A rare night in early February caught me off guard, but since I have this travel rig, I set up very quickly to do a couple of captures during a unprecidented clear night from start to finish!  I also reset further to the east later in the night and captured one other target, which is in process.

This was the first one of the night.  My biggest struggle was with the choice of subjects, since I hadn't much time.  I did have some fields set up prior on NINA, so went for this old reliable. I had done this as probably my first serious attempt at astrophotography, but this field makes this really completely novel for me.  The captue details are listed in the specs here, so no need to go into too much detail.  The exposures I choose when using the camera depend a lot on the general brightness of the objects and the brightest stars.  The camera has multiple capture modes that can be fit to better deal with things like bright nebulae or IFN, etc.  Here, I chose setting that features unity gain, but also the a very low read noise, very high dynamic range and a large full well.  The exposure time (here 45s) is always a tradeoff of seeing the most deep down within the dark areas vs the brighter stars, which I hope generally not to saturate.  In any case, I am pretty please with one of my first efforts, having gotten a satisfactory number of subs.  I am also thrilled that I got an unexpected shot of NGC2282, a pretty rarely targeted subject on AB.  Funny, is it actually resembles a bit the artifact that I discuss below, so I am glad I did not "clean" it off the image!  Also, not sure what the issue is here, but I find the mid-range size display of the image to look worse than the full sized image.  

An Investigation Into Star Artifact:
This will be an abbreviated description here because I will post a longer one in the Forums to bring in a wider audience.  But for those who don't visit the forums, you can see this, learn something and offer feedback as well. 

It has taken me a long time to process this image.  I did all the preprocessing right after capture, but because I had so much time, I wanted to investigate an issue that I have seen since this summer while trying out a number of different short focal length optics with fast f-ratios.  This included my WO ZS61, a rented Sigma f1.4 105mm lens, and two Rokinon 135mm lenses.  One can see the artifact best in Revision C of this post.  Because revisions on AB tend to look poor, I post a segment of that image directly here:

A clip from a mostly unprocessed master (the full stack), with STR and StarExterminator applied.  StarX, does a good job of leaving behind the offending ring halos, even some slight arcs.

In the optics that I mentioned above, I have been seeing this artifact consistently.  Though much less so in the refractor.  My forum post will get into more details, but I think I understand what is causing the problem and I think it is interesting. 

Observations:  1. I use an L-Pro LPS filter for all my imaging and when I use my QHY268MC camera, it also serves as an IR/UV cut filter, since the optical window of that camera does not have that feature, unlike its sister camera the ZWO 2600 color.  The L-Pro is an interference filter.  2.  Strikingly, I have found that almost every star that is associated with the ring halos are classified as Long Period Variable Stars!  (Or have LPV as a feature, such as an "S" star.) 3.  No, this artifact does not follow star brightness.  There are many bright stars that have no ring halos and dim stars with very strong ring halos.  4.  No the artifact is not dependent of background brightness or color.  (See faint ring halo just left of the center of the Rosette.)   5. Upon investigation of these stars in Simbad, and visualizing these stars in Aladin with a number of different wavelengths, I find a common thread that these stars are extremely bright in the NIR and Mid IR.  

Conclusions:  1.  I am not certain, but I believe that at the center of the problem is likely the L-Pro filter.  However, not to put full blame on a filter that has done very well for me, it is also the fact that with these optics:  2. very fast optics coupled with  3. very short focal lengths, means the very steep light cone (especially from the periphery of the objective) defeats the ability of the interference filter to eliminate the NIR signal, to which these CMOS cameras have significant sensitivity.  The angle of incidence of light onto interference filters is critical for performance.  4. But, why then do these fast, short optics create a ring of illumination?  Why wouldn't they just focus the NIR light onto the location of the star?  I believe that is because they are not designed too do that.  Especially for the SLR lenses in use today in Astrophotography, the market for these lenses is for terrestrial light or artificial light (designed for human visual sensitivities).  And that brings us back to the fact that unmodified SLRs and mirrorless cameras come with a very strict IR cut filter.  In fact, for those who use SLRs, modified for astro work, it is way more strict than to just NIR/IR.  See below:

Note the red response in the Red Bayer layer of a color camera.  We all know that SLRs, etc. are very poor at recording H alpha signal at 655nm, which is really well within the optical sensitivity of red light for humans.  So those SLR filters are really cutting a lot of the red spectrum!  I am not sure exactly why, but it may have to do with the origins of color film, and getting a "real" color balance in the product (film and digital).  One thing is for sure, narrowing the width of the spectrum in the most common cameras in the market makes it a hell of a lot easier for lens manufacturers to focus all those wavelengths onto a single spot.  Therefore, I now believe that these halos are "where"  they are because the lenses are not designed to focus the wavelengths of light that cause the ring halos.  5.  Why are these ring halos not colored, such as red?  If you look a the response curve above with the Bayer throughput overlayed, I have highlighted where the Bayer filters lose their ability to absorb light of certain wavelengths.  Essentially the Bayered sensor becomes agnostic to any light above 770nm and that begins even before that.  Because the light in the ring halo is long wavelength, all pixels see it and the halos will be some shade of grey.  I see that all the time when I use my NIR camera, which actually is a color camera, but when I put a 950nm cut filter in place, the result is grey.  6.  The stars that cause the issue a not just red stars.  Many red stars, even known red giants, do not leave these ring halos with my fast/short optics.  I believe that the Long Period Variable stars are particularly rich in the longer wavelengths, vs just red stars.  7.  Why did I not see this issue in my f2.2 RASA?  Well my RASA was not that short a focal length, but Celestron clearly states that the RASA 11 and 8 are designed to not pass much light beyond 700nm.  And that understood, is the cutoff that Celestron designed its focal capabilities for the scope.  The 13 inch RASA is specifically designed to have an extended useful spectral range from 700-900nm.  And that may have been more achievable for its longer focal length.

In any case, for these lenses, I am going to need a more conventional IR/UV cut filter!  And that bums me out, because I have come to rely so much on the light pollution reduction of that class of filters, which all seem to be interference-based construction.  It should be noted, that if I also was using or interested in any of the other multiband interference filters that are all the rage right now with OSC cameras, one should be looking to see if there is an issue in their images with these stars.

Finally, if I were really into finding and studying LP Variable stars, this would be the tool for hunting them down!

Anyway, I will repost this on the Forum, but if anyone else has struggled with this issue, let me know.  Also, if I am a novice bonehead, and "everyone already knows about this", please don't hesitate to let me know as well!