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Alt-Az 8" f/4.9 Musings

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#1 Phillip Creed

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Posted 24 July 2013 - 08:38 PM

I have an 8" f/4.9 OTA that is alt-az mounted on a Universal Astronomics Heavy-Duty Wood Surveyor's Tripod+Unistar Mount. This scope replaced a 6" f/5 achro and is an upgrade in every way conceivable, except for less FOV.

One thing I was amazed at was how FEW 6"+ reflectors I've seen on tripod alt-az mounts. Before I bought the 8" f/4.9 on this mount, I scoured the net for comparable set-ups and couldn't find any. But for $300 for an 8" scope, I figured it'd be an inexpensive experiment, so I put the 8" on this mount in 2010. It has quickly become my favorite scope.

My question is this--would a report on the 8" f/4.9 OTA and/or the alt-az system itself be of interest to enough folks out there to warrant a CN article, or are there a lot of folks out there with alt-az tripod-mounted 8" scopes that I just haven't run across?

There was a bit of a learning curve involved in making this work the way I wanted to, and there's a lot of quirks--as well as some hidden pros/cons of having a tripod-mounted reflector--I learned along the way. Just wondering if there'd be enough interest out there for such an article.

One thing that is nice (albeit just a tiny bit mean) is when a refractor owner sees me unfurl the tripod and the mount...only to see (gasp) one of those...those...those MIRRORED scopes on it.

Thoughts?

Clear Skies,
Phil

#2 rguasto

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Posted 24 July 2013 - 09:09 PM

:yay: :tomatodance: :elephdance: :hamsterdance:

#3 Jon Isaacs

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Posted 24 July 2013 - 09:53 PM

Thoughts?

Clear Skies,
Phil



I think a well written, thoughtful review is always welcome. I say go for it.

I tried my 8 inch F/5 on a StellarVue MG-2 mount which is a robust mount with slow motion controls. It was reasonably stable but reaching the slow motion controls was very awkward and so I went back to using the scope on the CG-5 ASGT.

The obvious reason that alt-az mounts with tripods are not common for larger Newtonians is that Dobsonian mounts work so well and allow for longer focal length OTAs.

Jon

#4 davidmcgo

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Posted 25 July 2013 - 02:16 PM

I use an 8" f6 Newtonian and a 10.25" Dall Kirkham on a DM6 with a Stellarvue TSL6 tripod. Mainly because I had the DiscMount for a refractor and it makes a good "grunt and heave" set up for lunar and planetary viewing at home when I don't wish to set up the German equatorial but still want some aperture. The longer Newtonian can be annoying near the zenith though since the rear end of the tube doesn't quite clear the legs.

The cass does not have that problem since the CG of the tube is only about a foot ahead of the back.

A Dob would not be that useful from my yard since I have a small space surrounded by a 1 story garage and 5 foot high fences. The tripod elevates the pivot for altitude and let's me get lower in the sky.

Dave

#5 Phillip Creed

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Posted 26 July 2013 - 03:50 PM

Tripod-mounted alt-az set-ups are really nice when you're scanning along the horizon and don't want to get down on your knees with a Dob.

As Jon points out, they're only suitable up to a certain focal length. As is, my 8" f/4.9 just--and I mean just--clears the tripod legs when I'm looking up at the zenith on the UA Unistar. I simply couldn't go beyond 1,000mm focal length, at least with a conventional reflector.

That being said, though, I would love it if some commercial firms started making lightweight 10"-12" f/3-f/4 tube Newts. A 20-lb tube Newt with a 12" f/3 Lockwood/Swayze mirror would be a Grab N' Go of Death. That's 12" of aperture combined with a full 2.0-deg TFOV w/6mm exit pupil with a Paracorr'ed 21mm Ethos, and it can all be carried in one go if you need to move it quickly, like when a key object is behind an unfortunately-placed tree. Basically, the idea is adopt Mel Bartel's ingenious "zip dob" concept in tube form for a conventional tripod alt-az mount.

Can't afford it, but I could at least dream.

