Wright 28

Recently I drove down to Phoenix, Arizona to spend time with my mom and family before she passed away. On the way down I made a quick stop in Northern Arizona at Lowell Observatory. I was meeting Stephen Levine, a Lowell staff astronomer at a warehouse on Mars Hill. In the warehouse was the Wright 28 telescope that had been in storage since October 2012. The American Association of Variable Star Observers was letting me take the telescope and try to put it back into service.

Wright 28 was donated in the early 2000’s by the late Paul Wright to be used as part of what was called the AAVSONet of telescopes. These are telescopes that are reserved for members to use for their variable star Projects.  The AAVSONet still exists but the telescopes are now considerably larger than the 11” telescope I picked up. Plus, converting a common amateur scope into a professional level instrument takes a lot of time, energy and sometimes money. The results are worth it, but if a former professional level instrument can be used instead, it saves everyone a lot of time and effort.

In October of 2012 the person hosting this telescope in the mountains of Southern New Mexico decided to get out of astronomy. He then asked for the telescope to be removed. Up until then Wright 28 had taken approximately 177,960 science frames (pictures), resulting in 4.9-million-star records. This was done using an archaic first-generation SBIG ST-7E CCD camera that was purchased originally in the 1990’s.

Stephen Levine drove to New Mexico, packed it up and transported it to Lowell in his car. Unfortunately, in transit to Lowell Observatory a lens (Corrector) at the front of the telescope got broken. The telescope was well packed and padded in a large wodden crate. The corrector shouldn't have brokken. After inquiries to have the manufacturer replace it would cost almost as much as just buying a another optical tube.

This is where I came in. I had a good corrector on a telescope with the same optics but a bad mirror. So, the idea was to replace the broken piece with my good piece to get one fully functioning telescope out of the deal. This would allow a very productive telescope to resume its role of gathering data on variable stars. The problem was that the manufacturer claims that you cannot use a corrector from one telescope on another due to “matching optics.”

If you call a manufacturer, they will tell you to ship the whole optical tube back to them and they will “match” the corrector to the mirror. Often it is just cheaper to buy a used optical tube than to try and replace the corrector by sending it back. The exception is when it is still under warranty. Otherwise, the cost of shipping and repair become prohibitive.

After doing some research it soon became clear that the manufacturers policies were not as cut and dried as they state. After talking to a business owner that makes parts for and repairs SC telescopes, as well as a former professional optician, it became clear that the issue was more complicated.  Back in the early days of Schmidt-Cassegrain manufacture the correctors, mirrors and secondaries were figured by hand while being tested to iron out any optical bugs so the claim was true. In recent years technology has changed some of these considerations.

Computer controlled shaping and polishing have made correctors cookie cutter items. They are pretty much all the same, with an occasional one rejected for a manufacturing flaw. It would seem that a corrector is chosen, placed in the optical train and then the secondary is tweaked if optical corrections are needed. Most of these corrections are for the mirror. This is because the corrector has no magnifying power making it optically very weak. Since these are now made by computer and tend to be similar coming out of the factory there is little to no difference between one corrector and another. "Matching" tends to be grabbing a corrector then testing it, turning the corrector until the best figure is obtained. No longer do they hand figure the corrector to match the mirror like was done on the early Celestrons. 

So, if you keep your secondary in the same orientation it normally doesn’t make all that much difference which corrector is used. Now in all fairness the optical standards for Wright 28 are low since it will only be used to do photometry. Unless it is way off, a few small optical flaws will make little difference when doing photometry.

Carefully marking the exact position and orientation of the secondary by using a ruler and paint stick was the first step in replacing the corrector. After carefully removing the broken glass the new corrector was bolted on carefull to not tighten the screws too much. Later that night it was tested and seemed to work prtty well. 

The secondary was too loose and when you collimated the scope the secondary assembly would rotate instead of the collimating screws. You can live with this but it also began to slide side to side. Obviously it needed adjustment. 

So the corrector was removed. There had been thin metal shims that kept the secondary from moving around in the center holes of the corrector. The secondary assembly has a plastic retaining cylinder that holds it snug against the corrector glass. This was tightened up several times to make sure the secondary was centered and snug. The corrector was placed back on the scope and the retaining ring tightened with a very small screw driver to keep it from being overtightened. One of the ways you can do a rough collimation check is to stand back and look at the "rings" in the tube. If all of them are symmetrical then it is in rough collimation. So stepping back I looked down the tube and heard a sickening crack! Two new cracks appeared in the corrector! I couldn't believe it. Why were the correctors breaking?

Pulling off the corrector retaining ring I noticed a small cork gasket had been added. It was about a third of the width of the retaining ring and was on the inside edge of the ring. There were several places where the "seat" for the corrector had been cut away and this gasket was putting undue pressure on the corrector in places where there was less support. Here was the problem. When the scope had been refurbished to make it a professional level instrument this gasket had been added to make sure that the glass corrector wouldn't slip in it's holder. An exacto knife and solvent took care of that problem! Now the retaining ring touched the corrector evenly all the way around so that there were no spots with more pressure on them than there should be. Sometimes the manufacturers knows what they are doing. 

I covered the cracks in the corrector with black electrical tape. Not such a good idea. It made the stars look very out of round. Removing the tape to expose the cracks brought back round star shapes. Turns out that because of the optical properties of the corrector as long as the cracks go from the middle to the edge you don't even notice them. This ought to help those of you who have cracked your correctors. 

Though the optics aren't perfect it is now back in service. I frequently take images of M 57 and then do photometry on the central star. Here is a compilation of 250 of those images taken with Wright 28.