Dobsonian or Alt-Az Mount for larger Reflectors?

[Jasonb]

Hi there,

First off, I need to stress that this is a hypothetical post, I don't have the money to be buying anything right now!

I currently have a 6" Skywatcher Reflector on a AZ5 Mount and a heavy duty steel tripod. It's pretty much a grab and go set up, I can easily get it in the car and I can set it up within a couple of minutes. The 6" Reflector with a RACI Finder Scope and EP is probably getting close to the capacity of the AZ5, though the heavy duty steel tripod helps with that.

I started wondering though, if I were ever to go larger (8" or maybe 10" max), what is the best mount to be using? I don't really want to go down the EQ mount route, I am perfectly happy with Alt-Az. But an 8" or 10" Reflector would be too much for the AZ5, so a new mount would be needed too, either a Sky-Tee 2 or, even more expensive, a Rowan Mount. 

Or I could look at going down the Dob route instead, to handle the 8" or 10" Scope. I'd be worried though about the Dob base being less forgiving about the ground it's been placed on, compared to a tripod, as it would need to be fairly flat, right?

I suppose when it comes down to it, my question really is about what's the best type of mount to be using for the larger/heavier scopes?

Thoughts please?

[John]

I've used an 8 inch F/6 Orion Optics newtonian on a Skytee II for a while and that worked quite nicely. I do wonder if a 10 inch, even if it is Orion Optics (which are lighter than the chinese equivalents), might be a little bit too bulky for such a mount though.

 

 

 

 

[Mr Spock]

The Dobsonian mount has feet underneath. You could always fit longer feet if required. Same for an EQ platform if you were to have one of those (see below)

 

[westmarch]

I’ve had a 10” Dob for a number of years and it is very stable. It will sit on most surfaces and, as Michael says, the short studs under the base will sink into soft ground and add stabiity. I have put it on a tarp if the ground is muddy.

John

Edited April 16, 2023 by westmarch

[Stu]

A dobsonian mount always seems to be the best option once you get up to larger sizes. I did try an Orion Optics 12” f6 (yes f6, not the more normal f5.3 or f4) on an AyoDigi II a few times before I got it on a dobsonian base and it was a bit crazy. Eyepiece height way up high and a bit cumbersome with plenty of vibration as it was only supported on one side and the Ali tube flexed quite a bit.

@Captain Scarlet uses a 12” newt with CF tube on his AZEQ6 so can give some good input I’m sure.

[John]

A decently designed dobsonian mount does a really good job of holding these larger optical tubes steady, even at really high magnifications. Goodness knows what sort of non-dob mount I would have needed to hold my 12 inch F/5.3 steadily at 300x - 400x - it would have been pretty massive, as well as costly I suspect.

 

[NGC 1502]

Agree with above. There’s no reason at all that a Dob mount should be unstable, it will have 3 very short feet, so if the ground is not perfectly level, no problem.  If the ground is too much on the slope, or a bit lumpy just place something solid under one of the feet.  Long thick grass or shrubby ground may be more problematic.

In fact a Dob mount is potentially more stable because the tube assembly is supported on both sides rather than one.

Ed.

[Stu]

33 minutes ago, NGC 1502 said:

In fact a Dob mount is potentially more stable because the tube assembly is supported on both sides rather than one.

I think that’s the key point isn’t it, support both sides really keeps the vibrations down and avoids flex from the tube or rings only being supported on one side.

[Captain Scarlet]

59 minutes ago, Stu said:

@Captain Scarlet uses a 12” newt with CF tube on his AZEQ6 so can give some good input I’m sure.

Yes indeed. I use an az-eq6 in alt-az mode for my 12” newt (strictly visual) and have done for about 5 years now. In current format, my biggest scope plus ancillaries weighs in at something over 20kg. I used to use the mount for my blue-steel-tube SW 300p which was ~28kg and even then it was perfectly happy, never a glitch.

I think the az100 esp with goto/motors would be even more capable.

I see you are, like me, in Ireland. Why don’t you nip down here to Baltimore and we can compare notes?

Cheers Magnus

 

[RobertI]

I tried a 10” Skywatcher Newt on my Skytee, albeit with a not very good tripod, and it was not useable. I think even with a very solid tripod a 10” wouldn’t be very stable. But with my 6” F8  and a 2” tripod it’s very stable and a good match in my experience.  

[Jasonb]

Wow, thanks for all the replies, some great information here. I appreciate people taking the time to share their knowledge and experience.

It sounds like I wouldn't have to worry about Dobs being less stable on uneven ground, in fact it sounds like Dobs are probably more stable, especially for heavier Newts. As @Captain Scarlet  says, the AZEQ6 is fine with his 12" Newt, but that's nearly £2,000 worth of Mount!

So, in general, it seems an 8" Newt you can get away with some of the relatively cheaper mounts (Sky Tee 2 etc.) but anything more than that is going to be a Dob or a *serious* mount. 

One thing a friend of mine raised, that I hadn't though of. With a Dob, the design gives you a low viewpoint, in terms of the mirror height. The primary is *much* lower that it would be with a Newt on a Mount, so if you're in a garden with a fence/wall around it, a lot of the lower sky will be invisible for you. Has anyone else experienced that? It's not like you can put a 10" Dob up on a table!

And Magnus, Baltimore is a little more distance than a "nip down", but I'd love to come visit some day/night, just to look at the scopes you have!

