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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Plastic sextant use

Author Topic: Plastic sextant use

 - posted April 09, 2020 04:11 PM      Profile for Lance           Edit/Delete Post 
I have a Davis Mk 25. Perhaps not the best choice for star shots - the Davis Mk 15 might be better with the traditional half silvered mirror, but I'm having fun. The index error seems to be a bit unpredictable, though. On a cold night, it seems to increase with time, together with increasing side error. Is this a common observation?
From: Hobart, Australia
David Burch

 - posted April 09, 2020 06:40 PM      Profile for David Burch           Edit/Delete Post 
No, it is not. How much did the temperature change from what temp to what temp, and how much did the index error change? and did you see this during one sight session or multiple times, and if so please list all the data you have on this.
From: Starpath, Seattle, WA

 - posted April 09, 2020 08:52 PM      Profile for Lance           Edit/Delete Post 
You've caught me out here. I wasn't systematic enough to record that.

I did 7 readings to try to assess the index error as soon as I went outside. It seemed to increase from about 2.5 to 3.5 'on' during the period of the seven index error observations, but I didn't record anything at the end of the session. Next time we get a nice calm, cold night, hopefully with no moon, I will write down the results at the start and finish.

Inside, the temperature was about 22C, while outside I think it was about 12C, certainly cold enough to fog my improvised artificial horizon mirror. My initial side error was small enough to make the star I was using merge into a sort of figure 8 at the start, but after 45 minutes in the cold the star and its reflected image were well separated, perhaps on the order of a couple of minutes, a very noticeable increase. That made me wonder if index error had changed.

BUT - another possibility is that despite my best efforts, my artificial horizon mirror was actually tilted into the west, giving me low readings to the east and southeast. From what you've said, that's starting to sound like a more reasonable explanation than thermal contraction of the sextant components, even if the cold air explains the change of side error.

I found my first set of sightings (Sirius) gave me 0.2 nm for 'a' (beginner's luck), but the other two sets of sightings gave LOPs much further to the west. The first of those two sets was the moon, lower limb, rising from about 17° to 18° over about 5 minutes during the sighting, a~7.2 nm. Then I looked at Rigil Kentaurus, rising from about 43° to about 43°45' over about 4 minutes, a~5.3 nm.

Each of the three sets of three heights fitted the calculated slope well.

From: Hobart, Australia
David Burch

 - posted April 10, 2020 08:12 AM      Profile for David Burch           Edit/Delete Post 
I recommend our book How to Use Plastic Sextants. It offers systematic ways to take the sights as well as recommending the instrument is in thermal equilibrium before starting.
From: Starpath, Seattle, WA

 - posted April 10, 2020 01:50 PM      Profile for Lance           Edit/Delete Post 
On order, hoping it will arrive soon! Thank you for your reply.
From: Hobart, Australia

 - posted April 11, 2020 08:51 PM      Profile for bruce           Edit/Delete Post 
"Davis Mk 15 might be better with the traditional half silvered mirror"

If you prefer the half-silvered mirror, replacement mirrors are available


Fits the Mk-15 and Mk-25


From: Everett, WA

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