Project 40 Is In the House
After more than 12 years of effort and many important milestones on the way, we have reached a new milestone on the way to our final destination. The Project 40 telescope is in the west wing of the Robert Ferguson Observatory. In early December 2015 Bill Russell’s 24-inch reflector was carefully and gently disassembled and stored in preparation for its new owner – is that you? Larry McCune, Mark Hillestad and Steve Follett then began the process of installing the telescope in its new home.
As we expected, Larry had to do some on site machine work to mate the telescope mount to the piers, drilling holes in the steel mount plates and into the piers to securely hold the telescope in place. Then the re-assembly of the telescope from its recent disassembly began, installing the three rocker bogey wheel assemblies on the mounting plates with positioning sheet between them that holds the centering shaft. On top of that the team dropped the azimuth ring attached to the thick plywood base also carrying the A-frame supports for the altitude bearings. Then the rocker box was installed in the altitude bearings and the struts and secondary cage were assembled on top of that. The primary mirror cell was slid into position and finally the optical elements were installed – the 40-inch primary mirror, recently coated by Viavi (JDSU/OCLI), the 16-inch tilted flat secondary mirror and, in a later session, the final optical assembly consisting of the diagonal mirror, the Paracorr coma corrector in the integrated focuser, and an eyepiece.
At this stage, it was time to balance the telescope and the team was surprised at how little additional counterbalance weight was required to get the telescope balanced at the two critical positions, with the telescope pointing to the horizon and then pointing to the zenith. As we continue to tune the telescope with the final components, the biggest being a finder telescope, we will go back to this step a number of times.
The image you see of the telescope propped up on a bench was how it looked at that point. Now the team installed the altitude and azimuth drives. Some final adjustments were made to the components, cutting off a bit of excess altitude drive shaft and providing additional bearing supports on that very long shaft. The azimuth drive likewise went in smoothly once we remembered how all the pieces fit together in that very tight space under the telescope. Checks were made for any interference between moving parts, but the work in testing the mount in Mark’s shop and driveway seemed to have paid off.
The drive uses servomotors and it measures the rotation of those motors as a way to roughly determine the location of the mount, and then compares that information to digital rotary encoders as a feedback mechanism. All of that is tuned with software supplied by SiTech, the maker of the drive system in a section call Tick Determination. We took the telescope through that process and everything validated for the altitude drive, but the azimuth drive was showing some problems. The symptoms were that we could hear the azimuth servomotor sounding differently as the mount was rotated. We spent a couple of night sessions characterizing the details of the way the mount was acting and diagnosed two issues. First, we measured the height of the azimuth ring as it rotated and found that it varied by a tenth of an inch and found that the variation appears to be caused by the mass of the telescope bearing down on the A-frame base and then on the plywood base. Even though there are force-spreading members bridging the A-frame base, the plywood was being bent down on either side, causing a sort of potato chip shape. And second, we were able to identify some “hills and valleys” as we moved the telescope in rotation by hand, and we think that this is caused by one or more of the bogey wheels (there are six, two in each rocker bogey assembly) being flattened or otherwise dinged, since the “hills and valleys” are separated by a single rotation of one of the bogey wheels.
In January, we fixed the potato chip base by adding some more steel to the cross member and inserting a shim to counteract the force that caused the shape change of the base plywood. And we replaced the bogey wheels – they are 6 years old and have, at times, rested for months with the telescope on top of them. We switched to nylon-filled wheels. With both of these fixes in place, the telescope moves much more smoothly in azimuth.
We decided that we should test the drive system in tracking mode to see if we could continue with testing, training and opening of the telescope despite the errors in the azimuth drive. Over short distances, the drive seemed to be behaving without issues. So on a night when we finally had clear enough sky to see a star or two, we collimated the optical components. This process lines up the components so that they are pointed exactly along the optical axis through the center of their figures and involves making small adjustments to the mirror holders of the primary and secondary mirrors to aim them more precisely as measured with a standard laser collimation tool.
Once collimation was completed and the telescope was aligned with a temporarily mounted Telrad, we pointed the telescope for the first time at two bright stars and synchronized the drive to those two points. With the telescope pointed to one of those stars, we were rewarded with sharp star images that were round inside and outside of focus, the standard star test that validates the correctness of the primary curve and the rest of the optical system. As a bonus, the stars across the field had no visible coma, even though the Paracorr coma corrector had not been fine adjusted. In a fast F/3.6 telescope like this one, you would expect to see coma, star images that look like small comets, at the edges of the field. The coma corrector eliminates those with some carefully designed optics. And finally, we were able to do all of this image evaluation work without once touching the drive controls – the telescope tracked for 20-30 minutes with the star in the center of the field.
Now we were ready for the real test. We impatiently waited for M42, the Orion Nebula to rise high enough to see, in what were really crummy skies. Jet contrails were puffing up in the moist sky into straight-as-an-arrow clouds that would drift over Orion and the rest of the sky, slowly obscuring and revealing the sky to us. We finally couldn’t wait anymore and pointed the telescope at the object, even though the observatory wall blocked half of the mirror aperture. We often tell others about our work here by describing the “Wow!” moment that we give to our visitors when they look through a telescope for the first time. I think I can say that Larry, Steve, Mark and I had a genuine “Wow!” moment of our own as we saw the Orion Nebula in a way that none of us had every seen before. The Trapezium was sharp and there were many more stars in that field than I remember ever seeing before. The clouds of the nebula filled the field and we could see color in the field, indicating that there was enough light coming through the eyepiece to trigger the color receptors of our eyes. Using the fine motion controls of the drive we could examine the full extent of the cloud, wandering through that stellar nursery like a walk on a new trail in the park.
The process of tuning the telescope, the training, and the documenting the way we will use this instrument to present the sky to our visitors has just begun.
This milestone, especially that moment when we saw that the images were all that we had hoped for over the last 12 years, is one that we will remember for a long time. But it is a mere stopping point on the longer trail we are on, to bring the Universe to our visitors, to enrich the experiences of our docents, and to inspire others to try to do what they are not quite sure they can do. Join us in the coming years to look through this new window to the stars.