I had the good fortune to test these Siebert filter adapters in Namibia last week and am extremely happy with the results!
My ultimate preference is for one filtered eye (predicted in this earlier thread), which provided the best all-around experience. I was easily able to "merge" the views.
A pair of broadband filters was also very pleasant.
I realize binocular (and eyepiece) experiences are fairly personal and vary widely, but details are below in case some of you are interested:
Latitude -23˚19' and 1400 meters altitude (Kiripotib). No artificial light pollution; zodiacal light extended to at least 40˚ at the end of twilight, dwindling and disappearing at solar midnight before quickly reappearing in the east. The glow of the central Milky Way extended to Antares and many(!) Messier objects were visible naked eye. The air was dry enough to cause visible sparks when running your hand across wool fabric. The dust of the savanna affected transparency, but otherwise the sky was clear.
Between our friends, we were able to test the following in my 7x35 Aculon:
–Two DGM NPBs (no discernable difference between the two filters).
–Astronomik and Lumicon OIIIs (very similar, though I noticed more subtle nebulosity in the Lumicon).
–Baader UHC-S and Orion Skyglow broadband filters (the latter offering very minor additional contrast).
–One TeleVue Bandmate II Hß.
I also tested my Star Analyser 200 on a lark. Besides enjoying the aesthetic novelty, I'm not convinced I will use it very often.
I kept a cleaning cloth handy since many objects were at high altitude and the Aculon's short eye relief caused some fogging.
All combinations were tested. I eliminated the Hß after failing to see the Cat's Paw (the same filter presented a satisfying view in my 4" frac). I unfortunately overlooked IC1283 in the same region. The Hß filter's reputation for limited use (at any aperture) won't discourage me from testing brighter Hß regions in Cygnus this August, as well as in Monoceros/Orion this winter.
I found panning the milky way with two OIII filters a strange but beautiful experience, and I spent quite some time getting lost. Eventually, though, I found some details noticeably absent. I normally have no problem with the "distortions" filters give to non-nebular objects (like the red stars through the NPB, which I actually enjoy), but the vistas were so wide in this case that I craved the natural view after a while.
Since I was in unfamiliar conditions, it was also difficult to determine the specific benefits of the filters themselves. This led me to try the single filtered objective. I immediately discovered the "blindness" sensation some folks describe when sweeping the sky using one OIII filter, which led me to closing the filtered eye while navigating.
This in turn led to blinking each eye back and forth, serving as a sort of "filter wheel" to test the differences. I wear contact lenses optimized for night use, so my eyes are more or less the same (minor anisocoria causes my left pupil to open slightly wider) and I don't have a dominant eye at the scope. This blinking was useful for comparing the minor differences between filters described above. I could easily determine the benefits and drawbacks of each view, and switched often before opening both eyes.
The merging of the views was nearly immediate and very compelling. I felt all of the benefits of the filter and none of the usual limitations. I was discussing this with another farm guest, who had experienced a similar merging while viewing the sun in H-alpha combined with some sort of coronagraph. I did not follow all the details since I'm not as familiar with solar observing, but I was pleased to hear another precedent for this effect.
The Carina nebula is a wonderful example. The Lumicon OIII gave the impression of more density to the inner folds / around the Homunculus and extended the wisps out much further. However, the field also contained the Wishing Well cluster (a sparkling and crisp "chip" of barely resolved stars @7x), the Gem cluster and a chunk of the dark nebula complex in the surrounding area. The star colors are very impressive and dark nebulae (one of my primary focus areas on this trip) are easily seen in these conditions. If you're willing to put up with the edge distortion (as I was) you can also frame Eta Carinae, Lambda Centauri and the Southern Pleiades, with immense dark clouds in between.
Those are the sort of wide field views I came for, and I felt the punch of the nebulosity in an otherwise clear, unfiltered view was absolutely worth it. This setup also enhanced the view of the Tarantula nebula/LMC, and I had similar experiences with more familiar areas (eg the Lagoon and Trifid). I tried for a few other unlikely sightings, which usually led me to my scope. I might expect some detection improvement in nebular patches of the SMC, but I did not test this with the binos.
Despite my determination to test each combination (all had their uses and charms), a single filter was the preferred view and I spent most of my time with this. As added benefits, I can share one filter adapter with another bino observer and I don't feel the need to purchase doubles of any filters I own.
These adapters hold firm to the binos and fit well. Inner diameter is 56mm, so they should fit any bino with a smaller objective housing. They cause no vignetting, with or without filters. Screw-on dew shields would be beneficial in my locale, which these adapters will have no problem accommodating.
The threads of one adapter needed a little finessing to work properly. The T6 aluminum adds 20% to the bino's light weight. I can imagine PLA or delrin with aluminum threads inserted like these would significantly reduce this, but in any case I hardly notice these drawbacks and am looking forward to many years of use.
Edited by Enkidu, 11 June 2019 - 03:40 AM.