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Minimum Aperture for Pluto?

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#1 Spaceman Spiff

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Posted 03 January 2021 - 03:49 PM

I know this question has been asked many times before, but hear me out. Pluto is getting fainter, albeit slowly, so the answer to this question is constantly changing. From articles and forums a couple of years old, I've heard it would take at least 6-8". Is this still an accurate estimate?


Edited by Spaceman Spiff, 03 January 2021 - 03:52 PM.


#2 junomike

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Posted 03 January 2021 - 03:53 PM

Depends on what you consider "seeing it".  I've seen It as a star in an 10" OTA but it took a 16" to see it as more of a disc.


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#3 Spaceman Spiff

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Posted 03 January 2021 - 03:54 PM

Thanks for the answer! I would consider seeing it as a faint unresolved point of light enough. So, 10" would be the minimum?


Edited by Spaceman Spiff, 03 January 2021 - 03:57 PM.

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#4 jim kuhns

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Posted 03 January 2021 - 04:08 PM

Yes Spaceman Spiff a 10" I agree is the minimum. I observed Pluto in a 25" f/5 Obsession years ago

and was easy to spot using a finder chart. to point the way. .


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#5 spaceoddity

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Posted 03 January 2021 - 04:11 PM

You will need a good star chart to verify as it will just look like a very faint star in most telescopes. 


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#6 scopewizard

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Posted 03 January 2021 - 04:14 PM

Depends on what you consider "seeing it".  I've seen It as a star in an 10" OTA but it took a 16" to see it as more of a disc.

I have seen Pluto many times for many years in a row. It is typically 0.10 arcsec in diameter.

A 16", like mine, would not resolve past 0.3 arcsecs in absolute best conditions.

Ganymede is 12 times 1.20 arcsecs in average diameter. I can easily see the disc in good seeing conditions.

Even in Mauna Kea, best seeing conditions are rarely better than 0.3 arcsec.


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#7 TOMDEY

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Posted 03 January 2021 - 04:26 PM

Depends on what you consider "seeing it".  I've seen It as a star in an 10" OTA but it took a 16" to see it as more of a disc.

I have seen Pluto many times for many years in a row. It is typically 0.10 arcsec in diameter.

A 16", like mine, would not resolve past 0.3 arcsecs in absolute best conditions.

Ganymede is 12 times 1.20 arcsecs in average diameter. I can easily see the disc in good seeing conditions.

Even in Mauna Kea, best seeing conditions are rarely better than 0.3 arcsec.

Yeah, there is no way an amateur telescope could possibly show Pluto as a disc. It's just going to look like a dot... maybe an aberrated dot... but still an entirely unresolved/unresolvable dot.    Tom


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#8 junomike

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Posted 03 January 2021 - 04:54 PM

I have seen Pluto many times for many years in a row. It is typically 0.10 arcsec in diameter.

A 16", like mine, would not resolve past 0.3 arcsecs in absolute best conditions.

Ganymede is 12 times 1.20 arcsecs in average diameter. I can easily see the disc in good seeing conditions.

Even in Mauna Kea, best seeing conditions are rarely better than 0.3 arcsec.

 

 

Yeah, there is no way an amateur telescope could possibly show Pluto as a disc. It's just going to look like a dot... maybe an aberrated dot... but still an entirely unresolved/unresolvable dot.    Tom

Maybe a large dot then.  It was a long time ago but I recall it being far better in the 16" then in the 10".


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#9 Allan Wade

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Posted 03 January 2021 - 07:37 PM

I’ve looked at Pluto in the 32” at near 1000x and it still doesn’t resolve as a disc to me. But I can understand what Mike is saying. At higher power it dances around in the seeing and can look bloated like a disc.

 

I’ve not tried for Pluto in a small scope, but there’s no reason I wouldn’t see it in my TOA130 from my dark site. I remember seeing it quite easily with direct vision in my old 12” from my Bortle 5 city yard.


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#10 Spaceman Spiff

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Posted 03 January 2021 - 10:55 PM

So, I probably wouldn't have a chance with my 8" dob?



#11 Allan Wade

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Posted 03 January 2021 - 11:25 PM

An experienced observer at a Bortle 1 site would see Pluto in an 8” dob. An expert observer would see over one magnitude deeper than a novice with an 8” scope. That is enough to shift Pluto from visible to not visible. I don’t know where you sit on that scale, but nothing beats practice and time at the eyepiece for improving your observing skill.


