franco although nothing like for the blue channel.
Yeah, looks like the blue channel is the main culprit.
This is ZWO's claim of the spectral response of the three Bayer components for the ASI294MC:
Notice that the Blue bayer pixels are very sensitive (ADU down less than 50% from the peak Blue ADU) at 400 nm. And Red is even worse at the other end of the spectrum. So I would expect the camera to be very sensitive to UV at one end, and IR at the other end.
Since the red image does not have a bloated star, perhaps your OTA is not as bad in the IR region. I would still make sure there is an IR-cut filter, however -- perhaps not as critical as UV cut for this particular OTA.
When they optimize the optical design for an "APO" scope, they make sure three wavelenths have the same focus. (Notice how out of focus your blue image is -- it is not just large halo... it is completely blurred. The red image is only slightly worse than the green.). The three wavelengths are usually at 656 nm (red), 536 nm (green) and 486 nm (blue). At the same time, other wavelengths are usually sacrificed unless you go through with more glass elements that adds more expense (also depends on the experise of the designer, but nowadays you can just let a computer program optimize it).
I suspect that your OTA is not meant for UV imaging (and humans can't see below 400nm anyway, only the sensors can), so they don't even attempt to even care about how far off from the focal plane the shorter wavelengths converge at. And it does not help that your color camera does not come with a UV-cut filter.
Since your OTA appears to be very sensitive to UV, I would recommend looking for good filters that cut in the UV region (and also IR region). Avoid filters that have little documentation, or are not manufactured by actual filter companies.
In your three images, check out some of the close double stars.. green has better separation, red is slightly worse, and blue is just terrible.
Chen
