Large Reflector on an Alt/Az mount?

Hi everyone,

Got a question for you all.

I bought a little 6 inch SN to try on my Stellarvue Alt/AZ mount. It was pretty cheap in cost, couldn't pass it up. I noticed when it is front heavy(meaning the back end is sticking out more from eyepieces and finderscopes) that the eyepiece height doesn't change all that much when going from horizontal to vertical. This makes it extremely comfortable and very easy to use. I can scan up and down with hardly moving in my chair.

My question is, does anybody have this in a bigger scope, like a 10 or 12 inch? If so, how does it work for you? Do you use counterweights to make it front heavy? Have you made a strong mount to hold it? Or does the eyepiece height change a whole lot to make it not worth it.

Thanks a bunch,

John

Sorry for the typos, I was talking on the phone when I posted this.

My question is, does anybody have this in a bigger scope, like a 10 or 12 inch?

In general, a 10 inch or 12 inch on an ALT-AZ mount will have an OTA long enough and a mirror heavy enough that the balance equation changes, in general the mirror end is the heavy end.

Thus the DOB... lots of 10 and 12 inchers on ALT-AZ mounts, they just don't use a tripod. The tripod required to stabilize a 10 or 12 incher is quite massive and it is unnecessary. Rather than supporting the OTA from one side and thus causing other balance issues, the DOB mount uses what amounts to a fork mount and thus balance is taken care with out the need for counter weights.

That 6 inch OTA may weigh about 10-15 lbs, a 10 inch will weigh more like 35 lbs, a 12 inch around 50 or more...

Dobs, they are quite wonderful in their simplicity.

Jon

john mclaughlin said:

Hi everyone,

Got a question for you all.

I bought a little 6 inch SN to try on my Stellarvue Alt/AZ mount. It was pretty cheap in cost, couldn't pass it up. I noticed when it is front heavy(meaning the back end is sticking out more from eyepieces and finderscopes) that the eyepiece height doesn't change all that much when going from horizontal to vertical. This makes it extremely comfortable and very easy to use. I can scan up and down with hardly moving in my chair.

My question is, does anybody have this in a bigger scope, like a 10 or 12 inch? If so, how does it work for you? Do you use counterweights to make it front heavy? Have you made a strong mount to hold it? Or does the eyepiece height change a whole lot to make it not worth it.

Thanks a bunch,

John

I have a short 6" mounted altaz too. You're right, the ep position moves in a pretty small arc, making it very easy to observe with. I use an 8" on the same mount as the six, but it is not really stable enough. The 8 is f5 too, but twice the weight, longer and centered farther from the center of rotation. So lots of rotational vibration in 8" combo which is a work in process. The 8 also has the benefit of a relatively small ep position arc. The mount is homemade fold up whatnot of adjustable height that will fit behind a seat in the car. I like to stand more often than sit.

For a 10 or 12" one might have to think about a pier mount, with a single beam open type ota, with a secondary cage at the top and a counterweight up the beam beyond that.
Or much taller more traditional dobsonian mount, with as open a truss system as possible with large altitude bearings up the ota as far as possible and spring type virtual counterweights.

Somebody marketed a newt a few years back with the focuser in the altitude bearing sorta. I believe is was an 8" with a very tall pier type mount. I don't recall the brand name.

John,
I recently picked up an Orion 8"/f4.9 Newt tube, and have been using it on a regular Unistar attached to a tripod consisting of the hub from an old Orion Skyview Deluxe GEM with woodwn legs from Hands-On Optics. The tube balances quite well and moves very easily in both axes. Since the Unistar requires the OTA to be mounted on the right side of the mount, I flipped the focuser so it was oriented on the right side of the tube rather than the left side. This also allows the observer to stand outside the tripod legs. The long moment arms of the Newt do induce noticeable jiggling, particularly when focusing, but it's not too bad -- maybe a second or two. The dovetail mount of the Unistar allows easy balance adjustment for light or heavy eyepieces.

Jim