Curved spider question

Hi all,

I'm planning out a 6" f/6 OTA and I was going to use one of those curved spiders from Protostar. I realize that there will still be diffraction effects from the spider but that it will be in a sort of halo around the target rather than spikes.

My question is, for planetary contrast purposes, does the curved spider actually make matters worse? Is a three-vane spider a better option? Or is it strictly an aesthetic preference? I never tried both in the same scope.

Thanks,

Mike

Mike, in my experience you will see no difference in planetary contrast between properly designed and executed curved and straight vane spiders, especially the single arch type sold by Protostar. I have tried both in my 8" f/8 Newt, a Novak straight four-vane and a Protostar single arch, and I see no difference in planetary detail. My guess is that they both generate about the same amount of total diffraction, but the single arch throws it around the entire field while with the straight vane it's concentrated in the spikes. Aesthetically, the view looks better to me without the spikes so I prefer the single arch.

Note that I said "properly designed and executed". Which means thin vanes, perfectly (or nearly so) aligned with the optical axis, single arch (curved vanes have more total length than an equal number of straight vanes), etc.

Do a search through the Telescope Making and Equipment Talk forums and you'll see numerous discussions on the subject. You'll also find that there is not a lot of consensus.

Doug