Well, it's now going on almost two years since I started this project, and I'm hoping there will soon be "light at the end of the tunnel" - pun intended!
Some of the most recent, and hopefully final, developments:
Back in October when I got some very brief great views before the weather turned bad, I did note a slight degradation of image quality when I used the front SM140 with the internal SM90. I suspected I might have a DERF 160 issue due to the colder fall temperatures, and the securing method I used (black automotive silicone). Sure enough, removing the DERF (still had the internal SM90 EFR in place), solved the problem - I had a much improved hi-contrast view. I consulted with Brian Stephens at Lunt, who agreed stiff silicone was likely the culprit and that most silicone's are not suitable for optical use, and recommended the same Loctite Superflex 575 RTV used to seal the collimator optics and lens cells (hard to find locally, had to order on-line:
https://www.zoro.com/loctite-rtv-silico ... lsrc=aw.ds).
First, I removed the old silicone and installed some additional safety retaining bolts and rubber washers; the washers do not actually touch the optic itself:
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I used a syringe and a cut-off 16 gauge IV catheter to squeeze in some blobs of the RTV between the filter and cell wall, similar to how an etalon plate is held in its cell:
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This should hopefully solve the issue of warping of the DERF with changing temperature, while offering a secure hold and prevent the remote chance of a failure from damaging the etalon itself. At some future point I may opt for a completely new DERF filter cell, but for now I hope this will work without untoward effects.
Next up was a better internal ERF and circular polarizer implementation. My original 105 mm B+W Kaesemann filter proved to be too small for the SM90 ERF filter cell I had widened up to 100 mm, as it had an actual OD of only 101 mm. So I acquired another B+W Kaesemann filter, this time the 112 mm, which has an actual filter diameter of 106 mm (almost identical to the Lunt ERF, which is 105 mm in OD)- which was perfect. Both B+W Kaesemann CP filters had to be sent to Schneider Optics in California in order to remove the filter from the filter cell, as there is no easy way to to this yourself without risking breaking the filter.
In order to have tilted filter components, I also ordered a Lunt 100 mm filter ERF cover, and had it machined to fit within the internal etalon mount front ring. I attached the SM90 ERF cell to the rear of this ring, and was careful to align both ERF filter cells so that they would have opposite tilts to minimize any chances of unwanted reflections:
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The completed ERF and CP equipped front etalon ring. The ERF precedes the CP in the optical path in order to protect the CP from excessive heat and possible damage when used in single stack mode.
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The completed internal etalon assembly, with ERF and CP, minus the vacuum assembly:
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So at this point, I'm hoping I've done just about all that I can to optimize the filter system and telescope's performance. It should now be completely suitable for both for single and double stack use. Time will tell, as now I have to wait for suitable weather conditions. Current temperatures are well below freezing, and I have a couple of feet of snow to contend with in the path to the observatory... This may be the only sun I see fore awhile ;-)
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Either that, or an exploratory trip to visit California when the weather improves there. In the meantime, maybe this thread needs to be moved to the "solar scope modifications" category!