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what alt az for an 8" f/5?

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#1 pmckeogh

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Posted 12 January 2013 - 12:41 PM

I've askied this question in the mount forum but have had only one responce.

What alt az mount will support a 25lbs, 8" f/5 reflector well enough to allow for good planetary veiwing?

I've looked into the unistar which I am considering and desert sky astro however the DSV-3 is too expensive. Is the anything else out there I should consider? I would prefer something with slow motion controls but I think I'm out of luck.

Thanks for the help
 

#2 KerryR

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Posted 12 January 2013 - 01:47 PM

I have 3 alt-az mounts:
1)Unistar on Universal Astro Heavy Duty Wood Surveyors tripod.
2)Stellarvue M7(similar to the more current M150) on a HUGE wooden tripod of unknown origin.
3)GyroII on a Tech2000 quick-draw pier

The Unistar and GyroII work passably with my Parks 6" f6 Astrolight, as well as some heavier refractors, but I have to be gentle while focusing, which is a bit tedious. Neither would work very well with an 8" f5. It could be done, but focusing would be VERY tedious, with best focus being easily missed at high power. It's amazing how much having a rigid mount improves high power observing, all because you can easily detect the point of best focus.

The M7 is the most stable. But, the mount is pretty huge, which sort of negates part of the benefit to alt-az mounts--light, fast and convenient.

The Unistar gets by without counterweighting, but heavier OTA's begin to strain the smoothness of the azimuth motion.

The GyroII requires a counterweight. This makes it more cumbersome to move and set up. If you've gotta use a counterweight, why not go EQ?

The M7 works without a counterweight, though I had to do some serious re-builds of the guts to get it to do so. Perhaps the M150 works off the shelf without a counterweight, but given the fact that the M7 was marketed as not needing a counterweight, I'd be skeptical.

The best of the 3 overall, which factors in lightness, simplicity, lack of counterweight, and smoothness of motions is the Unistar. It's the only one I can get out the door easily in one trip. But, planetary observing with an 8" f5 is going to strain it. MAYBE if the Unistar was on a REALLY beefy tripod, but, even then I'd be skeptical. Note that the Universal Astro Heavy Duty Wooden Surveyor tripod would likely be too tall.

The biggest issue I've run into is tube interference with the tripods near zenith. Because of the altitude of the ecliptic here in MI, this isn't an issue for planets. My guess, though, is that you'd want to be able to use the scope for other things on that mount, in which case you'll have to solve tube interference.

Interference can be solved by adding a pier extension, or by using a pier-type tripod, like the Quick Draw. But, the more distance there is between the top of the pyramid formed by the tripod and the scope, the more you introduce focus and guiding wobbles, so there's a trade off. Many pier extensions, such as those sold by Orion are EXTREMELY heavy, so they become inconvenient. I have a Stellarvue pier extension that's light, but it still adds to dampening times.

I tried the Unistar on my CG-5 tripod, with a 4" tall steel pipe pier extension. Dampening times were too long. Interference was too severe without the extension. Also tried the Orion extension, but this is so heavy that dampening time went up very substantially.

The least expensive, simple, most stable, with the smoothest movement, the shortest dampening times, and the least focus wiggles? A well-built Dobsonian mount. You can easily tweak the dimensions to put the ecliptic in view when you're seated with your back straight-- the best for patient planetary viewing. (This can be built with VERY simple tools-- see 'sidewalk telescope plans'.)
 

#3 Jon Isaacs

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Posted 12 January 2013 - 10:40 PM

The least expensive, simple, most stable, with the smoothest movement, the shortest dampening times, and the least focus wiggles? A well-built Dobsonian mount.



The only issue with a Dob mount for an 8inch F/5 is the eyepiece height, for me it was too low even with a Starbound chair. For me, the mount would have to be raised up several inches.

I have a StellarVue MG-2, it's adequate, not great but passable, for my 8 inch F/5 but it is awkward to use because I have to reach around the OTA to use the slow-motion controls. A dob mount is by far the best alt-az mount for a Newtonian..

Jon

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#4 EJN

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Posted 13 January 2013 - 02:01 AM

I agree the eyepiece height was too low on my 8" f/5 with a dob mount. The solution
was...a milk crate.

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#5 izar187

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Posted 13 January 2013 - 03:36 AM

My choice for an 8" f5 newt alt-az mount is a 2" pipe mount.
It breaks down into three basic pieces for fitting on the floor behind the seats in my vehicles.
Movements are silky smooth.

The vertical has an additional short section that can be threaded in place.
Tube rings on the ota allow for rotating the focuser position.
This combined with the short tube length yields the possibility to stand when desired.
With no stooping while observing, for any point in the sky.

With the short section out, the focuser can be rotated again to the best placement for comfortable seated observing.

My 8" scales in at 20 lbs.
The pipe mount is 40 lbs fully assembled.
It is used for other scopes as well.
 

#6 David Pavlich

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Posted 13 January 2013 - 04:27 PM

Since this is a duplicate post to the one in Mounts, I'm locking it. Use this link to continue the discussion in Mounts.

David
 






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