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Author
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Topic: bubble moon altitude corrections
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tkuhns
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posted August 21, 2024 05:14 PM
as like the sun;sd,dip,aug.are ignored.the n.alm.takes mean U&l corr.& subtracts 15'. my question is how to use the main corr.if at all which includes roughly 16'of sd & ref. & 50'of parallax.in n.alm.sight reductions. cel.nav.publications comment on the bubble but don't give examples of a sight reduction. i would like to see an example if possible. is planet ref. used ? it seems the u&l corr. is a small part of total parallax? thank you t.kuhns
From: tkuhns
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tkuhns
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posted August 29, 2024 10:59 AM
I am following up on my aug 21 inquiry on moon bubble artificial horizon altitude corrections. primarily how the main correction is used since it contains the s.d. which is ignored in artifical horizon reductions. I would be interested any publication that has moon altitude corrections for artificial horizons. thank you t. kuhns
From: tkuhns
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David Burch
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posted August 29, 2024 11:18 AM
I apologize for the delay. Going forward if we are ever more than 48h in responding you are welcome to call here 206-783-1414 or email [email protected] to let us know we missed a post.
Let me go over this and get right back to you.
From: Starpath, Seattle, WA
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David Burch
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posted August 29, 2024 11:21 AM
In the meantime, may I ask if you have detected any consistent offsets in moon sights with your bubble attachment.
From: Starpath, Seattle, WA
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David Burch
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posted August 29, 2024 12:13 PM
You are measuring the height of the center of the moon with that device. So SD does not enter in.
I would propose this (after IC correction):
You get an Hs at a time T
Look up refraction of a star at that height = ref
then Hs - ref = Ha
then look up the moon HP at time T
Then get the alt correction for for the moon at the Ha observed, and for the HP part average the L and U entries and add it to the main alt corr.
apply that to Ha to get Ho.
You can then check yourself with the values at www.starpath.com/usno
From: Starpath, Seattle, WA
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tkuhns
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posted September 01, 2024 05:08 PM
thank you for your reply. I will ans. your question of moon offsets first and alt. correction steps questions later. with the cassens and plath bubble horizon there are no offsets after it has been calibrated.to do this you need an accurate ht. of eye to get dip and water/ sky horizon like in regular sights. if ht. is 15ft; dip is -3.6'. then with index error at 0' subtract the dip back from 0 ; -3.6 would be 56.4' on the drum. this will raise the reflected horizon by the amount of the dip. then when the cross hairs are in the center of the bubble they should be right on the reflected horizon. if not there is a small screw on the front of the bubble. then just turn it cw or ccw to center the bubble on the horizon.then the bubble is calibrated and will accurately reflect your Hs reading.
From: tkuhns
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David Burch
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posted September 02, 2024 09:39 AM
Thanks for those details.
Yes, it is true that all such bubble sights need to be normalized, by adjusting it to get the right answer once, and then assuming it will be the same for subsequent sights, which I would guess works to within the expected accuracy of the instrument, or they could not be sold for $1,000 or more.
But I would guess that if you take the device off of the sextant and then put it back on, that you would have to repeat the calibration (normalization) process.
Is that what you find?
Also, what accuracy to you get from the device on subsequent sights once you have normalized it?
I mean for a star or planet? Leaving out any special concerns about moon sights.
________________
In passing, if there are no offsets, ie no errors, then it seems you must be doing the moon sight reductions properly? Or maybe I misunderstood your initiation question.
From: Starpath, Seattle, WA
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tkuhns
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posted September 03, 2024 09:43 AM
yes you can move the c&p bubble to a diff sextant without losing calib. I mark the screw with a black mark after calib noting the hour position ( 1-2-3 etc). I check the position when changing sextants. I do reconfirm it periodically if I am near water/ hor.this works well with the c&p horizon ultra and celestaire Astra 111 prof.the bubble needs a cord modification to use with celestaire. bubble Hs is not as accurate as hor.Hs. in hor. just let the body hit the hor. and hit time. the bubble requires multiple adjustments at the same time. first, get cross hairs in center of bubble,then the body in the center of the crosshairs. then hit time while a moment of no movement. pl.and stars are most accurate,sun more difficult. because of center motion. hor sights are good for<1mile bubble <5 miles. worse with moon. later thanks
From: tkuhns
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tkuhns
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posted September 04, 2024 10:29 AM
thank you for the proposal for bubble moon. I think ha= hs+/- i.e. - dip with ref in the main corr. using a bubble, ha often= hs when both are 0'. the confusion I have is when we add the moon n.alm main correction. which contains ref.& p.in a. that's o.k. but also s.d.& Aug. which should be ignored thereby lowering the main correction. to p.a.similar to the moon p.a. in the air almanac. is there any publication with bubble breakdowns like p-281 in the n. almost. thanks. t.kuhns
From: tkuhns
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David Burch
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posted September 04, 2024 12:48 PM
I provided a proposal for the sight reduction method for moon in a bubble measurement.
Have your tried that?
Also if you are getting bubble sights of the moon that are correct, are you using that procedure? If not you can try that.
If your moon sights are all accurate to ± 5 nmi (as I understand you to say) then there cannot be an error of the SD as that is ~15'
Also, may I suggest that you pursue any specialized questions about bubble sextants to the NavList group. You can reach that site and post there starting in fer3.com They have quite a few experts on bubble sights and can likely provide you with the definitive reference you are looking for.
From: Starpath, Seattle, WA
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tkuhns
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posted September 05, 2024 09:48 AM
I am using the procedure. I was getting accuracy of <5nm on the bubble with stars,planets and sun but not the moon which were > than 15'. now the error is much smaller. thank you.
From: tkuhns
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