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#12 Spaceman Spiff

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Posted 03 January 2021 - 11:59 PM

An experienced observer at a Bortle 1 site would see Pluto in an 8” dob. An expert observer would see over one magnitude deeper than a novice with an 8” scope. That is enough to shift Pluto from visible to not visible. I don’t know where you sit on that scale, but nothing beats practice and time at the eyepiece for improving your observing skill.

Thanks for the info! I’m not very experienced. From a Bortle 7 area, I can just barley see 13th magnitude stars with my telescope


Edited by Spaceman Spiff, 04 January 2021 - 12:11 AM.

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#13 Octans

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Posted 04 January 2021 - 12:05 AM

Maybe a large dot then.  It was a long time ago but I recall it being far better in the 16" then in the 10".

Pluto is ~0.08 arcsec in diameter, which is not much bigger than Betelgeuse at ~0.06 arcsec. That would be indistinguishable from a point source under any reasonable seeing conditions. However, Charon can get up to ~0.7 arcsec away from Pluto and appear as a lump on the side of Pluto under favorable conditions, making it look slightly extended.


Edited by Octans, 04 January 2021 - 12:05 AM.

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#14 Napp

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Posted 04 January 2021 - 12:14 AM

I saw Pluto in a 10 inch Newtonian a couple years ago.  It was very dim.  It took effort to find it and identify it.  You find the right star field, increase magnification, reorient in the star field and increase magnification.  You do this in incremental steps till you get down to Pluto being visible and verifying it’s the “star” of the proper magnitude not on the charts at the right position.  Thank goodness for Sky Safari going down to fainter than magnitude 15.


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#15 Spaceman Spiff

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Posted 04 January 2021 - 12:54 AM

So, chances are, I’d have to get a 10-12” telescope to definitively see it?



#16 Redbetter

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Posted 04 January 2021 - 04:10 AM

I have seen it with certainty several times in recent years with a 110mm ED refractor in Bortle 2 to 3 conditions.  Pluto is over half a magnitude brighter than my measured telescopic limiting magnitude with that scope, but it is also low in the sky which increases the difficulty.  I wouldn't have any reservations about trying it with a 4" apo in similarly dark & steady sky.  I have tried with an ED 80 in Bortle 1 conditions, but not succeeded--seeing was not steady enough to go deep that low in the sky, and I was falling at least 0.3 mag short of what I have done with that scope in the past.  I have also seen it with my 20" stopped down to 6", but in Bortle 1 to 2 conditions.

 

Most of this sort of threshold detection comes down to learning how to see stars at the limit of your aperture.  If you can detect stars moderately low in the sky down to 14.5 mag, you will be able to see Pluto since it is perhaps a tenth or two brighter than that.  For telescopic limiting magnitude the final 0.5 magnitude is very difficult/tenuous and is seen as occasional glints in the same location when you direct your gaze so that the most sensitive portions of your retina happen to catch them while in focus.  Once fully adapted at the eyepiece for the magnification/exit pupil employed,  observing the same area over several minutes can allow one to sort the "noise" from the real detections.  

 

You need a good finder chart down to 14.5 mag, and it doesn't hurt to use something like Wikisky later to check for stars somewhat dimmer than this, as you will likely see a few of them, and some can be near the same track/position or befuddle your pattern recognition.

 

A few years ago when my son was a complete novice with his 10" Dob I handed him the S&T chart for Pluto, explained how to employ it, and where to point his finder to start the hop sequence.  I turned to tougher DSO targets with my scope thinking that would keep him occupied for awhile.  He had it in about 15 minutes, but this was in Bortle 1 conditions.  I checked in his scope and he definitely had it; I had no trouble verifying the field and Pluto.


Edited by Redbetter, 04 January 2021 - 04:29 AM.

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#17 Allan Wade

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Posted 04 January 2021 - 04:11 AM

So, chances are, I’d have to get a 10-12” telescope to definitively see it?

Plus you would have to get well away from your Bortle 7 sky.


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#18 BrooksObs

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Posted 04 January 2021 - 11:14 AM

So, chances are, I’d have to get a 10-12” telescope to definitively see itCertainly

 

 

Not likely from your location with Bortle class 7 light polluted sky. Years back I easily saw it with my 12.5" Newtonian in Bortle 1-2 skies. But today I don't know if it would be within reach of my current 16" Dob due to my increasing light pollution level (currently class 6).

