Star Test – Chapter 2
Our experience with the focal point during Star Test – Chapter 1 resulted in making the pyramid spider (patent pending) a bit taller. You can see it here in this image as Larry rebuilt it, complete with the motofocuser. All but Larry arrived on Monday night, July 23, for the next round of testing – vacations do take precedence. This time we found we were a little too long, but that we could adjust the legs of the pyramid a half an inch closer and that was enough to give us some range of motion around the focal plane. We were aided by a convenient Moon to focus on, this time – can’t miss that target!
Before we started getting focused and pointed though, our experience with collimation taught us to do our rough and next level of fine collimation before we start doing video. Dickson’s collimating eyepiece was used to get lined up. We also tested, without the secondary this time, the deviation from collimation that we got when we changed the altitude of the OTA. At the focal plane, we saw the spot move from centered to about one-third inch off of center when moving from 30 degrees altitude to 60 degrees altitude. We attributed that to the pyramid spider, but we’ll want to do some more testing
We started on some low targets that we lined up by putting a visual eyepiece in place at prime focus, setting up the finder/refractor and then swapping the video eyepiece in. We were able to line up on Spica that way, and then, under SiTech control, move to Saturn, not far away. We did find Saturn (not in the field of the primary, but certainly in the refractor) and imaged it for a while, and even thought we were capturing the video, but my pilot error revealed we were just ‘ready’ to record and not recording.
Dickson brought along his illuminated reticle and using that on the refractor/finder saved the night. He was able to use a lower power eyepiece to find a star (Arcturus was our first target) and then to switch the illuminated reticle to more precisely center the object in the field, and that allowed us to find the star in the primary. It still required some search, but I think we’ll get better at this, and once we have a permanent mount in place, we should be able to depend on the SiTech drive to get us to our desired locations in the sky.
The star tests had no batwings (see Chapter 1) and inside focus, we could see the brightened edge indicating a turned edge that we already knew was there, and some spikes in one direction that we attributed to a light breeze blowing through the scope. The oval shape was not ideal, but the real surprise came inside focus where we got a drum shape with a roundish disk that had flat or slightly indented sides.
This is astigmatism, no doubt about it, but we knew that we had lost two of the ‘button’ supports on the primary mirror cell out of the 27 required, and they were lined up with the distortion, so we decided our next step was to fix the primary mirror cell at a work party the next Saturday. In the meantime, we cleaned up the video of our tests of Arcturus and Vega (nice and high and dark this time) and sent Star Test – Chapter 2 off to the Free-ATM experts list for evaluation. The experts concurred about some kind of astigmatism, but Mel Bartels also noted some “strong undercorrection particularly in the central zones with a modest turned edge and possibly modestly overcorrected midzones.”
On Saturday, July 28, Mark, Dickson, Steve and I gathered and removed the cell and mirror from the telescope, separated the mirror from the cell, and repaired the missing ‘button’ supports on the mirror cell. We needed to let the epoxy set so we left the scope stored with the cell in place but the mirror out and will do the next star test on Saturday, August 4. While we had the cell out of the scope, we examined all of its ability to move under the weight of the mirror and found some improvements to make with small adjustments and re-working how the collimation bolts were attached to the main support triangles of the cell. In this picture you can see Steve and Dickson masking the mirror’s edge with about one-half inch of masking tape to “remove” the turned edge.
At this point we are not making any predictions about how the next star tests will look – we’re just going to wait and see, and be prepared to take the primary back to Steve’s shop for some minor adjustments, if needed. But we might get surprised by a perfect star test, who knows?