Clear Skies,
Phil

#6 KerryR

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Posted 26 July 2013 - 03:59 PM

I too have the Unistar on a U.A. Heavy Duty Wooden tripd. I currently use it with a 6" f5 Newt. Works very well, with the ep at EXACTLY my eye height when pointed at the zenith.

Just about every other ota I've used on this setup, including a 6" f6 Newt hits the tripod and/or puts the ep too hight at zenith.

I like the idea of a fast 8", but I wonder if I'd run into the same problems with it as I have the other ota's I've tried.

A thorough review of your setup would be most welcome-- especially because it's one I'd consider using...

#7 Jon Isaacs

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Posted 26 July 2013 - 05:20 PM

That being said, though, I would love it if some commercial firms started making lightweight 10"-12" f/3-f/4 tube Newts. A 20-lb tube Newt with a 12" f/3 Lockwood/Swayze mirror would be a Grab N' Go of Death.


Probably no more so than a 12 inch F/4 on a Dob mount, probably less easily dealt with... Back when my 12.5 inch in F/4.06 was a tube dob, I timed myself setting it up. From the time I turned off the car's key, until the scope was sitting there, mounted, finders ready, chair alongside..

41 seconds.

It took me more longer to setup my TV Pronto.

I love widefield/richest views but you don't gain much going from a 31mm Nagler at F/4 to a 21mm Ethos at F/3.3 and what you do gain is beyond the 82 degree AFoV mark, if you consider the field inside the 82 degree circle, F/4 + Paracorr is wider.

I think F/4-F/5 is a nicer place to be than F/3-F4..

Jon

#8 TONGKW

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Posted 26 July 2013 - 09:19 PM

I have a Meade 6” f/5 Newtonian OTA. I use it for wide field viewing at stat parties with a Manfrotto 400 gear head. The set up works well and the view is quite good.
Unfortunately the Manfrotto 400 gear head was expensive 10 years ago and way too expensive now.

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#9 cuir

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Posted 27 July 2013 - 05:42 PM

when a refractor owner sees me unfurl the tripod and the mount...only to see (gasp) one of those...those...those MIRRORED scopes on it



:rofl2: I got the same reaction from a guy with his 102mm ED refractor and showed up with this........Er, well, my 10" water heater thingy.

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#10 izar187

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Posted 28 July 2013 - 01:31 AM

Short newts on alt-az mounts get my vote too. Enjoyed the joys of such for years.
Sometimes one just needs to think outside the boxes institutionalized by any hobby culture.
Equatorial or dobsonian were simply never the only ways to mount a newt.
All scope/mount combinations are a working set of compromises. Embracing rather than ignoring said, can yield very worthwhile results.
My short 4", 6" and 8" newts each allow for looking into an ep from horizon to zenith with little or no change in position. Seated, or rigged for standing.
No tracking, nor slow motion controls. But I would not exchange the whole sky, stoop-free ergonomics for all the tee in Scotland.

#11 Lamb0

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Posted 28 July 2013 - 05:24 AM

:FarmerRon: How 'bout an inverted fork for a 12" tube; say ~42" long... any good (& cheap) designs out there? :question:

I've got an old press fit pipe equatorial with solid steel 1.5" shafts to play with... ('cause it don't work "as is") :hmmmm:

#12 Jon Isaacs

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Posted 28 July 2013 - 06:06 AM

My short 4", 6" and 8" newts each allow for looking into an ep from horizon to zenith with little or no change in position. Seated, or rigged for standing.



An adjustable astro-chair resolves eyepiece position issues.

But in any event, since the balance/pivot point of the scope does not change because of the mount design, the difference in the eyepiece height from horizon to zenith doesn't change either..

For smaller scopes, they make sense, I think 8 inches is borderline...

I like alt-az mounts for Newtonians but putting a tripod back into the equation is counter productive, it adds complexity, a stiff, vibration free mount is more difficult.. manually tracking at high powers.. how stable, how free from vibration is it when one is observing doubles at 60x-80x/inch? Is real time focusing possible?

