Adam1234

Could you post a picture of your stacked and calibrated red image? And details of how you took the flats?

I (think) I may have fixed the issue with my filter wheel- I done an experiment this evening: 1) Took 2 sets of flats with no filter change, calibrated one set with the other - result: flats calibrated fine & cancelled each other out (already done this last time but done it again to make sure) 2) Took 2 sets of flats with a filter change between each set (going back to original filter before the 2nd set, calibrated one set with the other - result: as expected due to the misalignment on filter change, I saw the moon crater pattern 3) Re-calibrated the filter wheel 4) Took another 2 sets with a filter change between each set, calibrated one set with the other - result: flats calibrated ok and cancelled each other out This seems to suggest that re-calibrating the filter wheel may have done the job, so hopefully for future images I take, my flats correct for the dust motes properly - assuming that the calibration holds and it doesn't come out of alignment again any time soon. We shall see next time I manage to get out and image. Just in case, I think it will still be worth me cleaning the filters at the weekend. I'm not sure I'll be able to save the images I took before I noticed the issue, but I'll try and take a full set of flats and see. Probably will depend on whether the filter wheel came out of alignment before or after I took the images. At least it was only a nights worth of images I'll lose if not. But fingers crossed all is good for future images!

incredible

Welcome to the forum

Hi Tony, welcome to the forum

Beautiful image

Good idea, I'll try recalibrating first and if that doesn't work then I'll contact FLO who I bought it from and see if they can help

I'll try and avoid step 5 if at all possible 😀 I'm using the ZWO 1.25" LRGB Sii Ha Oiii filters that came as a bundle with the filter wheel and ASI1600, so shouldn't be an issue with the filters

Well, it looks to me like the filter positioning is certainly the culprit. As per @ollypenrice's suggestion, I made a master flat from one half of a set of flats and another master from the other half taken in the same run (50 flats in each half), and calibrated one with the other. This is my result with the L flats (filter with the most dust): Looks pretty flat to me with none of the 'moon crater' artefacts I'm seeing with the lights. The R, G & B flats all look corrected as well and look exactly like above so no point posting those. So I think that this confirms that my flat frames themselves are fine (reassuring in some respects), and that the issue is due to the filter wheel not returning to the same position with enough accuracy. Ok, now I need to figure out why it's not returning to the same position, and how I can fix this. Maybe I ought to try this: 1) Re-calibrate the filter wheel (I believe there is an option in the ascom driver) 2) Take a set of flats > change to a different filter > change back to the original filter > take another set of flats 3) Stack each set of flats and calibrate one master with the other 4) Jump with joy when the flats correct each other indicating problem has been solved; or 5) Start crying, chuck the filter wheel in the river and buy a new one (not ZWO) / seek further advice from the pros Adam

Hi Olly, thank you for your input, that will be a great test. I took 100 flats for each filter so plenty to split in half and create 2 masters from. I'll try that this evening and report back! 👍

Good shout, ill give that a go, and take another set of flats to see if that corrects the issue - probably unlikely if the filters were out of position when the lights were taken but it's worth doing. As another experiment, after recalibrating, I could take one set of flats say in R, change to a different filter, then back to R and take another set of flats, and see if the dust in the 2 sets of flats match, at least that would tell me if the recalibrating worked even if I can't correct my images. After that I think I will carefully clean the filters ( got some baader wonder fluid on it's way) and hope that i can keep dust out of the filterwheel in future!

I'm not sure exactly what technology they use to register the position, possibly a laser or some kind of sensor, I'd have to do a bit of digging. Also not sure what other manufacturers use. From what I have read online, the unidirectional setting is one direction, and is what I have seen recommended online.

Ok I took new flats outside last night to see if this had any effect. Also I adjusted the brightness on the laptop screen I used as a light source to increase exposure time to 8.25s for all filters (target ADU 25000). Calibrated the flats with matching darks. It does not appear that this has made any difference: As another experiment, I calibrated my previous set of flats taken last week with the new set of flats, with my theory being that they should cancel each other out if all is well with the filter wheel alignment, but as shown in the images below there are similar artefacts as in the light frames: Does this essentially prove that something is wrong with the filter wheel and that it is not returning to the exact same position each time? If this is the case, aside from just cleaning the filters (as inevitably dust will present itself again and I'll have the same issue), is there anything I can do about this? I have had a search around and lots of other people on the ZWO forum have had similar issues with this filter wheel but looking at the responses ZWO seem to be really ignorant to the issue and very unhelpful. Adam

Agree about calibrating near the meridian with dec near to 0. As soon as I started doing this my guiding was miles better going from over 1rms to about 0.5. I think also taking the time to ensure that my guide scope was in focus was a huge contributing factor too. Adam

