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.
My 12-1/2 inch scope employs a curved vane spider I constructed from 3/32" steel. It is only 2 vanes and is very sturdy. I have not detected any viration in it. It works well. I use it for a couple of reasons. 1. I wanted to see what a curved vane spider would really do. 2. It was easier to construct than a four vane spider. 3. I am too cheap to purchase a commercial 4 vane spider. I'd do it again but more because of the ease of construction. 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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3/32" steel is stout stuff, Brent. No wonder there's no vibration. Pretty thick vanes. On Thu, Mar 8, 2012 at 1:10 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
My 12-1/2 inch scope employs a curved vane spider I constructed from 3/32" steel. It is only 2 vanes and is very sturdy. I have not detected any viration in it. It works well. I use it for a couple of reasons. 1. I wanted to see what a curved vane spider would really do. 2. It was easier to construct than a four vane spider. 3. I am too cheap to purchase a commercial 4 vane spider. I'd do it again but more because of the ease of construction.
Along with commercial units using thick vanes, I've seen homemade curved vane spiders made from thick material. Wanting to avoid that, I went with the method of using stainless steel rulers. Bad idea. They are just too thin to not wiggle like the MaryJane Girls. Don't go the stainless steel ruler route if you want to make your own curved vane spiders. I learned that lesson! Mat -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Thursday, March 08, 2012 1:23 PM To: Utah Astronomy 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.
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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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.
_______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
Daniel posted about a generic "engineering" trend. Good observation, but it is, of course, totally dependant on design and materials. 99% (conservatively) of modern large-aperture amateur-made Newts are Dobs, and most utilize wooden construction for the secondary "cage". My caveat is that low-frequency vibrations can all too easily be traded for high-frequency vibrations in high-tension spider designs. Also, too much tension can easily pull a secondary cage made of wood out-of-round if the spider tension is too great, and the cage design isn't robust enough to handle the tension stress. The good news is, high-frequency vibrations aren't nearly as noticeable as low-frequency vibrations. Our brains are slow, lol. Wood is a very good material to use in structural elements, to dampen vibration. That's more good news. But... "Stiffness" in a design can contribute to higher-frequency vibration that, while not always noticeable visually, can be problematic when imaging, so be warned. Not as many folks imaging with large Dobs, so we can wipe our brows yet again. Moving on... A 3-vane spider will produce less overall diffraction than a 4-vane, while still allowing structural "stiffness" to be added to the secondary cage, all other things being equal. The tradeoff is that you end up with 6 diffraction spikes, instead of 4, as with a 4-vane spider, though each of those six is less intense than the 4-vane spikes. But allways make sure that your secondary cage can withstand the tension. I'd be willing to bet that if audited, many modern large Dobs with spiders in tension no-longer had perfectly round secondary cages. And thus, unequal tension and more secondary vibration than they were assembled with. A cardboard or composite tube will never be able to withstand long-term spider tension, without additional reinforcement not susceptible to long-term "creep". More anon. On Thu, Mar 8, 2012 at 3:38 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
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.
participants (4)
-
Brent Watson -
Chuck Hards -
daniel turner -
Hutchings, Mat (H USA)