The most accurate way to measure a sextant's index correction involves looking directly at the sun—which is not at all a part of normal cel nav. We normally look only to the horizon below it for regular sights, in which case the stock filters on the sextant do the job. To do the special "solar index correction," on the other hand, we look right at the sun with the sextant set to 0º 0', so we need exactly the type of filter used for the eclipse viewing. This type of filter foil is normally rather expensive. The price is lower when thousands of square yards are produced at one time, and many government agencies subsidize the production of them.
We discuss the technique and how to make the filters in both our books Celestial Navigation: A Complete Home Study Course and How to Use Plastic Sextants: with Applications to Metal Sextants and a Review of Sextant Piloting. It is the same method used by Lewis and Clark and other early land based explorers. The special filter we need for the sextant scope can be made from these glasses, more or less as explained in those books.
Work forms for the solar index correction with some brief level of instruction can be dowloaded as part of complimentary work forms package we have at the cel nav book support page.
9 comments:
David,
Can we not only use these glasses and no filters applied on the sextant
for determing the index error?
Or is this to simple thinking?
Regard,
HermanD
Herman, yes. that would be right. This applies to what we call solar method. The index is set to 0º 0' and that filter is over the telescope, with no leaks at all, and carefully taped in place so it cannot move. and then you follow the procedures. the other filters would all be out. we also have instructions in the plastic sextant book for making a paper sleeve to hold the filter. we are just using this inexpensive filter to replace the very expensive Bader solar filter we used.
David,
I ment more simple, putting the solarglasses on you nose
And using the sextant for determing index error without filters.
Or can direct sunlight in the telescope, damage the telescope.
Well in case off the most simple sextant the Davis MK3.
That has only a tube and flimsy filters.
Regards
HermanD
Interesting idea. We are in rain at the moment, but i will give this a try. (PS... not recommending this at all till we have a try here.) sun light will not hurt the sextant in any way. also i might note that you can buy the mk 15 telescope at davis for ~$20, then saw off the telescope mount on the mk3 with a dremmel tool or fine hack saw, then wrap the scope in industrial velcro fuzzy side and put a piece of the other side on the mk 3 frame and then have a mark 3 with a mark 15 scope, which makes for a really neat instrument. i think this is in our plastic sextant book.
David,
is the Sun already visible in your environment?
I ordered a Davis telescope, expect to receive it next week.
I carry glasses, to put the Eclipse glasses in front of my glasses
I had to cut a corner out of the eyelashes.
Now it fits perfect.
So I am ready, if you give approval.
regards,
HermanD
Sorry i did not try yet, but have all out on my desk. sun is just coming up here so might get a chance today. so far it seems a bit awkward.... but will report back. also i notice i did not use *industrial grade* velcro. not sure it if matters, but i had just used a couple strips of the household variety. we want the center of scope about center of mirror. also i noticed that i had drawn a line on the sextant to remind me of the alignment. Bback this afternoon if the sun does come out of the clouds. also it seems that the eclipse glasses i have, has a film large enough to make a filter cap for the small scope. it seems that might still be best procedure. we show step by step how to do that in the plastic sextant book.
OK. Finally got some sun! The answer is this does not work at all with the glasses on, and then looking into the sextant. If you get lucky you can point it the right way and see the suns, but hit and miss at best. It would be much better just to use the filters in the glasses to make an end cap filter. Then you can see what you are doing. With the total black glasses on you cannot see anything. I even tried taking the scope off of the sextant and could not find the sun. The answer is just carefully pull the glasses apart to get the filter and make an end cap version.
David,
Your are right, with the Eclipse glasses on your are blind.
Difficult to grab the sextant, and finding the correct grip
for using it.
I have a small peace of solarfilm from Germany this is used
one telecopes, they advise put the foil in front of the objective
NEVER on the oculair, because the sun can damage the coatings on
the internal lenses.
So it is clear now, solarfilm in front of the telescope, not in front
of your eyes.
Well it was one idea, but not usable.
HermanD
Yes for sure, the filter goes on the front of the telescope; the side farthest from the eye. With that confirmed, I might note that the $20 telescope we are putting on the Davis mark 3 probably does not have any expensive coating on it as would a higher quality model. (...but i should add that I do not know much about scope optics. It was my impression that the quality and process of coating the lenses is a key distinction of the good instruments.) And again, i refer to our plastic sextant book for instructions on how to make a nice fitting tube using cardboard and other tricks for assembly. I also recalled when checking this that you have to beef up the velcro thickness enough that the index arm slides under the scope. The ones we make have has two layers of velcro on the tube and one on the sextant. we also have forms on line (starpath.com/celnavbook) that can be used for the solar index correction that you can do with the filter.... one of our associates Lanny Petijean designed the form, which i think is based on a design from Lewis and Clark.
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