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SeanPatrick · 28-05-2019, 21:17

Quote:

Originally Posted by Stearmandriver

... maybe my eyes are just overly sensitive or something?

May just be. I don't recall ever having a problem with my Mk. 15. I haven't used it for a long time, but I'll dig it out tomorrow when the sun is up and see how it compares to my Astra.

Quote:

Originally Posted by Stearmandriver

I've got some eclipse-viewing glasses with foil lenses from the total eclipse a couple years ago. Think I'll cut the foil lens out and stick it on the lightest index mirror. THAT oughta do it!

That does seem like a good idea. I'd check to see if it causes any measurable error though. If it does - and the error is constant - you can correct for it just as you would for index error.

SeanPatrick · 28-05-2019, 21:32

Another idea would be to cut out a small circle and attach it to the telescope. (Perhaps inserted into the rubber cup on the eyepiece.) That would cause any error to be applied to both the horizon glass and reflected index mirror image - effectively cancelling it out. That is: assuming it is uniform across the film.

However, this would make taking sights using a natural horizon difficult, if not impossible. You'd be limited to using only an artificial horizon with that set up.

Stearmandriver · 28-05-2019, 22:14

Yeah, I thought instead of butchering those old glasses, I'd get a square of the Baader Astrosolar film, and shade one index shade and either one horizon shade also, or make a telescope cap of it to use for solar IC checks. I still feel like I must be imagining this... but it's cheap insurance.

SeanPatrick · 28-05-2019, 22:22

Better to be safe than sorry!

GrowleyMonster · 29-05-2019, 04:51

Quote:

Originally Posted by Stearmandriver

Yeah, I thought instead of butchering those old glasses, I'd get a square of the Baader Astrosolar film, and shade one index shade and either one horizon shade also, or make a telescope cap of it to use for solar IC checks. I still feel like I must be imagining this... but it's cheap insurance.

Can't hurt a thing! As long as you can see the sun properly you are good to go. I might add one thing though. Material not specifically designed for solar viewing COULD possibly give you the sensation of proper attenuation of the sun's radiation, but still allow excessive UV into your eye. Just shading against visible light is only half the battle. Some materials might not protect well against the full spectrum of solar radiation.

Of course a lot depends on how often you do sun observations. You can get away with a lot of abuse to your eyes or any other body part when it is isolated instances as opposed to constant repetition.

Back in the day, ship's officers would always do a sun shot during every day watch when a sight was possible. (In addition to a round of stars at morning and evening twilight.) After a career at sea, most still had normal vision though there were exceptions. But deficiencies in the ship's sextants (There were always two, and some guys would bring their own, as well.) would not go uncorrected for long. Now that we have come to rely so heavily on GPS and other satellite navigation systems, celestial navigation has almost become a spectator sport, with the observer the center of attention on the bridge wing, surrounded by looky-loos all offering comment and advice.

You will take a more accurate sight more quickly when you don't have to squint. You will have a more accurate sight when you see a well defined edge to the solar disc without the extra fuzziness from corona. So within reason, the darker, the better.

If you look for your sunglasses before going outdoors, and wouldn't dream of driving without them on, maybe you do have extra sensitivity to sunlight. But the variation really isn't all that much. Typical variation in welding shades for a given method and current range is only one shade number from the norm.

SeanPatrick · 29-05-2019, 14:04

I just took my Davis Mk.15 and Astra IIIb out for a "side by side" comparison.

With the Astra, the sun was completely invisible through all four index shades. Moving the darkest one gave a nice, crisp, clear image of a dull yellowish-green orb. Various combinations of shades produced different brightness levels (some uncomfortable), but the same color. Except when using only the first and last shade, which produced a fairly dim white orb. The view through the three horizon shades was very similar to the view through the first three index shades.

With the Davis, the sun appeared as a very dim reddish orb through all four index shades. Definitely not anywhere near too bright. Moving the grey filter increased the brightness only slightly. Moving one of the blue filters produced a noticeably brighter, yellow orb - but I wouldn't say it was uncomfortable to view. Replacing the blue filter and moving the orange one caused the image of the sun to be a much brighter blue - almost uncomfortably so. The view through the horizon shades was very similar to the Astra.

Stearmandriver, just to be sure: does your Davis have one grey index shade, one orange and two blue? Perhaps the wrong combination was installed, IDK.

Keep in mind that I wear sunglasses while driving - even on mostly overcast days. So, it sounds to me like you got one heck of a dud Mk.15.

Stearmandriver · 29-05-2019, 18:44

SeanPatrick, thanks much for the comparison.

I know the Mark 15 I've got has 4 index shades, but I don't remember the colors and I'm away for work tonight. I'll check tomorrow before I box it back up and send it in for exchange.

My 3 horizon shades produce an image similar to what you describe, but that's based on the view in my artificial horizon (bowl of water). I didn't look at the actual sun through the horizon shades.

