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Author
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Topic: Mark 15 - sun too bright even with all index shades
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adiemunsch
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posted August 18, 2024 11:09 AM
Hello fellow students of Cel nav.
I wanted to get some thoughts around what brightness to expect doing sun sights with a Mark 15.
Even with all index shades down, I find the brightness to be too much and I am worried about eye injury.
This is a second-hand sextant, genuine Davis instrument with no appearant modifications.
I have the same issues as has been documented on this page (screenshot of the video attached)
https://www.cruisersforum.com/forums/f121/davis-mark-15-sextant-is-this-normal-218577-2.html
The user who posted this had the same experience on his first and replacement Mark 15 after returning it because of the same issue.
The brightness is acceptable on the left side with the secondary reflection on the horizon glass.
At this point I am thinking of rigging a Baadar filter on one of the index shades to at least get some practice out of it.
Curious to hear abour your experiences about the Mark 15 and the brightness of sun sights,
Thanks!
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David Burch
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posted August 18, 2024 06:22 PM
This is an interesting point, and you reference someone else with similar observations. The next time we have a bright sun here we can take another look.
However this has not been a limitation I have experienced. We have rather extensive sun sight data with a Mark 15 documented in the book How to Use a Plastic Sextant. There are sights underway in the tropics and a long set of sights in Hawaii, both with very bright suns. The book even notes "The sun was bright and very hot, which was a factor underway and it was even very much hotter during the sights on land that followed." As i recall the issue of the heat was the main factor.
I cannot help but wonder if there were variations in the quality of the filters from one batch of units to the next.
In any event, you are seeing the effect, and the book does state "On a bright clear day with a high sun, you could easily use all shades, and on some instruments maybe even want more." This reflects the fact that i have indeed seen sextants, metal in this case, that did not have adequate filters.
This book also has detailed instructions for building a Baadar sun filter, but that discussion is for making one that goes over the telescope so we can do accurate index correction measurements using the solar method, also covered in the book.
But that won't help with the sights. You would have to try what you proposed of replacing or covering one of the stock index filters with a piece of the film. I will give that a try here the first sunny day we have. It will end up with a black side with the sun only, which will have an effect on how we do the sights, but should be doable.
It would also be good if we could figure some way to calibrate the shades you have to compare with what we have here on some old mark 15s and some new ones.
Lets start by asking for the colors you have in the index and horizon shades, numbering them 1, 2 , 3, with 1 being closest to the eye when in sight position.
If we are going to add another filter layer over one, it has to be high quality or the sights will be disturbed. This should not be an issue with a Baadar foil over lay.
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As for your main point, if the existing shades are all in place and you are not seeing around any edge of them, then chances of eye injury are small... but with that said, your picture looks much brighter than i might guess, and indeed if you have the impression of over brightness, then you are rightfully concerned.
Let's see what we can tell when we learn more.
From: Starpath, Seattle, WA
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David Burch
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posted August 18, 2024 06:43 PM
I just re-read the forum link you sent and looked at your image, which leads me to ask if you might have been looking toward the sun in that picture?
As opposed to looking toward the horizon and viewing the reflected view of the sun?
That difference can be night and day! The stock filters are not adequate for looking direct at the sun... We only look that way during the solar index method and that requires the Baadar filter PLUS the extra cardboard shade we describe in the book that prevents you from seeing around the edges.
Looking toward the sun it is *very easy* to have light get around the edge of a filter.
The video in that forum post is showing a sun that is getting around the edge of the shades. You can see it happen as the orientation changes just a bit.
It could be they were not in place, or something was misaligned or the scope removed?
https://www.cruisersforum.com/forums/f121/davis-mark-15-sextant-is-this-normal-218 577-2.html
From: Starpath, Seattle, WA
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David Burch
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posted August 18, 2024 06:50 PM
Just noticed that you are signed up for the cel nav course, so i am going to move this discussion to the Student Discussion forum and we can carry on there.
From: Starpath, Seattle, WA
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Topic Closed
