Kring I never need much gain to get a good selection of stars, with the gain higher under the Bin2 I do notice that stars can quickly halo and expand. I turned down my gain to make it where no stars had halos and the background was as dark as could be, I could still see a good amount of stars and this eliminated some of the fainter stars.
Hmmm, by halo do you mean one of two things? One (a) is that you see one or more distinct rings around a central star disk. The second (b) is a larger but dimmer disk surrounding the central star disk.
Both cases (a and b) become more visible as your dynamic range improves -- that is what happens when you use binning and you either use floating point, or using a sum to bin, in case of integers. Dynamic range really improves (a good thing) to the point it is easy to see defects of the guide system.
It is not that the halo was not there before, but just that the halo was below the noise before binning.
Case (a) -- the ring -- is simply diffraction optics at work. What you see is simply the Fourier Transform of a circular aperture. The result is a bunch of rings (named after Sir George Airy). This is from a program that I had written to investigate deconvolution methods; the left is the circular aperture function, and the right is the image plane, which is the square magnitude of the Fourier transform of the left:
If you squint, you should be able to see a series of rings radiating from the center on the right side -- that is the image plane, i.e., what your camera sensor sees. Now, when you defocus the telescope, the rings become stronger/extending farther -- so, if you see strong rings, it may just need better focus to get rid of this kind of halo.
Make or buy a Bahtinov mask for your guide scope if you can. You will be surprise how well they work.
The other kind of halo (b) is what you often see around very bright stars with narrow band filters. Check out photos of the Horsehead nebula, and more often than not you see that kind of halo around the star Alnitak. The cause of it is just poor optics and/or reflections inside poorly coated glass filters and lenses. At one time, I had bought some cheap optics that are sold as "guide scopes," a 130mm f.l. from QHY and a 120mm f.l. from ZWO, just to see how well they can do. The result is the halo of type (b); I have since thrown them away into my trash heap. Stick to reputable companies for scopes and filters.
Cheap optics just are not worth the time. The distorted images come form multiple aberration causes -- from spherical aberration, to coma, to who knows what else. One of the cause could come from a severe case of chromatic aberration. Where one wavelength is in focus (the central point) and the rest (the halo) is not in focus.
Fortunately, there is an easy fix for poor optics, and that is to never buy cheap optics.
I primarily use two guide scopes, made up of Borg components. One is the Borg 36ED (ED glass) with 200mm focal length when I need more lightweight, and my favorite one is the Borg 55FL (Fluorite glass) with 250 mm focal length. I also have a Borg 71FL (400mm f.l.) but have not need to resort to that yet. It easily weights twice that of the Borg 55FL. I use a Borg helical focuser too (Taiwan made, and much superior to the helical focusers from ZWO and SVBONY and the like).
Speaking of Bahtinov masks and Fourier transforms, this is what the Fourier Transform of a FarPoint brand Bahtinov mask looks like:
Recognize the pattern on the right?
And this is what adding a non-zero quadratic term to the aperture function (basically the Fraunhofer approximation of an off-focus aperture) looks like:
Chen
