Jupiter: Excellent views tonight

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    Posted by Robin Vann at 01:16 on 2010 Aug 11

    The best views I’ve had in my, so far, short career.I can see the NEB of course, and also a trace of some TB. Will report later after a good time at the eyepiece.I am also seeing Ganymede and Callisto as tiny discs for the first time.I am at 280x with a Baader Neodymium + IR Cut on my Skywatcher 150mm Newtonian.I can’t wait for my Orion (UK) SPX 200mm to arrive.Robin VannWarwickshireUK


    Posted by Robin Vann at 02:56 on 2010 Aug 11

    I suspect my assertion that I could see Ganymede and Callisto as tiny discs was not correct. They do look like tiny discs at 280x but I am thinking now that this is an optical issue or lack of experience; seeing 1 arcsecond objects as discs seems pretty unlikely at this magnification.I have now dropped the magnification to 150x where I feel the contrast is better; I am still using the Neodymium + IR cut, as this seems to ‘bring up’ the reddish details. I shall now detail the reddish areas I am identifying. Obviously they are seperated by the appropriate zones.I am seeing the NEB strongly and, somewhat less strongly, the SPR, which seems to occupy more latitude than it should: I think I’m seeing the SPR and one or more of the STB as a whole, without seeing anything of an STZ.There is also a glimpse of the NTB showing during moments of best seeing and very occasionally, a hint of the NNTB.Telescope: Skywatcher 150mm f/8Eyepiece: Baader Hyperion 8mmFilter: Baader Neodymium + IR Cut (Moon and Skyglow): obviously there is no moon troubling me, but the filter seems to be enhancing the reddish areas I’m discussing.I still can’t wait for my Orion (UK) SPX200 f/6 to arrive.Robin VannWarwickshire


    Posted by David Arditti at 14:53 on 2010 Aug 11

    I think your initial assertion, Robin, that you could see the moons as discs was almost certain correct. I would expect them to be resolved at 280x in a 150mm scope. I was looking last night (morning of the 11th) as well, with a 14" SCT and a 5" SCT with binoviewers. Obviously I could see far more detail in the 14", but the view through the 5" was interesting as well. I did not think to look to see if I could resolve the moons with the 5", but I will see next time. I suspect the answer would be yes, and even with a smaller aperture under good conditions. One thing I have learned (from imaging) is that Dawes’ Limit, developed for resolving double stars, does not apply to solid objects like moons and planetary detail. You can often resolve them below Dawes’ Limit. I took some images with the 14" (not yet processed).


    Posted by Robin Vann at 00:47 on 2010 Aug 12

    Thank you David, that’s reassuring.I’ve also been checking up on the Dawes’ limit. My book (a lunar observing guide) gives it as R = 120 / A where A is aperture in mm and R is in arcseconds. This would appear to give me a Dawes limit of 0.8 arcseconds with my 150mm aperture, assuming seeing being good enough; and for your 5", 0.96 arcseconds, which would also be sufficient to discern them as discs, seeing allowing.I would be interested in your experiments in this regard, and to know what formula you are using for Dawes’ limit.Robin

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