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Hi! I'm having some trouble getting my green and O3 filters to calibrate properly. The image attached is an example of what's happening. The image on the left is without flats, the one on the right used flats. Both images were run through Graxpert (which worked super well on the uncalibrated one on the left btw). After calibration, there is a large dark spot in the center which suggests that there's a bright spot in the center of my flats that's overcorrecting. The goofy thing that i can't really understand is that the other LR_BSH_ filters are not affected by this. It's only green and O3. So presumably it's a reflection somewhere in the train, but i don't understand why it's not affecting the other filters and why it wouldn't also be in my lights and thus calibrate out. Additionally, i do not see this when i move the same camera and EFW to my two refractors. Data from both of those is free of this problem. Gear being used: ZWO ASI294mm pro EdgeHD 8 with matching reducer and dew shield. Astronomik 1.25 filters - Deep sky LRGB, L3 lum, 6nm SHO OAG and appropriate spacers for backfocus How i take flats: A3 size tracing panel with 10 levels of brightness with a few sheets of printer paper taped onto it. I place the panel over the dew shield in the same positioning as i image. I always take new flats prior to a night of imaging. Camera is cooled to the same temp as my lights and corresponding dark flats are used. The dark flats were reshot recently and the behavior persisted between the different master dark flats. The things i've tried so far:
Any ideas on how to get this to correct properly? It appears that my train is fairly clean so at the moment, so i'm inclined to just not use flats and clone stamp out a few of the tiny problems. But that's not ideal.  |
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Hi Wesley, What do your master flats for the green and OIII look like? Can't see anything abnormal on them? Greg |
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Here's what the O3 flat looks like. And certainly a brightish spot is visible in the center. But what's it from? What could be causing it? Left - Normal Right - STF stretched to accent the bright spot.  |
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| To me, the flat worked, as can be seen on the image on the right perfectly removing the small spec that can be seen just lower right center. The EDGE will have very little optical gradient that is hardly noticeable when using the 294 All I can see is more contrast in the one on the right in areas that I would expect to see more contrast. After you remove your flat gradient, your STF stretch will look different, and so I expect that, and although I have never imaged this target, I personally see no issue here except MAYBE a slight normalization issue. I see your flat after I typed my previous comments - My opinion is the bright spot is caused from your reducer and that's a lot of gradient for the 294 based on my memory of using it. |
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Thank you for your feedback. I agree that in most of the image, the flat worked as intended. Several small motes and the vignetting was corrected. But the dark spot in the middle is certainly more than a normalization issue. So are you suggesting that that's just caused by the reducer and nothing can be done? That would be disappointing.  Here is another example where the problem is a bit more prominent. No processing done to this at all except default STF stretch.  |
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| When this happens to me, its because I haven't given the camera sensor enough time to cool and equilibrate. Rapid cooling at the start of a session can result in condensation on the sensor window. I have one of those ring sensor heaters from zwo on my 294. If I cool straight away to -20 and don't give it enough time I see this pattern. If I step down to 0 then -10 then -20 as I setup this doesn't happen. Of course for you this would show up in all your flats, not just your green and oiii. Anyway just a thought. |
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| OK, that indeed doesn't look normal, and Jeff stated the next thought that I had. Does all your frames appear free of dew when you did the blink test? If I were you, I would take some quality select lights and run them through both DSS (faster) and PI's Weighted Batch Processing (slower but better-quality stacks) script to obtain stacks for comparison. I'd be interested in seeing how they handled it. |
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| Does the same pattern emerge if you do a SKY flat / with tshirt on that green or Oiii filter at same exposure and ADU? That would be one way to determine if it was tha A3 flat panel or not. For me those Amazon panels were not evenly illuminated and I got terrible calibration. I know countless others that it works great, but for me and my 294 it was a no go. |
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Jeff Horn: This absolutely does look like that. But i've experienced frost on the sensor and dew on my optics many times. So i check for it a lot. lol I have to replace the dessicants in my camera about once a year due to the frost on the sensor thing. OK, that indeed doesn't look normal, and Jeff stated the next thought that I had. Does all your frames appear free of dew when you did the blink test? If I were you, I would take some quality select lights and run them through both DSS (faster) and PI's Weighted Batch Processing (slower but better-quality stacks) script to obtain stacks for comparison. I'd be interested in seeing how they handled it. I haven't run DSS in a while, but it's worth trying. I'll check this out! And yes, the subs all look normal compared to what i'd expect. I don't see any signs of dew. Jeff Horn: Oh i haven't tried this. I'll try different light sources. Sky flats, maybe an illuminated wall. Maybe it's something to do with the distance of the panel to the secondary somehow. Thanks! I'll explore this route. |
