The weather forecast for tonight indicated that it was partly cloudy. But with the speed the clouds were moving, this really just meant that it was totally clear for periods of a few minutes, followed by totally overcast for periods of a few minutes. It was quite odd.
Anyway, Bob had a take-home final for his class and I had nothing to do so I went over to his house and messed with the telescope while he was inside working. The goal of the evening was to get the polar alignment procedure kinks worked out. I started by finding the home position that the LX200 wanted when it was turned on such that its 2-star alignment would work and get the computer roughly in-sync with reality. When you select 2-star alignment, some text scrolls across the bottom of the autostar hand controller but because the refresh on the light elements is so bad, you can't read it. I suspect it tells you how to align the scope and fork, but I'll never know. The correct answer turned out to be RA:0° and DEC:90° [i.e. fork arms pointing north and horizontally opposed, OTA in line with the fork arms, corrector pointing north, finder scope on the bottom of the tube].
Once this "Polar Home" position was achieved, the scope could be turned on and its initial slews while in the 2-star alignment procedure would be in the right general direction. Of course, right when I got to this point, clouds moved in and I couldn't see any stars to do the actual alignment. I went inside for about 30 minutes and discussed rotating conics with Bob.
Coming back out to find that all of the clouds were gone was nice. I went through the two-star alignment, which only takes about two minutes, and then took a first stab at drift alignment. Drifting requires multiple 5-minute waits while the star is allowed to move off-course and then some correcting adjustments to the wedge. The method is straight-forward, but there was some question as to how much of a correction was needed for a given amount of drift. I started with very small corrections - if it drifted down two units on my reticle scale, then I would correct it over by 2 units, etc.
After several iterations with no apparent effect on the drift error, I cranked it up from a correction ratio of 1:1 to 30:1 and that was much better. Two iterations of 30:1 and the azimuth setting on the wedge was all set. Then it was the same thing on the elevation setting and that one only took a single iteration. And that was it! The wedge was aligned and there was no declination drift at all (at least in 5-minute drift tests). Finally! We have been trying to get to this point for ages.
The next step was to try some prime focus, through-the-telescope photography! But, of course, while setting this up, a heavy cloud bank moved in and it was totally overcast. We wasted about 40 minutes wardriving around the city (found 154 wireless access points). By the time we returned to Bob's house, it was clear again.
For the first try, I set up the optical train like this:
However, in this configuration there was insufficient backfocus; something had to be taken out of the chain. I decided that we needed the focuser to get accurate focus on the camera, so we ditched the focal reducer. With this optical train I was able to get the image of Rigel focused in the camera viewfinder as best as I could tell. next, we replaced the camera with the STI Stiletto focuser to get a perfect Ronchi-grating focus.
The Stiletto turned out to not be as plug & play as I was hoping for. The view looked like a big out-of-focus donut common to SCTs. There was a weird in-focus black line running through one side of the image, no idea what that was. The rest of the donut was dancing with seeing-related interference noise. Tuning the micro-focuser made no visible change in the pattern, except at the extremes where I could start to see some banding patterns. When Bob unlocked the telescope mirror and cranked it *way* out of focus, then the interference lines were obvious, but in the "near-focus" regime, the noise was so great that I couldn't make a judgement as to when the focus was perfect, and in fact I couldn't do any better with the stiletto than I could just looking through the viewfinder. Maybe this is just because the seeing was so bad here, I'm not sure. I'll have to call the STI guy and talk it over with him.
Anyway, once we got this settled it was cloudy again. And we had to wait about 10 minutes before we could take any pictures. The pictures were through some very thin clouds so all the star images are kinda fat.
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The Great Orion Nebula, M42, exposed for 29 seconds, ISO 100, ~3550mm EFL, f/10.
No guiding.
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The first shot I did was a 1 minute exposure, but it looked a little blury (probably due to a combination of the bad seeing, vibration problems we were experiencing, no counterweight system, and the total lack of guiding). I repeated the exposure with the 30 second shot above and then again with two minutes, below.
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The Great Orion Nebula, M42, exposed for 126 seconds, ISO 100, ~3550mm EFL, f/10.
No guiding.
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The two minute exposure shows signs of a significant change in orientation during the shot. Not sure what happened here. Anyway, both pictures look pretty bad but we weren't expecting anything great. It was almost 2:00am and it was a work night so we didn't bother trying to set up the STV autoguider. We'll save that for tomorrow night and/or the star party the day after tomorrow. The key point here is that we got prime focus photography up and running! Now it's just a matter of perfecting the technique.
Known issues that compromise photo quality:
- Vibration. Bob added a silicone liner to the anti-vibration pads he has and it has been curing for the past week. We need to test them out to see if they help.
- OTA Balance. We have no counterweight system for balanacing the telescope. We need a good adjustable one to handle various optical train and guidescope arrangements. Bob is going to buy one RSN.
- Lack of Guiding. We just haven't had time to experiment with the STV and get it working. In theory, this guider is supposed to be the best and should solve any problems in this regime once we get past the learning curve. Coming soon.
- Imperfect Focus. The STI was apparently compromised by poor seeing. Bob suggested that we make a mask for the OTA that reduces the aperture to something more modest, which will have the effect of increasing the seeing. Perhaps a Pickering-5 equivalent could be made.