This post was kept private for the purpose of offering supporting data and information to a forum post I made some time ago.  Now that I believe the issues have been resolve (not favorably, I should add!), I am posting this so that it is a permanent post to my page.  I will also post in the old forum post that I have added Revision "E" as an example of the optical difficulties I have faced to alert those who helped me at that time what the outcome is.  At the time I was seeking help from anyone who was interested or knowledgeable about the problems and possible solutions.  At this point, what I have learned is that I would have to put more $$ and optical intellectual input than I want to do at this time.  Also, there are details of some gear on this page that may interest others for reasons that have nothing to do with photometry.

ORIGINAL POST:
While this topic does not follow closely to aesthetic imaging (far from it!), it may be of interest to some I know have expressed interest here on AstroBin in my posts in the past.  Also, since the telescope (TS UNC) and fittings (Baader Steeltrack Diamond focuser) are useful for getting quality images of astro objects, this may be useful for some who could care less about NIR imaging and photometry but just want to image.  I have joined a group elsewhere that concentrates on photometry, and will certainly get more involved in that group.  But this story involves some optical problems that I hope to solve and some participants here may well be better able to help.  So I will not post this widely here, but use this page as an info dump for a forum post.  If for some reason, some here  feel this page would be more widely of interest, I will change the notification setting to make it widely available.  I also am doing this as a project for personal learning and do not want to just buy a solution, such as could be found on Innovations Foresight, the On Axis Guider, which I feel is a cool concept.

The photo shows a setup that in theory could allow me to image a field of view in NIR and VIS wavelengths simultaneously.  It makes use of the Baader Flip Mirror cell, which you can research if you are interested.  The initial design was to replace the mirror with a dichroic beam splitter.  The design is to use a splitter that has a cutoff of roughly ~750-800 nm.  Dichoics can be had that have tight cutoffs and can pass or reflect wavelengths above or below the cutoff at high efficiencies.  Often well above 95%.  These specialized mirrors are designed to work with a light incidence of 45 degrees (or other).  This is not so specific that the angle of the light cone coming into the splitter from an F4 telescope will matter much.  At least for the efficiency of parsing IR and VIS wavelengths.  Each camera will have additional filters to narrow the wavelengths being studied.  For example, my NIR/VIS camera will have a 950 nm edge filter, so it will only see anything above 950 nm.  (See revision C for an example 300 sec sub from this telescope and camera for a 950nm edge filter.)  The VIS side camera will likely be filtered to a Blue standard used by photometrists.  So I will only be sampling part of the collected visible light.  So far I have tried both "cold" mirrors (i.e. passing the IR band) and "hot" mirrors (.i.e. passing the visible bands.  I should state that the reflected images with either mirror type is very nice and clear and would be suitable for my purposes.

Now comes the problems:  
1. The through images, either NIR or VIS reveal a doubled image.  I am assuming that this is caused by internal reflections in the 5 mm thick fused silica substrates that the dichroic layers are deposited onto.  One of the mirrors I tested did have an anti-reflective coating added to the back of the mirror.  But this was not a solution.
2.  The images passing through the dichroic mirrors appear to suffer from astigmatism.  This was pointed out to me by @John Hayes whom I have contacted regarding these issues.  So thanks John!

John uses the Innovations Foresight ONAG system.  I may have more to say about the IF ONAG system later or within any discussion, but I get the feeling that my attempting to employ this within an f4 optical train may be pushing the limit.  But I have not given up just yet!  To that end, I have contacted the supplier of my dichroics (ThorLabs) to pose these issues to them.  I will follow up with any solutions.



FOLLOW-UP (10/27/22):  After much too long a time, I am following up with additional information that I have gotten in feedback from ThorLabs and also data I collected for the purpose of illustration to the aberrations I have with this system.  ThorLabs basically confirmed the two issues I highlighted above.  They suggested that a thinner filter would help, but within their product line, I would have to add other optics as well to properly correct the issue of multiple images.  To highlight the issue that I have, I have updated this page with a slide that shows what I am talking about.  It is in the Revision marked as "E".  Note the very strong astigmatism.  This being represented by the stars being stretched out very long!  Also, note that the star does not become just a simple line, as one might expect with the starlight going through a diffraction grating of a spectroscope.  Also, note that the line is not a simple line, but has two width maxima.  This I believe represents the doubled image.  You may note that the stars highlighted by the red arrows are very much shifted depending on the presence of the filter or not.  I consider this a small deviation, and my Baader Flip mirror has a horizontal ajustment for one camera, so this, at least, could be corrected.  But the other two issues are probably going to kill this project for me.  I would like to know how Innovation Foresight's OAG system solve these issue.  Though, I noted that even on their site, they offer an astigmatic corrector lens to help with some faster scopes.  My guess is that it is the speed of my system that is the biggest hurdle.

