Meade 102ED APO centering and colimation
Posted 23 September 2008 - 05:42 PM
I've owned it used for about 2 years now and had never been able to get the centering right. The ED lens always bottomed out on the lens cell before I could get it fully adjusted. Even when the diffration ring and airy disc, of the star test, looked good at lower powers (150X), when the power was pushed up (250X), noticable color smearing was present and there was astigmatic brightening in the ring. With even higher magnification(>400X), the airy disc, diffraction ring pattern broke into one half rings and one half coma flares. If I tried to push the ED glass further, the optic pinched. With the best centering achievable, without pinching, planetary and lunar images were good up to around 150X, with slight color smear, and everpresent, albeit mild, astigmatic expansions of slightly defocused images, and cross like, atmosphere induced diffration spikes on focused ones. I had resigned myself to the fact that I had ended up purchasing one of the poor specimen 102ED's out there. It was still pretty good; I still enjoyed it at powers up to 150X; stars at low power were still "tack" sharp... but deep down, I was feeling dissapointed.
So the story goes, until a saturday afternoon about a month ago. I was pondering the centering issue and decided that a combination of centering and colimation might help with the smearing issue. (I had used cheshire eyepieces and colimation had appeared perfect.) I went out and set up a piece of granite (with a lot of mica flakes) in the sun about 150" away. I focused in with about 200X one of the smaller mica "stars." I first played with the centering, to make sure that I was at the typical "edge" of adjustment, and then backed off slightly, in order to give the glass some breathing room. I then ran the power up to 400X and saw the typical half coma/ half multi diffraction ringed image. I then began tilting the colimation of the lens cell. At the first adjustment, I was given a bit of hope when the rings had almost surrounded the airy disc, and much of the color smear was gone. With one more adjustment, a single, light difraction ring, with perfect even illumination, was surrounding a perfectly round airy disc!!! I was stunned. My 102ED APO was showing a text book perfect airy disc and diffraction ring at 400X, with no color smear! The astigmatism, when moving through focus, was gone too!
That night, I was astounded that instead of blurry jumping moons of Jupiter, I saw tiny dancing discs. Jupiter itself seemed steadier, and brighter. The defocused image was much different as well. Slightly inside focus, I saw a faint lime green rim to the planet. Outside of focus, their was a pale, light maroon ring. In foucs, NO color was visible. I ramped up to 280X and still no false color was visible, even in the slightest. At that magnification, the image was softer then at 100X, but I still counted numerous cloud bands, and the edge of the planet was sharp in between atmospheric disturbances. The real proof came on the Moon though. No false color was visible on the rim! Not only did I notice, when moving through focus, that the image truly snapped with texture, it also snapped with color!!!! I'd seen the color snap before this... always slightly outside of focus! I also easily picked out the four main craterlets in Plato! Later I saw that the night was only rated a 4 out of 5 on the clear sky clock website.
My experiences over the past few weeks have been the same. I've seen stars in Jupiter's 100X field of view that had never show up before. I've been able to see Jupiter's moons pass in front of the planet. I even swear that I may have picked out a dark veritcal band on one of Jupiter's moons. I compared my scope to a friend's C8, and my scope blew it away on Jupiter! Hands down; out of the water!!!!
I'm so happy with this scope now, that I had to share this experience with all of you. Who knows, maybe all of these $650, "poor specimen" 102ED's are really just ugly ducklings waiting to be hatch; treasures waiting to be discovered!!!!!!
Posted 30 October 2008 - 08:32 AM
Fife Lake, Mi.
Posted 30 March 2009 - 12:37 PM
I wonder why the 750 was never upgraded with the new LXD electronics, motors, etc?
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