SUBSIM Radio Room Forums - View Single Post - [TEC] Telemeter scale in the german optics

Pisces · 06-15-08, 05:34 PM

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Originally Posted by Hitman

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How certain are you about that 6.25 degrees? Or I should say, I am curious how you derived at that.

I got it from a german WW2 Periscope Manual

The scale there was 10/16 of a degree per mark, and the "100" mark was 6.25 degrees and the example showed a ship at 250 metres. So I investigated a bit why that figure was chosen and soon understood that because tan 6.25 = 0,10 it made sense to get range figures. Actually, 5.75 degrees would be evn more accurat but apparently the rest of the scale wouldn't then be evenly distributed.

Be carefull there, tan 6.25 is 0.1095, so more like 0.11, not 0.1. Maybe the majority of german officers would not care of the differenc, but a german instrument-maker would not do this! There must be more than meets the eye about this 6.25 degrees value. Some clever reason. As far as I know, (which isn't much more than common sense) for practical use of that scale it doesn't really matter how many degrees it actually is, as long as it matches an easy to use mastheight/distance ratio. I mean, my second post above describes how I would have made it. But I'm not german!

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When looking at your sketch-a-scope

the thought popped up in my mind that the 10 might actually mean 1/10 radians, which is equal to 5.729577951 degrees (0.1*180/pi; and not too far off from your number, but in the ball-park if the SH3 scale is indeed off) If you are not familiar with a 'radian' (though I'm sure you know), it is the angle you get when the radius of a circle is wrapped around that circle. The tangent of one 10th radian is 0.100334672. So if according to your picture the distance E1=Mastheight*10, it could be rewritten the same as as Distance/Mastheight= 1/tangent(radian/10)= 1/ 0.100334672= 9.966644423.

If a contact measures up to be halve-way the 10-mark (and waterline) it is actually 1/20 of a radian, and again halve-way from that 1/40 of a radian. And with those numbers it rounds nicely to 20 and 40 also. The appearant linearity of the telemeter scale puzzles me though. It's not convenient for those 20, 40 numbers.

Actually, the sketch was done in an attempt to make it useful for SH3, therefore I kept the current subdivisions and wanted simply to modify the field of view of the periscope to ensure that the "10" mark matches 6.25 degrees and not 10 degrees.

But the historic periscope reticle looked like this one here (Please see if the subdivisions used make sense for you, your expertise in maths can probably provide a better solution

):

Picture of a real scope (Note that sometimes there seem to be three marks in the same space where there are four on the opposite side of the central cross):

This real life scope image shows there is indeed more than meets the eye. Good spotting.

On the vertical scale there are 4 spaces above the upper number 10 and 5 spaces below it, which is again repeated (NOT mirrored) below the middle centerline (I do not understand what you mean with 3 marks, did you miss-count?). And those divisions are matched on the horizontal scale. So your (later) sketch isn't an exact match. There must be some special user-instructions on how to use that peculiar division of spaces. I do not see how, other than that the scales could be/had to be rotated to get AOB. But that only explains why horizontal is the same as vertical, it still leaves 4 vs 5 spaces issue. I'm puzzled. Very! ... Maybe I need to sleep.

Hmm, can't you persuade a still living Uboot captain for an interview about this?

Just kidding.