[ATM] Make A Barlowed Laser Collimator?

["RodShea" <RodShea@comcast.net>]

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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