[John]

8 minutes ago, Jasonb said:

One thing a friend of mine raised, that I hadn't though of. With a Dob, the design gives you a low viewpoint, in terms of the mirror height. The primary is *much* lower that it would be with a Newt on a Mount, so if you're in a garden with a fence/wall around it, a lot of the lower sky will be invisible for you. Has anyone else experienced that? It's not like you can put a 10" Dob up on a table!

 

I have quite often experienced this as our garden is surrounded by fences, trees and houses. It is one of the reasons that I have a number of refractors on tall tripods.

With a number of the planets having been quite low in the sky over the past few years, I have found it much easier to get the refractors onto them, sometimes barely above the surrounding obstacles.

If I had a more open aspect though, the dobsonian arrangement would work just fine.

[Stu]

52 minutes ago, Jasonb said:

Has anyone else experienced that? It's not like you can put a 10" Dob up on a table!

I can’t for the life of me remember who, but someone on here did just that; built a small table to raise their dob (8” or 10”) so they could see over fences or hedges better.

[Space Hopper]

I've tried a 10" Newtonian on my manual alt-az, but it was a no go......too much mass, too high a centre of gravity, and it would have been a tip-over hazard in any sort of wind,

even with a Planet tripod.  The mass of the OTA also put too much strain on the saddle plates dovetail clamps.

This was an experiment though, and i'm sure an 8" equivalent tube would have worked safely, and without too much issue.

But i have no counterweight option, and am really better off with medium sized refractors / SCTs up to 11" max ; what my mount was designed for.

Something like an AZ100 though, with its counterweights and a sturdy tripod would no doubt work very well, and i think thats your best option if you want to stay with alt-az and observe with an 8 or 10" tube.

 

[Pixies]

1 hour ago, Jasonb said:

It's not like you can put a 10" Dob up on a table!

Preparing to try to split Rigel with an 8" dob:

Edited April 16, 2023 by Pixies

[NGC 1502]

2 hours ago, Jasonb said:

One thing a friend of mine raised, that I hadn't though of. With a Dob, the design gives you a low viewpoint, in terms of the mirror height. The primary is *much* lower that it would be with a Newt on a Mount, so if you're in a garden with a fence/wall around it, a lot of the lower sky will be invisible for you. Has anyone else experienced that? It's not like you can put a 10" Dob up on a table!

Indeed it’s possible to lose the lower areas of sky in the way you describe. However depending on the surrounding houses etc it can work to your advantage. For me if my scope is lower then my fences hides lights from nearby houses.

Several years back when Jupiter and Saturn were very low in the south then I used to put my Dob on a raised brick built flower bed to access those planets……

When I had an 8” Dob then on rare occasions I lifted that onto my workmate to access the lower sky. A bit wobbly but a wobbly view was better than no view!

Necessity is the mother of invention 😊

 

[Mr Spock]

I use the 4" to reach parts the Dob can't reach. As you can see in the above picture the Dob will only face south and west, with the 4" reaching east - as far as the house will allow.

[Guest]

If you are worried about uneven ground with a dob, a leveling board is a easy thing to make.  It gets the dob up off the ground and will give you a nice level surface.  

[mikeDnight]

1 hour ago, Mr Spock said:

I use the 4" to reach parts the Dob can't reach. As you can see in the above picture the Dob will only face south and west, with the 4" reaching east - as far as the house will allow.

That reminds me of an old Heineken advert from the 70's or 80's. "Refreshes the parts other beer's canot reach!"

IF ONLY TAKAHASHI MADE BEER!  🍺🍺🍺

Edited April 17, 2023 by mikeDnight

[Mr Spock]

22 minutes ago, mikeDnight said:

IF ONLY TAKAHASHI MADE BEER!  🍺🍺🍺

It would probably be the best... sceptic green in the world  

[Jasonb]

Once again, thanks for the replies, they're much appreciated!

[PeterC65]

From my own research into bigger aperture scopes, comparing a Newtonian on an AZ mount versus a Dobsonian, the eyepiece height is about 500mm higher with the AZ mount. At the zenith that makes it about 2m high for a 12" Newtonian on a AZ mount versus 1.5m for a 12" Dobsonian (1.75m and 1.25m respectively for a 10"). For comparison, with my 6" Newtonian on an AZ mount the zenith height of the eyepiece is 1.5m and I find that just right for observing while perched on an observing stool (set one notch below its maximum height).

I think if you were using a 12" Newtonian on an AZ mount you would need to not extend the tripod legs and keep it as low as possible.

 

[Louis D]

I've found Dobsonian mounts very forgiving of slight slope because the center of gravity is directly over the azimuth rotational axis.  Alt-az mounts are not at all forgiving of slight slope unless you're using a counterweight on the other side because the center of gravity is way off to one side of the azimuth rotational axis.  This necessitates re-leveling the mount in the dark anytime you need to pick it up and move it to another location to dodge observing obstacles like buildings or foliage unless your entire observing area has the same slope and you keep your tripod oriented exactly the same from place to place.

For Newtonians with focal lengths of one meter or longer, I generally recommend the Dobsonian mount over the alt-az mount in my experience.  For shorter, just the opposite.

[Louis D]

To raise up a Dob, lots of folks on SGL use a water butt stand:

 

 

[Graham Darke]

My Orion 12 inch f5.3 on its dob mount and my az-eq6. Best of both worlds here. Up and running in seconds on the dob mount for quick visual sessions and I’ve used it for planetary imaging with the driven mount. It just clears the tripod legs when pointed near the zenith.