I keep getting this ring of stray light that I cannot eliminate through any method. I have tried different scopes, filters, filter wheels, filter masks, OAG and no OAG, and I've thoroughly eliminated any light leaks. That only leaves the 2600MM as a potential source.
Has anyone seen anything like this?
I see the same circular gradient when using ZWO's tilt adjuster on my 533MC. I haven't experimented with different combinations of gear though.
robert-j-strain Do you use the screws to connect the filter wheel to the camera or a ring to screw the camera onto it? Using the screws is a know source of light leak.
Does it show up on your calibration frames (darks, flats, flat dark/dark flat)?
Make sure there are no open screw holes on the filter wheel. Put electrical tape over any unused holes to check.
The newer EFW ships with black gaskets that are supposed to go between the EFW and the camera and well as the EFW and the OAG-L. You can purchase them separately from ZWO (or other places like Agena)
wvreeven Have tried it both ways - adapter and screws. Still present. Have tried a different brand filter wheel, as well. I have eliminated every light leak I can find with tape or be changing hardware to narrow it down.
I have changed enough and tried enough different combinations that the only common hardware has been the 2600.
Corsair It does not. Not visible in darks or flats. Flats are usually 2-4 sec with a Flatmaster, but have tried sky and regular LED panel.
hyiger I have the rings. Still present.
I have only seen this type of artifact with optics.
Kevin_A I've done two different scopes and it's been in both.
Block the exposed motor housing on the back of the EFW (except for the end of the motor spindle itself). It is known to be a source of light leaks as well as the screw holes.
Mapleleaf Did that a long time ago.
Have you tried without all the filterwheel etc… straight to scope flattener? Can you even try a different camera or borrow a friends. In reality a sensor cannot do that, the sensor glass could but not the sensor unless you see that circle when looking at the camera directy. Check any spacers you have kept while swapping the scopes as maybe one tube is internally shiny. I spray all my 21 and 16.5mm spacers with matte flat black on their insides as I have had internal reflections from them.
That's my concern ... that it's the sensor glass.
I haven't tried it with no filter wheel, but I have used two different brands, which are internally different, and two different brands of filters.
I have changed every spacer though.
robert-j-strain The left image is a master light that has been calibrated, and the right one is the single light frame without any calibration, right?
If so, can you please do this test? as the ring seems only to appear in the master light from the screenshot.
1 Stack all light without any calibration fields to see if the problem appears.
2 Check the master flat, does it have the ring?
This is interesting as I am seeing something similar with my new ASI2600MM Pro + 36mm X 7 position wheel. Here is a single 120s light frame, a single flat, and a calibrated master. Ignoring the dust motes, you can see a ring where it is brighter on the outside than inside the ring.
I suspect a light leak, but have not tested this directly with any experiments (will this weekend when I am near my rig). Note that it is largely corrected in calibration (the gradient that remains is probably light pollution) so is not the kiss of death, but it does not seem like it should be there. 
Yes, I use unmounted filters, but all have the masks in place. I was able to determine that some of the problem was from a light leak through the unused screw holes on the filter wheel case. I covered those with black tape which helped. But, it did not seem to fully fix the problemI. I wonder if there is still light reflecting around inside the wheel, that then goes through the unused filter locations. I only have 4 filters in the 7 position wheel right now.
Y5EGDT6T I wonder if there is still light reflecting around inside the wheel, that then goes through the unused filter locations.
There is one other place you might want to check, since it is a chronic problem with all of ZWO's filter wheel.
The stepper motor is at one end of the protrusion of the EFW. It looks like a white circular disk. There is no O-ring or light seal of any sort between the metal enclosure and the motor itself. Tape the gap up with black electrical tape so that the entire circumference is completely blocked. Like this:
As shown in the image, don't tape the brass bearing itself.
If there is still a light leak, do the same thing to the gap in the USB Type B connector.
Chen
I finally figured this issue out, and want to report it so otherscould benefit and also to be fair to ZWO - the issue was not primarily a light leak with a ZWO product. Unfilled screw holes on the front of the ZWO filter wheel were causing some issues, which were fixed with some electrical tape... but this did not fully fix the issues. After extensive testing, I found no other light leaks involving a ZWO product. This included the junction between the camera and the filter wheel and around the filter wheel motor.
Extensive experimentation with a lot of configurations and spacers finally revealed that internal reflections from my Stellarview field flattener was the issue. Again, to be fair, this flattener was sold as being optimized for DSLR cameras (for which it worked great for me). I started using is with the ASI2600 and ZWO filter wheel, proper back focus, but perhaps because of the different internal geometry it was causing weird effects. No hit on Stellarvue, they state that it was designed for DSLR, so no one to blame here.
Just goes to show - every rig is different and getting every piece of equipment to work with every other piece still takes some thought and work!
