A Learning Week

Creating something new is messy.  I’ve talked in this blog many times about the iterative nature of our design for Project 40 and all of the moments when we have run into a problem that has caused us to backup, figure what to do next, try a few different things, and then settle on a solution that gets on to the next bump.  Each time we learn something new about the technology, and not a little about ourselves. This week, it was the turn of the optical design to give us a lesson.

Last week we were jubilant about seeing the telescope mount come together for the first time. There is nothing like seeing an idea that has inhabited our imaginations take form in the space before us.  Last Saturday we gathered for the first star test of the uncoated primary mirror. As I reported earlier, we purchased the flat diagonal mirror and we just got the coated secondary back, so we now had all of the optical components nearing completion for the first time. Assembling them into the telescope, designed to hold them all, and star testing the primary before we coat it, was such an important milestone, we were literally trembling to see it. The hope was that we would see an image, slightly out of focus, that would confirm for us that the primary was as good as we think it is, and with no astigmatism.

The first setup of any telescope under construction can be a painstaking process. Steve Follett is our lead and moved us through that process, adjusting the primary, the secondary and the flat to get things centered and on line. He brought the design specs with him, for reference, and we went to place the secondary mirror at the recommended distance from the primary and discovered an error. The calculations we had done nearly two years ago to specify the primary-secondary distance had gotten missed in our communications and the secondary curve was chosen at a point five inches further away than the mount design was built. We’ d built in some adjustment, but not that much, and we could not find focus.

Now, Cassegrain focus is an interesting beast. Our Naysmyth design is simply a Cassegrain with the light path folded to an eyepiece off to the side instead of passing through a hole in the primary.  Many of you have experience with a Cassegrain in the form of the Schmidt-Cassegrains owned by many amateurs, and know that the focuser is a knob on the back of the primary that moves the primary mirror a very tiny amount. That works because the distance between the primary and the secondary mirror in a Cassegrain very sensitively controls the location of the focal plane. For some commercial Cassegrains, a change of 1 mm in that distance can change the location of the focal plane by 10 mm – a 10x mechanical gain.  We knew all this and began to try and figure out where the focus was happening by looking for something like focus at ridiculous distances from the eyepiece.

All of our attempts to find the focus on that Saturday night failed.  I have to say, Larry, Steve, Mark, Dickson and I were heartbroken.  We certainly hadn’t given up, but the thought that we had gotten the calculations on the optics that wrong was really confusing.  The optical system was behaving as if the light cone was not a cone at all, but a collimated beam that would not be brought to focus without a telescope in the focuser tube.  By the time we shut down that night, we decided to go home and re-do the calculations to make sure we understood what we thought we understood, and to figure out the best way to extend the location where the secondary is mounted, without making the tube too long for the observatory. In short, our plans for Project 40 were in serious jeopardy. There was already talk of making a new secondary mirror if need be.

Sunday was spent going over web sites, ray tracing program results, calculations and emails back and forth for how best to extend the secondary cage mounting point.  By the end of the day on Sunday, I had produced a calculation that showed our secondary could not possibly work (it turned out to be wrong), while Larry worked out an extension of the secondary cage.

On Monday, Larry and Mark put the new secondary cage extension on the telescope. The original design of the telescope mount called for the secondary to be mounted at 108″ from the primary, with some adjustment higher and lower than that. The curve on the secondary was put on the glass assuming a 113″ distance. For this design, we’d calculated that the distance multiplier would be about 15x. A 5″ difference in the secondary-primary distance would have pushed the focal plane 75″ away from the eyepiece, if those calculations were correct. The new secondary extension allowed us to move the secondary up to 119″ away from the primary.

This temporary secondary cage extension moved the spider out by 11 inches.

Mark reported, though, that even with the extension, he and Larry could not bring trees 400 yards away into focus. Did we need to move the secondary farther away? Were the trees too close to focus the telescope? This really didn’t make any sense – we had to be doing something wrong.  We planned to get together again on Tuesday evening to continue troubleshooting.

Before sunset on Tuesday, Mark, Larry and Steve rolled the telescope out of Mark’s garage. They pointed the telescope at the Sun and used that bright star test source to search for the focus, and found it – 8′ outside of the focuser with no eyepiece in place.  Before the test could be repeated with a new calculated focal distance (96″/15 means we move the secondary 6.4″ further away) the Sun went down.  That evening, we tried to focus on trees and stars and were once again frustrated – no sensible focal plane.  The good news for that night was that we could find the focus. Standing eight feet away from the telescope, holding an eyepiece, was not going to be workable for the public, though.  We decided that the Sun was needed to locate this thing.  We also decided to ask for help, so I posted our quandary to the AltAzimuth Yahoo group that specializes in 1-meter and larger telescopes.

Just after noon on Wednesday, Steve and Mark met and tried again. They were once again able to find the focus point of the telescope by pointing at the Sun, and, projecting the focal plane on a white background, could see sunspots. By making small movements and finding the next focal plane location, they found that the actual multiplier for this optical system was more like 30x. And they finally achieved focus at the desired location on the focuser with the secondary at 115-11/16″ from the primary.  Relief, followed by joy!  In an instant, the world made sense again. By that time, we’d gotten a response from Dave Rowe on the AltAzimuth board confirming our calculations were in the ballpark and with advice on finding focus.

We gathered again on Wednesday evening, this time with a much-changed mood in the air. It took most of the evening to get the scope lined up with temporary finders attached to the mounting frame so that we could find a star in the eyepiece. We finally did find Vega, but the seeing was so poor that we were unable to definitively call the primary mirror ready-to-coat.  More testing will follow until we are certain enough to move forward. But the satisfaction of understanding something we didn’t understand before, and of finding the place where Nature, not our calculations, said our focal plane must be, created a glow that I know will last me for a long time. This won’t be our last challenge. But it will be a memorable one.

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1 comment so far

  1. Ted Judah on

    Wonderful post. I have watched this project from a safe distance for some time and I have to say I was on the edge of my seat for this one.


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