Can the use of a Sextant cause eye damage?

[model 10]

Originally Posted by Cpt Pat

I suppose I have to change out all the red lights in my Cessna and every other airplane and helicopter I fly.

Cessna changed to blue post lights in about the mid 80's.

[Snowpetrel]

Originally Posted by boat_alexandra

Maybe a digital camera can work (I know it's electronic) but no risk of eye damage.. and it can potentially take much more accurate sights.

It seems to me that an app could be made to adapt a phone's digital camera into a half decent emergency sextant. You would need the right filter over the lens in the sun area, or some sort of software filter, and a way to calibrate it. But it could combine the calculations and plot easily enough. Perhaps it would work best with stars?

Originally Posted by Chuck Hawley

I think that teaching yourself celestial navigation is a wonderful way to understand the order of the universe, and it's the closest that many atheists (myself included) will come to believing there is a Higher Power.

To be able to take some dusty tables, a $20 Casio watch, and a $50 plastic sextant, and to be able to find your position anywhere in the world is a very satisfying pursuit, and not necessarily related to one's paranoia. It's a beautiful thing, especially a three-star evening sight.

Chuck

Thanks Chuck, that's a beautiful description. I like how as a skill it connects me to the universe, and also to all the great navigators and seamen from the past.

[iabmatos]

Originally Posted by Snowpetrel

It seems to me that an app could be made to adapt a phone's digital camera into a half decent emergency sextant. You would need the right filter over the lens in the sun area, or some sort of software filter, and a way to calibrate it. But it could combine the calculations and plot easily enough. Perhaps it would work best with stars?

For Android smartphones, have a look: SkyPortal, by Celestron

[Snowpetrel]

Originally Posted by hodgmo

“Ive often wonder if an adjustable polarising lens could be used for quickly fine tuning the shade level.”

The US Navy tried this, and (at least) some David White Mark II sextants used adjustable polarizers rather than colored glass for the index and horizon filters. Here’s a picture of one: https://photos.app.goo.gl/UwBGMEygVHchMb3A9

A US Navy comparative test of sextants (1958-1961) resulted in a report (“PRECISION CELESTIAL NAVIGATION EXPERIMENTS”) that includes some discussion of the use of polarizers for daylight star observations: http://www.dtic.mil/dtic/tr/fulltext/u2/259559.pdf

Thanks, very interesting stuff. And it may become relevant again one day, as in any future war navigation systems are very likely to be the first targets.

[Time Theory]

Thank you all for the valuable information. Based on your experience, I think it is safe to say that there is very little risk of eye damage if the Sextant is used properly and the filters are not compromised.

Thanks again for your help.

[Olddan1943]

Folks, here is a good source for new and used sextons.

NEW SEXTANTS

Enjoy, enjoy. SMILE

[a64pilot]

I’ll only offer up that at every Solar eclipse in my life, all the news agencies etc tell you you can view the eclipse safely with I believe a number 14 or higher welders shade.

However a 14 is incredibly dark shade and very unlikely you’ll find one in your shop, as a contract welder in the oil field as a kid I think I wore a #9.

So yes a welding shade is safe, if it’s a 14. You know I think a 14 may be the darkest there is?

https://www.usatoday.com/story/news/...ely/578679001/

[GrowleyMonster]

Originally Posted by a64pilot

I’ll only offer up that at every Solar eclipse in my life, all the news agencies etc tell you you can view the eclipse safely with I believe a number 14 or higher welders shade.

However a 14 is incredibly dark shade and very unlikely you’ll find one in your shop, as a contract welder in the oil field as a kid I think I wore a #9.

So yes a welding shade is safe, if it’s a 14. You know I think a 14 may be the darkest there is?

https://www.usatoday.com/story/news/...ely/578679001/

I saw a 12 once. I, too, usually use a 9 for arc welding, occasionally a 10. I have never seen a 14, and I can't imagine being able to see an eclipse through one.

[Znightflyer]

Originally Posted by Adelie

I’m not sure that arguing with an eye doc about their area of expertise is a winning move.




