6" Refractor vs 10-12" Dobsonian

[chrispj]

I'm sure this question has been asked many times before in many similar forms so I have to apologise in advance!

 

I've recently dived into this past-time and rapidly concluded similar to bikes, surfboards and any other of my hobbies the ideal number of telescopes (& eyepieces) is n+1... My experience to date is with the pair of 4" refractors, which I've been enjoying splitting doubles, looking at the moon, and the more well known and brighter objects I can see from my south facing garden. I'm very much looking forward to seeing Saturn later in the year and the other objects the changing skies will bring my way. 

 

But, I'm finding one thing I'd like to see is galaxies and other DSOs, which I've had quite limited success so far (for example I'm reasonably sure I could see M13 but it was barely a smudge). I'm uncertain what the limits of what I should expect to see through the ST102 are, but aperture seems to be king so I presume I'll need to go bigger. But how big? Will a 6" fast refractor improve things significantly, or do I need to put in another garden store and go for a 10" or 12" Dobson? I've shied away from reflectors so far for simplicity sake.

 

The other question is what effect higher quality glass has (I am clueless) - is the main benefit for resolving more detail on planets for example and more aesthetically pleasing images, or is there an element of compensation for aperture with better light transmission?

 

 

 

 

[PeterW]

For faint galaxies aperture is probably the answer. High quality refractors are great for detail, but 10-12”reflector would be cheaper and be better for galaxies and alloW more power on globulars etc. You’ll need to be aware they can be surprisingly large (depending on design). Get a collimator and don’t fear using it. It’s quick to adjust if it needs it.

Peter

[Louis D]

A long focal length, 6" refractor is just about enough to start resolving large, close globular clusters like M13 at 200x and above.  However, an 8" Dob will do it better while being cheaper and easier to mount (it's built in for the Dob).  A 10" Dob will obviously be even better.  Aperture rules for both resolution and light gathering.  Dark sky sites are a must for galaxies.  Even then, don't expect them to look like astrophotographs, they don't.  You have to have tempered expectations.

Higher quality glass reduces false color or fringing that robs images of contrast.  This is mostly important for discerning low contrast details on planets.

[AlexK]

I spent several years observing hands on with the professional high end 150mm Zeiss Coude refractor and with the high end Russian long refractor AVR-3 (also 150mm). Both stationary installed on their stock high-end motorized equatorial piers under the sliding roof of the local youth astronomy club's observatory. I can tell you without a single doubt: views with my current very-average-optics 12" Dob are very long way better in every regard on planets and DSOs (weather permitting)...

[neil phillips]

You cant beat the physics of resolution i am afraid. Contrast is a different question and most often highly inflated by the refractor guys. 12" optics unless real bad quality will show more tiny craters on the moons surface. Compared to 6" unobstructed glass. Wishful thinking will not rewrite physics. Agree with Alex above

[Peter Drew]

You need both.     🙂

[RT65CB-SWL]

10 minutes ago, Peter Drew said:

You need both.     🙂

...and to quote the old adage/saying: "...aperture is king!" 😁

[Deadlake]

How long does a 10" reflector take to cool down?

I was thinking along the lines of a C9.25 or a GS0 10 F12 Cassegrain which I can put on a goto mount.

Ps: For deep sky aperture is indeed the limiting factor, for anything planets and doubles use a refractor.

[dweller25]

3 minutes ago, Deadlake said:

How long does a 10" reflector take to cool down?

I was thinking along the lines of a C9.25 or a GS0 10 F12 Cassegrain which I can put on a goto mount.

Ps: For deep sky aperture is indeed the limiting factor, for anything planets and doubles use a refractor.

With a rear mounted fan a 10” Newt will cool significantly faster than a C9.25 and is significantly cheaper too

[dweller25]

Hello Chris, a good 12” Dob would be the way to go for DSO’s - they are great on the planets too.

Skywatcher Dobs are good but heavy, Orion Optics make great mirrors and are a little lighter.

[Deadlake]

Just now, dweller25 said:

With a rear mounted fan a 10” Newt will cool significantly faster than a C9.25 and is significantly cheaper too

But not as fast at the GSO. I take the point for example those TS Boren power newts are very attractive, however for using on a GEM the focuser is located at the wrong place on the scope.

Apart from weight I think the GSO 10" has it.

[John]

14 minutes ago, Deadlake said:

How long does a 10" reflector take to cool down?

I was thinking along the lines of a C9.25 or a GS0 10 F12 Cassegrain which I can put on a goto mount.

Ps: For deep sky aperture is indeed the limiting factor, for anything planets and doubles use a refractor.

My solid tube 12 inch dob cools quite quickly. It can be used to observe at lowish powers on DSO's practically straight out of the house. After around 30 minutes high power views are really pretty stable and by 40 minutes I can really push the magnification to look for that fine lunar / planetary detail.

My 130mm triplet refractor seems to have a similar cooling time. The smaller aperture refractors are somewhat quicker as you would expect. The Tak 100 seems to need very little cooling - I've split quite tight double stars with that scope straight out of the house.

I keep all my scopes in the house.

 

Edited May 13, 2021 by John

[KP82]

For DSOs especially galaxies and globulars, there is no contest between large dobs and any readily available refractors (6" or less). Even my mere 8" will do better let alone a 12".

However sky quality from where you observe is also important. For example if you observe from your garden in the middle of a big city, the benefit of the larger aperture will be limited. One of the advantages of a 4 - 5" refractor is that you can easily take it to dark sites and they will show better results than a 10 - 12" in a severely lighth polluted garden. But if you live in a Bortle 4 or darker rural area, getting the dob is a no brainer.

