Use spider web. This is right stuff, used for exactly that purpose in the past.
http://www.surveyhistory.org/cross-hairs1.htm
http://en.wikipedia.org/wiki/Reticle
http://www.practicalmachinist.com/vb.../t-126349.html
I was using the glass core from the optical cable, not easy to find these days because they are now made of plastic.
The good thing about it is that due to refraction, it behaves as double strand.