I failed to mention that the diffraction caused by a spider is not eliminated by a curved vane. It is only minimized visually. The curved vane does affect resolution, especially if it is thick. Visually you don't see it as much because the angle of the vane changes constantly as a function of its position in the aperture. This makes the spike seem to dissappear, but its effects are still there. For those who have not seen my 10 inch scope, I purposefully constructed it with the idea of minimizing the effects of the secondary and spider. It has no spider, and he secondary is only 7/8 inch minor axis. It was purpose built to see how close I could get to a refractor like image. Effective aperture of tis scope is 9 1/8 inches. Comparing this scope to Siegfried's Clark refractor, I would say I almost succeeded. Under excellent conditions at the Wedge, the Clark still had slightly higher resolution images (only slightly), but my 10" had no secondary color. Richard Suiter's book is an excellent reference for these questions. From: Brent Watson <brentjwatson@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 3:02 PM Subject: Re: [Utah-astronomy] solar filter The secondary does decrease effective aperture and therefore resolution. As mentioned before the effect can be calculated by subtracting the secondary diameter from the primary diameter. That yields the effective aperture. Also, don't forget that the larger aperture scopes work at their peak resolution on fewer nights than smaller aperture because it is harder to get a good column of atmosphere in larger sizes. This may be some of the reason for the smaller imported scopes showing a better image than the SPOC scopes. All in all, the larger lines will form behind the larger scopes. The 22" always had the biggest lines. The more interest in the scope, the more interest generated in the public. From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:51 PM Subject: Re: [Utah-astronomy] solar filter My comments on the obstruction were about the spider, specifically the straight armed cross spiders that dominate dobs. Curved spiders eliminate the diffraction spike effect. Aside from blocking a little light, the secondary has no effect as an obstruction. Having said that, many people aren't bothered one bit by diffraction spikes, and some even like them. So it largely comes down to personal preference. I personally have never had the opportunity to see side by side the difference between a dob with an off axis filter and one with a full aperture filter so perhaps my input is a little unbalanced. :) Josh On Tue, Mar 6, 2012 at 12:22 PM, daniel turner <outwest112@yahoo.com> wrote:
The central obstruction boogy man is a long held myth. If you take the size of an aperture and subtract the size of the obstruction that gives you the size of an equivalent unobstructed telescope. So a 10 inch SCT with a 3 inch secondary will perform at the level of a 7 inch refractor. I've seen this when Roger Fry's 10 Meade is compared with Sigfried Jachman's AstroPhysics. The best white light view of the sun that I have ever seen was from Ed Erikson SCT with a full aperture Orion solar filter.
DT
________________________________ From: Josh <mountaindrifter@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tuesday, March 6, 2012 12:05 PM Subject: Re: [Utah-astronomy] solar filter
I'm not that familiar with the design of an SCT, but it's my understanding that any telescope that has a spider or similar obstruction will benefit from an off axis mask. It will remove the diffraction spikes from the image. If you have a curved spider than you don't have diffraction spikes any way, so there's no benefit in that case other than reducing the amount of light that enters the telescope. Another thing to keep in mind is that any time you reduce the aperture of a telescope you also reduce the resolving power. However, with solar scopes, 4" is plenty for great detail.
On Tue, Mar 6, 2012 at 10:59 AM, Dunn, David <David.Dunn@supervalu.com> wrote:
I don't know all the reasons for doing the off-axis but that is what I have done with my previous filters. I do know that the Sun is too bright and just like you would mask your telescope down to get a better view of Mars, masking it down to 4" will help with keeping the light from washing out the detail when viewing the Sun.
I am planning to buy 1/2 meter of the Baader solar film and cut it into 4.92" squares. I plan to sell them for about $4. That will handle the cost of the film and some poster board to protect the sheets.
Thanks, David Dunn
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Kim Sent: Tuesday, March 06, 2012 10:50 AM To: 'Utah Astronomy' Subject: [Utah-astronomy] solar filter
Anyone have thoughts about advantages/disadvantages of using a full-aperture filter versus an off-axis filter on a SCT? I'm trying to decide whether to purchase or make a full-aperture filter. If I purchase a sheet of film and make an off-axis filter I'll have enough film left over for one or two pairs of binoculars. I'm always for saving a buck or two - or 25 or 30 (for another piece of film).
Kim
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_______________________________________________ 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".