6" Refractor vs 10-12" Dobsonian

[AlexK]

Re Dobson cooling. It should not be too much of an issue on the Islands as the temperature swipes are not that drastic there. I see it's like 20C to 10C (day/night) in the summer. So with the cooler fan it should be like 20 min. If that's still seems like a serious problem for your flow you don't have to wait, mod your OTA with the surface layer blow fan(s) and/or front duct ring. Folks swear that running it when observing is fixing the problem nearly instantly.

Re atmospheric conditions and the large aperture. That's true, smallish aperture will show a smallish planetary disk kind of more neatly and for kinda longer. But what's the goal? Enjoy the neatness or see tough details? With my 12" I saw fine details in individual Jupiter belts clouds, as well as streams around the GRS! Yes for a second or two in like 30 minutes of manually guiding it uninterrupted at 640x, but that was truly striking, spacecraft flyby like experience, enough to forget about refractors and GEMs forever!

Re GoTo. Better skip that doubtful feature on a furniture-particle-board fork mount which is typical for commercial 12" GoTo reflectors. It's never 100% reliable and a chore to use immediately and to service it over time. Needless to say it's rendering all benefits of the Dobsonian design to zero. So I believe it's just a dead(ly) weight . Yes, it still has certain appeal for a knowledgeable (or warned) newbie because of the tracking possibility though. Especially having in mind that the real Dobsonian mount (which is a breeze to guide manually) is a really rare find nowadays anyway as every China engineer has "innovative" "improvements" ideas of their own (including that GoTo gimmick). But still, for less or comparable money you can have an direct-focus-imaging-capable tracking added eventually with the EQ Platform (DIY, custom order, or even OTS). Wooden GoTo mount can't help even with the piggyback imaging.
The trivial star hopping is way more reliable at pointing than that GoTo as soon as you get a properly designed for that flow digital star chart on a handheld device and practice with it a bit.
And to give you a perspective: the manual telescopes pointing revolution, giving you a nearly effortless 100% reliable pointing possibility is just around the corner (I might be even not the first announcing it).

[faulksy]

18 hours ago, Louis D said:

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.

i don,t agree 100% about looking like photographs.

a well known astrophotographer of here and who is a moderator looked through my dob a few years ago. and on the objects we viewed he said apart from the lack of color, they looked as good as any photo he had ever seen. this was a good night seem to remember the omega nebula was one of them and m101 and 51

Edited May 13, 2021 by faulksy

[Carbon Brush]

46 minutes ago, ollypenrice said:

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.

This sounds very like a scope I saw perhaps 2005-ish, near the Nottinghamshire/Derbyshire border. It was assembled and ready to go. The owner had just taken the lens for recoating so he couldn't demonstrate it.
But it was a really amazing looking instrument. I have never seen anything like it before or since.

[AlexK]

55 minutes ago, ollypenrice said:

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

Yeah, that Zeiss Coude is a true modern marvel of the optomechanics of 70-es. But by now, its mechanical base aging renders it less beneficial for the intended scientific tasks. E.g. the detectors miniaturization revolution outweighs the need of professional maintenance (expensive and time consuming). So it's being replaced at observatories to cheaper to own modern solutions with even more capabilities. Good for amateurs willing to take the job of restoring that technical wonder!  It was especially awesome for imaging, photometry, spectroscopy, Sun observing (manual sketching, heavy H-alpha stacks).

Anyone not aware of the Coude scheme: its focal plane is routed to the Polar axis by two flat mirrors, so the view is permanently and precisely fixed to the ground, thus your eye (or any other large and super heavy opto/mechanical and/or electronic instruments of the 70es or older) stays immobile and focused down to sub-microns while you move the other end of the optical axis around the sky:

Though at our youth observatory it was idling for years in a row due to mechanical issues started to pop up here and there due to the lack of scheduled professional mechanical maintenance. I believe by now it's just sitting in the storage partially disassembled.

