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Here's an autostretched, cropped image of M81 after WBP in Pixinsight with 60 x 180 sec frames, with flats, darks and biases. Equipment used: ASI2600 MC Pro, Unity gain (100), Celestron 9.25 SCT, .63x FR, IR Cut Filter. I had to grossly crop the image to remove even uglier edges due to the APS-C camera format, given the smallish image circle of the telescope/FR combination. Imaging in Bortle 7 skies. What could be causing all the background swirls and smudges? I haven't even tried background neutralization or other post-processing steps yet. There was no dew accumulation. Is there any hope for this image? Thanks in advance for assistance on this. Glenn  |
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Your flat correction is not good. There are uncorrected dust mote all over the place. Go back and blink through your calibrated data and it will jump right out. Next you have to fix the calibration but I can’t say much about how to do that without more information. Good luck… John |
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Hi Glenn, Yes, as John indicated, your Flats need some revamping - those are certainly dust 'donuts' and possible smudges somewhere on your imaging train. Can you give some more info on your Flats method? There probably isn't hope for this set of images, but getting this problem under control will benefit all subsequent sets. - - Steve |
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Wow, that's an amazing artifact! I have almost the exact same optical train (no additional FR in mine, just the built in flattener). Looks like maybe dust on two surfaces. The smaller donuts are probably dust on the camera's glass window. The bigger ones may be on the filter or FR a few millimeters in front. None are on the sensor itself...those are generally smaller, dark, and ugly. They don't defocus to the shape of the primary. Dark donuts are new specks not recorded in the flat. Light donuts are imaged in your flat, but the specks that caused the shadows are gone. And the donut with the drop shadow in the upper left is a speck that has shifted slightly. But I have no idea what the oval white cloud is...unless M81 is about to absorb another galaxy! Did you have any dew heaters running? If they were off, including the window heater on the 2600, maybe condensation exaggerated things a bit. The whole image appears to have smear over it in addition to the donuts. Otherwise, nice shot of M81! I bet a careful cleaning and new cal frames will make the target look terrific! |
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Steve Solon and Terry Chatterton: Steve, I take flats with a light box and cover it with some tracing paper to dim it somewhat, but I can never seem to get it right. I need a beer method. I have tried to clean the imaging train and it looks clear enough, but getting all the smudges out can be a problem, especially on the C9.25 collector plate. Glenn |
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Dave Rust: Dave, I'm running the Celestron dew heater ring on my C9.25. I connect it to one of my ASIAIR 12v power outs, but I have no way to control it, so maybe it's running hot. Could that be a source of some of the artifacts? I also have the ASI2600 anti-dew power on. One other issue could be the IR Cut filter. Do I need one for galaxies in my light-polluted skies with the ASI2600? I read somewhere that the protective glass on the camera is a cut filter. Glenn |
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The plate is very forgiving, At that first stage, the light hasn't been focused yet and the surface area is huge. On occasion, I've even completed a shoot satisfactorily with 15% of the plate frosted over. So I wouldn't think that's the culprit. Yeah, I have the same issue with the light box. Almost every setting is too bright. I've had success with a white T-shirt first stretched across and held with a rubber band. Another effective method is to shoot the sky while it's still blue, right before it's dark enough to make out any stars. And a shirt can be added here, too, to dim things so you can catch the light before it begins to fade more quickly. |
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I think the camera window is also an IR cut filter. I use a pollution filter (HEUIB-II) and a Ha-OIII-Sa filter (Triad Ultra) with good results...and IR has not been an issue with those, either. So, unless there is light leakage, say, from a filter drawer, and/or condensation (which I've not seen yet on a filter), I don't think filters would cause what you are seeing. I also ran the ring off ASI. I did not see any abberation from using full-time power. But my battery bank sure noticed! I did later get a lower cost pulse-width power controller, which helped. But, to make sure, I now run an extension cord when I want a full 8 hours out of the rig. |