 

BrooksObs


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#19 Voyager 3

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Posted 05 January 2021 - 09:10 AM

Not likely from your location with Bortle class 7 light polluted sky. Years back I easily saw it with my 12.5" Newtonian in Bortle 1-2 skies. But today I don't know if it would be within reach of my current 16" Dob due to my increasing light pollution level (currently class 6).

BrooksObs

But still you are at the top of the heap as experience level goes ... So you may have a chance .

#20 Sketcher

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Posted 05 January 2021 - 01:10 PM

You should try with what you have.

 

Just finding the right spot to look in, matching the stars in your field of view with those on a Pluto finder chart, and experimenting to find the best magnification (for you, your sky and your telescope) for showing the faintest stars would help in improving your astronomical expertise -- regardless of whether or not you succeed in seeing Pluto.

 

And once you actually succeed in finding Pluto (perhaps with a larger telescope from a darker location), the next step is to make a sketch of Pluto along with field stars (at least those that lie forward along the path of Pluto's orbit) so that you can make a follow-up observation on the next clear night in order to verify, by Pluto's obvious motion, that you did indeed find Pluto.

 

Back in the old days, when I first started observing/looking for Pluto, the published minimum required apertures tended to be about double that which some observers actually needed (and half of what others needed) -- depending, of course, on sky darkness, experience, etc.  So, minimum required aperture is one of those things that isn't going to be the same for all people.  People are different in their abilities.  Telescopes are different in their capabilities.  Pluto's altitude in one's sky will make a difference.  And differing levels of sky darkness and transparency can make for some humongous differences.

 

So get out there and give it a try.  There's nothing to lose and everything to gain.


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#21 Redbetter

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Posted 05 January 2021 - 02:29 PM

One of the common themes on CN runs something like "what is the minimum aperture to see X from Times Square?"  There is no good answer for what it will take to see something from heavily light polluted conditions because they are so variable and there is feedback with any haze, transparency issues, and glare is a big factor as well.  One can give a much better estimate of the minimum aperture to see something from dark rural sky--it doesn't have to be pristine but something yielding about 21 MPSAS will get a person in the ballpark.  

 

The standard suggestion for verifying something is Pluto says that one should view it over two nights (not necessarily back-to-back nights, but close enough together to demonstrate the position has changed.)  However, one can actually accomplish much the same thing a month or so later by revisiting the original field and trying to find the same speck of light in the same position. Obviously this helps if one has made a sketch or notes that explain the position relative to the chart used.  If you can see stars just as dim or dimmer than Pluto was, and there is nothing where Pluto was before, then you had it.  On the other hand if you see a star in the same spot you thought was Pluto...you didn't have it before. 

 

The above is a check I have done for various moving targets, particularly with a marginal sighting near the limits of the scope or when multiple candidates of similar brightness were suspected near an anticipated position.  I have used this as added verification even when I observed something two or more times over different nights.  I have done this for asteroids (which are comparatively bright) and I have done if for Pluto, Makemake, and Haumea.  Ironically, I haven't had trouble with verifying the dimmer targets above, it has been the asteroids that have had greater positional uncertainty and resulted in some misses when I didn't have both a sufficiently deep chart and a good (accurate) track.  Famous last words:  "It is 10th magnitude, I won't have trouble identifying it in this field"--followed by not seeing anything of the proper magnitude in the near vicinity, or several objects of unexpectedly similar appearing magnitude.  I have noticed at times that some of the software and online ephemerides disagree substantially with one another on positions, and this has left me looking for things where they were not.


Edited by Redbetter, 05 January 2021 - 03:13 PM.

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#22 grif 678

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Posted 08 January 2021 - 10:32 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.


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#23 Redbetter

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Posted 09 January 2021 - 08:12 AM

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.

They are simply wrong.


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#24 GeneT

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Posted 10 January 2021 - 01:27 AM

However, Charon can get up to ~0.7 arcsec away from Pluto and appear as a lump on the side of Pluto under favorable conditions, making it look slightly extended.

I would love to see this.



#25 Octans

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Posted 10 January 2021 - 01:54 AM

Here's a nice image by Terry Lovejoy from a couple months ago that gives a idea of what Pluto+Charon could look like in a relatively big telescope: https://twitter.com/...523551347101697 It's very subtle.


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