After all the articles that have been written in this blog over the years about the 11″ CPC (Temple 28) it would seem that it ought to be running flawlessly! This is not the case. In the past it has run well enough to get some pretty good images but it has never really worked like it is supposed to.
Several months ago I noticed that one y axis was no longer working in autoguiding. Now in my defense I don’t get to open up the telescopes very often here in the midwest. In fact I went 2 full months without one clear night! It is very hard to troubleshoot something when you can only test it out every couple of months!
In past blogs I mentioned wanting to take a sledgehammer to the scope several times. This was another one of those time. In fact I was mad enough that I decided to just take off the camera and autoguider and just use it as a visual scope. My stubborn nature soon reasserted itself and I began to troubleshoot…again!
We moved to Missouri in 2017 and one of the first things I realized was that the the conditions in Missouri really limited the number of stars that the internal guider could see. This is a very small chip and if you got 3-5 stars in it that was a win! So after getting a wedge and adjusting the gears I put on an external guider. Made it much easier to find a suitable guide star.
A short focal length guider was what was tried first. It was lightweight and I thought it should work pretty well. But it seemed the focal length was not long enough to keep up with the guiding chores of an 11″ F/6.3. There were still occasional images that would trail the stars. An Orion ST-80 was added with a ZWO 120MM camera to guide. This seemed to work well. At the time though, I was having issues with my control software so I ran most of the tests just using Maxim DL 5. In maxim you can still track even if your errors mount and you get above a certain error range. So it appeared to work except for some out of round stars from time to time. For ever 3 images I got 1 with trailed stars.
I finally got CCDCommander to work with the setup and that is where the issue with the Y axis became apparent. I had the error set to .75 of a pixel. So if the errors go outside of this value it will wait to start the next exposure until they numbers go below .75. After an image or two the program would try and correct the errors then stop! When you used PHD you could easily see that the north axis wasn’t working at all and this was causing the stoppage.
So…played around with balancing and it still didn’t do the trick. Tightened up the set screw on the clutch assembly, that fixed some play but didn’t fix the issue. Found an actual weight and slider bar and added that. This way you can just slide the weight up and down on the bar to balance the tube. Still didn’t fix it. This all took around 3 months because of weather.
After some thought I realized that the last time it had really guided well was in Northern New Mexico using an internal guider. It would still crash after a period of time because the gears were incorrectly adjusted when I got it and they would slip during an observation. Still the internal guider chip worked pretty well.
So I removed the uber heavy ST-80 and rebalanced the scope. After orienting the ST-8E and calibrating the autoguiding system it was ready for a test. There was a period of around 2 hours before the almost full moon came up. With moisture, high clouds and gusty wind good images seemed to be out of the question but it would serve to test the guider.
For about 20 images I imaged M 42, then moved to M 44. With a total of around 70 images the stars were round in most of them. There was some trailing in the first hours due to wind gusts around 15 mph. Still it is a start.
Now before I get excited (been here a few times already, then your emotions are dashed by harsh reality) there needs to be further testing. A 2 hour run is not really enough to declare victory! The fact that CCD commander did not crash and burn during the 2 hours is a positive. I also went out to the telescope, pulled the DEC gear and found the scope was still way out of balance. The gears fit so tight on the shaft that they won’t free wheel even when you loosen the clutch knobs. You have to take the gear completely off to achieve really good balance.
If this is the solution to the issue then it is most likely too much weight with the ST-80. I have seen other CPC’s with 2-3 extra scopes mounted on their tubes. However, with this scope it mounting an external guider has not worked very well. Temple 28 had it’s point of balance moved to accommodate a heavy camera when a carbon fiber tube was added. So the typical procedures for a regular CPC just don’t seem to apply. In fact I have had to add weights to the front of the scope, on top and bottom even without the heavy external guider. This is with the change in the point of balance.
The sky tonight has unexpectedly cleared. So test number 2 is going on as I write. The image of M 3 is the result of the first two, 60 second images taken using the internal autoguider. Overall the run is on # 10 and so far the errors have been acceptable. If I was going to do “pretty” pictures this tracking would probably not be acceptable. However, for photometry this should work well.
The program was set to take 160 images then move to M 57 before dawn. Then the mount would park and calibration images be taken. Before I could post this blog a 10 mph wind came up with 15 mph gusts and that does it for testing tonight!
Like it has been for years now, this is still a work in progress. Stay tuned for more updates!