Which of these two scopes for planetary use?

[great_bear]

Take two, brand new, Sky-Watcher scopes, and assume the build and optical quality is identical for both. Which makes for the best visual planetary scope of the two?

1. Sky-Watcher Skyliner Dobsonian 6" F8
   
2. Sky-Watcher Skyliner Dobsonian 8" F6
   

The 6" has a higher focal ratio, and the 8" is at the size where thermal issues in the absence of a fan might affect image stability and fine detail. On the flipside, 8" is a step up in aperture compared to 6".

So - which do you think would be best visually on planets?

[Pete Presland]

it would be the 8" for me,aperture is king!

[spaceboy]

Ditto

[swamp thing]

Yeah I would go with the 8"

The extra aperture = better resolution, exactly what you want on the Planets.

[Bizibilder]

As the focal lengths are the same I would opt for aparture/resolution for the few nights when you can use it. The 8" wins.

[Moonshane]

I am going to disagree. based on my experience with my 6" f11 I'd go for the 6" if this is for planetary / double star etc viewing only. I have also had a 12" f5.3 and have a 16" f4 and there's generally no comparison. the 6" wins on planets for most of the time. VERY occasionally more aperture matters when the seeing is excellent but more often than not the apparent increased contrast with the slower FR means that more detail is visible.

this will be closer of course with the 6" f8 and the 8" f6 but for planetary I'd go for the 6".

When I stop down my 16" with an aperture mask (170mm f11 aperture unobstructed) the difference is very apparent on the moon and doubles (not tried it on a planet yet).

obviously if viewing targets were widened then aperture wins every time.

[neil phillips]

I am going to disagree. based on my experience with my 6" f11 I'd go for the 6" if this is for planetary / double star etc viewing only. I have also had a 12" f5.3 and have a 16" f4 and there's generally no comparison. the 6" wins on planets for most of the time. VERY occasionally more aperture matters when the seeing is excellent but more often than not the apparent increased contrast with the slower FR means that more detail is visible.

this will be closer of course with the 6" f8 and the 8" f6 but for planetary I'd go for the 6".

When I stop down my 16" with an aperture mask (170mm f11 aperture unobstructed) the difference is very apparent on the moon and doubles (not tried it on a planet yet).

obviously if viewing targets were widened then aperture wins every time.

This may be true visually but most certainly not for imaging applications. If imaging will ever be considered the 8" will win hands down. Greater resolution. less gain needed for the ccd. Fans can be fitted fairly easily thats not a concern.

[Moonshane]

I agree Neil but Jeremy did specify visually. I'd also agree and recommend a fan for all dobs no matter what size.

[great_bear]

Thanks for the views so far - please keep them coming guys - particularly folks with active experience of both (or similar).

(oh, and yes - it's purely visual that I am discussing here...)

When I stop down my 16" with an aperture mask (170mm f11 aperture unobstructed) the difference is very apparent on the moon and doubles (not tried it on a planet yet).

That's interesting - although I would thing stopping down isn't quite the same; I'd have thought you get the benefit of the "smaller column of air" (if there's any truth in that hypothesis, which I've always doubted ) but would still suffer the thermal management issues of a big tube.

i.e. I'd expect a stopped down big dob to perform worse than a dob made in that particular size.

Although in your case Shane, I'm assuming you've got fans etc. all on the go in your 16" so I guess there's no/few thermal issues?

[SiriusB]

Hi Great Bear,

Just a few thoughts.

At one time i think an 8"/8.5"/8.75" f6..ish Newt was pretty standard fare for the keen visual planetary observer.

Seeing at your locale would influence my choice,How is it?

At mine it would have to be the 6" at least as far as planetary observing went, but i'd still choose the 8" overall.

My old Nortons says 8" is about the min size for "serious" observing of Saturn & Mars.

Worth bearing in mind when you stop a newt down off axis you're subject to the designs off axis abberations. A Newt of modest focal ratio has a diffraction limited field that is relatively small, - possibly not so great for critical planetary viewing then.?

[Moonshane]

I think the fact that the aperture is a little more and the hole is at the top on a well cooled mirror and flocked tube will help, plus it's a better figure (1/8 pv in the 16" and 1/6 pv in the 6"). I don't actually have a fan fitted currently but that's next. I'll report back when I have had a chance of a go at Jupiter - hopefully before I die - there's so much blummin' cloud!

[Moonshane]

Worth bearing in mind when you stop a newt down off axis you're subject to the designs off axis abberations. A Newt of modest focal ratio has a diffraction limited field that is relatively small, - possibly not so great for critical planetary viewing then.?

true but I am using a paracorr which I understand creates the same DLF as an f8 scope. all I can say is what I see through the stopped down aperture and that's very pleasing at least on the moon and doubles.