flat field error

Ray Hansen

I'm getting a flat field error that looks a little familiar, but I can't find it in my notes. I talked to ZWO support about it and was told they'd never seen it before and it certainly wouldn't be a camera problem. The attached links are the files as processed in WBPP. First is the master light, second the master flat, third is a folder with the input files. The flats are sky flats taken with Sharpcap. I also tried the tee shirt, dish towel, and sheet paper. I took the camera/flattener assembly indoors and used an LED monitor and a Pegasus light panel. I also took some flats with Maxim and NINA . All results were essentially the same.I would appreciate feedback from anyone with an obvious answer.

https://drive.google.com/file/d/1rH-CTKqGYfH7Bx0tPH0mmfTfn6FFPI2p/view?usp=sharing

https://drive.google.com/file/d/12ogikY6yiWGx5RKk8SZD3G07oXmEEvqF/view?usp=sharing

https://drive.google.com/drive/folders/1eBQ-rN9JiPufxvjZ9dPo2fTjy1CecRb6?usp=sharing

BlockHead

Hi Ray,

I looked at your data and there is something fundamentally wrong. Those flats do not match the lights in any way. The flats themselves are definitely an issue. Your Ha data looks fine.  It certainly looks like real out-of-focus obstructions with your flats. I don't understand it at all. If I had to guess..I would say you have light leaks... extra light getting to your sensor. When you take flats... it should be DARK in the room. 

I am surprised you would get the same result with Sky Flats. 

Bottom line... when you let light through your telescope at night and looking at the sky it is different than your flats... you need to solve that issue first. 

I do not think this is the same effect as the 294 (which in Red light shows variations in the sensor surface). 

Were you able to calibrate images with a Red Filter? Green and Blue? (OIII?)

-the Blockhead

BlockHead

Let me add..it does not appear you are dithering your frames based on the pattern.

In addition, masterlight frame was shifted considerably compared to the input frame I looked at. This means the reference frame that was chosen is likely not a good one.

-the Blockhead

Ray Hansen

Hi Adam,

Thanks so much for looking at this. I was suspecting the same. The scope is a 180mm refractor, so when I saw the WBPP results I brought the camera with flattener indoors and started testing with everything I have available. In total, I used two other sets of NB filters, one set of RGB, and several old neutral density and UV/IR filters. Not a single one reproduced the problem. When I put the original Ha on, the flat corruption was back. So I'm going to assume the cause is somehow the filter/camera combo, although that doesn't totally make sense since the lights appear flat as I move the cursor around the field. Anyway, I have a convenient way around the problem for now.

RH

Ray Hansen

Just a followup. I had convinced myself originally the filters were the culprit. I had taken the flattener off to get a better look at the surfaces, and the images I collected at that point were OK. There was something about the flattener coatings that impaired transmission. Something I could not see visually, but that the 16-bit camera could see easily. I spoke with Stellarvue and they sent a replacement unit the next day. Everything looks fine now.