Edited May 13, 2021 by AlexK

[mikeDnight]

Well I'm going to disagree with the relative views of M13 as seen through various apertures. If a telescope gave such a poor view of M13 as depicted by the alleged 6" telescope image, I'd take a hammer to that telescope as its not fit for purpose. The image of M13 that claims to be the view through a 12.5" reflector is how I see M13 in my 100mm apo refractor when reasonably well dark adapted, except that in the refractor the stars are way sharper. And a 6" refractor is a formidable instrument on M13 and many other deep sky targets. Even some of the brighter galaxies can be extremely beautiful in a 6" refractor, so unless there's a desire to go deep in search of targets on the border of visibility, I much prefer a 6" refractor over a 10" reflector as a visual deep sky instrument.

[faulksy]

its nice that we all agree that no matter what, aperture is king

[John]

The views of the bright globular clusters with my refractors (100mm to 130mm) are nice, sometimes very nice. The views with my 12 inch dob can be jaw dropping.

I have owned a few 6 inch refractors as well but again, no competition for the 12 inch dob on deep sky objects from them.

While I do observe deep sky objects with my refractors, the dob is by far my preferred instrument for such visual astronomy.

I guess newcomers reading this thread might wonder why our experiences and preferences vary - the lack of consensus must make reaching a decision on instrument choice challenging 

It's just how it is with such discussions though. Once we can have star parties and outreach events again, the best thing is to get along to one and find out for yourself what floats your boat

 

[ollypenrice]

8 hours ago, faulksy said:

its nice that we all agree that no matter what, aperture is king

I'm not sure that we do! It depends what the observer wants. The OP wants to track down faint galaxies from an imperfect site and, for this purpose, aperture does matter most, I think. But it's not all about faint galaxies, viz...

 

9 hours ago, mikeDnight said:

Well I'm going to disagree with the relative views of M13 as seen through various apertures. If a telescope gave such a poor view of M13 as depicted by the alleged 6" telescope image, I'd take a hammer to that telescope as its not fit for purpose. The image of M13 that claims to be the view through a 12.5" reflector is how I see M13 in my 100mm apo refractor when reasonably well dark adapted, except that in the refractor the stars are way sharper. And a 6" refractor is a formidable instrument on M13 and many other deep sky targets. Even some of the brighter galaxies can be extremely beautiful in a 6" refractor, so unless there's a desire to go deep in search of targets on the border of visibility, I much prefer a 6" refractor over a 10" reflector as a visual deep sky instrument.

I agree. The view ascribed to a 6 inch is precisely the opposite of the reality in a large refractor. The image shows a blurred polo-mint-like glow around the core and the essence of the refractor view is the resolution of stars.

Where I prefer a 6 inch refractor (if it's a good one) over a 10 inch reflector is on stars or wider fields in which stars define the quality of the view. I love observing through our large refractor because of the quality of the view, the tightness of the stars, the darkness of the background. I do think the view is more beautiful in a fine refractor but that was not what the OP was asking about.

Olly

Edited May 14, 2021 by ollypenrice

[Deadlake]

21 hours ago, chrispj said:

 It's reasonably dark where I am 

Do you know the SQM where you observe? Some people think 21.8 SQM is dark and 21 SQM is dark'ish, while other count themselves luck with a sky around 20 SQM.

Edited May 14, 2021 by Deadlake

[scotty38]

@mikeDnight and @ollypenrice I stand by my comment that that's how M13 looks through my 6". Just to reiterate though:

1. It's an achromat (Altair Starwave 152 f5.9)

2. On the occasions I've had to use it it's not been what we'd call dark (I've only had it a couple of weeks)

2. I know my low light vision isn't the best

I made the point of mentioning these things for all the obvious reasons but given your comments can you give examples of what constitutes a good 6" refractor please?

Edited May 14, 2021 by scotty38

[NGC 1502]

Lots of good advice already posted.

I’d like to agree with some of the comments regarding the quality of the view of M13 through various apertures. Last Saturday night I was at one of my local clubs dark sites with my 10” Dob. Once full astronomical darkness arrived M13 looked far better than than those comparison views.  At 200x the view was superb, black background sky with a blizzard of stars, chains of stars radiating from the edge of the main cluster......

