Sextant - sun filter problem

skyflyer

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I have acquired a 1957 Carl Plath sextant with - I assume - all original parts; nothing seems to be replaced or missing.

However, when flicking the filters over I have a problem.

The (reflected' image on the right hand side is fully covered,(ie darkened) but on the left side (the direct image viewed through the non-silvered mirror) the filter does not cover the entire field of view, leaving a partially unfiltered view

When in the final stages of a sight, this is not a problem since at worst you are looking at the sea with sunlight reflecting from it.

However when starting a sight by pointing the entire instrument at the sun in order to 'bring the sun down', you can easily catch the full glare of the sun around the edge of the filter.

Am I doing something wrong? Is it to do with the sighting scope? There is only one with the instrument and it gives 3x magnification, which I assume is correct or was it designed for star sights? If so why the filters?

If there are any experts with a view I could post a photo in due course!

Cheers
 

barnaclephill

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The (reflected' image on the right hand side is fully covered,(ie darkened) but on the left side (the direct image viewed through the non-silvered mirror) the filter does not cover the entire field of view, leaving a partially unfiltered view

When in the final stages of a sight, this is not a problem since at worst you are looking at the sea with sunlight reflecting from it.

However when starting a sight by pointing the entire instrument at the sun in order to 'bring the sun down', you can easily catch the full glare of the sun around the edge of the filter.

Am I doing something wrong? Is it to do with the sighting scope? There is only one with the instrument and it gives 3x magnification, which I assume is correct or was it designed for star sights? If so why the filters?

Here's the instructional link first of all: http://www.davisnet.com/marine/products/mar_product_docs.asp?pnum=00011

I haven't used my plastic one for a few years and it's cloudy today, but here goes... Someone will be able to correct me.

First of all I think you are not using any filter on the left side, the side that has the horizon. This will eliminate/reduce glare from the sea (or artificial horizon). You are correct in using shades on the right hand side, where you have the sun.

IIRC I put a shade on the right hand side, and had the sextant pre-set at an approximate angle, being approx the angle above the horizon, thus keeping the horizon part on the horizon while I fished around for the celestial body (it might be safer for you to practice with the moon first). Then I would bring the celestial body to the horizon, upper or lower limb as appropriate.

I assume that the 3X scope is for stars/planets, since the Mk3 Davis sextant doesn't use a scope (maybe because it's also the budget model, but a scope hasn't been necessary for me for sun nor moon shots).

Good luck with the instructions. I was happy with the instructions and a 5-year almanac, getting results generally within 3-8 miles accuracy. If not, write back here on the forum & I'll have to think some more.
 

sarabande

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there are always two sets of shades; the index mirror shades, and the horizon mirror shades. The latter are at the front of the instrument, the former towards the top.


They are usually a different shape I = square, H = circular, so that you can tell the difference if you want to adjust the shading while using the telescope.
 
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stewart9901

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what sort of index error was the sextant showing? any problem with the scope would be very readily exposed by checking that. the first time i use any sextant i check all the correctable errors and adjust as required.
I have found on a few sextants that wear and tear on the shades lead to them moving to easily and exposing one edge of the mirror, could this be an issue?
I dont have any experience with this particular type, is the scope removable?
 

actionoptics

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plath sextant

My Plath Yachtsman has a filter in front of the half mirror which is in line with the eyepiece telescope so if I want to point directly at the sun to bring it down to the horizon, the clear half is protected by this filter. It can then of course be swung out of the way to view the horizon.
 

skyflyer

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First of all I think you are not using any filter on the left side, the side that has the horizon

No, I am!

The problem is that the field of view through the unsilvered part is greater than the size of the filter, so that there is a very small amount of unfiltered light at the top and bottom edge. The filters are suitably stiff so can be positioned anywhere and will hold; if I move it to to obstruct the top 'gap' then the bottom gap becomes bigger and vice versa!

There is no index error and all the mirrors have been set up perfectly.

I think the "bring the sun down" method - as recommended in many books including Tom Cunliffe's, is not appropriate for this sextant. As another poster has said, it is probably safer to look at the horizon and then 'find' the sun. (it's pretty obvious!). The 'bringing down' method works better for planets and stars I suspect.

sextant.jpg


I am planning to restore this properly in due course, but was getting the hang of Astro first! One solution (but inelegant) would be to tape or paint the top and bottom of the clear 'crescent' to reduce the size. As I am planning to make a new mirror when i restore it, it might not be so difficult.
 

BobPrell

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is it possible that the horizon mirror has been replaced sometime in the past with one that is smaller than original?


Try this -----
hold the sextantout to one side of your head, with the telescope pointed roughly towards the horizon. watch the horizon mirror from the side, rather than thru the telescope.

Move the index until you see light from the index mirror illuminating the H mirror. Then the sextant will be set at about the required angle.

Then look thru the telescope and in my experience, it will be set within a couple of degrees of the angle to measure.
 

skyflyer

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Try this -----
hold the sextantout to one side of your head, with the telescope pointed roughly towards the horizon. watch the horizon mirror from the side, rather than thru the telescope.

Move the index until you see light from the index mirror illuminating the H mirror. Then the sextant will be set at about the required angle.

Then look thru the telescope and in my experience, it will be set within a couple of degrees of the angle to measure.

Brilliant idea - works a treat - thanks a lot!
 
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