The view of the sun through the 4 index shades does produce a deep red colored sun, but it IS bright... had me on the edge of squinting and like I said, I had an afterimage of the sun in my right eye for a few minutes after taking 4 sun shots. I also tend to wear sunglasses, but I spend a lot of time outside and work as an airline pilot, so it's not like my eyes aren't used to seeing a bright sky.

So... I don't know. I forget if I mentioned it, but there were a couple other odd things about this sextant, like a bit of wear on the gear grooves on the bottom of the arc, and an obvious thumbprint on the horizon mirror... makes me wonder if I got a refurb that wasn't quite refurbed enough

.

Well, I'll report back. Thanks again.

Stearmandriver · 30-05-2019, 19:06

All right, checked my Mark 15 (and then sent it back for exchange). 4 index shades, 3 horizon shades. The index shades are: one grey, one orange, two blue. They seem to be in good condition. I shot a video clip with my phone through the sextant before boxing it up:

https://www.youtube.com/watch?v=sYscZ_K12sA

This is with the sun brought down to the horizon already, so you're not seeing a direct view of the sun through the horizon glass. You'll notice that as I rock the sextant back and forth, the sun image shifts from the right side of the horizon mirror to the left... on the left side, I guess we're just seeing a secondary reflection of the sun on the unsilvered horizon glass. But you can see that the sun is bright enough on the right side of the horizon mirror to blow out the camera, and dimmer on the left. The sun on the horizon mirror on my Mark 3 is MUCH dimmer than this.

We'll see what the new one does...

SeanPatrick · 30-05-2019, 21:11

Wow! Yes, that seems much brighter than what I saw through mine. I don't know what's going on there, but something is definitely wrong.

I thought about taking some photos/video through mine, but didn't. I'll try to post one or the other soon* so you can see what you should expect from a "normal" sextant.

*It's raining cats and dogs here right now. There's a severe thunderstorm warning, a hazardous weather outlook and a special weather statement. That ... and it's about midnight - so, no sun right now anyway.

Hopefully the weather will be better tomorrow.

SeanPatrick · 01-06-2019, 18:02

Well, I tried to take a pic through my Mk.15 scope, but wasn't able to hold my phone, the sextant and get it all lined up on the sun.

How you or anyone is able to do it is beyond me. Maybe I'm just a klutz. Sorry.

cygnussailor · 02-06-2019, 04:34

Quote:

Originally Posted by Stearmandriver

As expected, prompt response from Celestaire with a shipping label for the exchange, on the first business day since my email.

Growley, interesting idea about swapping shades. I have trouble believing that a device sold with the actual intent of using it to stare at the sun doesn't have enough shades, but I think you're right. At first I thought I was probably imagining my eye irritation out of paranoia, but no... it's definitely there. Unless I randomly get a darker combination of index shades on the replacement, I'll not be looking at the sun again until I fix this. Cannot believe, just from a legal standpoint, that they'd sell a sextant with this issue... maybe my eyes are just overly sensitive or something? They never have been... this is just bizarre!

Anyway, good idea on moving shades around, but I think I've got an even safer idea: I've got some eclipse-viewing glasses with foil lenses from the total eclipse a couple years ago. Think I'll cut the foil lens out and stick it on the lightest index mirror. THAT oughta do it!

ANY properly designed sextant should have enough shades such that if all of them are in place at the same time you would not be able to see the sun or the horizon. If your sextant shows an image of the sun that is too bright to be comfortable to view even with all the shades in place, then there is a major problem!!! Are all of your shades different density AND color?

Stearmandriver · 02-06-2019, 20:36

Quote:

Originally Posted by cygnussailor

ANY properly designed sextant should have enough shades such that if all of them are in place at the same time you would not be able to see the sun or the horizon. If your sextant shows an image of the sun that is too bright to be comfortable to view even with all the shades in place, then there is a major problem!!! Are all of your shades different density AND color?

Thanks for the confirmation, that's kind of what I thought. They are different colors, not sure about density.

I wish I'd tried to look through them when they were all folded out away from the sextant and seen how bright the sun was, before I sent it back. I'm wondering if the mirrors were somehow so out of alignment that the reflected image from index to horizon mirror was somehow making it around the index shades, but that hardly seems possible, especially since I had adjusted the mirrors to remove perpendicularity error, side error, and almost all index error. Ah well, I'll update you when I get the replacement.

secrabtree · 04-06-2019, 17:43

A coiple of hints about plastic sextants.
1 - Learn to do the math to correct index error, etc. rather than adjusting the sextant each time. The plastic will wear.
2 - One can get better accuracy with the metal sextants IF he is on a stable platform high above the water (distance to horizon is greater and the variability of wave height has less significance. I've found that the errors introduced by being on a small boat (mine was a Cal 39) makes the accuracy that my father got on a merchant ship unattainable by me on my Cal.