Solar SIG!Siebert 1.3X Optical Corrector: Enabling binoviewing on Coronado’s Personal Solar Telescope

Jim Richberg

The Coronado Personal Solar Telescope (PST) is a wonderful way to look at the sun in Hydrogen-alpha, and has produced a legion of new “solar addicts” in the short time since its release. One of the limitations of this scope compared to Coronado’s higher end products was its inability to come to focus with binoviewers, even with fairly significant Barlowing. I’d been talking to Harry Siebert about his new optical correctors. I was in search of an option that offered low magnification and required no modification to the PST– even one as trivial as unscrewing the blocking filter/eyepiece holder (which can’t be done on my PST anyway, as it is cemented in place.) Harry thought about it and came up with a relatively tall 1.3X corrector, and sent me what I believe was the prototype unit. As such, mine looks a bit different than the production units are likely to, but I’ll cut to the chase and tell you right now, IT WORKS!!!!

Note: this is *not*, repeat not, the 1.5X PST OCA Harry has listed on his web site; I have no experience with that particular unit.

Here are my impressions after several solar and one nocturnal viewing session.

1. Although I have owned various binoviewers for five years now, I am not a “bino nut” and in fact greatly prefer ‘Cyclops mode’ viewing. That still may be the case for night time observing, but this OCA worked well enough on my Black Night binoviewers that I may switch my low and medium power solar observing exclusively to bino mode. (At higher powers and under conditions of mediocre seeing, mono viewing still rules for the brightest, crispest solar image.)

Note that the image is inverted compared to that of the un-bino’d PST.

2. My PST renders particularly sharp images (Greg Piepol and I have estimated it at <0.7 Angstrom, and it consistently beat my Maxscope 40), but it has always had “sweet spots” within the FOV that would be in better focus compared to the rest of the field. I’d gotten used to moving the scope slightly to center whatever portion of the solar disc I was interested in within this region of best focus. (This was one of the advantages the Coronado Maxscope has over the PST, by the way– a much “flatter” focused FOV.) Harry had said that the OCA would help flatten the FOV. I was frankly dubious, but it proved to be true. This was a totally unexpected result, and was possibly the most persuasive single benefit of binoviewing for me.

3. Light loss: Insignificant at low to medium power, moderate at high power. I measured this by looking at the faintest prominences visible through a good (high light transmission) eyepiece in mono mode, then looking for the same wisps through the bino. I knew intellectually that there was light loss because of all the extra elements in the optical path with a binoviewer, but as a practical matter the drop off was so slight at low powers that I was unable to decide definitively whether or not I was seeing the effects of reduced illumination. (Part of this was due to the fact that my bellows-style eye cups offer 100% baffling of environmental light in mono mode, but “leak” light–which reduces contrast– when used in bino mode due to the different position of the eye socket over the eyepiece.) At medium powers the drop in light was beginning to manifest itself as dimming of the image and attendant softening of focus; at high powers (40X and above) this was definitely visible. At the highest bino’d magnification I tried the image, while still usable, was noticeably inferior to that of a single eyepiece in terms of resolution and brightness, although the bino’d view continued to have the advantage of uniformity of clarity across the FOV (compared to the “tunnel vision” nature of the mono view described in #2 above.)

4. At first I thought the magnification of the binos was greater than 1.3X, but after comparing it to monocular views, I confirmed that it is 1.3X or something close.

5. Since this is Harry’s “zero modification” unit, all of the optical correction takes place above the PST, and the unit you’ll have to stick in your binoviewer is is much like a very long Barlow. (Mine add 3.75″ of height to my binoviewers; the production units are supposed to be considerably shorter) Your binoviewers will be sitting well above your PST; depending on your mount, this might make for some stability or balance issues. Harry plans to shorten the length of the OCA on the production units by folding or bending the light path within the OCA; I opted to have my unit made “straight through” because I’d like to be able to use it at night (it’s lower powered than my current 1.8X OCA), and want to avoid the additional light loss from the extra optical elements that will be involved in shortening the path.

