Can the use of a Sextant cause eye damage?

[Time Theory]

I was thinking of trying to learn Celestial Navigation and purchase a Davis MK 3 Sextant for practice. Do you know if using a Sextant properly over an extended period of time can cause permanent eye damage?

Do you think the Davis MK 3 provides adequate filters to protect the eyes?

Thank you for your help.

[Adelie]

I believe the filters are safe. I have the same model so putting my money where my mouth is.

Where are you? It would be easier in the long term to add something more specific in you profile.

[Adelie]

I believe that Ebsco (?) sextants had problems with degrading filters. But in that case is apparent because the sun became bright enough to hurt your eye before damage was done.

[barnakiel]

You only look at the sun for short time only. If you use any filters to limit the amt of light to a normal / comfortable level, there will be no damage.


If you have any condition, discuss this up first with your doc. If your eyes are fine even brief squinting into the sun at noon does no damage, just blinds you for a brief moment. Nature designed us this way because we are daytime animals.


Cheers,
b.

[seandepagnier]

Any suggestions for replacing sun filters? Mine went opaque.

[Olddan1943]

Originally Posted by Time Theory

I was thinking of trying to learn Celestial Navigation and purchase a Davis MK 3 Sextant for practice. Do you know if using a Sextant properly over an extended period of time can cause permanent eye damage?

Do you think the Davis MK 3 provides adequate filters to protect the eyes?

Thank you for your help.

If you fail to use the filters, you can cause damage to your eye.

[nwdiver]

Do night star and planet shots only.......

[hellosailor]

Alexandra-
Replace *what* filters? If you knew the diameters and densities, you might be able to use common photographic filters. Some sextants only have dark (green, etc.) filters, others also include a variable polarizing filter that can replace a whole set. Sizes will differ.
Someplace like Celestaire or Robt. White & Co. will possibly know what you need.

To the OP: Unless you're buying cosmetic junk intended for wall plaques, "proper use" is to swing ALL of the sun filters in place before attempting a sight, and then to remove them one at a time until you can see the sun. But if you're familiar with what you need, you can start with your own 'usual'.

You might be better off looking for a class at a local planetarium or seaport museum, if you have concerns about these things.

[noelex 77]

Eye damage is almost always caused by the non visible portions of the spectrum.

With any filter the concern is that the visible portion of the spectrum can be attenuated more than the UV and IR portions. In this case looking at the sun can be comfortable, but the unseen radiation can cause significant damage.

This is easily prevented with well designed filters, but it is impossible to tell by looking at, or through the filters if they provide adequate protection.

I think cases of damage while using a sextant are rare, but you are dependent on the manufacturer producing a suitable filter. When purchasing a sextant second hand there is the concern that the filters have been replaced perhaps with a DIY alternative.

[Adelie]

Originally Posted by boat_alexandra

Any suggestions for replacing sun filters? Mine went opaque.

Welding shades from a local welding supplier.

[noelex 77]

Welding produces high levels of UV radiation, particularly UVC. Sunlight has a very different spectral distribution. There is almost no UVC (at sea level) for example.

Welding filters will almost always provide adequate UV protection, but the IR is less of concern when welding. Traditionally welding filters had an IR cut off around 680-700nm which is probably fine for viewing the sun, but more modern welding filters have extended the IR cut off (because this does not cause much damage welding) and extending the spectrum helps visibility. Modern welding filters have an IR cut off around 760nm which I doubt provides adequate retinal protection for viewing the sun.

The bottom line is using a filter designed for a specialised purpose such as welding and assuming it will be fine for viewing the sun because it is dark enough is not necessarily correct.

[DennisDW]

Hi Time Theory,
I'm an eye doctor. Everyone's right. With proper filters you'll be fine. You will not have any problems or permanent damage otherwise. The one comment I would wish to modify is that all the visible spectra of solar radiation can be extremely damaging if you stare at the sun. The cornea and crystalline lens filter out most UV and IR however in doing so they are "sacrificial tissues". In the case of welder's flash, the corneal epithelium gets burned but recovers quickly and at the other end of the spectrum we see glass blower's cataracts from IR.

As an aside, Red lights for night vision preservation is a long standing myth, not supported in retinal anatomy. It may shield the rods in your peripheral vision somewhat but we don't use that for fine acuity. Low level white light is OK and patching one eye keeps it fully dark adapted while your in that light. Then take it off back in the dark and you'll have no loss. In 15 or 20 min the other eye will be back to full adaptation too.
Cheers all,
Dennis

[noelex 77]

Originally Posted by DennisDW

As an aside, Red lights for night vision preservation is a long standing myth, not supported in retinal anatomy.

I think we would be better to agree to disagree on that point Dennis .

[denverd0n]

You can disagree with well-established scientific fact if you want, but I prefer to accept what is demonstrably true.


It is a fact that a white light, of the same luminosity as a red light, will affect your night vision to exactly the same degree. The main reason the myth of red lights exist is because red "night lights" are almost always of a low luminosity. People then compare these to white lights of a much higher luminosity, and come to the conclusion that the red light is better for night vision. No, it is the lower luminosity light that is better than the higher luminosity light. The color has nothing to do with it.

[GrowleyMonster]

Proper use of a sextant (with no alterations such as discussed above) by a reputable maker is not going to damage a healthy eye. The precautions are obvious. Set up a Sun shot with all filters inline first, then one by one move them out of the optical path until you can clearly see both Sun and horizon. Still paranoid? Shoot only at twilight and forget about Sun lines altogether. But that would be a shame, because a noon running fix is really easy and dependable.


Davis sells thousands of those plastic sextants a year. If the filters were inadequate, someone would probably have sued them into bankruptcy by now. While I have never actually used one of them, I am pretty sure they are perfectly safe to use.