Night vision is a very complex subject and a single graph is not going to be sufficient basis to support any side in an argument. Especially since such a graph means you are arguing from first principles rather than empiracly.

Talk to A64pilot about this. He had a long discourse in some thread in the last year. If I recall correctly the USArmy went to white lights for instruments in helicopters after doing a lot of research.

I’ve got a paper on my laptop about this I’ll post later after work.

Aviation manufacturers stopped using red lightning for illumination in cockpits in the 70s. Just saying 🤔

[skenn_ie]

Not if you use the filters properly and replace them as required. Polarising filters make it easier to locate the sun in overcast conditions.

[SeanPatrick]

Originally Posted by Snowpetrel

It seems to me that an app could be made to adapt a phone's digital camera into a half decent emergency sextant.

Several apps have implemented this, but the reality is that a phone's sensors are just not accurate/sensitive enough (yet). You just can't get a reliable measurement within the tolerances required for celestial navigation.

[noelex 77]

Originally Posted by Greg Purinton

Aviation manufacturers stopped using red lightning for illumination in cockpits in the 70s. Just saying ��

They did so because modern planes require comprehensive information that needs to be processed quickly. This cannot be done with red illumination.

Older planes used simple instrumentation, airspeed, compass, turn and bank and AH. The information from instruments such as these is simple enough that red illumination could be used and the pilot would have the best dark adaption.

There seems to be a belief that the lighting was changed away from red because red light does not work to preserve dark adaption. This is wrong. It was a change in requirements, specifically the need to process complex information rapidly that resulted in the change.

Creeping into an anchorage trying to make out crab pots is not something pilots do.

Yachts are one of the very few environments where very high levels of dark adaptation are sometimes required for safe operation. If you do want to process other information, such as the depth instrument, red illumination is the best, indeed the only way information from instrumentation can be received while preserving high levels of dark adaptation. The WW2 aircraft designers knew this.

The fundemental physiology of the eye has not changed in the intervening years.

[GrowleyMonster]

Originally Posted by skenn_ie

Not if you use the filters properly and replace them as required. Polarising filters make it easier to locate the sun in overcast conditions.

I am not certain, but I think possibly the use of a polarizing filter to "see" the sun through overcast would result in an unreliable observation. The semidiameter might appear smaller. Maybe shooting both upper and lower limb, and averaging Hs and shot times, would overcome this, but I would rather wait for an unobscured view of the sun. That doesn't mean there is anything wrong with polarized filters, but for that particular use I am not sure it is a good thing. I could be wrong, though.



Reminds me, though... back in the day it was common aboard ship to send a Deck Cadet down from the bridge to ask the Capt to please get the "cloud filter" from the ship's safe so he could get his daily celestial tasks done. Sort of like other errands like fetching a bucket of steam or some "relative bearing grease". Anyway in that context a pair of rotatable polarizing glasses coaxially mounted together would indeed be a cloud filter!

[jzaraschi]

Can't vouch for the quality of a cheap sextants plastic filters. They are after all a cheap practice unit or as a back-up in a ditch bag. I would not recommend for long duration voagages for several reasons.

1. The plastic expands and contracts when in or out of the sun, making them most inaccurate. 2. They are easily damaged.
If you are serious about celestial navigation, buy a good one.

[sailorladd]

I used a sextant for seven years while cruising around the Atlantic and Pacific as well as the Arafura Sea, and I mainly used it in the day time, twice a day was my normal routine, at around 9 or 10 am and again at local noon to get a fix, I used to take three shots each of the sun on each of these occasions, and was using an ancient bronze Weems and Plath I believe it was, which had sun filters for all degrees of sun brightness.
just flip over a filter and look through it, and you would find one (or a combination) of two that was comfortable and safe to use, and basically you are bringing a reflected image of the sun down to the horizon...

Now some twenty years later I have had no trouble whatsoever with my eyes apart from needing the usual reading glasses associated with getting older (and saltier What I HAVE had, and would absolutely do differently if I had my time over, would be to USE sunscreen!! and do not expose my skin to as much sun as I did in the past. I am now dealing with a pretty bad case of skin cancer, but that is a discussion for another day...