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I awoke at 6.15am on New Year's Day and saw the sky was quite clear. With the Moon and Jupiter just to the west of south it was probably worth going out. Clear skies being in short supply I went without breakfast and just took a cup of coffee outside.Having dragged the pier over to my usual observing position I was set up by 6.30. The Moon's surface proved to be covered in rapid, though not particularly violent, thermal agitation [again.] Jupiter was at about 36 degrees high with the Moon slightly east and below at only about 33 degrees altitude.
After the usual hassle of dragging the massive pier and struggling to get the heavy OTA into the rings I was actually too warm in my big duvet jacket. So I had to remove my hat and jacket for a while to to cool off. Though it was far from warm with my hands getting cold whenever I touched bare metal. The car roof was still white with overnight frost at first. My inverted, plastic water butt mounting protector proved to be quite a handy table for eyepieces and accessories.
I homed in on Plato, as I usually do, in the hope of seeing some minor craters. It was not to be though as the Lunar features were slightly soft. Eventually the Moon became dimmer and I discovered the objective had misted over when I removed the eyepiece and looked though the open focuser. The OTA had come from unheated accommodation with temperatures inside and out reading 37F, 3C on my digital thermometer.
I had not fitted the full dewshield since dewing over had not occurred before today with only the stumpy permanent dewshield fitted. After fitting the longer dewshield I wiped the lens gently with a microfiber lens cloth and thankfully it remained clear after that. It would be handy to be able to keep the objective indoors and fit it to the OTA only for observation. Though this would require a complete redesign of the counter-cell and might well introduce collimation problems.
With the slightly soft images I was getting again I decided to use a thin feeler gauge blade to knife-edge the objective on a bright star. Examining the star first, with an eyepiece, had showed soft edges both inside and outside of focus. With any rings rather lost in the colourful mush.
The knife-edge required the 5" of straight extensions to be able to get the focus at the surface of the 2-1.25" adapter. The nearer I came to focus the more convex became the shadow! Ideally one wants a straight, advancing edge. Then even darkening at focus. With the direction of the shadow movement reversing outside focus.
This is not quite so straightforward with an achromat because the different wavelengths [colours] each fall at different focal planes. This softens the shadow compared with a concave mirror and causes different colours to appear. Filters can help but I had no desire to get so involved with no bright star being particularly well placed for testing. I was half crouched, my back was aching and I was struggling to keep the star centered. So I didn't persevere with my testing for very long.
Perhaps I should set up an artificial test star at the far end of the drive to test the objective in comfort. A ball bearing or small Christmas ornament at 100 yards in bright sunshine should do. With the OTA horizontal and solidly supported on a workbench the "star" can be centered and kept there. There would be no thermal effects from the ground in weak sunshine with the drive itself being shaded by a tall hedge.
Back to the Moon and Jupiter and both remained rather soft. I couldn't tell whether the roof was responsible for the continuing thermal effects because it was now just below the objects I was trying to observe. Could the steel telescope tube be having some thermal effects on the image? It seemed unlikely unless I had brought the OTA out from the warmer indoors.
With slight fringing still visible at best focus I decided to try a 6" stop in front of the lens. It took only a few moments to cut out a cardboard packaging circle and then make a 150mm aperture in the middle with a craft knife. The Moon certainly became more monochromatic as a 6" f/19 [R35 equivalent] after the stop was fitted into the dewshield. Though it did not make much difference to the sharpness of the view. As usual, I rotated back and forth through my selection of secondhand, Meade 4000 Plossls. Neither Jupiter nor the Moon would take high powers and I topped out at about 175x without any increase in visible detail. The Fringe Killer filter just yellowed and dimmed the image without any increase in detail.
It quickly clouded over at 7.45 so it was time to pack up and go in for breakfast. The Moon made only one brief appearance after that but the sky was already brightening rapidly.
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