1st Planetary scope: Refractor vs Dobsonian

[blusky]

Hi all,

I was lead to believe that a 4" refractor would give a sharper and crisper image of planets when compared to a 5" Dobson. However I can stretch the budget to about 8" Dobson vs perhaps 5" refractor.

Not looking to do any imaging, hence tracking is not required (however this may change).

Appreciate if you can tilt my opinion one way or another: a large aperture of a manual altazimuth Dobson vs clarity + daytime use of a smaller diameter refractor.

Many thanks!

[todd8137]

be in a dob owner a would say the 8"

[Moonshane]

assuming you mean an achro refractor then it's got to be the 8" dob.

it would be closer with an ED/APO refractor but the dob still has the edge as aperture gives more resolution.

[Andymac79]

Refractor for me but way more expensive to avoid the CA issues

Andy

[nicnac]

the dobsonian.

but get the skyliner (solid tube) then you can mount it on an EQ later if/when you get into imaging

[RikM]

If you mean Achro rather than Apo then I either own or have owned one of each of scopes listed (plus a 5" Mak). An 8" f/6 Dob in good collimation eats the others for breakfast.

Edited October 31, 2011 by RikM

[Andymac79]

Big dob may indeed gather more light. A frac will give you more 'connection' with what you see, Just a better view in my view

Andy

[RikM]

If you are talking about a ~5" Apo like a 120ED, then yes they do give very nice views, but the 8" f/6 Dob will still show you more detail and at 1/4 or less the price.

I think the views in the 5" f/9.4 achro refractor I have are good, and on some nights they really are very good, but I have done quite a few side-by-side comparisons with my 10" Dob over the past few months and as a result...the frac is back in it's case in the shed and the Dob is by the back door ready for action whenever there is a break in the clouds.

Edited October 31, 2011 by RikM

[brantuk]

"Not looking to do any imaging, hence tracking is not required (however this may change)."

Go for the dob to start. You'd get an achro in the same price range but a 5" appo (more suitable for imaging) will cost substantially more. If you do decide to have a stab at imaging then a short tube wide field appo will be the thing to go for. But you'll need a half decent EQ mount for it as well

[cantab]

I'm looking at a similar choice regarding my first scope, though at a smaller scale with a lower budget. Depending on exactly how much I have to spend, it could be 2.8" frac vs 4" mm dob (the Orion Skyscanner), or 3.5" frac vs 5" dob (the Skywatcher Heritage Flextube). I figure LP is going to spoil DSO views so I want a scope good for planets; does the extra aperture on a dob outweigh the "suitability" of a long-tube refractor for planetary targets?

[rory]

I'm looking at a similar choice regarding my first scope, though at a smaller scale with a lower budget. Depending on exactly how much I have to spend, it could be 2.8" frac vs 4" mm dob (the Orion Skyscanner), or 3.5" frac vs 5" dob (the Skywatcher Heritage Flextube). I figure LP is going to spoil DSO views so I want a scope good for planets; does the extra aperture on a dob outweigh the "suitability" of a long-tube refractor for planetary targets?

i would'nt go for the 4mm dob ! however portability would be good,could fit in your pocket .

[cantab]

LOL. I knew I'd muck things up when I decided to switch the units to inches from mm! And this forum preserves my mistake forever, no eternal edit here.

Edited November 1, 2011 by cantab

[blusky]

i would'nt go for the 4mm dob ! however portability would be good,could fit in your pocket .

Yes, indeed, and won't break the bank at all!

Here:

Buy Pocket Telescope and Microscope, Red online at JohnLewis.com - John Lewis

[blusky]

the dobsonian.

but get the skyliner (solid tube) then you can mount it on an EQ later if/when you get into imaging

Ah, was considering the extendable as well. I would guess the light pollution affects the extendable ones more since they are more open, no? Any particular models I should be considering/walking/running away? Thanks...

[Orion_the_Hunter]

I also own an 8" Dob. It's a fantastic all round 'scope which is great at Planetary observing. Part of the reason I find it good for planets is that I find it easy to track Planets and keep them in the eyepiece.

[Damo636]

Another vote for the 8'' dob.

[michael.h.f.wilkinson]

A well-figured, well-collimated 8" reflector will give more detail on planets than a 5" scope (even an apo), even though the contrast of the fine details may be a bit lower.

[John]

Ah, was considering the extendable as well. I would guess the light pollution affects the extendable ones more since they are more open, no? Any particular models I should be considering/walking/running away? Thanks...

Most people will use a light shroud with a truss / pole tubed newtonian to stop stray light getting into the optical tube. In apertures of 12" or more they make sense because they help with storage / transport of the scope but in apertures of 10" or less I reckon a solid tube is the way to go.

The Flextube Skywatcher dobsonians (awful name by the way - the last thing you want a scope tube to do is flex ) are actually a little heavier than their solid tube counterparts but, for some, the ability of the tube to collapse down is helpful for transport / storage.

Edited November 1, 2011 by John

[rightguard]

the dobsonian.

but get the skyliner (solid tube) then you can mount it on an EQ later if/when you get into imaging

This.

Also remember that in astronomy, no matter what those 'frac guys tell you.. aperture is king

An 8 inch dob will blow away a 5 or 6 inch newt or 'frac, EVERYTIME.

[michael.h.f.wilkinson]

This.

Also remember that in astronomy, no matter what those 'frac guys tell you.. aperture is king

An 8 inch dob will blow away a 5 or 6 inch newt or 'frac, EVERYTIME.

Agreed, but one caveat: fast refractors rule on very wide-field objects

[rory]

This.

Also remember that in astronomy, no matter what those 'frac guys tell you.. aperture is king

An 8 inch dob will blow away a 5 or 6 inch newt or 'frac, EVERYTIME.

A dob effectivley is a newtonian isn't it ? So a 8" will be better than a 6" !

[rightguard]

A dob effectivley is a newtonian isn't it ? So a 8" will be better than a 6" !

yep both are reflectors, they only differ in the mount

[RikM]

f-ratio is important. A 6" f/11 might very well best an 8" f/5 on most typical nights observing planets.

[michael.h.f.wilkinson]

f-ratio is important. A 6" f/11 might very well best an 8" f/5 on most typical nights observing planets.

A slower scope is better, but central obstruction need not be such a problem. My C8 with its 32% central obstruction (10% light loss) is sharper than my old 6" F/8. It is a bit softer at the same exit pupil.

[umadog]

"f-ratio is important. A 6" f/11 might very well best an 8" f/5 on most typical nights observing planets"

Focal ratio is not relevant it's the size of the obstruction that matters. So long as the secondary obstruction is under 20% of the primary diameter by area, the scope behaves like an unobstructed instrument. More than 20% and you start to see its effects. The effect you see isn't due to light loss it's due to increased diffraction caused by the large circumference of the secondary. This decreases contrast. However, there's no reason a 25% or even 30% obstructed scope can't perform very well. Why? If the scope is already a large aperture instrument with good optics then even with a hefty central obstruction it can still show superior contrast and detail. Optical quality and aperture matter more than focal ratio. I see this every time I observe Jupiter at f/4.

Focal ratio isn't the only thing which influences secondary obstruction size. The design of the instrument matters also. For example, my current f/4 has an obstruction of 19.5%. I used to have an Orion XX12, which is f/5, and that had an obstruction of 23%.

Here's Jupiter at f/4.5: www.cloudynights.com/ubbthreads/attachments/4889931-j2011-10-29_12-31_rgb_tba.jpg I guess it's been Barlowed. Hence the f/23.7 reference.

Edited November 1, 2011 by umadog