Strange Mottling Effect in Master Images - Any Recommendations?

Caleb Sexton

Hi all,

I'm looking for some guidance and insight here. Over my past two astro projects, I've found the emergence of mottling in my master frames after using WBPP. This is most evident in my S2 filter. I've reshot my S2 flats twice now with the same outcome. The target time using NINA is 4 seconds for both the flat and flat dark images used for calibration and shooting for around 30k ADU. 

After using DBE in one test and Gradient Correction in another, I wonder what might be driving a similar outcome. I've attached a few images of the S2 filter zoomed in to note the effect, as well as, a full view of the frame with Autostretch applied after to reallllly highlight the mottling effect and a few of the master S2 frame without GC applied. I've also attached a copy of the S2 master flat as well.

Any insight and guidance on steps to address this would be greatly appreciated. I have all the gear still up, connected, and in the same protected place.  in the event, I need to reshoot my flats. A few options have come to mind: 

• Attempt to reshoot the flat frame at less than 4 seconds to see if that makes a difference. 
• Attempt to tune the flat frame based on other guides I've seen Adam provide. 
  

I'm pretty sure this is due to some over/under-correcting that is taking place, but I'm looking for feedback, guidance, and recommendations on how best to proceed. It's a frustrating problem to sort through, for sure. 

I'm also more than happy to send along the actual data file, but I'll have to attach it later when I get to my machine that has it stored there.

Equipment details used for acquisition:

• Askar FMA230 telescope
• Antila 3nm Ha, O3, and S2 filters
• Deepsky Dad's automated flat panel

BlockHead

First thing to always look at is the pedestal.

Did you check for zeros to see if your pedestal is optimal?

Just make certain we do not have a compounding issue.

It does look like your flat shows some mottling itself. I believe there can be an issue with short focal length telescopes and certain NB filters. You can get an interference pattern because of the glass in front of your sensor (or something like this). You did not mention the camera you are using... but this as been an issue for some ZWO AIS 294? . I think it dealt with the fact the coating on the window/sensor was not quite even. 

You are correct that changing the brightness of the flat should have no effect.

-the Blockhead

Caleb Sexton

Oh shoot! I totally forgot to mention the camera used, derp. It's an ASI 183mm Pro. Interestingly, I do have a 294, but it's on my other telescope at the moment, and, so far, hasn't encountered that issue there, luckily, haha. 

I've attached screenshots of the WBPP setup. I did add a pedestal value of 200, but wasn't sure if that could be the issue or not.

Thanks for the help Adam. If it's useful I can send along the the master files too, but I know you're incredibly busy.

Kindly,