Wright 28

Wright 28. Celestron 11″ SCT mounted on a Celestron CGE with a Moonlite focuser.
 
 
 
 
 

In December 2022 I drove down to Phoenix, AZ for a funeral. On my way there I stopped and picked up Wright 28 at Lowell Observatory. 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 telescopes are reserved for members of the American Association of Variable Star Observers 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. 

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. 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 on an equally archaic Celestron CGE mount. The camera was a Parallel Port camera and took 30 seconds for every image upload. I have one just like it and it still works!

Astronomer Stephen Levine of Lowell Observatory drove to New Mexico, packed it up in a solid, padded wooden case, and transported it to the observatory in his car. Unfortunately, in transit to Lowell Observatory, a lens at the front of the telescope (The corrector plate) was shattered. After inquiries to the manufacturer, it would cost more to replace it than just buying another used optical tube. 

This is where I came in. I had a good corrector on a telescope with the same optics but a bad mirror (or so I thought). So, the idea was to replace the broken corrector with my good corrector 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 claimed that you cannot use a corrector from one telescope on another due to the need to  “match 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 nother 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 who makes parts for this type of telescope, then 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 pretty cookie-cutter items. They are all much all the same, with an occasional one rejected for a flaw. It would seem that a corrector is chosen and then the secondary is tweaked if optical corrections are needed. Most of the corrections will be because of an imperfection in the mirror. So, if you keep your secondary in the same orientation it 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 this type of imaging. 

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. The corrector was removed from the old tube and cleaned. It was then placed on Wright 28 and the original secondary was put back on with care to not screw it on too tightly. 

In testing the scope later that night I noticed a bit of astigmatism. It was 27 degrees with 20 mph winds. Needless to say I was not out very long. In a later test, it was noticed that it was out of collimation just a smidge. Tweaking it a bit caused a major reduction in the astigmatism, but it needed further work. This second test was also in marginal conditions with thin fast-moving clouds that made stars play hide and seek with the telescope. It turns out that collimating a SCT is not my strong point. 

After many nights of clouds, rain and genuinely bad weather I finally got better collimation. I think it is in an acceptable place now. Unfortunately, the camera settings were off and it did not save any of the images from the test session! Somehow the tube that screws onto the secondary and corrector worked loose and I had to remove the corrector and tighten it up a bit.

Upon reassembling the telescope and tightening the corrector ring (the ring that holds the corrector in place) with a small screwdriver it was ready for rough collimation. I stepped back and looked down the tube and I heard a gut heaving crack! The new corrector was cracked again! So much for that telescope!

You can clearly see the 3 cracks. However, when you look through the scope you don’t notice anything not even diffraction spikes on bright stars!

 

After taking a few days to calm down and take a deep breath I sent out some emails. One was to Robert Piekiel, author of “Celestron: The Early Years.” Robert is one of the true experts on SC telescopes. His comment was to ignore it as long as the cracks radiated from the center to the edge. Because of the optical properties of the corrector the cracks would not affect the image very much, other than some star spikes like on a reflector. He also mentioned that if I needed to that I could use clear epoxy and glue it back together.

Here was the problem with the corrector. If the cork had been as wide as the corrector holder it probably would have been ok. Unfortunately, it wasn’t and that just created unnecessary stress on the corrector.

 

I took out the cracked corrector and noticed something that should not have been there…a thin cork ring of material on the corrector holder. This apparently put pressure on the glass and since a portion of the felt that the corrector plate is seated on had been removed (no idea why!) even a light pressure could serve to crack it. So I removed that cork and restored it to its original condition. This added cork was the cause of a shattered corrector and a cracked one!

Well, I had to take the corrector off again one more time and it fell into 3 pieces! So I used some clear epoxy and glued it back together. Works great! Again it went back into the tube for testing.

The Corrector works ok. But somehow it now won’t get completely collimated. You run out of screws on one axis. So I tried to get it lined out and found that it just will not get into perfect alignment! So…I bought a Meade 10″ optical tube and put it on the CGE mount. Works great with much less hassle!  After years of messing around with a balky mount and then a balky optical tube, it was time to wave the white flag and do something else! 

Fast forward about 6 months. I made up some DIY mirror cleaner, alcohol, and a drop of dish soap, and sprayed it on the mirror in the tube that the corrector originally had come out of. I watched with amazement as most of the tarnish and grime washed off! This was kind of unbelievable compared to what it looked like previously. So I put the corrector back on its proper tube, replaced the secondary with the one that originally came out of the telescope in the first place, and am now waiting for warmer weather to work on it all. Hopefully, it can be used for visual work if nothing else.

 Unfortunately, it will need some work on the tube assembly if it is going to be used for imaging. The screws that hold the tube onto the mount need to be reinforced to eliminate the play in the declination axis. There may be life in the old girl yet!

M 42 with the Meade 10″ and SBIG STF-8300 Camera on the Celestron CGE mount.