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Author
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Topic: Astra IIIB Deluxe vs Astra III Professional
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G Jackson
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posted March 20, 2025 07:50 PM
Anything that I can find online about the performance difference between these two is very generic. Directly from Celestaire’s website regarding the Professional-
“Its construction is similar to the Tamaya sextants in that it has a bronze arc fused to an aluminum frame. The bronze arc can be manufactured to a higher tolerance than aluminum, which gives the sextant higher accuracy.”
Does anyone have any actual accuracy numbers associated with these two that can be used to justify the extra cost?
To StarPath: Do you think the bronze arc is worth the extra cost? I will be using the sextant to learn CelNav, and then primarily as a backup to GPS.
Thanks!
From: Wilmington, NC, USA
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David Burch
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posted March 21, 2025 09:49 AM
We recommend the Astra IIIb with traditional mirror.
If you have done just a few sights or thousands of sights, you will not do any better navigation with a different sextant, even if it costs 3 or 4 times more.
We have extensive discussion of cel nav accuracy in our textbook Celestial Navigation and there are numerous real ocean passage examples and special analysis techniques in our book Hawaii by Sextant.
From: Starpath, Seattle, WA
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N
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posted July 05, 2026 04:03 PM
The reply does not actually address the question of whether the bronze arc would be better (at the cost of $200 and 8 oz/0.5 lb extra).
Since I am wondering, and expect others to wonder as well now that the Astra IIIB Professional (featuring a bronze arc fused to the aluminum frame, like the Tamaya) is available, I am posting the resources/links below.
TL; DR:
A. Looks like the USN trains people using something like an Astra IIIB Pro, with the bronze arc.
B. (Minor) accuracy difference probably unimportant in practice.
C. The Starpath suggestion to hold a jug of water or milk up to your nose for a while and see how it feels after holding it a while seems like the ideal test absent any other information. For reference,
* Astra IIIB Deluxe is 42 oz / 2lb 10 oz (2 pints, 10 oz water)
vs
Astra IIIB Pro (50 oz / 3lb 2 oz, so an extra 8 oz of water)
vs
Davis Mk 25 (plastic) (15oz = not quite 1 lb, about 1 pint of water).
Links and discussion:
1.
From: https://en.wikipedia.org/wiki/Celestial_navigation (Image 5 -- Starpath site won't allow posting image link w/parentheses in it)
(Wikipedia / Celestial Navigation)
Image shows a US Navy misdhipwoman using what looks absolutely like an Astra IIIB Pro (but a military part #) – look at the bottom of the sextant to see what appears to be a bronze arc fused to an aluminum frame, which is what the IIIB Pro has and the IIIB Deluxe does not.
2.
From: https://www.celestaire.com/selecting-a-marine-sextant/
"WEIGHT: Sextants are available with their major metal parts made of either aluminum, bronze or brass. The alloys of these metals are well suitable for use at sea. Some people feel that the heavier weight of a bronze sextant provides greater steadiness and hence more accurate readings, especially if it is windy. Others find that the lightweight models are less tiring to their wrist and arm and that the reduced fatigue gives better results. As the observer develops proficiency and speed in sight taking, fatigue becomes less of a factor. Lightweight plastic models can be difficult to use facing into a stiff wind because they tend to “flutter”."
[Aside -- this flutter is for a 1 lb plastic sextant; even the Astra IIIB Deluxe is already almost 3x this weight].
3.
From: https://www.starpath.com/courses/celestial/choosing_a_sextant.htm
"When it comes to the actual physical taking of the sights, the plastic sextant is actually much easier to handle simply because it is so much lighter. Even the strongest arm gets tired when holding a 3 or 4 pound sextant at arm's length for even just a minute or two. Hold a half-gallon of milk (4 lbs) up to your nose with one hand for a minute to get a feeling for the effect. (The Astra IIIb is an aluminum alloy sextant that weighs about 2.7 pound, the next step up is a brass alloy Tamaya or Plath instrument that weighs closer to 4 lbs)
So for some users, a plastic sextant could be preferred for just that reason alone. It does not matter at all if an instrument is inherently more accurate if it is too heavy to use. We come back to this weight issue again later on.
…
Furthermore, the weight difference of 2.7 pounds for an Astra versus 4 pounds for others is a big factor in ease of use. The heavier sextants are harder to use. When your arm gets tired, your sights suffer the pain."
4.
From: https://www.practical-sailor.com/safety-seamanship/sextants-do-we-really-have-to/
"Most navigators like a sextant with some “heft.” Mass helps to steady the instrument. If, however, extending the sextant strains your arms and you’re rushing your sights to be free of its weight, then you’ve got too much heft. Modern sextants range from 2 to just over 4 pounds. Finding one that’s got substance enough to be steady but is light enough not to be taxing is a definite quest."
5. The extra accuracy due to the bronze arc seems unnecessary in practice; seems like question would be more of durability/smoothness of motion/possible corrosion due to dissimilar metals, none of which are discussed much anywhere.