I have one of those rocket blowers actually - it did nothing to the dust on the front lens, but good shout to try that on the filters first before using any wipes/fluid, as my filters are mounted I can see myself using a wipes and just moving the dust to the sides rather than removing it completely 🙃 Ok, so I think my plan of action at the moment is to retake my flats outside (when it's not raining) so the ambient temperature is similar to that of when I took the lights, and hope that the movement between lights and flat was caused by some contraction/expansion of the optical train caused by the difference in the ambient temperature, and if possible if there is some clear sky (🤣🤣🤣) focus each filter before taking the flats to eliminate any focus issue. Then if the problem persists, strip down and clean filters. Fingers crossed that the apparent movement is not caused by the filter wheel not returning to the same position, as I've no idea how to solve that without buying an expensive filter wheel...🤞

Thanks, I'll look into the wonder fluid - I think I've seen it on FLO

As a side note - if I can't sort out the problem, I will probably have to clean my optics - objective lens and filters. I have a load of Zeiss wipes at home, would these be safe enough to use? https://www.amazon.co.uk/ZEISS-Lens-Wipes-Pack-200/dp/B00IKGH2TI

That is a good point about ambient temperature - it was sub-zero outside when I took the lights (it is winter after all), and even though I cooled the sensor to the same temperature as the lights, the ambient temperature was ~18C inside when I took the flats. That is obviously quite a difference, but at least is something I can test by re-taking some flats outside (as long as it is not raining!) I really hope that this is the reason and not that the filter wheel isn't aligning each time the filter is changed as that may be more problematic! Thanks! Fingers crossed I get the problem sorted! I won't get to finish M33 this year as it getting a bit on the low side and too close to streetlights, but there's always next year!

If it helps here are the Master flats L > R > G >B, autostretched

Hi All... could someone please help me identify why my flats aren't correcting properly and advise what I can do to improve the correction? More specifically, I have a series of dust motes, some of which have been corrected, but others are just leaving large ringed artefacts that look sort of like craters on the moon. Here are some stacked images to show you what I mean: (NGC2403) R filter - calibrated with darks and flats (with dark flat calibration) (NGC2403) R filter - calibrated with darks only - No flats (NGC2403) G filter - calibrated with darks and flats (with dark flat calibration) (NGC2403) G filter - calibrated with darks only - No flats (NGC2403) B filter - calibrated with darks and flats (with dark flat calibration) (NGC2403) B filter - calibrated with darks only - No flats (M33) L filter - calibrated with darks and flats (with dark flat calibration) (M33) L filter - calibrated with darks only - No flats Some notes to help the diagnosis (and hopefully the resolution): Equipment: SW Evostar ED80, ZWO ASI1600mm pro, APT for image capture and flats aid, ZWO electronic filter wheel (Set to unidirectional) FYI images autostretched & uncropped 100 Flat frames for each filter taken with APT flat aid, target ADU 25,000, using the white t-shirt method and white laptop screen All dark frames (light darks and flat darks) are matched in exposure length to the respective lights and flats Flats were taken the following day after imaging All equipment remained on scope in the same position between lights and flat, i.e. did not remove or rotate camera etc Calibrated, registered & stacked in Pixinsight Darks/Flats/Flat darks taken in the dark to ensure no light leak Some of my thoughts: I thought it could it be where I took all the flats the next day, and therefore the filters weren't in the exact same focus position as the light frames which may cause different sized dust donuts in the lights vs flats. However, I'm not so sure as 1) I've never had this problem before when taking flats the next day, and have successfully taken flats with all filters in one go before, and it seems most people do this 2) The last filter I used on the night was the L filter, so if this was the case you would expect this to have the best correction, but this is probably the worst affected. In fact I ran ABE on the L image it is absolutely horrible and really shows the artefacts (see image below), but ABE on the other images did not make them worse at all 3) I have an EAF so I couldn't have accidently changed the focus by a considerable amount Would trying for longer flat exposure times work? The reason I ask this is because the best correction in my opinion (although maybe not perfect - the rings may possibly still be very very faintly visible if I look closely) is with the B filter, with which the flat frames exposure time was the longest at 7.34375s. Flat exposure time for L & G = 5.30469s, and R = 5.98438 (M33) L filter - calibrated with darks and flats (with dark flat calibration)_ABE Any advice will be greatly appreciated as being in the South UK I don't get many clear nights as it is, and I would rather not have to bin images I do get on these rare clear skies! Thanks in advance Adam

Holy cow that is in an incredible sight! Well done!

Nice image!

I'll give it a look 👍

Wonder if it might be worth me getting a cheap laptop at some point to use purely for imaging

I'll be definitely trying that!!!