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I’m having trouble with lights and flats having this bright patch in the centre. Has anyone experienced this before?
ZWO FF65 refractor, EFW mini, 533MC Pro.
I’ve looked for light leaks and internal reflections but can’t find any. The optics are all clean and in perfect condition, nothing on the filters or sensor window
Maybe dew on the sensor glass, but just a wild guess.
If it's on flats and lights, what does a calibrated frame look like?
Craig Dixon · Aug 23, 2026, 12:18 PM
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ZWO FF65 refractor, EFW mini, 533MC Pro.
I’ve looked for light leaks and internal reflections but can’t find any. The optics are all clean and in perfect condition, nothing on the filters or sensor window
Has this just suddenly changed or have they always looked like that? Are you having trouble with calibration? If not, I’d stop flat peeking.
I know for certain this isn’t dew and it’s not calibrating out properly. I’ve had the kit for years but I only recently moved this camera and filter wheel to this scope. I’ve completed one project with this configuration so far but am fighting the data due to the bad calibration. I’ve also tried re-shooting flats many times using many different methods but this bright centre is persistent.
That’s puzzling because even if the new back end is to blame, you’re taking flats with the same setup so it shouldn’t really matter. Everything points to something changing between taking your flats and taking image data.
Agreed but this bright patch is also appearing in light frames. The most obvious place to look is the filter but it looks fine so I’m stumped
Okay so here’s what I’ve discovered. The filter is an Astronomik L3 so I tried a ZWO UV/IR. The problem persists but isn’t as bad as Google AI was suggesting the filter was too close to the sensor, causing light to bounce. The camera was screwed directly into the filter wheel. I’ve just added a 21mm spacer between the camera and EFW, which solves the problem. HOWEVER, this causes major problems with other filters
I had a similar problem with a Celestron C6 and the ZWO 533mc when I changed from a simple but wiggly T-nose adapter to a stiffer Celestron extension tube (to address slight sensor tilt). I could not solve the central brightness issue with changing a filter wheel for a filter drawer and back (with and without filters) . Wasted several weeks of imaging and troubleshooting on this problem. When I gave up and removed the stiffer/longer Celestron extension and returned to the wiggly T-nose to attach to the OTA, the central lighting issue (on the lights and flats) disappeared. There was never a back focus issue so I’m thinking internal reflections. I’ve since transitioned to a 2600MC and all is still good. The issue with the T-nose is that it’s secured with two small thumb screws and is hard to tighten, as well as the slight wiggle. Hope this helps - these internal lighting issues are frustrating.
Craig Dixon · Aug 23, 2026 at 03:36 PM
Okay so here’s what I’ve discovered. The filter is an Astronomik L3 so I tried a ZWO UV/IR. The problem persists but isn’t as bad as Google AI was suggesting the filter was too close to the sensor, causing light to bounce. The camera was screwed directly into the filter wheel. I’ve just added a 21mm spacer between the camera and EFW, which solves the problem. HOWEVER, this causes major problems with other filters
Remember that flat calibration only corrects for multiplicative errors like vignetting and radiometric field dimming. Stray light is additive; not multiplicative, so it won’t be removed by calibration. Changing the spacing to the filters probably isn’t solving the root problem. It is most likely spreading the stray light so that the problem isn’t as obvious. Have you inspected the inside of your components to make sure that you don’t have stray reflections? Remember that it doesn’t take much to cause this sort of problem. Are you taking your flats in a perfectly dark location so that you know that there aren’t any light leaks? It does not take very much stray light to cause this sort of problem.
- John
i read somewhere that some :) filter wheel had an internal ir light which (i think) was used for detecting when the wheel was in position correctly.
some had/or developed a fault, letting this light leak into the optics.
though your effect looks very centered and even.
I’m pretty sure this isn’t a light leak as I’ve never had any light leak issues before and it’s a good quality refractor so not an open system. I’ve also tried shooting some flats in a dark room with a towel completely covering the entire scope and I still get this issue.