Jon

#13 Phillip Creed

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Posted 28 July 2013 - 04:13 PM

I too have the Unistar on a U.A. Heavy Duty Wooden tripd. I currently use it with a 6" f5 Newt. Works very well, with the ep at EXACTLY my eye height when pointed at the zenith.

Just about every other ota I've used on this setup, including a 6" f6 Newt hits the tripod and/or puts the ep too hight at zenith.

I like the idea of a fast 8", but I wonder if I'd run into the same problems with it as I have the other ota's I've tried.

A thorough review of your setup would be most welcome-- especially because it's one I'd consider using...


I'm writing it up right now. I'm hoping to have it sent over to the powers that be at CN for review sometime next month.

One thing I learned early on is that, within reason, try to keep excess weight off the upper end of the tube. One way I removed some weight at the focuser end was to use a green laser pointer in lieu of a 50mm finder scope. That helped a little bit. I still have the tube in a manner such that the bottom just baaaaaarely misses the tripod legs no matter where it's pointed.

Some might be worried about damping times on the mount, which is a legitmate concern. The set-up I have as-is works fine to about 400X; that's a 10mm Delos + 4X Powermate. It's only about 2-3 seconds of damping time, which I find acceptable. In Ohio, it's normally the atmosphere the keeps me in the 200X range. A good night allows me to whip out the 4X Powermate on the 14mm Delos (286X). The stock focuser isn't the best; I've looked through better ones and I've certainly owned worse. It's fine for up to 200X; after that, you'll want something better, but it's still good enough to "get you on the interstate" if you're into planets and the seeing allows 300+ power.

The viewing height is what really, really attracted me to this kind of set-up. I started getting back problems about 5 years ago, so I don't like crouching or kneeling if the object's along the horizon, which is often the case if you've got a dob.

Yes, I'm paying more vs. getting an 8" dob, but the comfort level of a decent eyepiece height is worth it. All three main components--the Unistar Basic mount, the UA Heavy-Duty Wood Surveyor's Tripod and the 8" f/4.9--can be purchased for about $200 each on the used market. I've even seen the 8" f/4.9 OTA go for $150 used. For that price point, I couldn't resist trying an experiment, and I'm glad I did. It was great knowing I was going to boost the aperture and *MAKE* money when I changed telescopes (I already had the mount and tripod), because my old 6" f/5 achro OTA sold for more on the *used* market than the *retail* value of the 8" f/4.9 Newt. Plus, I was able to sell off my refractor-specific items, like the dielectric diagonal and the minus-violet filter.

My tube weighs about 17 lbs. w/o rings. There's nothing about it designed to be "lightweight"; it's just a conventional Newt. Going larger than 8" on this kind of set-up would start getting tricky in terms of stability and the tube hitting the tripod unless (this is largely the deal-killer) you have a 10" - 12" f/3 tube with a lightweight mirror. Screaming-fast, lightweight f/3 mirrors were science-fiction for the longest time, but no more.

I once saw a 10" f/3 tube dob on Astromart with a Steve Swayze mirror. I don't know how much the tube weighed, but I was sorely, sorely tempted to buy it, but I knew with the speed and quality of that mirror it was out of my price league. Basically, a case of one knowing they've got champagne tastes, but only beer money.

Clear Skies,
Phil

#14 izar187

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Posted 28 July 2013 - 05:43 PM

One of the ways to work around a tube hitting the tripod issue is to use something more like a pier. The pier can be permanently positioned at the observing site, as in backyard, cottage/cabin, or a friends or family members farm. Or the pier (or pier like rig) can be portable, in that it knocks down into smaller components for compact storage. My 8 (f/4.8) can sit upright in the seats of either of my vehicles, with it's three piece pier tucked on the floor behind a seat.

#15 izar187

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Posted 28 July 2013 - 06:27 PM

My short 4", 6" and 8" newts each allow for looking into an ep from horizon to zenith with little or no change in position. Seated, or rigged for standing.



An adjustable astro-chair resolves eyepiece position issues.

