Sensor non-uniformity with IMX571 mono in S and H?

My best advice is to stop looking at your flats and start looking at your images.

I see non uniformity in my narrow band flats with my IMX455 but flats remove it perfectly.   This stuff is simply FPN caused by PRNU, which is what flats correct. If you properly calibrate your data, none of this should print through into your images.  So, how do your calibrated images look?

John

Good luck, it's always exciting!

One of my cameras had similar issues that differed from brighter/dimmer targets and/or one filter to another.

The sensor and window appeared very clean under examination in normal diffuse daylight, but at night when I illuminated it with a very bright point source (a cavers' LED headlamp from some feet away), a subtle leftover cleaning smear across the sensor became more obvious.

For what it's worth, when a sensor needs to be cleaned beyond what can be blown off, I've had good success with,

1. A newly unwrapped microfiber cloth with very light contact (i.e., I loosely fold the cloth for contact without direct finger pressure) after fogging the sensor with one's breath; or, for a more stubborn contaminant,

2. Use of a Pec Pad optical wipe (by Photosol Inc.; Amazon.com) on which one light spray of ROR optical cleaning solution (by V-Max Products; Amazon.com). This has been 100% successful for both sensor and glass cleaning over the years.  It leaves no residue and does not attack metallic coatings. 

These methods work fine with filters.

As an aside, living in the humid tropics, fungus on lens surfaces can become a problem. Many otherwise fine lenses are sold at steep discounts on resale sites online due to internal fungus. These can be disassembled and restored to a like new condition with a hydrogen peroxide wash, and there are a number of how-to videos on YouTube detailing exactly how to properly disassemble and restore particular lenses this way. Restoring and reselling some lenses can pay for your astro hobby!

What kind of scope is it?  That looks like a baffle light leak or something to me more than "contaminated" flats, but maybe not.

Kyle Goodwin:
What kind of scope is it?  That looks like a baffle light leak or something to me more than "contaminated" flats, but maybe not.

*** It is a 127mm Triplet refractor. I retook flats indoors and the result is way better but there are still those rings on it. I also removed the lens hood but still the same rings.

What exact filters do you use (brand, size, mounted or unmounted)? I recently stumbled upon a thread where it was assumed that some mounted filters tend to create such circular reflections. I have a ZWO ASI 2600 MM (same sensor) and use it with 36 mm unmounted filters with no such problems.