NavList:

Dear Frank,

This posting of yours about the telescope errors I noticed is easiest to reply to for the moment, before I examine all the other points you made about the Lunars and Graphs, which I will have to address later when I have time.
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I am not sure, and may be doing you a disservice, but you seem to be getting mixed up between prismatic effects of a lens system which displace an apparent axis of light rays, which is what I am referring to; and collimating errors which displace the light rays by pointing the telescope central axis in the 'wrong' direction.

The sextant used was a Zeiss VEB Freiberger bought new in the mid 1980s. Maximum calibratoin error is only 21 seconds of arc. The telescope is unmarked for maker or magnification but I am fairly sure it is an 'off the shelf' 8 x 30 unit bought in by Zeiss -probably Russian. It has very good optics.

I am quite sure the telescope is collimated correctly with the sextant frame and index mirror. It is parallel with the frame; and the index mirror has been adjusted (first adjustment) for perpendicularity with the frame in the usual manner. I admit I have not checked with a laser light source for an exact collimation but collimation error is not the effect I am seeing. What I am seeing is what you yourself describe when you say:-

"......Now rock the sextant to the left so that objects are on the extreme left side of the field of view. They should separate slightly. Next shift them to the right side of the field of view. They should separate again slightly (by the same amount and NOT overlap). If you get this symmetrical separation on both sides of the field of view, then you probably do NOT (my emphasis - Douglas) have any issues with collimation......."

I am sure this is prismatic effect of the lens system.
(Note: nothing to do with the prisms inside the telescope)

Any lens system will divert the apparent light rays axis when one moves away from the optical centre. This is why it is a symetrical separation on either side of the telescope FOV - or up or down - anywhere in fact away from the optical centre of the lens system. In a properly set-up telescope the optical axis should, of course be exactly the same as the physical axis centre of the lenses too.

Prism - induced by looking through the 'wrong' part of your spectacles, or when the lenses are incorrectly made or cut and glazed, is a serious problem for spectacle wear causing 'eye-strain' at best or even diplopia (doubling) at worst. I had not noticed it before in a telescope system but using these fine tolerances of measurements between stars/Moon has made it apparent to me.

In any event - it is a clear and and present 'danger' and I noticed it was a good minute or two of arc from the edge of the field to the centre and so a serious error-inducing problem for a lunar distance sight.

I noticed it as the sextant drooped as my arm tired and I found the image falling over to the telescope edge of its field-of-view - tried the other way and found the same.

Only answer (no matter whether it is collimation or prismatic effect): make sure the sight is central in the FOV.

Douglas Denny.
Chichester. England.

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Original Posting:

Douglas you wrote:
"It is noticable that any off-centre light rays are refracted slightly by prismatic effect, being displaced slightly. The star aligned on the limb of the Moon is no longer on the limb if the image in the telescope is allowed to drift to the side of the field of view: move back to the centre again and it is back in alignment. The effect is small, but about one or two minute of arc which is serious with lunar distances."

This is probably a normal effect of the collimation of the sextant's telescope. But you MAY have collimation error if this is large, and from your desscription, it is. To test it: line up the limb of the Moon and some other body at the largest angle you can manage (use Jupiter or the Sun for best accuracy) so that they are exactly touching at the center of the field. Now rock the sextant to the left so that objects are on the extreme left side of the field of view. They should separate slightly. Next shift them to the right side of the field of view. They should separate again slightly (by the same amount and NOT overlap). If you get this symmetrical separation on both sides of the field of view, then you probably do not have any issues with collimation. But if the bodies separate on one side and overlap on the other, or separate by noticeably different amounts, then you have collimation error, and this will throw off your observations in a systematic way. Most of the old navigation manuals described this and emphasized the importance of checking telescope collimation. Luckily, collimation is easy to test and adjust using a variety of simple tabletop techniques. You can also directly observe collimation problems using these methods. Details upon request.

-FER
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