In any case, I have decided that I will send the information that I posted here to ThorLabs to see if they can offer a simple solution, but really what I am asking for is well beyond what ThorLabs is in the business of making these wonderful dichroic splitters.  If a simple solution presents itself in the future, I may reopen up my account on this project, but considering the cost of my new f4 12 inch scope, it makes no sense too spend more money on this than it would be to just get another scope and dedicate it to the wavelength I want to add to the analysis.  There are some nice mounts that I was already considering that could handle two 12 inch scopes.  And considering my current health limitations, I will be taking a bit of a break in going full bore in this direction until I am sure I will be around to deal with the data, in any case!

END of FOLLOW-UP



THE FOLLOWING WAS IN THE ORIGINAL POST, WHICH I KEPT PRIVATE UNTIL I COULD UPDATE THIS WITH A RESOLUTION:
I have included two other images as revisions on this page.  B is an image of my telescope setup with the splitting imager mounted on the focuser.  For those interested in the TS UNC, here you go!  I have the Baader Steeltrack diamond focuser mounted as it was delivered.  I am happy with it.  It basically came stock with the telescope because last year TS could not get their focuser in house to make their telescopes and they substituted the Baader unit at no cost.  You guys can tell me if that was a plus because I have no experience with the TS focusers.  So far I am happy with the Pegasus power unit.  Saves me many bricks and power lines.  I have no issues with backfocus with the Baader Flip mirror setup.  The cameras can be adjusted to a high degree and if I needed too, I can slide either camera well into the body of the Flip Mirror cell to give me more backfocus.  That is unnecessary.  BTW, even if I wanted to use the IF ONAG designed for C-sized sensors, I beleive I would not have the back focus to do so.  My IR camera (the QHY5III462C) has a very small sensor.  That limits my field of view with this telescope.  On the other hand, it also allows me to use 1 inch circular dichroic mirrors in the Flip Mirror cell.  That certainly simplifies things and lowers the cost (if this should work!).  The VIS camera will be swapped out with a unit that has the same sensor and pixel dimensions.  I will not spring for the money for that until I know I have the solutions to my problems.

Revision C is an image that shows exactly what my field of view is for the photometry I want to do.  Unlike past images of this sort that I have posted, this image is with the 12 inch TS UNC telescope with the IR camera.  You might ask, why do I need to image VIS and NIR simultaneously?  As far as photometry is concerned, I am not interested in recording the photometric rise and fall of well known variable stars, or even of known exoplanetary systems.  I am interested in discovery.  Yes, even if that means I spend the rest of my days looking at an unmoving collection of boring stars!  What I show in revision C is a sampling of the known Young Stellar Objects to be found in this area of the Elephant Trunk Nebula.  In fact there are many more close by just outside of my field of view.  YSO, of various names and types are interesting, because they tend to still have their tightly associated circumstellar disks, clouds and nascent planets.  These sorts of structures tend to enhance the NIR signals from these stars, even ones that are quite blue.  As stars, they are also pre-main sequence, meaning that they have not begun to fuse hydrogen yet.  So they are very very young.  These sorts of stars have a greater chance of giving up clues as to what is going on in their environments.  And in this particular area, academic studies have shown that photometric variations can occur on the timescale as short as seconds and minutes.  Granted, these studies have made use of orbiting IR telescopes, but hey!...  If available to me, imaging these stars in two bands at the same time guarantees that if I see anything, I will basically time-stamp both the VIS and IR signals accurately.  And seeing identical or differential signals at a VIS wavelength vs a NIR wavelength can say a lot about what is likely going on around a star.  Revision C is only one example of such an area of interest that has a high concentration of YSO.  So there sky is rich with this stuff!

Finally, if this ever gets posted generally, I apologize to the Spectroscopy group for this showing up there.  Though I suppose the concept of gaining information through more than one band to learn about the structure of an astro object kind of falls into that category.  However, I did search for Photometry and no such group appears to my knowledge.

Alan