Funny you should mention spokes. That is what I used on the 22 inch - a bicycle rim on the inside of the tube and the secondary supported by eight spokes. That was very stiff. From: daniel turner <outwest112@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Thursday, March 8, 2012 3:21 PM Subject: Re: [Utah-astronomy] Diffraction effects of secondaries and spiders (Was: solar filter) The four vane design allows you to put tension on the vanes like spokes on a bicycle wheel. This gives the upper tube assembly of larger scopes a lot of stiffness at a reduced weight. The bigger the scope the more you are driven to the four vane design. DT ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Thursday, March 8, 2012 11:23 AM Subject: [Utah-astronomy] Diffraction effects of secondaries and spiders (Was: solar filter) All of the curved vane commercial spiders I have seen use very thick vanes, as a way to eliminate vibration. They actually decrease the throughput to the primary over a straight-vaned spider and increases overall diffraction. As you point out, Mat, it's a personal aesthetic to not want to see spikes, but the telescope is in truth hobbled a bit when it comes to fine planetary detail and resolution, when using a thick-vaned curved spider. My 4.25" f/5 Newt has a home-made secondary holder that I've never really been happy with. I'll probably replace it with a single-stalk holder, or incorporate an optical window (I have a couple of surplus windows that I think originally were intended for Edmund Astroscans) I originally built my 6" f/8 Newt with a single-stalk secondary holder, a brass rod. It blocks no more light than a conventional 4-vane spider, and I only see two diffraction spikes, 180-degrees apart. That telescope has a 1.25" low-profile helical focuser but I am now upgrading to a 2" wyoroc Crayford. I have to replace the single-stalk secondary holder because it was attached via a mounting hole in the focuser that is going to disappear. I'm going to go with a conventional 4-vane spider and a spring-loaded home made secondary holder. I'll just have 4 dimmer spikes instead of two brighter ones. Everything I've ever built larger than 6" has always used a 4 vane spider. On Thu, Mar 8, 2012 at 8:53 AM, Hutchings, Mat (H USA) < mat.hutchings@siemens.com> wrote:
We made a curved vane spider which consisted of one ~170 degree vane to which the secondary hub was mounted. The views were really great without the spikes and I really liked it. However, that vane wiggled like Santa Claus's belly any time the scope was touched. I worked, and worked to improve the design and did make some strides, but it just didn't perform well enough for me.
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