[ATM] Make A Barlowed Laser Collimator?

[Scott Ewart]

Rod,
    I built my own collimator similar to an EZ-Collimator.  In the 
angled viewing surface with the hole in it, I inserted a very small 
sleeve made of aluminum.  I did this to make the beam smaller and 
rounder (the beams from cheap lasers are a bit elongated).  This 
causes significant scatter off the inside of the hole.  At first I 
didn't like the idea of this, until I started using it.
    The scattered light makes the primary's central mark more visible 
in difficult lighting conditions while collimating the secondary. 
Then when I go to align the primary, the reflected scatter shows the 
silhouette of the central marking ring on the angled viewing surface, 
just like a barlow would, except that you still have the central point 
of light.  No need to remove the laser to screw on or add a barlow.
    This has the further added feature of being able to show very 
clearly the source of any collimation change due to changing altitude. 
Does the point of light move within the shadow of the ring?  It's the 
secondary moving.  Do the ring shadow and point move together?  It's 
the primary moving.  Simple and elegant.

Scott


----- Original Message ----- 
From: "RodShea" <RodShea at comcast.net>
To: <atm at atmlist.net>
Sent: Saturday, March 31, 2007 7:38 PM
Subject: [ATM] Make A Barlowed Laser Collimator?


|I have a red 635 nm laser collimator with a hologram that screws into 
its "business end" with the same thread that is used in a 1 1/4" 
filter.  That is, if I unscrew the hologram holder, I can screw in an 
1 1/4" filter.  I'm thinking that a small plano-concave lens or 
negative achromatic doublet mounted (somehow!) in a filter "barrel" or 
"ring" would produce a diverging beam.  I also figure I could machine 
an angled surface plug, similar to a BLUG, that could fit, 
permanently, into the filter ring, perhaps sandwiching the lens in 
place.
|
| This would allow me to unscrew the hologram ,then screw on the 
diverging lens assembly and use the laser for Barlowed laser 
collimation, using the angled surface to see what I'm doing while 
collimating.  This would be a lot more compact than an 1 1/4" Barlow, 
and probably lighter, since a full diameter lens, and ? the doublet, 
would not be needed.
|
| Am I right on this, and can anyone tell me approximately what focal 
length lens I want, or point me to a site that would help me calculate 
it?  The scopes I would use it on are Newtonian, with focal lengths of 
between 60" and 100".
|
| Thanks,
|
| Rod
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