Unlike the 7 threads on the worm of a Plath and most others, the CHO-T has a worm with 15 threads. But more importantly, unlike the fixed pressure of the worm against the rack, the CHO-T, much like the Freiberger, has a very sophisticated and highly adjustable pressure system. These are two very important features in achieving the silky smooth (near zero backlash) micrometer drum motion needed to achieve the 6 seconds of arc accuracy which this instrument boasts.
The extremely close tolerance of the index journal, with its diameter of about 3 to 4 times that of most other designs, greatly contributes to the stability and rigidity of the index arm movement.
Reducing the weight of a sextant is important to the practical accuracy of the instrument. However, in most cases, a reduction in weight is often achieved at the sacrifice of rigidity. Not so with the CHO-T.
As Bill Morris points out in his highly authoritative treatise, the CHO-T sextant frame scored the best in terms of weight/rigidity of the 9 sextants tested of both bronze and aluminum alloy. By the way, if you are interested in owning any of the top-priced sextant models, you definitely want to invest in his book. It's totally devoted to renovation, maintenance and adjustment and is much more in-depth than his website info.
Unlike the back-silvered index mirrors of all other models, the CHO-T index mirror is a first-surface reflector. This eliminates the high-angle, double-bounce refraction error and reduces the star glare that all other sextants suffer.
The micrometer drum does not have a vernier scale. None is needed. The CHO-T makes use of a fully adjustable magnifier loupe that services both the main and micrometer scales. When you look through the loupe at the micrometer scale there is no doubt in your mind what the tenths reading is.
There is no battery-operated lighting system. None is needed. The inside of the magnifier loupe is coated with a luminescent material that provides sufficient light to read the scales.
One of the shortfalls of most sextants has to do with telescope collimation. You cannot simply slap a nonadjustable telescope into its mount without accepting some degree of accuracy loss. The Keplerian telescope of the CHO-T can not only be adjusted for collimation, but it has parallel cross-hairs in the field of view to simplify this process.
The sextant with the Keplerian telescope weighs just under 3.5 pounds.
All in all, when you look at the simplicity and common sense of all of these improvements, some of which could have been implemented 200 years ago, you really scratch your head and wonder why this was all left to the Soviets in the very dawn of sextant manufacturing and design. So, now here's what I have done to bring this sextant up to factory release.
First, a word to the wise about buying one of these sextants, or a Freiberger for that matter, from the pawnbrokers who are simply selling them as is. I have yet to buy a CHO-T that was not all but frozen from long hardened grease. It often takes two to make one sweetheart (because of a defective, non-repairable major component) and a full refurbish effort requires a minimum of eight hours of work. CHO-T sextants were largely military and merchant marine issue. As such, they rarely found their way into the hands of owners who were willing to give them ongoing TLC. If it doesn't indicate refurbished, it's something you'll have to figure out on your own.
That being said, I have completely broken down and re-lubricated this sextant to the last bolt and spring. I have squared the index mirror to the frame, adjusted out side error of the horizon mirror and reduced the index error to less than a half minute of arc on either side of zero on a star-on-star sighting.
I have cleaned and lubricated both telescopes and collimated the Keplerian telescope.
I have also cleaned and lubricated the magnifier loupe and I have put a fresh thick coating of strontium-90 (just kidding - 88) aluminate paint on the loupe interior.
The mirrors are in near perfect condition as are the shade lenses, although the index mirror has some micro-scratches which you really have to look closely to see. Also included is your choice of the standard spare index mirror (although back-silvered) or a spare first surface index mirror which I manufacture myself.
Overall, the sextant has some cosmetic un-niceties in that it has seen its share of time at sea. However, it is mechanically flawless and the components seem all original.
The sextant comes with a 3G iPod loaded with Starpilot (a very powerful celestial navigation program) the nautical almanac to the year 2050 and several other apps which are useful in celestial. The iPod has a screen protector. It comes with a charging wire. The battery in the iPod is not new, but it seems to hold a charge for a significant amount of time.
In addition, I am including an ultraviolet flashlight for charging the magnifier loupe. With a three-second blast from the UV light, the strontium aluminate paint is charged for about 15 minutes.
If you want to clear the lunar distance to make accurate sights to calculate GMT, this is your first choice of sextant above all others. The Freiberger is close in accuracy, and makes a good second choice for this purpose.
The books in the pictures are not included, but if you are a novice, I will part with a copy of Mariner's Celestial Navigation by Crawford for $15 extra.
Disregard the time limit eBay suggests for shipping. These instruments need to be tweaked out of storage, which requires a series of good celestial sights. So, depending on sky coverage, this could take up to a week.
I have sold many sextants on eBay, both marine and aviation, but I have been out of action for more than a year now. So, if you want to see my past reviews you will have to dig for them a bit by pasting this URL into your search bar: https://www.ebay.com/fdbk/feedback_profile/balloonron?user_context=BUYER and clicking on "Received as seller."
You can see my regular day job by searching customnine balloons.