Hi All,
The QHY8 is good value for a cooled CCD but it has some issues, one of which is an intense circular flare around bright stars in long exposures. This can mess up images of the HH and Pleiades to name but two... and we obviously bought a cooled CCD so we could take long exposures huh?
Discussion with other IISers led me to the cause: it's an internal reflection.
Incoming light reflects off the CCD cover slip onto the front filter, then subsequently back into the CCD.
Simply removing the front filter is not a solution, as this filter is also a seal to prevent moist air entering the CCD chamber. Remove the filter and the CCD will ice up within seconds.
The solution is to move the filter as far away from the CCD as practical, and/or find a filter with superior antireflective coatings.
There's a limit to how far we can move the filter. Move it too far and the air volume inside the QHY8 increases to the point that the cold finger cannot remove the moisture and the CCD ices up. Perhaps a desiccant plug is called for but that's another subject entirely.
I tried two solutions and both work OK.
1) Replace the QHY8 nosepiece with a 2-inch T-adapter and a Baader UV/IR filter. Image attached. The Baader filter is supposed to good A/R coatings, and the 2-inch nosepiece moves it away from the CCD.
This solution was good optically, the flare was reduced and had a larger diameter. But I had issues with air leaks and CCD icing. Possibly the 2-inch nosepiece held too much air for the QHY8 cold finger to remove. Also, the 2-inch nosepiece with the Baader filter attached was too long to fit properly into the focuser, it didn't sit flush and I had camera tilt issues. So...
2) Add T-extenders between the QHY8 body and the original nosepiece. Image attached. I got the T-adapters from a Meade variable T-adapter that came with a 3.3 focal reducer. This is my best solution so far. Flare is greatly reduced in intensity, although it's larger in diameter. The assembly is reasonably airtight and the camera fits accurately into the focuser.
Images attached:
1) QHY8 with standard nosepiece and T-extender in the background
2) QHY8 with 2-inch T-adapter nosepiece and Baader UV/IR filter.
3) QHY8 with T-extenders between the body and the standard nosepiece.
4) HH image with standard nosepiece
5) HH image with extended nosepiece
NOTE the ugly blue PVC tape around the QHY8 body is an attempt to seal the housing against dew.
Cheers,
Rob