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I spent some time on the refractor today. Starting with the refitting of the trailer wheels to get back the necessary height for the straight tubed OTA. I also worked on the Vixen focuser to remove the last bit of shake when the focuser was rotated in the back plate. This required a thicker, plastic packing ring to take up the former slack. I wanted to line the inside of the focuser compression ring with felt to reduce friction but had none. Then it occurred to me that non-woven, disposable cleaning cloths have a very similar consistency. I now have the focuser firmly mounted and collimatable via stiff compression springs and the long retaining bolts inside the backplate.
The problem with my Orion Cheshire eyepiece is that its body is considerably undersized. So it rattles in all of the 2-1.25" adapters I own.[3] Unfortunately the rattle is too much for accuracy even with a compression ring type adapter. Yet the gap is too small for tape to be used as packing. The whole point of a Cheshire eyepiece is accurate optical alignment. Yet it feels as if I am using a length of string to measure alignment precisely. A collet style compression adapter would help but it has occurred to me that I could add further radial screws to the single one usually provided. A single screw will push the Cheshire off-axis but tilt remains an unknown.
The problem with my Orion Cheshire eyepiece is that its body is considerably undersized. So it rattles in all of the 2-1.25" adapters I own.[3] Unfortunately the rattle is too much for accuracy even with a compression ring type adapter. Yet the gap is too small for tape to be used as packing. The whole point of a Cheshire eyepiece is accurate optical alignment. Yet it feels as if I am using a length of string to measure alignment precisely. A collet style compression adapter would help but it has occurred to me that I could add further radial screws to the single one usually provided. A single screw will push the Cheshire off-axis but tilt remains an unknown.
In fact I have yet to properly collimate the iStar to a high standard thanks to a number of factors. Not least having two OTAs to work on as I developed the folded form alongside the classical straight tube model. The former focuser slop has been one long hurdle but the sheer size of the OTA makes adjustment to the push-pull objective cell a time consuming business. Until now, I have been relying on the f/12 character of the lens to relax my demands on its perfect alignment. If I were to collimate the lens and focuser on the workbench I am still unsure if it would retain alignment after being lifted bodily onto the mount. Its great size and weight preclude gentle handling! Then it has to be taken down again at the end of the observing session and stored on its nose. [Again and again, over time.] One of the reasons for the desirability of an observatory is not having to remove the telescope from its mounting. Which should leave the optics free of mechanical shock. The answer, for the moment, is to repeatedly check the alignment once I feel the Cheshire can be trusted.
Once again I left the refractor out for several hours before I started observing Jupiter just after 10pm. At the lowest power of 68x, with the 32mm while searching for Jupiter without a finder, I could see promising detail in the belts. Increasing power through the usual steps ended with a rotation through the powers available. 220x with the 9.7mm was slightly too much tonight so I settled on 178x with the 12mm. Detail was absent due to thermally agitated seeing but I wanted to practice just staring at the planet in the hope of a brief clarity. There was none to be had tonight. I stopped the aperture down to 150mm and this seemed to help. I also tried the 'Fringe Killer' and noticed fleeting detail in the blank areas of the planet. This filter brought no greater detail in the belts and produced strong yellow cast and considerable dimming. Though the eye eventually forgets about the yellowing. Just as it does when wearing yellow safety glasses.
There was no sign of the GRS or any moon shadows this evening. Though I sensed texture in the belts it always evaded resolution. The limb seemed to be boiling gently but lowering the power to 144x did not help in seeing any greater detail. It just made the planet smaller which required more concentration. It's odd how low were the powers I used with my 6" f/8 Celestron refractor. I rarely pushed that over 120x even with the Baader 'Fringe Killer.' I used to amuse myself by seeing how low a power I could use and still see the Cassini Division on Saturn. It was still possible below 50x but such was the strain that it felt as if my eyeball was being sucked out by the eyepiece! The higher powers of the iStar have me spoilt. Or perhaps they are making me lazy?
The roof of my home had been basking in bright sunshine all day as the air temperature reached 60F. The roof lay directly below Jupiter but moving the pier would not have helped. Jupiter has moved too far west and I would need to drag the entire instrument down the drive past the house. This would have exposed me to the wash of poorly designed headlights of passing traffic. It's amazing what car manufacturers can get away with for the sake of styling, isn't it? Remember when headlights were round and cast a 'proper' beam without lighting up everything else in the entire area?
There was no sign of the GRS or any moon shadows this evening. Though I sensed texture in the belts it always evaded resolution. The limb seemed to be boiling gently but lowering the power to 144x did not help in seeing any greater detail. It just made the planet smaller which required more concentration. It's odd how low were the powers I used with my 6" f/8 Celestron refractor. I rarely pushed that over 120x even with the Baader 'Fringe Killer.' I used to amuse myself by seeing how low a power I could use and still see the Cassini Division on Saturn. It was still possible below 50x but such was the strain that it felt as if my eyeball was being sucked out by the eyepiece! The higher powers of the iStar have me spoilt. Or perhaps they are making me lazy?
The roof of my home had been basking in bright sunshine all day as the air temperature reached 60F. The roof lay directly below Jupiter but moving the pier would not have helped. Jupiter has moved too far west and I would need to drag the entire instrument down the drive past the house. This would have exposed me to the wash of poorly designed headlights of passing traffic. It's amazing what car manufacturers can get away with for the sake of styling, isn't it? Remember when headlights were round and cast a 'proper' beam without lighting up everything else in the entire area?
Out of focus views on a bright star was the usual vivid "bonfire" of rapidly moving colours. The Ronchi eyepiece merely confirmed the bad seeing conditions. After an hour and a quarter of struggling with the poor 'seeing' I packed up.
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4 comments:
Chris, I had exactly the same problem with my Cheshire eyepiece - it rattled around far more than the errors I was seeking to eliminate. I think I wrapped a little Sellotape round it just to pad its shank out a little. I found that the Cheshire got me close, then a star test was used to finish the job - but I agree that's tricky with a long tube. I also tried using 5V laser pencils, and found that once really trued up to be square-on to a flat plate which rested on the objective's outer ring, this gave a useful indication of whether the beam emerged centrally from the focuser tube or not. BW Andrew
Hi Andrew
I don't know whether to be relieved yours was the same or disappointed with their quality control.
The scene is Orion headquarters: "I know, let's make some badly fitting measuring instruments."
You must have thinner Cellotape than mine. I thought I might try plumbers PTFE tape.
I am excused lasers on health and safety grounds. Too many aircraft passing overhead and I'm bound to get it wrong! ;-)
Regards
Chris
Just a further thought, based on your description of the geography of the garden and the poor seeing: as far as planetary observing is concerned, I would think the heat rising from the roof would impair the seeing far more than passing car headlamps. So I would be tempted to drag the scope to the south of the garden and have a black cloth over my head to eliminate headlight glare, just for the sake of hopefully viewing through stiller air. A
Hi Andrew
That roof has been a burden since I put it on myself, working alone, nearly 20 years ago. The only other potential sites for observing on either side of the house capture the sweep of passing headlights and neighbour's security lights. Then there are the woods to our West, the barn to the east, the choking smoke from both East and West, the overhanging, dripping birch trees.. I'm beginning to sound like the sad robot in Hitchhiker's Guide. ;-)
Regards
Chris
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