Ok, so i recently got this camera and it seems like i can t use it to shoot the horsehead. I m so dissapointed Below a STF stretch of a 20x300s stack shot on my FSQ85 and optolong h-alpha 7nm filter. Is it really that hopeless?
There are a few specific objects that are problematic with the ASI600 due to it's microlens diffraction. Well, more specifically, a few specific stars. Alnitak is enemy #1, basically. And this is not just the case with the ASI1600, although it tends to be a bit worse due to the microlens reflection, but it is an issue with scopes that have higher blue dispersion (you end up with a bright blue cast over most of the image in color combinations), scopes that have big fast apertures (you usually get obvious aperture-shaped reflections), etc. Alnitak is THE star that annoys a majority of imagers with a very wide range of equipment. A close second is Gamma Cas, another very bright star in Cassiopeia near the Ghost of Cass nebula. It, too, is ultra bright and has been a problem for many imagers with a wide range of equipment. It just gets a bit worse with the ASI1600 due to the microlens diffraction. There are some other stars out there that are similarly problematic, although usually not to the degree of these two, however it is a minority of objects that are affected.
As with any scope that has problems with Alnitak or Gamma Cass, one solution is to orient the image such that the problematic star is out of the frame. With reflection and dispersion issues, this is only partially successful since a reflection can still occur off of other elements in the train, and can still be a problem so long as the star is within the imaging circle (which is sometimes many times the area of the sensor itself). With the ASI1600, all you need is for the star to be JUST off the frame, and the issue will disappear entirely. So that gives you more options.
The interesting thing about this effect is it is actually now well understood thanks to the knowledge and experience of guys like Mark (sharkmelly) and Frank (freestar8n). I think Frank was actually working on a tool that could be used to cancel or at least mitigate the effect of the diffraction due to the knowledge those two gained while learning about the effect when it occurred with other kinds of cameras (no, the ASI1600 is not the only camera to experience the effect!) It seems it can be well-modeled, which means at some point there may be a solution to the problem. (Food for thought).