24th December 2015: Jupiter beckons again.

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Jupiter was glowing brightly to the south again as I rose at 6.30 am. By the time I was fully set up it was nearly 7.00 and Jupiter was still only about 32-33 degrees high according to Stellarium. The planet was moving along just above the roof ridge seen from my usual flat observing spot. So I decided to drag the entire pier and telescope down the drive to bring Jupiter into clear sky.

The optical effect, if any, was not dramatic, but I was now in direct line with the wind being channeled past the house. Observing with one's eye watering and the telescope shaking was not an improvement over my more sheltered area north of the house. Yet again there was a strong thermal overlay on the Jovian belts with them disappearing completely at times. From long experience I know I usually need at least 40 degrees altitude for a decent view of the planets almost regardless of the seeing conditions.

I persevered and pushed the power up to 175x with the Baader Fringe Killer in place. Though the filter had little benefit and only seemed to dim and yellow the image. I rotated through various eyepieces to see which might be best but there was little to choose between them. The point of best focus was soft and best judged as lying somewhere between a magenta or a green fringe around the planet. A far cry from recent lunar views where the focus is much tighter. Two dark Jovian belts kept teasing with increased detail but it was never resolved.

I still haven't got around to making a 150mm, 6" mask for the 7" iStar R35 objective. 84/6" x 1.35 = f/19. It would be interesting to see if stopping down the 7" would actually improve the view. There would be some loss of aperture but the colour correction would be further improved from the present f/16.2. The CA ratio changes from 2.3 to 3.15 which moves a 6" f/12 R35 well into Sidgwick's well corrected range. When focusing with the full 180mm aperture the image goes quite free of false colour and devoid of fringing at best focus.

As previously discussed, I need more mounting stability in the wind and much less weight to carry or drag about. The OTA is very hard work to carry out from storage and then lift up into the open rings in stages. Moving the entire instrument always means traveling slightly uphill and the sheer weight is very noticeable despite the pneumatic tyres. It may be that the whole instrument is rocking on the flexible tyre walls during wind gusts. I suppose the wheels could be duplicated for lower rolling resistance on soft ground and less flexing in the wind. Solid "puncture proof" tyres are available but from long experience I know they they will only sink deeper. Despite decades of compaction the graveled area I have to work from is never really firm and becomes worse in winter and after rain. Any plans to move to a better site, with a clear southern view, have never materialized. 

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