Light circle in center of image

Could you switch on "neutralize BG" on the panel on the right? Just to get a clearer picture. Are you using calibration files (bias, flats, darkflats, darks)?

@vincent-mod

Hi Vincent, thanks for the reply. I'm not using any calibration in these images but the results are similar with calibration. Attached is the image with neutralize BG set in normalization.

A friend though I had a some dew on my lens but this has happened both times trying no filter and I checked there was no dew - always have the dew heaters on in this humid weather. But it made me think, could it be IR light from the dew heater, since I don't have an IR filter in these shots?

Good question, but hard to answer. Calibration data would show a bit more possibly. I would definitely look into taking those as I can also see amp-glow on the top right, which is dealt with with darks. It could be dew, maybe, as it's in the center of the image. At least I would take a very close look in the entire system for IR or other possible light sources.

Thanks, I’ll start from scratch and remove everything including FF and dew heaters and see what I find. 

@astrogee Same camera, similar scope, same problem.

I've been very suspicious of dew/frost on camera window and even bought a $20 ZWO camera dew heater even though ZWO says it's not required because heat from the cooler is routed to the window. Didn't seem to help.

Was suspicious of a light leak in the image train, but everything is a screw fit now and the problem persisted.

Last two sessions I hung an opaque curtain to block two local light sources. Problem solved!

Apparently, that light was bouncing around in the dew shield and found it's way into scope and camera.

All that said, I'm still suspicious of dew or frost in the camera. I've been careful to cool to zero and then after a bit cool to -10ºC. So maybe that helped?

I don't cool to -20ºC because light pollution noise far exceeds camera noise even at -10.

Posted by: @tailspin45

All that said, I'm still suspicious of dew or frost in the camera. I've been careful to cool to zero and then after a bit cool to -10ºC. So maybe that helped?

Thanks @tailspin45, I think in this case, it should be somewhat sealed by the field flattener. But I'll keep an eye on camera dew. Lots of experiments to do on this one 🙂

@astrogee I have a 0.6x reducer screwed in too; all the connections are threaded. But I still don't think it's air-tight.

I tried replacing the desiccant tablets too, but that didn't help either.