After about three weeks of cloudy skies we finally had a totally clear day today so we brought the telescope down to the Overlook in White Rock despite the 86% full moon. The purpose of the trip was to test out a bunch of the new gear that had arrived since we last did some observing (focal reducer, ballhead, illuminated reticle eyepiece). Unfortunately, we both forgot to bring the instructions for drift polar alignment with us and couldn't reconstruct the correct steps on site. Not having a copy of the instructions in the log book or something was a mistake and I've corrected the equipment checklist to reflect this.
Also, we discovered that with the focal reducer on the back of the OTA, followed by the microfocuser and the 2" diagonal, the optical train extends too far back and the declination can't go beyond about 80 degrees without the diagonal running into the base of the fork. One solution to this is to add the focal reducer after alignment and not take pictures of anything through the tube that has a declination higher than 80 degrees... I hope there is a better solution somewhere. Perhaps we can lose the diagonal for prime focus photography but I'm not sure we'll be able to get far enough back for proper focus. Anyway, further experimentation is required here.
Next problem: We didn't bring the vibration pads because the silicone liner Bob installed hadn't cured yet, and the telescope was noticeably shaky from just walking near it, to the tune of about 1mm of image movement at the focal plane (as measured on the reticle). Hopefully the vibration pads, when completed, will damp this out.
Because we're new at this and we had to work through the problems listed above, it took a long time (over and hour) to get set up and a rough alignment with Polaris. By this time, of course, clouds had started to come in. They were really light clouds, but the big moon was making them bright and ruining the opportunity for any long-exposure photography. Instead, we started with a series of lens aberration tests on the 50mm f/1.4 lens. We mounted the camera to the ballhead and took repeated two-minute exposures of Orion, stopping the aperture down one half step every exposure.
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The constellation Orion taken with a Canon 300-D and a 50mm lens set at f/1.4 for two minutes.
This is the least-cloudy picture we got all night.
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Of course, this was a mistake as I should have lengthened the exposure by 1.4x each stop, I neglected to do this for some reason. Also, the clouds moved in steadily and towards the end of our eight-exposure test they were all over the frame dimming out certain portions, etc.
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Some stars from the corner of the image of Orion, taken at various f-stops
with the 50mm f/1.4 lens. Intermittent clouds account for the changes in
brightness. This test was flawed as all exposures were at the same shutter
speed rather than accounting for the reduced light gathering of smaller
apertures with longer exposures.
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Despite these problems, it is pretty clear in this test strip that the aberration on the bright star is all but gone by f/2.5. To be safe, I'll probably set a lower limit on the f/stop for astrophotography use of this lens to f/2.8. Also, bear in mind that this aberration is only apparent in the corners and the center of the image looked great even at f/1.4.
At some point I'd like to re-do this test with a good dark sky, no moon, and no clouds, and with proper exposure compensation for the changing apertures. However, I expect the results to be the same. Also, I need to repeat this test with the 28mm lens and the 100-300mm zoom lens at several zoom points.
Anyway it was very cold this evening (~15 F degrees) and with the clouds we called it a night before doing the other lenses.