Star Test – Chapter 1
We naively made our first attempts to star test the primary mirror with the secondary mirror in place. Perhaps we were just full of hope rather than naive – you’ll have to be the judge. Shortly after we established that we did, indeed, have an optical system, we gathered to try the telescope out on a bright star to see how it was performing.
Small telescope users will be familiar with the star test. You focus the telescope on a bright star, and then move the system a bit in and out of focus. We use it with our scopes to check collimation – the shadow of the secondary mirror should be centered in the diffraction ring, and the ring itself should be nice and round. Telescope makers know that the star test will tell you if the mirror’s surface has the right shape, and that it can be a fairly sensitive indicator of turned edges and astigmatism, in addition to collimation of the system.
Our first attempts gave us what we started calling batwings (thanks, Larry!). It was certainly clear that we would not get an immediate thumbs up on the primary so we could send it off to get coated. We guessed that the secondary could have been deformed by our temporary mounting method (double sided foam tape) or the curve of the secondary mirror itself. We decided we needed to test at prime focus.
The focal length of the primary mirror is 3,664 mm, putting it beyond the secondary cage by about 300 mm or more. We thought about diagonals but decided to create a kind of pyramid spider to hold a temporary eyepiece holder. Larry crafted the addition and got it installed on the secondary cage, and provided a motorized focuser, to boot. During the afternoon, he and Mark measured and re-measured and decided that the first pyramid sides were too long, so they cut them down to the desired size.
The full team of Dickson Yeager, Steve Follett, Mark Hillestad and Larry McCune gathered that evening and started by doing some rough collimation and then tried to get a focus. Nothing terrestrial was visible from Mark’s driveway that was far enough away to reach focus, so we waited for twilight. With an eyepiece in the focuser, it became clear that we needed more distance to reach focus. We found two extension tubes and the combined tubes allowed an eyepiece to be adjusted into the focal plane at prime. The irony wasn’t missed by any of us, of course. With the extension tubes in place, we checked and found that we could see about the inner two-thirds of the mirror – the rest was vignetted. Not great, but still worth testing.
Our earlier star tests were purely visual. If they had shown us a perfect optical system, we would probably have left it at that. Knowing that we needed the diagnostics that the star test would give us, we decided to try and record the test on video. VMOA owns a Meade Electronic Eyepiece that has an 8mm square imaging sensor and electronics to produce a signal on an RCA cable suitable for recording. We later calculated that with that size image at prime focus we had an image size of about 7 arcminutes. That was the cause of all the time we then spent trying to find things.
Even though we have a 2″ refractor piggybacked on the mounting to use as a finder (and it with a tiny finder of its own), it was impossible to locate an object in such a small field. We got flat-out lucky to find Vega as twilight was beginning – searching the area we happened to see a very out of focus Vega move across the video monitor and were able to walk back to it and focus on it. And it wasn’t inspiring. The batwings were back, extended in one direction inside focus, and outside focus in the other direction.
We were certainly out of collimation and we began the slow task of getting it into collimation. That procedure consisted of Steve adjusting the collimation bolts on the back of the primary mirror, while I watched the video monitor and adjusted the pointing of the telescope with the guide speed adjustments. I should say that we did lock onto Vega in The Sky and the mount was tracking very well throughout the evening. That said, every touch of the collimation bolt threatened us with loss of the star, so we moved slowly – a little collimation improvement, a little re-centering. Finally, after about 15 minutes of this, we moved the collimation enough to lose Vega from the field, and were then unable to recover it.
We knew that it would be difficult to find an object given this experience and our collimation was not quite there yet. So we looked for a star that was low in altitude. That allowed us to replace the video eyepiece with a visual one and to climb a ladder and stick our head and body in front of the primary to help us find Antares. Once we had it locked for tracking and centered, we swapped the video eyepiece back in and we were back in business, sort of. As you would expect, low altitude makes for not very good seeing. We did continue to make progress with collimation, but in analyzing the images later, we found that we were able to improve our image size at focus only from 32 arcseconds to 16 arcseconds. Barely passable seeing should be about 6 arcseconds, but we were just looking through too much atmosphere, and we were not going to find anything much higher in altitude without a head at prime focus. And then we collimated Antares right out of the field. We satisfied our curiosity by imaging Mars for a while and then called it a night.
You can see the whole video (7m 24s) here: Star Test Video – Chapter 1. You may need to download it to view it – QuickTime and iTunes should both show it, as will any player that will play an MP4 format video. We played this video with commentary for the docent team at our last meeting and made plans for Star Testing – Chapter 2.