Minimum aperture for seeing GRS?

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    John Rogers

    People occasionally ask me what is the minimum equipment necessary to see the Great Red Spot on Jupiter, now that it is smaller than in previous decades, but very red and high-contrast.  And I need to ask you, members, as I don’t do much visual observing nowadays.  With what small aperture and magnification could you see the GRS in 2017, and was it harder or easier to see than in previous decades?  Thanks in advance!

    David Arditti
    In the last year I have been showing Jupiter to the general public with a 100mm ED glass refractor with a magnification of 80-160, and when the GRS is on the disk, and it has been pointed out to them, 95% of them have been able to see it. This would be in the poor seeing we have been getting at the low altitude we have for Jupiter. So this gives a measure for people with no experience of looking though telescopes at all.
    For more experienced observers I have little doubt a 75mm refractor would be enough, under typical conditions, with a magnification of 80.
    I would say the GRS is easier to see now than it was in past decades. I think in the past a 100mm refractor or 150mm reflector would have been the absolute minimum, with good conditions.
    I do have 80mm and 66mm refactors as well, so now the question has been posed, I’ll see if I and others can see the spot with those this year.
    John Rogers

    Thanks David.

    Paul G. Abel

    Hi John,

    In the past I was able to make it out easily in a 4 inch telescope. In the last few years, I have seen it without too much difficulty in a 90mm refractor. The use of a W#80 (light blue) helps to make it easier to see.



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