Struggles and Progress Towards a VIS/NIR Rig for Photometry. Gear: Baader Flip Mirror Cell, Dichroic mirrors/filters, etc.

Equipment Reflectors Optics
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Alan Brunelle avatar
Alan Brunelle avatar
Hi All,

I know that this topic tends to fall outside the scope of most interests here on AstroBin, but I do know that a few individuals who post here, do show some photometry on occasion...  However, the problems that I am having have more to do with optics and I am posting here because I believe that there are a good number of individuals who have a very high understanding of optics and potential solutions.

I have on my collections staging area a page that shows the equipment I am trying to get to work and a more detailed description of my problems.  While the page falls under "gear", I feel that it really does not fit the general interest, so I am not posting it generally.  So if you are interested in this, go to this link and you should gain access to the images and the text:  https://astrob.in/g9a5ee/0/ 

The bottom line is that I am trying to split my images from my f4 305 UNC reflector into VIS and NIR using a dichroic mirror (45 degree) and while the reflected images are perfectly fine, the pass-through images have astigmatism and also seem to have a ghost image offset from the main image.  i.e. 2 images in one!  @John Hayes has already pointed out to me what I am likely seeing.  My system very closely resembles  the Innovations Foresight On Axis Guiding system, which John is very familiar with.  And on that site, there is some mention of astigmatism and correction.  But not of the reflected image.  So I am wondering if there are any optics experts out there who might suggest a solution or a correction.

Thanks,
Alan
andrea tasselli avatar
andrea tasselli avatar
I have looked at the page in question and I can't find much in terms of either astigmatic image or reflections (which MUST exist for a converging beam at an angle). Besides, given that the primary if not only aim is photometry it doesn't really matter whether the IR image is astigmatic or not since PM is about counting photons and not much else, isn't it? AFAIK, cylindrical surface can produce astimatism but somehow I don't think this is the issue here. Further analysis would require images of the out of focus pupil either side of focus, 4-6 wavelengths should do for a fast system like yours.
Alan Brunelle avatar
Alan Brunelle avatar
Thank Andrea.  None of the images or revisions actually show any of the defects I discuss.  I thought that I had saved some such images, but cannot find them.  If later this week I get the rig out, I will certainly record some images.  The one image of the Elephant Trunk area is actually a reflected image, with no defects other than the unguided nature of the exposure.

You are correct, I can live with a bit of astigmatism.  In fact perfect focus of these images will be unnecessary nor unwelcome.  As long as I am not trying to resolve two stars that are very close together.  As you may be suggesting, John Hayes did say that the solution to the astigmatism would be to employ a cylindrical corrector lens.  I have a query into ThorLabs as to the nature of such a lens for an f4 system.  But you are correct, I am not looking necessarily for perfection in that case.  I will try to get the images this week to see if that sheds light on my defects.  But my guess is the duplicated images are simply the internal reflections of the image within the 5 mm glass substrate (prism).  What is odd, is that this effect is not apparently an issue with the Innovations Foresight ONAG system.  And that may be because of my short f.l.  But astigmatism is something that IF addresses for the through images, and they actually sell a corrector for that.  

Maybe a solution is to have a dichoic filter on a thin film substrate?

Alan
andrea tasselli avatar
andrea tasselli avatar
Hi Alan,

I did the calculations for a 5mm beam-splitter and it yields ~30 microns of spread due to astigmatism. If the beam-splitter were more of a thin plate, say 2mm, it would yield an entirely acceptable value of 12 microns, i.e. 4 pixels for your camera. I haven't added spherical aberration nor coma to the calculations though. There is suggestion that a slight wedging of the beam-splitter, at around 0.47 deg would correct astigmatism but I didn't check it. Obviously you could add a thin weak lens to correct for all aberrations, made in BK7.
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andrea tasselli avatar
andrea tasselli avatar
Alan Brunelle:
Maybe a solution is to have a dichoic filter on a thin film substrate?

Yes, but anything less than 2mm would do just fine.
Well Written
Alan Brunelle avatar
Alan Brunelle avatar
andrea tasselli:
Hi Alan,

I did the calculations for a 5mm beam-splitter and it yields ~30 microns of spread due to astigmatism. If the beam-splitter were more of a thin plate, say 2mm, it would yield an entirely acceptable value of 12 microns, i.e. 4 pixels for your camera. I haven't added spherical aberration nor coma to the calculations though. There is suggestion that a slight wedging of the beam-splitter, at around 0.47 deg would correct astigmatism but I didn't check it. Obviously you could add a thin weak lens to correct for all aberrations, made in BK7.

