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Best aperture for planetary viewing


What's the best aperture for planetary observation?  

2 members have voted

  1. 1. What's the best aperture for planetary observation?

    • Under 5 inches
    • 5-6 inches
    • 6-8 inches
    • 8-10 inches
    • More than 10 inches

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If you could choose only one aperture for the best views of planets, what would you choose, given typical UK seeing?

The theory goes larger scopes are more sensitive to seeing, but resolve more in the good moments, so what's the happy medium for you?


Edited by Andrew*
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I've voted "more than 10" owing to the consistently good views of planets through my 12" f/6 David Lukehurst.

Interestingly, there seems to be a very strong preference for 8-10".

Any comments?


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I haven't voted because every time I use a different instrument I change my mind! When the 10 inch SCT is good it is very good. The 5.5 inch apo is very good far more often. Very occasionally the big Dob is the winner...

If I were to go for just one, I suppose the 5.5 apo.


Edited by ollypenrice
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I've voted 8"- 10". Whilst my 4" refractor will always give a really good view the Skymax 180 I'm using gives an excellent view most of the time and with increased resolution etc.

I know that if I was stuck with the 4" (what a hard life!) I'd always be hankering after the increased aperture...


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I've chosen 6-8" because the best planetary views I have had from home have been with an 8" f/6 Meade Dob.

The best view I have ever had though was through Astronut's SCT at SGL5.

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I'll challenge anyone's 8-10" Newt to a 'Saturn Shootout' with my 133mm f/12 refractor... I've never seen a scope that small beat my Apomax!


Edited by Ad Astra
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I've tried all sorts of scopes on the planets ....

4" APO

5" APO

6" Newtonian

7" Maksutov

8" Newtonian

9.25 SCT

10" Newtonian

My conclusions ....

1. The refractors give the 'nicest' images - sharp and contrasty.

2. Large central obstructions do not help with planetary views.

3. A good 10" Newtonian outperforms a 5" APO on the planets and is relatively cheap.

4. 'Poor seeing' can be overcome by putting a fan on the back of a newt to remove the boundary layer off the mirror surface.

BUT - the 10" Newt did my back in so I'll stick with the 5" APO.


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A trend I've seen on this forum is back injury.

Dweller25, you've done a lot of planetary sketching, so would you really say the 5" APO gets close to the performance of a 10" newt? I'm asking you because I've seen your sketches and you obviously pay close attention to the hard-to-see details on planets. I would have thought that going down to half the aperture would sacrafice too much resolution.

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  • 2 weeks later...

if the poll is that were are only allowed to view planets then I'd go for my 6" f11 newt. this outperformed my 12" f5.3 dob on almost every occasion. it has more contrast, sharpness and detail on average.

BUT and it's a big but (I like big buts and I cannot lie etc....) when the good seeing really kicked in the 12" gave literally jaw dropping photo quality images with more brightness and sharpness in the best moments of seeing than the 6". it also of course showed more moons than the 6". you know, those views that make you gasp and make loud(ish) proclamations in the middle of the night and you start explaining to the cat 'how awesome that was'.

if we are allowed to include other objects (other than doubles and lunar) then of course the 12" wins hands down and if I had to keep one it would be the latter.

I am putting together a 16" f4 with 1/8 PV and strehl of 0.984. Although not a typical planetary candidate, it will be interesting to see how it compares on planets with the 6" and how I recall the 12".

I also have a 8.5" f8 mirror set and will be interesting to see how this performs when converted into a dob. not sure of the figure/quality of that though.

Edited by Moonshane
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IMO I think I'd have to say that depending on seeing conditions 120x to 200x on most Planets works well. But when Mars is worth looking at it seems that it requires a little more , say 200x to 250x . This is what I have experienced .

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Well my 14" dob gives me better planetary views than any other scope I've owned (Helios 6" reflector, Meade 6" Achromat, WO Megrez 88FD). I think the Megrez could prove better than I've seen from it but I need to get some decent very short f/l planetary EPs. My shortest, a 7mm Nagler, only gives 71x in its 498mm f/l.

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