Info on curved spiders

[Satchmo]

I've always been a fan of curved spider vanes for secondary mirror holders, heres some good reading.

Some info on curved spiders here

A commercial vendor with more good info here

[Blue Skies (Jacquie)]

Thanks for those links. The pic in the second link is good and says a lot!

[janoskiss (Steve H)]

The best geometry for curved spiders one that minimises spider vane length (to maximise contrast) subject to the constraint that the vanes cover orientations spanning 180 degrees (to completely eliminate diffraction spikes; span not 360 because a 180 degree rotated vane is still running along the same direction). There are also practical considerations, e.g. single vane spider does not provide enough rigidity. Efficient curved spiders can be found on some Russian Cassegrains. Some of the TAL scopes have them. 3 vanes, each covering a 60-degrees.

[Satchmo]

Originally Posted by janoskiss

. Some of the TAL scopes have them. 3 vanes, each covering a 60-degrees.

I guess there are two schools of thought there. You can minimise spider vane length at the expense of uneven distribution of diffracted light and stability issues, or go for even and non overlapping spider diffraction and rigidity .

The above TAL type you describe, satisfies the latter conditions. This is definately the optimal design as the diffracted light of each vane appears as a pie shaped wedge opossite the vane of twice its circular angle. Thus 3X 120 degree fans give maximum dispersion of diffracted light with no overlap causing hot spots and most even distribution. All other designs remove the spikes but do not give an even energy distribution over the field, or around the star. A 60 degree angle vane also has hardly any more length than a straight vane.
60 degree vanes can typically be bent 90 degree arc for added strength with only 60 degeree in the light path. Dick Suiters analysis of spider diffraction suggests that spider vane thickness can be taken to 1/128 of aperture without any significant effect on the contrast ratio ( MTF) so a 10" scope for instance can use 2mm thick curved vanes and have a very rigid spider.