Having been disappointed, yet again by Jupiter's lack of clarity, I then checked the collimation with the Cheshire e.p. This confirmed it was really quite good with the bright, reflected spots overlapping closely.
The presence of two bright stars, one to the east and the other to the west, suggested I conduct some star tests. A very colourful experience that proved to be with violent thermal agitation! Almost as if a vivid stream of colour was rushing past the multiple fine rings and badly frayed edges each side of focus. It was difficult to be certain that the brightly coloured mess actually had a focal point where the star contracted to an intense, spangled, constantly moving, intense yellowish dot. The green and magenta were certainly pretty but it was no surprise that Jupiter was another disappointment.
I brought out the Neumann Ronchi test eyepiece for the very first time and was delighted [at first] to see dead straight lines outside focus. The problems started as I neared focus and the lines rapidly spread out [as expected]. The outer lines began to bend sharply around the edges of the lens as if testing a steep parabola at the center of curvature. Again the vivid colours showed that it was an achromat and not a mirror. Inside focus the curvature was absent and the lines remained straight but greatly expanded as expected. Nearer focus there was a single black line across the center and two, multi-hued outer lines.
Not ideal conditions for testing due to the strong thermal currents. The sun had shone brightly all day from a clear blue sky. Though thin cloud had passed over in the early evening it had gone by 10pm. Could the OTA have retained some heat despite falling into shadow by 5pm?Five hours to cool 10F? I doubt it. Should I install a fan and ventilation port to help cool the thin, steel tube? Metal is a very good conductor so the tube had probably cooled well enough in the 10F drop over several hours. The lens elements are quite massive but surely four to five hours of exposure to the cooling air is enough to follow those few degrees?
I think I have proved to my own satisfaction that the folded design does not detract from whatever the lens can show in good seeing conditions. Each OTA needs a different height of pier and very different fixing methods to the same mounting saddle. Though I had no great difficulty lifting the long and heavy OTA back down at 11.30pm, the lift back up next time is not something to which I look forwards. So I shall rebuild the folded OTA and seriously reinforce the [temporary] fixing crossbars to reduce flexure. I could even use a keyhole bayonet fitting to the saddle crossbars to reduce the present balancing act on the beer crates and stepladders.
The folded OTA is rather bulky and certainly no lightweight, but promises easier handling. Particularly as the objective lens can be fitted via its bayonet rather than being part of the overall weight of the OTA. Overcoming the psychological hurdle to going out to observe always requires the least potential effort ahead. At least I [think I] have proved that the straight tube is not a night and day improvement over a folded optical path. Further star testing and the Ronchi test EP will follow when conditions improve. Meanwhile I shall continue to practice my solar photography.
Click on any image for an enlargement.
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