I did try the OCA at night on my 16″ Dob, and found that optically it worked well and provided a useful alternative to the higher power correctors that most binoviewers require to achieve focus on Newtonian telescopes. When using this corrector in a nocturnal binoviewing application, though, it’s full length becomes apparent, and the eyepieces of your bino’d scope may be fully twelve inches away from the optical tube. If your focuser hardware isn’t up to the task (I’ve got a JMI, and use a beefy Mercury Support Systems compression ring 2″-1.25″ adapter, so mine worked fine), you may find the unit sags alarmingly. Again, to be fair, this OCA wasn’t designed for standard nighttime binoviewing, so you can look at the ability to get low magnification night views of clusters and DSO’s as a ‘freebie’.

In closing, I’ll reiterate that this OCA works as advertised, and makes “no-modification” binoviewing a reality on the PST.

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Other PST binoviewer issues not specific to this OCA

Mount considerations and keeping your eyes centered: This is an issue due to the idiosyncrasies of the PST and its non-movable diagonal. If you don’t have your PST on a setup where the diagonal is always facing more-or-less straight up (i.e. unless you’re using rotating rings, a Dob mount, or something similar), you may find yourself observing with the eyepieces canted at odd and sometimes uncomfortable angles. My PST is piggybacked on the mounting rings of a white light solar scope, so I find myself doing these contortions frequently. I discovered in my testing of the binoviewers that they are much more sensitive to head placement than a single eyepiece. Due to the odd angle the binos were at a number of times during my sessions, my head placement shifted enough that I was in fact only receiving light through one side of the binoviewers. If you’ve got a mount like this, I would recommend doing the “blink test” periodically (i.e., close each eye in turn) during binoviewing to make sure that you haven’t canted your head enough to slip back into mono mode.

Ambient light at the eyepiece. One of the biggest and often overlooked factors in solar resolution is the impact of environmental light flooding in at the eyepiece. (This is also a factor in nocturnal viewing, especially from light polluted areas!) I have found that bellows-style eyepiece cups work phenomenally well in single eye/mono mode viewing on the PST, especially if you don’t wear glasses and can press your eye socket down into the rubber. This can produce a virtually perfect light seal. Unfortunately, due to the interaction between the physiology of the face and the geometry of binoviewers, these same eyepiece cups don’t produce the same seal when used in binos. “Winged” eyepiece cups can reduce the worst of the intruding light, but still let significant quantities in from the non-winged directions. I experimented with making winged bellows style eyepiece cups, and produced something that worked fairly well but vignetted the views. Oops! I found that for the best views (highest contrast, darkest background) during my solar binoviewing session, I reverted to the old tried and true method of covering my head and the eyepiece holders with a dark cloth or towel. This is tolerable in December, but decidedly less appealing in summer. I’ll keep experimenting with “on the eyepiece” solutions– if you’ve got something that works, please drop me a note! (jrichberg@bigfoot.com)

Optical cleanliness: Eyepiece glass tends to accumulate lots of debris– “eyelash oils”, flakes of skin, bits of hair, etc. Eyepieces can hold a considerable amount of this junk without affecting their nighttime performance, and I know some observers who prefer to put up with this cosmetic debris rather than taking *any* chance of adversely affecting the coatings of their eyepieces by cleaning them. Because of the comparatively much greater illumination of the field of view in solar viewing, however, any bit of foreign matter shows up as a defocused blur or spotting if on the image of the solar disc. If you’re the type who cleans their eyepieces for solar use (I go for *gentle* swabbing with high quality Q-tips moistened in distilled water), be aware that in solar viewing you’ve now got THREE times as many optical surfaces to keep clean. (Twice the number of eyepieces, plus the top and bottom surfaces of the binoviewer itself.)

Clear and sunny skies!
Jim