It’s a light leak, 99.999%. Whether is closed or not is immaterial if you have internal reflections which tantamount to a light leak.
andrea tasselli · Aug 24, 2026 at 12:23 PM
It’s a light leak, 99.999%. Whether is closed or not is immaterial if you have internal reflections which tantamount to a light leak.
if it were a light leak, I’d expect it to disappear when imaging in a completely dark room with the scope covered in a towel though.
You then still have a light leak, from the FW positioning LEDs.
Thanks, I’ll investigate this asap. if it’s the LED in the filter wheel, presumably, I can’t just tape this up though. as the wheel won’t work properly. I had the EFW on another scope working perfectly so if this is the fault, it must have developed since I swapped gear.
You can check by putting an IR filter after the FW, if you have one.
Craig Dixon · Aug 24, 2026, 01:04 PM
Thanks, I’ll investigate this asap. if it’s the LED in the filter wheel, presumably, I can’t just tape this up though. as the wheel won’t work properly. I had the EFW on another scope working perfectly so if this is the fault, it must have developed since I swapped gear.
Before taking flats, put your filter in position then power off the filterwheel and unplug. This should switch off any internal LED's and remove as a potential light source. Reshoot your flats and see if there's any improvement.
BTW, the ZWO EFW mini can take 1.25” mounted or 31mm unmounted filters. Which ones are you using?
Richard · Aug 24, 2026 at 01:43 PM
Craig Dixon · Aug 24, 2026, 01:04 PM
Thanks, I’ll investigate this asap. if it’s the LED in the filter wheel, presumably, I can’t just tape this up though. as the wheel won’t work properly. I had the EFW on another scope working perfectly so if this is the fault, it must have developed since I swapped gear.
Before taking flats, put your filter in position then power off the filterwheel and unplug. This should switch off any internal LED's and remove as a potential light source. Reshoot your flats and see if there's any improvement.
BTW, the ZWO EFW mini can take 1.25” mounted or 31mm unmounted filters. Which ones are you using?
Thanks, will do. I’m using 1.25” mounted filters in this one
On the FW ir illumination issue. I believe that was a concern with certain ToupTek wheels. In fact, I had one with the issue and it caused an effect more like a very heavy gradient, nothing at all like what the OP is seeing.
Craig Dixon · Aug 24, 2026 at 11:12 AM
I’m pretty sure this isn’t a light leak as I’ve never had any light leak issues before and it’s a good quality refractor so not an open system. I’ve also tried shooting some flats in a dark room with a towel completely covering the entire scope and I still get this issue.
Surprisingly, most astro-gear is not light tight—including cameras. Given the symmetric nature of what you have in your flats, I suspect that the problem is from a diffuse internal reflection. There are a couple of things you can do to try to identify where it’s coming from. First pull your camera, point your scope out a window or into the sky and put your eye in roughly the same location as the sensor and look backwards through the system. Study all of the surfaces that you can see to see if you can spot any reflections. Remember that it doesn’t take much to cause this sort of a problem. Then take black flocking paper and line wherever you suspect a problem. Then go test it to see if it worked. The problem is being caused by stray light—almost certainly being reflected from an internal surface. You just have to find where it is coming from.
John
I’ve done some more testing:
Move filter wheel to UV/IR and then turn it off - no change
Remove filter wheel and fit filter in-line using a M48-1.25” in-line adapter - no change
Remove filter completely- problem solved
This surely suggests that the filter is the problem. It’s an Astronomik L3 that I’ve used for years on different scopes and had no problems with.
Very interesting, thanks for sharing. I was sure it was dew when i saw the image
Google seems to think there might be a light leak in the camera itself. I’m planning on putting another camera on tonight to see what effect that has and the. I’ll put everything on another refractor to see what effect that has
No chance you've recently cleaned that filter?
I’ve cleaned it since having these problems but it’s spotless