is managed during acquisition and with the flats

but, with a galaxy with "clear background", you can do something in linear :
- you split your image in 2 : star_only and starless (the original image must be the sum of the 2)
- is then much more easier to use DBE or ABE with much more samples (dont forget to tick the "Normalise" box in order to keep the same value for the mean sky background) ==> the basics is that, a part of some galaxies, the sky background MUST be even, so you can push with the parameters :cool:
- you can even use clone stamp if you wish
...
- at the end, you add up the 2 files, stars_only and starless_corrected

This is a kind of "ugly workaround" but it does the job and it allows not to throw away your precious acquisitions :)
this is basically what was mentionning @pfile but in starless mode, so it is much easier
 
Djibi said:
hello @mvrecko
ideally, this is managed during acquisition and with the flats

but, with a galaxy with "clear background", you can do something in linear :
- you split your image in 2 : star_only and starless (the original image must be the sum of the 2)
- is then much more easier to use DBE or ABE with much more samples (dont forget to tick the "Normalise" box in order to keep the same value for the mean sky background) ==> the basics is that, a part of some galaxies, the sky background MUST be even, so you can push with the parameters :cool:
- you can even use clone stamp if you wish
...
- at the end, you add up the 2 files, stars_only and starless_corrected

This is a kind of "ugly workaround" but it does the job and it allows not to throw away your precious acquisitions :)
this is basically what was mentionning @pfile but in starless mode, so it is much easier
Click to expand...

yeah, this is true, these modern star removal tools make this so much easier.

i would use "screen mode" in BXT and then use the screen blending expression they suggest in the tooltip to merge the images.
 
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