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On 15/01/2014 20:48, Frank Reed wrote:
> Yes, and as I mentioned in a post last night, a really useful trick is
> to use objects (could still be dice) that are different colors. Marker
> caps, legos, and other small toys work great for this.

LEGO bricks, that's a great idea! They have a nice sharp edge (so the
top face is clearly defined) and I hear they have very precise dimensions.

> You also wrote:
> "* It's not correct to use the top _edge_ of the dice. One must be
> looking along the top plane."
>
> I'm not sure I understand the distinction you're drawing here. You sight
> in line with the top edge of the two objects. Do you mean that one
> should not be sighting from "above" trying to get the top faces of the
> two dice to appear aligned? If that's what you meant, then yes, that's
> an important distinction. The goal is to get the two top edges to align
> in a straight line when viewed from exactly the height of the dice above
> the arc.

Yes, I meant 'edge' as the line segment for the intersection between 2
faces of the dice. Rather than 'top plane' I should have said 'top
face'. This is one of the things I did wrong - I was matching up the
edges without checking the height from which I was looking.

> Yeah, I don't understand those peculiar shapes. They may have been
> modeled on a bit of testing gear created for the optics lab. Clearly
> simpler objects work just fine.

I found several posts in the archive, made by Alexandre Eremenko in 2004
about this same situation, e.g.
http://fer3.com/arc/m2.aspx/Adjusting-perpedicularity-Index-Mirror-Eremenko-oct-2004-w18186
http://fer3.com/arc/m2.aspx/Perpendicularity-SNOT-Eremenko-oct-2004-w18298
Since the SNO-T has a very offset index mirror, it is great that they
put the correct procedure in the manual, but sad that they then shipped
with these 'diopters' that don't have a top surface. Strange.

Today I have mocked up a sextant arc and an index mirror in POV-Ray, a
ray tracer, and it has confirmed what I described in my earlier post.
It's a really fun tool to play with and surprisingly easy to use (the
objects are defined in a text file with a simple syntax). I can
introduce errors in alignment and increase the mirror offset and see the
result on the reflection.

> By the way, it's far better to call that Russian sextant a "SNO-T".

Will do. I copied it from the PDF posted earlier today. I guess the
importer preferred a less snotty sounding acronym for this noble instrument.

Thank you very much for your reply,

Luc

   

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