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Minimum Aperture for Pluto?

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#26 Allan Wade

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Posted 10 January 2021 - 06:22 PM

I have a couple books by very experienced well known observers, and they both say that to be able to see it, and know that you are seeing it, a 12 inch scope would be needed.

I’ve long since learnt that the skill required to write books about observing is in no way related to observing skill. I’ve seen Pluto easily in my old 12” from my city home. I remember that night, because I showed a mate who had never looked through a telescope before and he could see Pluto as well.


Some of the expert observers here on Cloudy Nights would get Pluto in a 6” or perhaps less from a Bortle 1 sky.

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#27 Sheol


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Posted 12 January 2021 - 07:37 PM

                                  In all honesty, larger than anything I own. Or will ever own. Unless seeing a tiny, very dim "star" maybe move against the background sky over a period of a day or two.


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#28 Rutilus



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Posted 13 January 2021 - 06:38 AM

If Pluto was at a higher altitude from my location, I would be able to observe it in scopes well below 12 inch aperture.

Some 25 years ago I observed it with a 14 inch scope, at the time it was bright enough to be seen with a 4 inch refractor

from dark skies. Here is a drawing I made showing its movement over several nights with the large scope.

Attached Thumbnails

  • Pluto-cn-X.jpg

Edited by Rutilus, 13 January 2021 - 06:39 AM.

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#29 Sheol


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Posted 13 January 2021 - 07:38 PM

                                              Oops, my bad. I spoke in error. I really meant as a recognizable disc. I know you can see a dim little star-like spot. Maybe my properly collimated Orion XX12i Truss Dob COULD see little Pluto. BTW, you are so abused I saw New Horizons' pics, that is NOT a *Minor Planet* ! But that is another thread and discussion.


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#30 Redbetter


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Posted 13 January 2021 - 09:05 PM

Pluto is too small and distant to show as anything but a stellar point in even the largest apertures, except that Charon might be resolvable separately from the planet, if the seeing is nearly perfect with a very large aperture.  Allan has been trying for Charon with the 32" but has not had the seeing for it so far.


Pluto reaches about 0.1 arc seconds diameter near opposition.   That will be near the limit of resolution with a 48" aperture or so, but Pluto's surface brightness is low since it is past Neptune which is about 4 magnitude lower surface brightness than Jupiter.  That means that the magnified image for resolution would reduce surface brightness well into the mesopic range, making resolution of a disk about the size of the spurious disk quite difficult visually, even in perfect seeing.  


As for aperture required to see Pluto, it still just comes down to how deep a person can go at a given elevation in the sky with their scope and conditions.  Pluto is perhaps 14.4 magnitude and I can reach ~15.0 with a 4.33" scope in relatively steady Bortle 2/3 skies.  But that is for objects higher in the sky.  I lose a few tenths with Pluto lower in the sky.  This has been critical when I have tried with a 3.15" scope in Bortle 1 conditions, since the sky wasn't steady enough down low.  I have reached 14.3 with that ED80 scope higher in the sky in Bortle 2/3 conditions, but was getting stuck at 14.0 around Pluto's position lower in the sky.


I have no doubt I could see Pluto with a 4" ED refractor from my normal observing sites in good conditions, and I suspect I could scrape by with a 90 ED, but every tenth magnitude makes it tougher.  I would probably need to catch it closest to opposition to take advantage of any Seeliger effect.  There is also some known variation in brightness depending on which face is toward us (albedo variation) so there is some luck involved...unless someone takes a look at the albedo variation and ties it to the orbital position to find the maxima in the 153 hour period.  

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#31 Tyson M

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Posted 14 January 2021 - 04:43 PM

Hmmmm July 15th for its closest approach and its very low- east of the Teapot in Sagittarius. Going to be a tough target at mag 14.3

Edited by Tyson M, 14 January 2021 - 04:45 PM.

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