strange banding on my OIII filter

[glowingturnip]

any idea what's going on here ?

 

These are OIII stacks from separate targets, taken at separate times of year, and both show quite strong banding from top to bottom, strongest in the centre, and echoes of it in the same shape to either side. 

They've had flats subtracted (flats taken after each session, stretched sample shown underneath, no banding) and have had DBE done.   The effect is most noticeable with stars subtracted as here, and with a strong stretch.

I think I can just about see the same pattern in the SII for the one target, though not for the other and not in the Ha:

My equipment is as per the sig, and the filters are Astronomik 12nm (cheapskate, I know).

Obviously one test I can do is to give the filter a quarter-turn next time I'm out to see if the pattern moves, which I'll do, but if that really is the same pattern I'm seeing in the SII above, then it won't be that.

Any ideas ?

[Laurin Dave]

What do your flats look like, is it on them in the same pattern?  is it visible on any other channels after an extreme stretch (ie the same sort as for Oiii and Sii)..  The curves and the fact they are in, what seems to me the same place on both filters makes me wonder whether a) there's a reflection somewhere or b) whether something is rubbing the filters ..  I'd take them out and give them a clean .. re-do your flats and see if t goes away

Dave

[glowingturnip]

hmm, I think you might have a point re something rubbing against the filters - the marks do look like they could be concentric about a point somewhere off to the left of frame.  I had a mini-panic a couple of years back after changing filters - I suddenly started getting huge long blooming-like streaks off all the stars, turns out that the ribbon connector from camera to filter wheel (I've got the internal 5-filter version) had become displaced and was fouling against the shutter plane.  It could well be that that ribbon cable is still causing issues, rubbing against the filters out of shot as they rotate past.  I shall investigate.  Hope they're not scratched.

I think I can rule out reflections - I shoot in a rural location and can control all the light around me, so no stray light at all, moonless, and those two were taken of different targets at different times of year, so different orientations.

Oddly I can't find the same pattern in the flats at all (there's an example flat segment in the first pic above) whatever kind of weird stretch I try to put on them.  Can't figure out why it doesn't show in the flats - can the filters have different transmission properties for very dim light over a long period vs relatively bright light for a few seconds ?  Maybe a good thing that the flats aren't correcting this, or I'd be none the wiser.

I think I got away with it on my CED214 below, but I'm not particularly hopeful for the SH2-206 I'm currently processing - it doesn't seem a very interesting target anyway, at least with my data, and now I've got a damaged channel - maybe I should go Ha monochrome.

 

[glowingturnip]

just an update on this for anyone experiencing similar - had a look inside the filter wheel, and gave them all a proper clean. 

Doesn't look as though anything was rubbing, but it could well be that the filter optics were pinched - the filter looked like it was slightly skew, and the screws holding it in were pretty tight.  I'm not a fan of how the filters are held in in that Moravian filter wheel, relies a lot on those little washers which don't sit level when the screws are tightened.

Anyway, made sure they're all mounted properly, but haven't managed to have an outing since, so don't know if I've resolved it yet

[glowingturnip]

Apologies for bumping an old thread - Covid has meant that I've been locked down away from all my kit for most of the year - I hadn't actually imaged anything since October last year until I had a go last week.

I've still got this banding issue unfortunately.  I shot a new target, and could see the exact same banding pattern on my OIII and SII channels this time too.

So that's three targets, all different parts of the sky and different times of the year, and all showing the exact same pattern in the exact same part of the frame as it comes out of the camera.  My flats don't show the pattern at all, and I redid my master darks and bias and don't see the pattern there either.  The pattern is visible in the raw lights, and the flats don't remove it.

I've had all the filters out prior to the last attempt - all are clean and don't have any scratches, and unless I was extremely unlucky, the filters would have gone in with a slightly different orientation, and yet the pattern orientation hasn't moved.  That, plus the pattern being visible in both OIII and SII leads me to deduce that it's not the filters.  Nothing is fouling inside the camera - the shutter plane can rotate freely, and the filter wheel isn't rubbing on anything.

 

It's a Newtonian with a coma corrector - so the only optics in the way are the two mirrors and the CC.  It has an OAG

 

I'm thinking:

- light-spill/internal reflection - possible, and wouldn't appear on the calibn frames, but difficult to see how, I'm very careful with any external light when I'm imaging, and given that the three attempts were different times of year and different  sky positions and yet the pattern is always the same, then it's unlikely to be reflection down the open end of the tube.  I always cover the mirror end of the tube with black-out cloth so it can't be from there - maybe it's some specific light spill around the OAG, but difficult to see how

- pinched optics on the coma corrector - it is screwed in very tight since to get the spacing right I actually have very little thread left to screw it on with and don't want the camera falling off (I have a safety tether), but if it was that, then presumably the flats would correct it

- sensor issue - obviously my worst scenario, but then again the flats should correct it

- warped/pinched mirrors - again the flats should correct it, would have thought the effect would be more blurred too

Can't think what else it would be, so given the above I guess I have to be looking for a light spill - any other ideas ?

 

I should get another chance to test soon (in Spain, so looks like I'm going to have to quarantine afterwards) - guess will try: rotate camera 90 degrees, see if it moves; try without CC; try with/without a big black cloth over everything but the end; with end covered but rest uncovered.  Anything else to try ?

 

Cheers,

 

Stuart

[glowingturnip]

jetting off on Friday, hoping to get a bit more imaging - quick nudge to see if anyone has seen anything like this before ?

 

Cheers

[glowingturnip]

It was pinched optics in the coma corrector from being screwed on too tight (not that anyone cares, it seems 😢 )

Still not sure why the flats didn't pick it up though.  Anyway, did a run of OIII on the Crescent last night, not a hint of banding