[Rod]

Originally Posted by alistairsam

Hi Rod,

I've been thinking of wire spiders as well. how do you know what the max tension is before the string snaps?
is a 3 ended wire spider common/advisable?
when you centre the secondary holder, is it just at the geometrical centre, is the secondary offset at the secondary mirror?
how do you ensure the secondary holder is perfectly aligned to the axis of the ota?

Hi Alistair

There is really no need to overtighten the strings. I tend to tighten them until I here a bright note when I pluck the string.

The problem with a three ended spider in most configurations that it is more susceptible to twisting. Having the veins connect at two opposite points prevents twisting. You can make a three vein spider that won't twist if you use Mel Bartels' design as shown here:

http://www.bbastrodesigns.com/trilateral.html

My first wire spider used Mel's design and it worked very well. The latest one I have built shows less diffraction spikes.

One of the reasons I went with the square upper end and the truss arrangement I did was so that it would be easier to align the optical and mechanical axes of the scope. I felt doing so would improve goto accuracy. The truss tubes only bend in one direction unlike when you use split blocks. The square upper end makes it easier to see if the spider is off centre. The other thing you can do is drop a plum bob down through the centre of the spider hub when the scope is vertical and adjust the truss mounts until the spider is centred. I did this with my previous scope and got good results by adding shims under the split blocks.

Hope that answers your questions. Wire spiders are cheap and really no harder to build. Just make a simple jig to thread the wires. Wiring this one up literally took only minutes. Mel's design takes a little longer (at least for me) but it isn't hard either.

Rod.