The following is something that I just posted with my last image, since it is relevant to the data generated and processed. I am partially reposting here, and changing where appropriate to those who may not see my image
Rosette West Widefield - In OSC - Lesson Learned Regarding Light Pollution Filter and Short & Fast Optics!
For roughly eight months now, I have been seeing rings/halos around some of the stars in images that I have generated with newer optics. These are a WO ZS61, a rented Sigma f1.4 105mm lens, and two Rokinon 135mm lenses, one of which I kept as a permanent arrow in my quiver. At first, I just assumed that it was brighter stars and optical defects or possibly having to do with backspacing or the like. It is much more rare in my refractor, though one I saw, I thought that it was an unknown planetary nebula! However, as you can see in the image below, which is a cropped part of my image from the post above, it is much more common in my Rokinon 135 and even more so in the rental Sigma f1.4:
Figure 1. 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.
After seeing this repeated, independently of any changes in backfocus or other such adjustments I could make, I started to realize that there was no logical reason that some of these stars had the rings and some did not. Brightness did not matter, color did not matter, background did not matter. If it was just a simple optical issue, then it would be expected to be with all stars of a particular class, i.e. bright stars, red or blue stars, etc. No such luck.
Long Period Variable Stars: I finally went onto Aladin and found some of these stars and most were listed in Simbad. In certain images, each and every one of the stars that had such ring halos were Long Period Variable Stars. In other images, almost every one was, but some were listed as S stars or some other classification that ultimately falls under similarities to LPV stars. I have since found a few that are listed only as stars with no indication of LPV. However, the correlation is so strong that I feel that these stars, if studied more deeply would be found to fall into this catagory. What is common to all of these stars is that if one looks at them in imaging with NIR/IR or Mid IR cameras (easy to do on Simbad), these stars all jump out as having very high light fluxes in the NIR-MIR. In fact, probably much higher in those wavelengths that in the visible. That goes for those stars that are listed as just stars. However, stars that are listed as Red Giants do not cause ring halos. Even though red giants do have strong IR signals. Hopefully that is interesting to anyone who cares!
A listing of some relevant facts: 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 this camera does not have this feature, unlike its sister camera the ZWO 2600 color. The L-Pro is an interference filter. 2. I have used my L-Pro filter for a number of years on different, larger scopes very successfully as a IR/UV cut filter. In fact, terrestrially, using this filter on my QHY camera yields very natural colors for trees, houses etc., which absolutely is required to have the L-Pro installed to do so. 3. To repeat, I have found that almost every star that is associated with the ring halos are classified as Long Period Variable Stars! 4. 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. 5. The artifact is not dependent of background brightness or color. (See faint ring halo just left of the center of the Rosette.) 6. 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 otherwise 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) may well defeat 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 to 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, these filters are 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 and not just the IR! 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 ring 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? The answer is 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 mono (gray). 6. The stars that cause the issue are 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 abilities/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 easily achievable for its longer focal length.
I have a very speculative theory on why LPV stars may have this issue. It seems that these stars are near the ends of their lives and they are variable because they are ejecting large quantities of gas and dust. Maybe like what is going on with Betelgeuse right now. This means that there is a large amount of dust in the local region of these stars and because dust is close, it can get heated enough to emit very large amount of infrared light. Much more than a red giant star can alone. How or why this is so unique to this type of star is beyond my current understanding, since I would have thought that situation could be met by other types of stars. It may just be that LPV stars are much more common.
In any case, for these new short, fast 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.
If anyone who is new to using these short, fast optics and are having these same problems, I hope that this is helpful. And other than knowing to choose the correct type of IR/UV cut filter for their OSC camera, if anyone knows of any other reasons or similar issues, it would be nice to get some feedback.
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!
Best,
Alan
Rosette West Widefield - In OSC - Lesson Learned Regarding Light Pollution Filter and Short & Fast Optics!
For roughly eight months now, I have been seeing rings/halos around some of the stars in images that I have generated with newer optics. These are a WO ZS61, a rented Sigma f1.4 105mm lens, and two Rokinon 135mm lenses, one of which I kept as a permanent arrow in my quiver. At first, I just assumed that it was brighter stars and optical defects or possibly having to do with backspacing or the like. It is much more rare in my refractor, though one I saw, I thought that it was an unknown planetary nebula! However, as you can see in the image below, which is a cropped part of my image from the post above, it is much more common in my Rokinon 135 and even more so in the rental Sigma f1.4:
Figure 1. 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.
After seeing this repeated, independently of any changes in backfocus or other such adjustments I could make, I started to realize that there was no logical reason that some of these stars had the rings and some did not. Brightness did not matter, color did not matter, background did not matter. If it was just a simple optical issue, then it would be expected to be with all stars of a particular class, i.e. bright stars, red or blue stars, etc. No such luck.
Long Period Variable Stars: I finally went onto Aladin and found some of these stars and most were listed in Simbad. In certain images, each and every one of the stars that had such ring halos were Long Period Variable Stars. In other images, almost every one was, but some were listed as S stars or some other classification that ultimately falls under similarities to LPV stars. I have since found a few that are listed only as stars with no indication of LPV. However, the correlation is so strong that I feel that these stars, if studied more deeply would be found to fall into this catagory. What is common to all of these stars is that if one looks at them in imaging with NIR/IR or Mid IR cameras (easy to do on Simbad), these stars all jump out as having very high light fluxes in the NIR-MIR. In fact, probably much higher in those wavelengths that in the visible. That goes for those stars that are listed as just stars. However, stars that are listed as Red Giants do not cause ring halos. Even though red giants do have strong IR signals. Hopefully that is interesting to anyone who cares!
A listing of some relevant facts: 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 this camera does not have this feature, unlike its sister camera the ZWO 2600 color. The L-Pro is an interference filter. 2. I have used my L-Pro filter for a number of years on different, larger scopes very successfully as a IR/UV cut filter. In fact, terrestrially, using this filter on my QHY camera yields very natural colors for trees, houses etc., which absolutely is required to have the L-Pro installed to do so. 3. To repeat, I have found that almost every star that is associated with the ring halos are classified as Long Period Variable Stars! 4. 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. 5. The artifact is not dependent of background brightness or color. (See faint ring halo just left of the center of the Rosette.) 6. 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 otherwise 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) may well defeat 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 to 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, these filters are 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 and not just the IR! 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 ring 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? The answer is 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 mono (gray). 6. The stars that cause the issue are 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 abilities/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 easily achievable for its longer focal length.
I have a very speculative theory on why LPV stars may have this issue. It seems that these stars are near the ends of their lives and they are variable because they are ejecting large quantities of gas and dust. Maybe like what is going on with Betelgeuse right now. This means that there is a large amount of dust in the local region of these stars and because dust is close, it can get heated enough to emit very large amount of infrared light. Much more than a red giant star can alone. How or why this is so unique to this type of star is beyond my current understanding, since I would have thought that situation could be met by other types of stars. It may just be that LPV stars are much more common.
In any case, for these new short, fast 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.
If anyone who is new to using these short, fast optics and are having these same problems, I hope that this is helpful. And other than knowing to choose the correct type of IR/UV cut filter for their OSC camera, if anyone knows of any other reasons or similar issues, it would be nice to get some feedback.
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!
Best,
Alan
