Vixen VC200L Triangular Star Re-mediation

I purchased a Vixen VC200L in the early summer of 2006. My objective in this purchase was to obtain a telescope for imaging that would have less mirror flop and a flatter field than my C-11 and would give a shorter focal length to ease guiding and tracking requirements.
Upon receipt of my new scope I quickly discovered that at the native focal length of 1800mm I could readily detect non-round stars even in 1/4 second exposures. These stars appears to me as a pear shape or sometimes as more of a triangle. I could not detect them visually even at 200X. My initial reaction was that this must be the result of some sort of pinching so I downloaded the newly updated collimation instructions for the VC200L and performed a careful collimation of all three adjustable axes and I made sure that both the primary and secondary collimation screws were just snug and not too tight.
After this adjustment I captures some test exposures of five seconds and compared the star images to my previous foul ones and was happy to see pretty good star images as shown in this 330X blowup.
For the following couple of months I was occupied with summer vacation and then I received the Vixen focal reducer and did some imaging at f/6.4 with some success.
In mid August when I went back to trying to image with the Vixen at f/9 I found to my great horror that the mis-shapened stars had returned. At this point I did a comprehensive review of the posting on the vc200l Yahoo! group from 1998 to present and found that at least two or three times a year as new users came on-board there would be a discussion about triangular stars. Here follows a brief summary of what I found,
  1. Paul Sweeney reported triangular stars back in 2000. Herm Perez told him to make sure the mirror could move 0.5 to 1mm. He later reported having spent a lot of time with collimation and did improve his images but still saw some residual triangular shapes.
  2. Erik Bryssinck reported triangular stars in CCD work but later concluded that it was a camera problem and not the optics.
  3. Harry Pulley says that he once tightened a collimation screw so much that he couldn't get it out but it didn't cause pinching.
  4. Rick Bowden says that tube flex is a big problem and that mounting a heavy guide scope on the Vixen can cause excessive flexure.
  5. Antonio Fernandez bought a new (green tube) VC200L and saw pinched optics so he returned it and bought an older used one and has no problem.
  6. Ferran switched to a C-9.25 after trying three different VC200L's. He never achieved round stars.
  7. John Mirtle says that he has no problem with poor star shape and his Vixen holds collimation very well.
  8. John T. (lunacie) says that critical focus was his issue with poor star shape.
  9. Anthony Tripodi says that very accurate collimation is essential and that tube flexure caused image "softness" in his case. He also suggested that I contact Scott at Vixen NA.
  10. Rick Stevens says that when the primary collimation screws are too tight then he gets triangular shaped stars.
  11. Filippo says that his "pear" shaped stars were caused by over-tight retaining ring screws.
Much of these posting made sense to me but I quickly realized that I was not going to be able to diagnose any problem without getting inside the scope to see what it all looks like.
I undertook a complete tear-down of my Vixen VC200L as described here. I noticed that the mirror could not move in the mirror cell so I adjusted the mirror shims to allow for movement prescribed by Herm Perez in (1). I also made sure that the three retaining block were not compressing the mirror in any way as described in (11). These block are, however, made of dense rubber and machined to allow a space for mirror movement so I think it unlikely that these would cause any pinching unless severely over tightened.
In inspecting the mechanism I also noted that the three point push-pull collimation of the primary is very well designed. The mirror cell has no bearing on any point but these three push-pull pairs. I scarcely see how any modest (or much more than modest) force applied to the primary collimation screws could cause pinching with this design. This agrees with Harry's experience (3).
I performed a meticulous indoor collimation of all three components following the Vixen procedure followed by a careful star collimation at 200X.
Unfortunately when brought fully to focus this 330X blowup from a Canon 350D frames showed the same triangular stars that I had seen before the tear-down and adjustment. Notice too that this is not an equilateral triangle but rather seem to have two orthogonal legs like a right-triangle.
Following Anthony's advice (9) I contacted Scott at Vixen N.A. technical support and found him to be very personable and sympathetic but unfortunately he could not suggest any actions that I had not already tried.
I spent several long evenings testing different things, doing iterative collimations and loosening the secondary screws to no avail. I decided to try out a software package called MetaGuide.
This image of Vega shows the typical pear shape as rendered by MetaGuide. The 2X Televue barlow was used to give an effective f/18 focal ratio.
On my second or third evening out with MetaGuide I came to my first clear conclusion. Tube currents may have invalidated most of my previous observations. It's been very hot lately and I have not been giving sufficient warm-up time in the evenings. I turned my tube to different parts of the sky and observed the tell-tale plume (always pointing up) as seen in this image.
After a few cloudy nights I was able to go back out and re-collimate with a properly thermally equalized telescope. It was also about 10-15 degrees cooler than it had been the previous week.
MetaGuide gives you a live video view of the star at which you are looking. So with typical mediocre seeing in my part of the country I can look at the scintillation of the star at high power. It looks sort of like an amoeba squirming around on a Petri dish. This turned out to be important. While adjusting focus of the webcam with MetaGuide I noticed that as focus moved in and out the star squirmed up and out along lines corresponding to the diffraction spikes caused by the spider. Here is a 10MB AVI file if you want to see what this looks like.
I made really subtle tweaks to my collimation using MetaGuide and this image shows the current 330X blowup of a star. Still not as good as my initial "dumb luck" results.
As a final sanity check I decided to make an offset mask to remove the effect of the spider. Of course, this also quadruples the focal ratio and the small aperture reduces the effect of seeing as well. Still, If I could have stars like this with full 8" aperture I'd be happy!
Note also that I found the aperture mask very handy for visual collimation as well. With a bright star it's much easier to do airy disk collimation using the mask and it's even possibly on nights of mediocre seeing.

Here are my conclusions

  1. Pinching is not my problem and the Vixen is well designed in this regard.
  2. A fat spider is bad and adds sensitivity to seeing conditions.
  3. Any slight miscollimation enhances effect #2 asymmetrically and lead to triangular stars (right-triangles).
  4. Superb collimation and/or excellent seeing can reduce the effects.
  5. Some people, especially owners of older models, don't seem to have this problem at all. I can't explain why.

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