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DeejayP999

6" f8 or 8" f6 for Planetary?

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Hello everyone.

I recently sold my SkyWatcher 150PL (on the EQ 3-2 mount) and am looking to replace it with a Dob.

My use is purely visual, Lunar & Planetary and I was very satisfied with the planetary views put up by the 150PL (but not so keen on the mount).

I'm now considering the 150PL on a Dob mount or, possibly, the 200p.

I like the 200p because it's, well... Bigger!

However, I do wonder if the longer focal length and relatively smaller central obstruction of the 6" makes it more suitable for my use (especially with more basic eyepieces).

Decisions, decisions...

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I think the 8" F/6 would be even better than the 6" F/8. My old 200P dob used to put up great lunar and planetary views :smiley:

This report from Neil English is worth a read:

http://neilenglish.net/test-diving-the-sywatcher-skyliner200p/

Neil is a great refractor fan and mentions that the 200P even managed to surpass the Skywatcher 180mm maksutov-cassegrain on planetary views (Neil calls it a 17cm but it's the 180 mak that he is referring to)

F/6 is not too tough on eyepieces - I seem to remember plossls doing pretty well in mine.

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The flat on the supplied 200mm/f.6 will probably be bigger than it needs to be for planetary observations. We'll get better contrast for planetary if we keep the flat to the smallest practical size.

There's a calculation for working out the size of the minor axis for a planetary flat, but someone else might be able to tell you. I have a chart somewhere giving the lists of flat sizes for given focal lengths, I'll see if I can find it.. ..

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The flat on the supplied 200mm/f.6 will probably be bigger than it needs to be for planetary observations. We'll get better contrast for planetary if we keep the flat to the smallest practical size.

There's a calculation for working out the size of the minor axis for a planetary flat, but someone else might be able to tell you. I have a chart somewhere giving the lists of flat sizes for given focal lengths, I'll see if I can find it.. ..

20% of the primary is the generally stated "ideal" diameter for the secondary for high resolution work.

The Skywatcher 150PL has a 36mm secondary which is 24% of the primary diameter. The 200P dobsonian has a 50mm secondary which is 25%. Neither completely optimal but I'd suggest that the additional 34% aperture of the 200P would more than make up for the tiny difference in % secondary size.

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I had a 6" F/8 then went to a 8" F/6 which is the same length as the 6" but slightly wider, it out-performed the 6" in all areas.

If you can afford it the 8" is the way to go.

Edited by dweller25
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In agreement with all above, but always tempted to go bigger still with a dobsonian (within comfortable storage and handling limits).   OO's VX10L has always looked very tempting for planetary and meets the 20% rule for secondary size.    They also used to make a 200mm/F8 'planetary newtonian', with an 18% obstruction - though this size seems to be very out of vogue with the big manufacturers.  

Edit - Review of OO 8"/F8 Dobsonian

Edited by SnakeyJ
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In agreement with all above, but always tempted to go bigger still with a dobsonian (within comfortable storage and handling limits).   OO's VX10L has always looked very tempting for planetary and meets the 20% rule for secondary size.    They also used to make a 200mm/F8 'planetary newtonian', with an 18% obstruction - though this size seems to be very out of vogue with the big manufacturers.  

Edit - Review of OO 8"/F8 Dobsonian

My 12" F/5.3 Orion Optics dob has a 21% obstruction - nearly there :smiley:

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my 170mm f11 dobfractor has a zero obstruction.  Seriously though, I love my 6" f11 dobsonian but agree that use on an equitorial platform transforms use at higher mags. I do despite what is said in the 8" f8 review find that aperture brings more detail even though yes you have to wait longer for the moments of seeing with more aperture. I'd say the 8" f6 too. BUT I'd get the 8" f8 if you can find one.

post-17776-133877758944.jpg

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According to Sigwick's chart of flat sizes, the minor axis of the flat on a 200mm/f.6 Newtonian for planetary work should be about 34mm.

There will be light loss of about 0.5 mag on the edge of the field, but it doesn't matter for planetary viewing, where maximum contrast is required.

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I keep kicking myself as i had the chance of an 8" F7 a while back, i should have got it but to many scopes

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200mm f6 is a nice size for planetary :)

I've always wanted a 250mm f8 with an undersized secondary. I always felt that would be the perfect planetary scope.

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very true re 250mm f8 but you might need a ladder to view through the eyepiece.

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Many thanks for the input - looks like the 8" is the favourite.

I've also just noticed that the 8" comes with a Crayford focuser instead of the standard R&P on the 6". That swings it even more in favour of the 8" for me.

However the Revelation (GSO) 8" f6 comes with a dual speed Crayford and a roller bearing mount for similar money.

But how good are the GSO mirrors vs Skywatcher? I've read mixed reviews about the GSO - but I've never seen anything negative written about the Skywatcher's optics.

Hmmm... :smiley:

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Hi,eons ago my wife brought me a F8,8.50 newt as a first scope how disappointed I was,I always wanted an F6,but the view of the planets and moon,just stunning,still got the scope lives in the corner of the shed......Des

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Whatever ever the reasons, First Light Optics still insist that the Skywatcher-Skyliner 200P is.........The UKs most popular Dobsonian telescope. :grin:

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I missed out on an eBay auction a few years ago for a 1/10th wave f8 Orion Optics 8" Dob. Looked lovely and went for about £350 I think. Still regret it!

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I missed out on an eBay auction a few years ago for a 1/10th wave f8 Orion Optics 8" Dob. Looked lovely and went for about £350 I think. Still regret it!

That would have been a superb planetary scope !

I'm always amazed at how little Orion Optics scopes go for sometimes, considering their reasonably high retail price :icon_scratch:

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That would have been a superb planetary scope !

I'm always amazed at how little Orion Optics scopes go for sometimes, considering their reasonably high retail price :icon_scratch:

I think that may be a legacy of their older stuff which was mechanically suspect, and mirrors not certified/tested. I think they chuck out some good stuff now and the used prices should really be higher, but they don't seem to rise for some reason.

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Yes the current OOUK VX range is perhaps a much improved package compared to former older versions. I use a VX 8" F6 1/10 wave mirror dobsonian. Terrific and versatile scope that will provide years of pleasurable use and that is the key, if you aim to retain and get fairly frequent use of the scope for the long haul, no need to get concerned about depreciation. The depreciation factor may itself become less of an issue for sellers as the reputation of the new breed gains in momentum.  

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My 12" F/5.3 was from the SPX range which preceded the current VX range. It's primary is 1/9th wave PV and it's Strehl is .987 according to the Zygo report. Quite decent for £250.00 :smiley:

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I've had a couple of 'wow' moments on Jupiter with my OO model. 2nd hand and with the mirror cert, they really are rather good.

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My 12" F/5.3 was from the SPX range which preceded the current VX range. It's primary is 1/9th wave PV and it's Strehl is .987 according to the Zygo report. Quite decent for £250.00 :smiley:

I've had a couple of 'wow' moments on Jupiter with my OO model. 2nd hand and with the mirror cert, they really are rather good.

Yes a win win situation, my VX14 dobsonian 1/10 wave optics was second hand, the VX8L new at a small discount, good scopes either way.

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