Ed.

[ollypenrice]

1 hour ago, scotty38 said:

@mikeDnight and @ollypenrice I stand by my comment that that's how M13 looks through my 6". Just to reiterate though:

1. It's an achromat (Altair Starwave 152 f5.9)

2. On the occasions I've had to use it it's not been what we'd call dark (I've only had it a couple of weeks)

2. I know my low light vision isn't the best

I made the point of mentioning these things for all the obvious reasons but given your comments can you give examples of what constitutes a good 6" refractor please?

It's very difficult to do that on stellar objects because the eye and the camera respond quite differently to stars. You can process an image of a nebula so that it gives a fairly accurate impression of the eyepiece view but clusters are notoriously difficult to image well. A classic case would be the Trapezium in M42. It is very easy to see four well separated stars at the eyepiece of even a very modest telescope but, as anyone who has imaged M42 will confirm, this is not so easy with a camera. Likewise the eyepiece sparkle of the Double Cluster is very hard to render in an image. The camera cannot easily control stellar size in the way the eye can, so closely placed stars run together. The great thing about a good apochromatic refractor is that it produces very pinpoint stars, the opposite of the way cameras behaves, which was my original point.

I'd rather describe my experience of M13 in instruments I've used:

20 inch F4 Dobsonian. Instantly spectacular with very bright stars resolved to the core and a magnification allowing the cluster to fill a widefield, high power eyepiece. The mirror I had was not diffraction limited so the stars were not truly pinpoint but they were still well resolved. Darkish bakground.

14 inch LX200 SCT: Slightly less spectacular due to mildly reduced stellar brightness but tighter stars still resolved to the core. Background perhaps a tad lighter than in the Dob. I never had both at the same time so that's from memory. (Note: I also had a 10 inch SCT at one time. The 14 inch is much better on stars.)

5.5 inch TEC140 apo refractor: Slightly less spectacular again because of further reduced stellar brightness and an image optimized at lower magnification in order to preserve brightness. However the stars are the tightest, the background the darkest and the impression of sharpness to the core comparable with the others. Outlying stars look sharpest in this instrument to my eye.

Trapezium not easy in the camera. This had a lot of fancy processing: https://www.astrobin.com/full/380941/0/

Olly

 

 

 

[scotty38]

5 minutes ago, ollypenrice said:

It's very difficult to do that on stellar objects because the eye and the camera respond quite differently to stars. You can process an image of a nebula so that it gives a fairly accurate impression of the eyepiece view but clusters are notoriously difficult to image well. A classic case would be the Trapezium in M42. It is very easy to see four well separated stars at the eyepiece of even a very modest telescope but, as anyone who has imaged M42 will confirm, this is not so easy with a camera. Likewise the eyepiece sparkle of the Double Cluster is very hard to render in an image. The camera cannot easily control stellar size in the way the eye can, so closely placed stars run together. The great thing about a good apochromatic refractor is that it produces very pinpoint stars, the opposite of the way cameras behaves, which was my original point.

I'd rather describe my experience of M13 in instruments I've used:

20 inch F4 Dobsonian. Instantly spectacular with very bright stars resolved to the core and a magnification allowing the cluster to fill a widefield, high power eyepiece. The mirror I had was not diffraction limited so the stars were not truly pinpoint but they were still well resolved. Darkish bakground.

14 inch LX200 SCT: Slightly less spectacular due to mildly reduced stellar brightness but tighter stars still resolved to the core. Background perhaps a tad lighter than in the Dob. I never had both at the same time so that's from memory. (Note: I also had a 10 inch SCT at one time. The 14 inch is much better on stars.)

5.5 inch TEC140 apo refractor: Slightly less spectacular again because of further reduced stellar brightness and an image optimized at lower magnification in order to preserve brightness. However the stars are the tightest, the background the darkest and the impression of sharpness to the core comparable with the others. Outlying stars look sharpest in this instrument to my eye.

