Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

6" f8 or 8" f6 for Planetary?

DeejayP999

Recommended Posts

DeejayP999

DeejayP999

Posted
Posted

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...

Link to comment
Share on other sites
  • Replies 25
  • Created
  • Last Reply
John

John

Posted
Posted

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.

Link to comment
Share on other sites
Merlin

Merlin

Posted
Posted

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.. ..

Link to comment
Share on other sites
John

John

Posted
Posted

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.

Link to comment
Share on other sites
SnakeyJ

SnakeyJ

Posted
Posted

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

Link to comment
Share on other sites
John

John

Posted
Posted

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:

Link to comment
Share on other sites
Moonshane

Moonshane

Posted
Posted

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

Link to comment
Share on other sites
Merlin

Merlin

Posted
Posted

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.

Link to comment
Share on other sites
DeejayP999

DeejayP999

Posted
  • Author
Posted

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:

Link to comment
Share on other sites
John

John

Posted
Posted

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:

Link to comment
Share on other sites
Stu

Stu

Posted
Posted

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.

Link to comment
Share on other sites
scarp15

scarp15

Posted
Posted

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.  

Link to comment
Share on other sites
scarp15

scarp15

Posted
Posted

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.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.