[ATM] Subject: Building 2 mirror RACI for bi-refractor

[Peter Abrahams <telscope@europa.com>]

>I am building a Matsumoto EMS system in a box:
>http://www.page.sannet.ne.jp/mazmoto/index-e.htm
>With only 2 mirrors you can get a RACI (right angle correct image) .  A
>pair, and you can build a bi-refractor.
>------------------------------

Two-mirror image erectors work nicely; but they have a notable 
drawback compared with 4-reflection systems like Porro prisms.
The image rotates as you rotate the EMS system.  The image in a 4 
mirror system does not rotate when you rotate the prism housing.
In practice, this means that an additional collimation step must be 
undertaken with each setup; to ensure that the left image has the 
exact vertical orientation as the right image.  Most users 'eyeball' 
this, and like all eyeballed collimation, it is typically a bit off, 
leading to unusual & unpleasant visual effects with higher 
magnification.  No doubt a collimation tool could be devised to allow 
precise alignment.  The EMS unit I tried at RTMC (on a commercial 
binoscope, for sale) was definitely off in rotation alignment -- this 
was 6 or 7 years ago, I think.
But I hope you pursue this project, these are an interesting means to 
build a binocular telescope.
--Peter


=============================
Peter Abrahams   telscope@europa.com
The history of the telescope and the binocular:
     http://home.europa.com/~telscope/binotele.htm



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