Trapezium not easy in the camera. This had a lot of fancy processing: https://www.astrobin.com/full/380941/0/

Olly

 

 

 

Thanks Olly, appreciated and will read through shortly as just need to nip out. Just to be clear though I'm only referring to visual with this scope. I have the Altair and a Stellamira ED 80 for visual but my imaging is done with a WO GT81.

[chrispj]

2 hours ago, Deadlake said:

Do you know the SQM where you observe? Some people think 21.8 SQM is dark and 21 SQM is dark'ish, while other count themselves luck with a sky around 20 SQM.

It's around 20.8 according to lightpollutionmap.info. If I pop up to the common behind the town I could apparently get to around 21.2.

[chrispj]

Incidentally, this is fascinating reading, thank you all for your contributions! I must look into if there is an astronomy club nearby (I know there is the observatory at Sidmouth but not so much happening there recently for obvious reasons).

[Deadlake]

While I was looking at some thing else, this came up. Since we are discussing aperture, contrast and reflectivity thought I put it in to the mix:

Why Zambuto?

 

 

 

[Deadlake]

42 minutes ago, chrispj said:

It's around 20.8 according to lightpollutionmap.info. If I pop up to the common behind the town I could apparently get to around 21.2.

You don't really have any issues with light pollution, relative to a lot of observers.

From an aperture perspective you can choose what you would like I feel.

[ollypenrice]

2 hours ago, scotty38 said:

Thanks Olly, appreciated and will read through shortly as just need to nip out. Just to be clear though I'm only referring to visual with this scope. I have the Altair and a Stellamira ED 80 for visual but my imaging is done with a WO GT81.

To be clear, I realize you're talking about visual observing and my reference to imaging is simply to say that I have never been able to replicate a visual cluster view in an image. If we were to show the OP the difference between a 6 inch and a 10 inch on a faint fuzzy I think we could do it.

I suspect Obsession are guilty of having made a globular the benchmark but we'll forgive them for other reasons! 😁

Olly

Edited May 14, 2021 by ollypenrice

[scotty38]

ok got it, thanks!

[JeremyS]

4 hours ago, scotty38 said:

 can you give examples of what constitutes a good 6" refractor please?

Yes, @mikeDnight’s Tak FC 100 DZ

 

😊

 

[Mr Spock]

I've viewed a 0.7" separation double with my C9.25. Two airy disks clearly separated by space.

0.7" is smaller than the airy disk of a 150mm refractor. Just thought I'd mention it 

[jetstream]

5 hours ago, Deadlake said:

21 SQM is dark'ish

21 SQM is dark.

[scotty38]

 

15 minutes ago, JeremyS said:

Yes, @mikeDnight’s Tak FC 100 DZ

 

😊

 

Ok I'll run with this then.... All other things being equal then Mike's 6" FC100 🙂 would give me a nice sharply resolved M13 versus the fuzzy ball ( I see right now) in a real 6" achromat, correct?

Edited May 14, 2021 by scotty38

[vlaiv]

Just a thought - seeing does have impact on resolution.

In poor seeing / not optimal conditions - detail in globular cluster can suffer as much as planetary detail.

6" refractor in good seeing will outperform 12" dob in poor seeing, although 12" will have both more light grasp and better resolving power.

 

[jetstream]

Re: colour- my 15" dob the other night presented a bright blue Izar split, on par with my refractors.  In general the fracs can deliver better color- but the fast dobs operate at a larger eye illumination for a given mag, where the refractors typically "dim" things down effectively (exit pupil wise) for a given mag. It is a myth that reflectors dont show colour (very) well.

Re: star clusters. Aperture is king and there are no magic scopes of any kind including refractors. I had the TSA120 and the 15" dob both out the other night and there was zero, and I mean zero comparison between the two. The 15" blew it out of the water on M13 and M3- M3's view is stunning in larger scopes. Btw, I got NELM of 7 in Ursa Minor.

Of course on large nebula, or large field views of M31 the TSA pulls ahead.

How about Carolines Rose? Anyone going to say a 100mm,120mm or 6" frac is going to beat the views through a 10" or larger dob? My 15" (so does my 10") eclipses the view of my fracs on this. How about M37? ^^