A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Douglas Denny
Date: 2010 Mar 5, 15:53 -0800
You wrote (on the subject of irradiation):
"I am sorry to place such a noxious fuming piece of material evidence under your noses that perhaps you had rather was not there, giving rise to a screwing up of the nose and distorted features as if sucking a lemon, but it does deserve more comment methinks other than the expected poo-poo-ing and harumphing of George Huxtable .......
I can't find the poo-pooing and may have missed it.
What is clear is that the results of Haines and Allen simply do not fit with what is found in practical navigation. We never find errors due to irradiation of up to 15 arcminutes when bringing the sun's lower limb down to the horizon. The paper by Randle and Lampkin (post number 12094)does give results that more closely match reality, of up to 50 seconds error. It also appears to show that by using filters in such a way as to reduce contrast ratio, the irradiation error can also be reduced. This is in accord with one of Haines and Allen's findings.
You also wrote:
"As I said - mention subjective, (but real), phenomena of human physiology to physicists and astronomers, (with the scientific evidence to go with it in this case).. and their eyes glaze over ....."
I'm not sure that this isn't poisoning the well a little. As someone with more training in the biological than physical sciences, my eyes often glaze over once discussion moves from the realms of elementary algebra and trigonometry to the higher realms of mathematics, so I can well understand others finding physiology a bit challenging, with all its biological variability to be accounted for. It has been said that Nature doesn't cheat, but sometimes it can seem that way.
Pukenui New Zealand
Bill thank you for your input.
George harumphed a bit..it was inevitable. See Post#12127. George thinks some of the paper is "crazy"
One or two further points:
The reason a full 15 minutes of arc is not necessarily perceived in observations generally I would suggest is because the observers are effectively self-adjusting the index error of their sextants and hence calibrating themselves. i.e. there is an element of 'observer calibration' as suggested in the paper as the only means of dealing with irradiation, - without their realising it.
The authors say:-
"...It is impossible to eliminate irradiation from the eye but its effect in angular measurements may be reduced by changes in the in the optics of the measurement system or by optimising the stimulus conditions. If the irradiation effect cannot be reduced below allowable tolerance limits, corrections must be made to the recorded measurements. to do this the observer must be calibrated...."
In other words if a sextant is calibrated with the sun brought down to a far horizon at sea level; or by touching limbs of the sun - and adjusting for zero index error the observer's sextant is then 'calibrated' with an inbuilt correction for the sun's irradiation component. If however, they calibrate the sextant with a star brought together to coincidence (or two stars), then the irradiation component will be different.
I should expect the minimum irradiation component with the two star calibration for index error of the sextant (preferably stars as faint as is practicable to make the adjustment), and this checked with a calculation of the real as opposed to the observed angular distance between the two stars. (A method endorsed by some people here I note).
My point is though that even with an 'observer calibration' (intentional or inadvertent) the paper shows irradiation is different for different tasks: star to star; star to Moon (lunar); Sun to horizon; Moon to horizon. To seek perfection the observer themselves would need calibrating for each task.
The authors say:-
"... corrections for irradiation will be particularly important when lunar and planetary diameters must be determined for possible use in mid-course guidance calculations..." (For Space navigation).
Which is of particular relevance to the lunar distance aficionados here.
I am not sure where you find this conclusion Bill about shades reducing 'contrast ratio' .... you say:-
"It also appears to show that by using filters in such a way as to reduce contrast ratio, the irradiation error can also be reduced".
I thought it said the opposite in the conclusions in the Abstract on the front page: vis: "....the detrimental effects of irradiation are _not_ reduced by using optical filters across the entire field of view to reduce the illuminance of the extended source..."
Which rings true as it is shown in the paper the irradiation is dependent on the contrast _ratio_ which will not change by filters being introduced.
All in all, I should have expected this paper to be a veritable 'bombshell' introduced into this forum as it indicates clearly that the inherently very high accuracy for angular measurement from a sextant (in terms of its engineering and fundamental optics) can be so degraded with this simple to understand physiological effect, yet it has caused hardly a ripple. It seems to be being ignored - which is Ok if you are convinced your observations are really to within a tenth of a minute of absolute, and can actually do it. I do not think I can do it even with the best sextant in the world.
It is a subject which certainly deserves much better understanding and discussion. I for one will look forward to others digging-out those other references from NASA on irradiation to see if the results of this Haines and Allen paper are justified and/or corroborated.
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