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I’m doing LRGB imaging with a 120mm refractor with a reducer. I’ve got a blotch showing up on my subs, however, only on the R,G,B filters, not the L. The blotch is in the exact same place and shape on all three filters. Under normal circumstances I’d be looking for the problem on the camera sensor or other lenses in the light path. The fact that it doesn’t show up on the L filter has me puzzled. I’d like to fix/clean this situation but I don’t know where to look. Suggestions would be appreciated. I’m going to be adding an Ha filter to the system shortly so I’ll have everything apart in the process. Hank |
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| Could you give us examples? |
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  The top one is the Luminance and the bottom is Green. The problem is the one on the left side. |
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The top one is the Luminance and the bottom is Green. The problem is the one on the left side. Actually, it's everywhere. You got dust motes on the filter. And you don't just clean it, you take flats to get rid of (and many other things besides). Do you? |
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| I wouldn't bother taking everything apart, just take flats. I've had dust somewhere in my OTA for months and flats compltely eliminates the blotches. |
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| To be honest I hardly clean my sensor at all anymore unless the dust is super agregious. I just make sure to take really good flats and it's as if there's no dust at all. |
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Not to worry, dust motes are everywhere, and there is just no way to avoid these guys. the optical surfaces, all of them, are super sensitive, and most any cleaning operation has the potential to introduce "micro scratches" - invisible to the naked eye, even with a 10X loupe. dust motes- your blotches- are obvious, but image deterioration through micro scratches are much more insidious - the reduce image contrast significantly... so its best to open the optical train very rarely... dust motes can and will move occasionally , so we have to reshoot flat frames repeatedly, some shoot their flats at the end of every imaging session. you will find plenty of info on the web how to do this, there are several different techniques. the fact there are different ways means there is no accepted "best way": your results will vary, there is also a learning curve associated with this, and while you will find you can reliably (almost 😉) correct dust mote artifactss, issues like vignetting are much more difficult to eliminate consistently. a popular "beginner mistake" would be to shoot darks with different gain/ offset values than you lights. that is NOT an issue with flat frames. Also, https://astronomy.tools/calculators/dust_reflection_calculator allows you to calculate where any given artifact sits in your optics, hope this helps! |
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| It kinda looks like it’s on the luminance too. It’s just to a lesser degree. |
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Dang! I see at least two more. The typical donut shape. And about the right size of dust shadow when it's lying on the glass pane in front of the sensor. Sure don't know why the left one is so dark, but it is the same size as the others. It might just be an extra thick piece of crud. |
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If it appears in the exact same place on three different filters the the problem cannot be the filters. What are the chances of having 3 identical pieces of crud in the same place on three filters? It most likely is on the cover glass of the chip. Geoff |
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The less visible parts look to me like dust. The really dark spot reminds me more an dew than on dust. Maybe you should check for this, too. But I could be wrong on this. I do agree with @Geoff . It must be something on the sensor or the camera glass. Because the spots are quite sharp at the edges, I would have a look at the sensor itself first. With the increasing distance of the dust mots to the sensor, they should be less sharp at the edges. This is another reason, why I may guess the darker one is dew on the protection glass of the camera. It is really fuzzy. If it is dew, it should change a bit over time. So maybe you should try to take some images over a longer time range and look for changes. Or try to take an image uncooled and then cool the camera and try it again. This is even better. If it does not change, I'm wrong. Hope you solve your problem. CS Christian |
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| Thanks, folks. I’ll pull the camera and check the protective glass cover. |