Corrosion (probably not a big deal): see comments re stainless setscrews here: https://www.practical-sailor.com/safety-seamanship/sextants-do-we-really-have-to/
"Some top-of-the-line sextants are made with brass armatures, but it’s more common today to see frames made of aluminum. This raises the topic of maintenance and the spectre of dissimilar metals. Even though you will obviously do your best to keep your sextant dry, small boat use makes it likely that sooner or later it will get wet. Rinsing with fresh water and patting the sextant dry with a clean cloth is generally all that’s required, but pay particular attention to those spots (like the stainless steel screws holding the mirror frame to the aluminum arm) where dissimilar metals are in contact." [N.b.: Astra IIIB Pro has a bronze, not brass arc]
Accuracy: https://www.starpath.com/courses/celestial/choosing_a_sextant.htm "you will not do any better navigation at all with a $2000 sextant than you will with the Astra ... the main factors in achieving that accuracy is what sights you take and usually most important, how you correct for the motion of the boat between sights." I.e., boat motion between sights, etc., dwarfs the 5-10" of accuracy you pick up with the bronze arc.
PS: Fun, but unrelated links:
* Great read on backlash/sextants: https://sextantbook.com/?s=astra
" C Plath seems to have got things correct from the beginning (in about 1909). Manufacturers, like W Ludolph, Tamaya and Astra who imitated his design principle did well to do so."
* Interesting comparison/discussion of various sextants: https://reginasailing.com/ocean-sailing/buying-sextant/
* As opposed tot he wood box these come in, one can get a waterproof plastic box for the sextant from many sources (e.g. Pelikan); Celestaire sells a Pelikan-style box w/custom foam available to better secure the sextant. I have no idea how important such a box really is. And the plastic box alone costs as much as the plastic Davis Mk 25 sextant...
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N
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posted July 05, 2026 04:46 PM
PS: Accuracy / Vernier:
TL; DR: Doesn't matter. Deluxe model may actually be easier to read than Professional.
Re: the original question, the extra bronze arc on the IIB Pro adds 0.5 lb, $200,
--> provides +/- 5" better accuracy (15" (Pro) vs 20" (Deluxe)) , and
--> shifts the Vernier scale to 0.1' (Professional) from 0.2' (Deluxe).
Note that 0.2' of arc (Deluxe) is 12" of arc, so is sufficient for either the 20" of guaranteed accuracy for the Deluxe or for the 15" of guaranteed accuracy for the Professional... the finer vernier for the Pro model would only matter if one is actually doing better than spec (easy to imagine -- spec is worst case). But see previous post for Starpath's comment that accuracy of position fix is not in practice limited by this level of sextant measurement accuracy. I suspect that the 0.1' vernier is harder to read, especially in low light on a pitching deck, than the 0.2' one (which has ~ half as many divisions) -- certainly with standard (non-sextant) micrometers finer verniers take a bit more attention and squinting to read properly (assuming same barrel size for micrometer.)
Aside: Images of the IIIB Deluxe and III Professional show... the same 0.2' vernier. SO I am unsure if the vernier spec below might be a typo. Compare the vernier in the images to, e.g., that for the Cassens & Plath Polaris sextant, which shows the vernier with 0.1' divisions (so 11 marks, instead of 6). If the vernier is actually the same, then the only difference between the Deluxe and Pro is weight and 5" of accuracy spec.
Astra IIIB Deluxe spec is
Arc -5° + 130°, radius 153mm
Accuracy ± 20″, throughout the arc
Micrometer Drum 1′ scale, vernier to 0.2′
Horizon Mirror 51mm diameter
Index Mirrors 56x42mm
Shades 4 index – 3 horizon
Weight 2 lbs., 10 oz
Astra III Professional spec is
Arc -5° + 130°, radius 153mm
Accuracy ± 15″, throughout the arc
Micrometer Drum 1′ scale, vernier to 0.1′
Horizon Mirror 51mm diameter
Index Mirrors 56x42mm
Shades 4 index – 3 horizon
Weight 3 lbs., 2 oz
Specs from:
Deluxe: https://www.celestaire.com/product/astra-iiib-sextant/
Professional: https://www.celestaire.com/product/astra-iii-professional-sextant/
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N
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posted July 06, 2026 12:31 PM
Celestaire confirms that the Astra III Professional and IIIB Deluxe both have the same 0.2" vernier scale (differing spec was just a typo.)
Thus the two sextants are completely identical except for the bronze arc on the Professional, which adds 8 oz and an additional +/-5" of accuracy.
The bronze is apparently somewhat more durable/robust if bumped/dinged. Apparently navies/military use a lot of these sextants, and order the Professional with the bronze arc presumably for the added robustness.
Matching David's comments above, for more typical users it's not clear there is an important benefit to the bronze arc (unless you just want to have it.)
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David Burch
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posted July 07, 2026 08:48 AM
Moving this discussion to the Student Forum for further discussion.
From: Starpath, Seattle, WA
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Topic Closed