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| By the way, tell me more about the "beer method." That sounds like something I'd like to try! ;-) |
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Glenn, I have had some donuts show up like that before, working through my flats process. It was nowhere near this many, but I was able to "fix" the image by running Starnet/StarXTerminator, then using PixInsight Clone Stamp to remove the artifacts before recombining. -Chris |
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Hello Glenn, I'll make a few suggestions and basically in line with what has been said above about flats / your flats. This is what works for me for successful dust mote mitigation: 1. Get a decent flat light panel, like https://nanliteus.com/nanlite-lumipad-25-high-output-dimmable-adjustable-bicolor-slim-soft-light-ac-battery-powered-led-panel/ . Both the color and intensity can be adjusted. 2. Get some 12x12" translucent acrylic sheets. I have three (3) "under" my light panel. 3. When you are ready to shoot Flats and Dark Flats, switch your ASIAIR (back) to Preview mode and turn on the Histogram. 4. Adjust the light panel intensity such that a test shot of 3 or 5 seconds results in a median histogram value (its technical name escapes me) of right around 20,000. Once you are there, set your Autorun Flats to the 3 or 5 seconds and take thirty (30) Flat frames. 5. Reset the Autorun for Darks and shoot the same length (3 or 5 seconds) and quantity. Pixinsight will match them in WBP. Doing this takes the guesswork out of any calculating. Beats me what the other artifact is, but who knows? With a reasonably clean / clear optical path, it too may just go away with proper Flats/Dark Flats. Well wishes, Jay |
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Glenn Silverman: Glenn, The corrector plate is at the system stop so the condition of the corrector will not produce dust motes. Neither will dust on the primary or the secondary mirrors. Dirt or other contamination on the field lenses may cause a problem but most motes will come from dirt (or surface defects) on any filters, reducer components (if you have one) and/or the sensor window. Those need to be kept extremely clean and that's where you should be the most OCD in your cleaning efforts. Jay Hovnanian: Jay, There is no need to take so many flats. Properly exposed, Flats have a very high SNR so their influence on the statistics of the final image is quite small. Taking more flats than necessary won't hurt anything but you really don't need more than around 8-16 to make a good flat master. The reason that's relevant is that for some EL flat panels, it can be hard to properly expose NB flats in the red (e.g. Ha and SII) without using pretty long exposures (sometimes many minutes), which can be pretty time consuming as you run through all of your filters. John |
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Dave Rust: Dave, If I had a beer, maybe I wouldn’t mind the smudges so much. What I really need is a “better” method. glenn |
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Hi Glen, You mentioned, that you have to crop the Image because of the edges. Are the edges to bright or to dark? this could point to problems with the darks. Older darks or darks with different Gain/Offset can cause problems. the other question are the flats from the same night? To troubleshoot i would process a subset of the light frames to speed up processing 1. no bias, no darks, no dark flats just the lights and the same for the flats the compare if the dust is on the same spot and size 2. lights + flats (no bias, no darks, no dark flats) is the result free of dust spots? 3. the same only with bias 4. the same only with darks CS Detlef |
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I beleive the issue is a bad flat callabration, doing a visual inspection of a dark or bias I think is pointless as they are not optic realeted. Darks and bias can be done without the camera being attached to the scope. This is a flat issue in my opinion, take care in clenning filters and the FR. |
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| Most dust donuts are undercorrected but others are overcorrected. This indicates light leaking inside the dark or bias subs. |
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Hi, Glen! guess what? I just experienced symptoms similar to yours. It turned out to be an almost invisible film on the camera window. One recent night I took a shortcut and cleaned the glass with a Zeiss eyeglass tissue with embedded cleaning solution. Last night I re-cleaned with a tissue and a drop of pure alcohol included in a telescope cleaning kit. Symptoms disappeared. |
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Dave, Thanks for the heads up. I’ll give your cleaning tips a try. Glenn |