I appreciate your doing the calculations.  It is very helpful.  Your points are well taken.  It is interesting that the hot and cold mirrors that I have been using to prove this system are advertised to have a 30 armin wedge built into the fused silica substrate.  It is advertized to minimize interference effects.  I will have to look into this because my mounting could have had a critical impact on the image.  I will have to rotate the filter to get the most out of the wedge.  The slightly more expensive long-pass and short-pass filters I was going to settle on in the end do not mention such a wedge.  But they do have a proper antireflection coating on the back of the substrate.  I will have to ask if this wedge is something that is present on the better dichroics or are available from this company.

If I am forced to use a corrector lens, I will likely end up using a short pass filter for the device.  That way I can get the best optical performance from the NIR side (reflected image) of the system without the need or worry about any introduced glass elements.  That will leave me to correct the VIS side of device.  My goal is to have a 100% reflectance imaging train for the NIR side of things, if possible.  I have come to distrust the transmittance from optics from comercial telescope manufacturers.  For things like correctors, etc.  

I have not seen any spherical or coma abberations within the small fields of view for these small sensors.  With this telescope, I definitely see a strong coma with my C-sized sensor camera.  As expected.  For that kind of imaging I have used a coma corrector to some positive effect.  But I am still working on the perfect back focus for that since I have only had that image train out for one session.  I am close, but not perfect. However, the sensor for the NIR camera is so small, it really doesn not come close to being touched by the coma'd field.  Even if it did, it would be so slight I would accept it as I will do for some slight astigmatism.

The dichroic I have currently have mounted is a long-pass and I hope to get some imaging on it if I get some clear skies later this week.  At that time I can post some of the images. 

As you suggest, I may have some solutions for the astigmatism, or at least some means to accept the result, or combination of correction and acceptance.  So that leaves me with the internal reflected image which I believe that I am seeing as well.  Let me image the pass-though signal this week and I will post it to help in the understanding.  For all I know, it may well be just a reflection off of the 950 nm edge filter I use on the camera!   

I am not willing to give up just yet on this project.  That Innovations Foresight have seemed to solved the issues gives me hope that I can succeed.  If I fail, I guess I will just get a filter wheel and do consecutive imaging...

Thanks,
Alan
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D. Jung avatar
D. Jung avatar
Tldr, but in the laser lab we usually used thin pellicles for this type of beam splitting
https://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=898
Alan Brunelle avatar
Alan Brunelle avatar
D. Jung:
Tldr, but in the laser lab we usually used thin pellicles for this type of beam splitting
https://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=898

Thanks!

I looked at this link.  Nice to see that there are options.  Certainly efficiency is a factor.  But it is nice to see mentioned the concept of ghost images and solutions that eliminate them.
D. Jung avatar
D. Jung avatar
Alan Brunelle:
D. Jung:
Tldr, but in the laser lab we usually used thin pellicles for this type of beam splitting
https://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=898

Thanks!

I looked at this link.  Nice to see that there are options.  Certainly efficiency is a factor.  But it is nice to see mentioned the concept of ghost images and solutions that eliminate them.

These are made for "monochromatic" laser light, so not optimized for brought band light sources. However, I'm not sure you will be able to find anything better off the shelf.
Alan Brunelle avatar
Alan Brunelle avatar
Hi All,

I wanted to let those of you who offered help and showed interest that I have received input from ThorLabs to my questions.  I have also updated and formally posted the webpage that I had left private with a revision, "E", to show you the degree of aberration that I have been dealing with.  Given the answers that I got from ThorLab, and the fact that it would cost me a good deal of money to trial and error this thing, I have put this type of solution on hold.   @D. Jung, a thin pellicle was suggested by them as a partial solution.  However, unless I contracted them to make me a custom splitter (even if they would), they do not offer such a unit.  @andrea tasselli, the ThorLabs higher end splitter/filter is actually a bit thinner.  I may actually spring for that to give it a try.  But you may be interested in the data I present in Revision E to see if what I am getting follows your calculations.  The astigmatism, even if ignoring the duplicated star images, seem quite gross!   Other problems may be in action as seen in these images.  

Honestly, my health issues also factor into this currently.  But as far as money is concerned, a solution that may be simplest is to simply duplicate the f4, 12 inch.  It is really not that expensive.  And the mount I was planning to use could handle two 12 inch OTAs.  The short-term solution is to get a filter wheel and not do concurrent photometry, but that kind of kills the point of what I was trying to do.  In any case the posted, updated page can be found here:

Struggles and Progress Towards a VIS/NIR Rig. TS UNC 305 and other Gear.

And a shout out to @John Hayes to alert him to my updates, since he has been so very helpful with my challenges.

Thanks!
Alan
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