|
Author
|
Topic: Index Correction using the Sun?
|
Joseph Christian
|
posted April 06, 2005 08:49 PM
Recently read and appied Lanny Petitjean's procedure for correcting IC and side error. I knew I was ON @8 mins. Corrected side error easily enough and starting my lower adjustment to just touch the Sun's LL and measured 27.5 mins.ON. Adjusting up, thur the Sun,stopping just above the Sun's UL' I measured 32.5 mins.OFF and subtracting from 60, result was 27.5 . According to Lanny, to check my measurements, I divided the sum of both readings by 4 to compare with that days Sun Semi-diameter of 16. My anwser 13.75? I'm thinking I need to preset my index to Sun SD ON, and adjust out gap between reflected and direct view to correct problem? Any suggestions?Please.
|
|
David Burch
|
posted April 08, 2005 03:56 PM
To apply this procedure (solar IC method) you must first remove all side error. We have a discussion of that point here.
Since your estimated IC is large, once the side error is removed, then I would start with a first adjustment to align the mirrors about right. You can typically set this to about 1' or so accuracy this way. Set sextant to 0° 0' and then looking to the sun with both shades in place, turn the index adjustment until the two views of the sun overlap each other as best you can.
This process may alter the side adjustment—ie move one of the sun views to the left or right— so as needed tweak the side adjustment screw until the suns are vertically in line again.
Once you have that set up, then you can go back and repeat the solar IC procedure to find out what exactly you have... and maybe report back what you find.
Your estimated 8' On the scale is a large IC, which should be reduced in any event. The beauty of this method is that you are both measuring the IC of the instrument and simultaneously checking that you did it right by measuring the SD of the sun. If the SD of the sun that you get this way does not match the true value at the time, then there is some error in the sight readings... either in the lower to upper measurement or the upper to lower measurement.
As i interpret your numbers [in the terminology of the form we have in the article linked above]you have 27.5 on and 60-32.5 = 27.5 off, so i would conclude from these data that the IC = on-off/2 = 0.0'. BUT when you check the SD you get 27.5 + 32.5 = 60.0 / 4 = 15.0. If the value from the almanac at the time of the sights was 16.0', as you mentioned, then the sum of your two measurement are off by 4', which i think can mean the IC is 2.0'. In other words, it does not look like your IC is 8' On, but closer to 2' — but i do not think we can tell if this is on or off since we do not know which measurement was in error. I will have to think on this some more.
Does that help any? I agree, something is not right.
From: Starpath, Seattle, WA
|
|
|