Edited May 13, 2021 by KP82

[RT65CB-SWL]

1 minute ago, Deadlake said:

How long does a 10" reflector take to cool down?

I was thinking along the lines of a C9.25 or a GS0 10 F12 Cassegrain which I can put on a goto mount.

Ps: For deep sky aperture is indeed the limiting factor, for anything planets and doubles use a refractor.

I give my C6/SCT and 're-modded' ETX105 about 30-40 minutes minimum.

Maksutov's and SCT's have long focal lengths and maybe a limiting factor when imaging DSO's on a goto mount. You can use a FF/FR* with the SCT and bring to f6.3 using an f10 'scope, though not sure what it would be on a Maksutov. My ETX105 is f14 without a focal reducer and there is not a dedicated FF/FR for them [that I am aware of] but it still would be 'slow'.

 

* FF/FR = field flattener/focal reducer.

[Chris]

For faint fuzzy's it's all about the aperture: 

 

[scotty38]

20 minutes ago, Chris said:

For faint fuzzy's it's all about the aperture: 

 

I'm in a Bortle 4 and have a 6" Achromat but given it doesn't get dark at the minute and my low level vision is not 100% the 6" view above is not that far off what I see for M13. The illustration is slightly better than what I see but taking into account my "issues" seems fair to me.

[Deadlake]

Does anyone know how for a reflector to account for reflectance the mirror versus aperture, e.g.

• EdgeHD has around 82% reflectance
• GSO 10" F12 around 96% reflectance

Edit: should of googled.

https://www.cloudynights.com/articles/cat/articles/optical-theory/aperture-comparision-r1901

 

Edited May 13, 2021 by Deadlake

[Carbon Brush]

Modern reflector coatings are very good. There is a bit of variation between manufacturers. But the GSO 96% is typical.
Don't forget you lose at bit with every reflection. With a newt there are only 2 mirrors. With a compound/complex scope there may be more surfaces involved.
In terms of what you get for the ££ spent, you can't beat a big reflector. I have owned several in the 8" to 10" range over the years.
My take is that 10" is about the biggest for easy carrying, transport, etc. Though this is a personal view.
HTH, David.

[chrispj]

Thanks all,  it will be a big mirror next then! It's reasonably dark where I am (as long as my neighbour's security light isn't triggered) so hopefully I wouldn't be needing to take it anywhere very often. I'll keep an eye on the classifieds for when i get back from my next work trip (otherwise i would probably pounce on the 250p goto @Carbon Brush has up just now).

 

[Mr Spock]

I just went through the same choice process. I have a 250mm Goto Dob on order

It doesn't matter how good a refractor is, or what some claim they can see, as has already been said, ye canna change the laws of physics.

I used to have a 250mm Newt on an EQ6 before it came too unwieldy. I switched to a C9.25 which was a drop in contrast and quality.

The C9.25 also too a long time to cool down despite being stored in a cool porch. The new Dob is going in a shed so should cool really quick when put outside.

[Deadlake]

7 minutes ago, Mr Spock said:

I just went through the same choice process. I have a 250mm Goto Dob on order

It doesn't matter how good a refractor is, or what some claim they can see, as has already been said, ye canna change the laws of physics.

I used to have a 250mm Newt on an EQ6 before it came too unwieldy. I switched to a C9.25 which was a drop in contrast and quality.

The C9.25 also too a long time to cool down despite being stored in a cool porch. The new Dob is going in a shed so should cool really quick when put outside.

Thanks thats confirmed my thoughts, the C9.25 is off the list. Open or no tube it is. 

Edited May 13, 2021 by Deadlake

[Louis D]

3 hours ago, Deadlake said:

Ps: For deep sky aperture is indeed the limiting factor, for anything planets and doubles use a refractor.

If you get a premium, hand figured mirror, large Dobs blow the doors off the planetary views through any refractor under 8" that I've ever looked through.  At star parties, the views of Jupiter and Saturn through high end TECs, Taks, and AP scopes simply lacked the fine detail seen through 12" and up Dobs sporting Zambuto and other fine mirrors.  Perhaps under UK skies your refractors consistently beat large Newts, but not everywhere is this the case.

Perhaps for vastly unequal doubles the unobstructed refractor rules.  However, if you make an off axis aperture mask for a 16" Newt, you've got a roughly 6" unobstructed instrument now that should compete very favorably.

[Louis D]

3 hours ago, KP82 said:

One of the advantages of a 4 - 5" refractor is that you can easily take it to dark sites and they will show better results than a 10 - 12" in a severely lighth polluted garden.

If I'm going to the trouble of going to a star party or weekend camping at a dark site, I'm taking the big guns with me to make the most of those dark skies.

[Spile]

I have and am very happy to recommend the 8" Dobsonian (or bigger) but do be realistic in terms of DSO fuzzies. There are however plenty of wonderful targets that are definitely worth looking at.

[ollypenrice]

16 hours ago, AlexK said:

I spent several years observing hands on with the professional high end 150mm Zeiss Coude refractor and with the high end Russian long refractor AVR-3 (also 150mm). Both stationary installed on their stock high-end motorized equatorial piers under the sliding roof of the local youth astronomy club's observatory. I can tell you without a single doubt: views with my current very-average-optics 12" Dob are very long way better in every regard on planets and DSOs (weather permitting)...

Sorry for the sidetrack but this grabbed my attention. Friends in my UK astronomical society actually bought a complete but dismantled Zeiss coudé six inch about 20 years ago. It had been used at the University of St Andrews in Scotland. I dare say it will have been very similar to the one you used. I only ever saw the lens but they did get it up and running.

I agree with you on the big reflector beating it, though, for DS observation of faint targets. And big Dobs are incredible value.

Olly