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| This certainly is an usual and puzzling issue. I have Baader NB filters (both 1.25" and 2", for F/7 and Hyperstar F/1.9) respectively and never have had issues with flats and my ASI1600MM. By any chance do you have another camera you test out? Strange that it only occurs with your green and OIII filters. I'll have to give this some more thought as you've done everything I would have done. Have you taken out each filter and inspected them and maybe put them in a different filter slot? Hope you solve this soon! Clear skies. |
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I had the same issue, like Jeff, but slightly different. By the end of my session the camera protective glass would accumulate dew, which caused a dark spot on my subs or flats. There was a very simple explanation in my case. I didn't turn the camera's dew heater on, a rookie mistake, but now i know better. In your case however, there is a fairly easy way to test this, so at least to definitively eliminate as a possibility, test out your flats in the begging of your session before turning the cooling on. If you don't get the circle in the middle, then you know it's probably something to do with the dew towards the camera side. As for why it might be appearing only on your green and o3 filters, my guess and this is based on no scientific data, if your flat panel is like mine, the light coming out of it is more on the green and blue spectrum, and thus the effect might be amplified in those channels. Cs |
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I actually do have a new camera. I just got a player one Poseidon M with new filters. So this will be a good test once the gear induced clouds go away. I also suspect that the flat panel I’m using is strong in the blue-green which could be causing the problem. So I’m going to first try sky flats or similar. Or maybe a different flat panel. I’ll update when I’m able! Thanks for the ideas! |
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You might also try setting a pedestal when you run WBPP when stacking narrowband filters as sometimes there can be over-subtraction with your darks which could be contributing to the issues you are having when it divides the flats. One way to check is to first using pixelmath simply subtract one of your dark frames from the light frame. Then is you want to see all of the 0 value pixels first convert your recently dark subtracted light frame into an RGB image. Next make sure in pixel math that the "use single RGB/K expression" is un-checked. in one of the channels put in the following expression : iif($T==0,1,$T) then in the other two channels just insert $T as the expression and then drag the triangle over the light frame in question. If you are indeed over subtracting you should see all of the pixels in color that went to 0. As dark subtraction happens before calibrating the flats to the lights your problem could simply be that your darks are being over subtracted, making a bunch of 0 value pixels before the flats are divided. In order to set a pedestal in WBPP, (most likely it will be somewhere around 100-250 depending on your camera, I have a ASI294MM pro and usually use around 125 for a pedestal), load all of your frames (darks, lights, flats, dark flats or bias') into WBPP then navigate to the calibration tab and select your narrowband light filter set that may be over-subtracting and on the side panel you will see a place where you can write in a pedestal. Hopefully this will help with your issue. |
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So it's been completely overcast here since i received it. But i have the poseidon M installed and did some testing. Here is a single 3s O3 flat at around 18k adu. Upper left - untouched Upper right - Standard STF Bottom - Boosted STF At least to my eye, i see no indication of the bright center reflection spot that's present in my previous one. So i'm super hopeful that this will solve my problem. Though it's looking like it might be a couple weeks before i have a chance to test it with new data. Here's what has changed: New P1 phoenix 7/2" EFW filled with Astronomik 2" filters. The filters "should" be the same as the 1.25's in my other EFW but you know how that goes. The old filters are 2 years old in case age may matter to coatings. New P1 Poseidon M primary camera OAG prism minorly adjusted to back it out of the new larger frame. Everything else, including the flat panel i was using before is the same.  |
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| UPDATE: I can now confirm that my green and O3 channel are no longer having the over-correction problem. YAY! We had 1 clear night last week with bad seeing. But was enough to get a couple hours in green and an hour in O3 to properly test things. What changed: New primary camera: 294mm -> Player One Poseidon M New Filters: Astronomik 1.25" -> 2" New Flat panel - the new one has less of a blue leaning tint. I'm not 100% on what the culprit truly was since i changed more than one variable. I moved that cam/efw to my smaller Askar fma135 and didn't see the overcorrection. But i also used a different flat panel. So if i had to guess the cause, i'd say it's a combo of the EdgeHD's secondary mirror, bluish flat panel, with a reflection problem from the 294 or 1.25" filters Thank you to everyone for their suggestions while i problem-solved this! 1 hour of O3 on Thor's Helmet with corresponding master flat. No sign of overcorrection and no sign of bright spot on master flat.   |