But in any event, since the balance/pivot point of the scope does not change because of the mount design, the difference in the eyepiece height from horizon to zenith doesn't change either..

For smaller scopes, they make sense, I think 8 inches is borderline...

I like alt-az mounts for Newtonians but putting a tripod back into the equation is counter productive, it adds complexity, a stiff, vibration free mount is more difficult.. manually tracking at high powers.. how stable, how free from vibration is it when one is observing doubles at 60x-80x/inch? Is real time focusing possible?

Jon



An adjustable astro chair is not needed at all for one of these flavor scopes. A best position can be chosen for the rotation of the focuser. The tripod or pier height set for the particular scope in use, sure. But then no adjustment in butt altitude is required. Any old stool or chair of choice will work.

The real borderline point is the ota length. And to some extent the flexure of the mount, as the high frequency kind will wind up most in the focuser view.
If the ota is long enough to dobsonian mount, then do so.

But if the newt tube is +/- 36" or less, then an alt-az mounting on a tripod or pier has some real advantages. Setting aside the bulk of the dobsonian rocker box, the ability to rotate the ota/focuser into optimum position, and small range in travel of the eyepiece in altitude, these should not be underrated.

Is it easy to under-mount an alt-az mounted newt? Well, sure it is. Just as easy as it is to under-mount a scope on an equatorial mount. Some retailers have been doing that since forever.

#16 Jon Isaacs

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Posted 28 July 2013 - 07:12 PM

The viewing height is what really, really attracted me to this kind of set-up. I started getting back problems about 5 years ago, so I don't like crouching or kneeling if the object's along the horizon, which is often the case if you've got a dob.



So, what chair were you using? Chairs are cheaper than mounts...

Jon

#17 Jon Isaacs

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Posted 28 July 2013 - 07:23 PM

An adjustable astro chair is not needed at all for one of these flavor scopes. A best position can be chosen for the rotation of the focuser. The tripod or pier height set for the particular scope in use, sure. But then no adjustment in butt altitude is required. Any old stool or chair of choice will work.

But if the newt tube is +/- 36" or less, then an alt-az mounting on a tripod or pier has some real advantages. Setting aside the bulk of the dobsonian rocker box, the ability to rotate the ota/focuser into optimum position, and small range in travel of the eyepiece in altitude, these should not be underrated.



Honestly, I don't get it. All the work, expense and effort, the compromised stability of the mount, all to avoid an adjustable astro-chair that, will even with these shorter tubes, will make viewing more comfortable??

I bought a Starbound chair about 12 years ago. Of all the accessories and junk I own, it's the only one that gets used each and every night, regardless of scope I am using. I would sooner part with my good eyepieces than the Starbound chair.

The one thing that Phillip might include in his writeup is that some believe that keeping the mirror further from the ground is beneficial at higher magnifications, less affected by the ground heat.

Jon

#18 Phillip Creed

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Posted 28 July 2013 - 08:36 PM

Jon,

Excellent point. It is *definitely* an advantage to have the mirror further from the ground. That's especially true if there's any kind of breeze, as the elevated position of the mirror will catch more wind vs. a dob-mounted scope. The mirror's pretty light as is (I think it's only about 1"-thick and weighs 4-5 lbs.), so I'm usually limited by the atmospheric conditions after about an hour passes by.

The one thing I don't care for on the 8" f/4.9 OTA is the occasional dewing of the secondary mirror, as the tube doesn't extend far enough in front of it to prevent dewing on humid nights. I bought an Astrozap dew zapper solely to tackle that issue.

Clear Skies,
Phil

#19 izar187

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Posted 29 July 2013 - 02:40 AM


An adjustable astro chair is not needed at all for one of these flavor scopes. A best position can be chosen for the rotation of the focuser. The tripod or pier height set for the particular scope in use, sure. But then no adjustment in butt altitude is required. Any old stool or chair of choice will work.

But if the newt tube is +/- 36" or less, then an alt-az mounting on a tripod or pier has some real advantages. Setting aside the bulk of the dobsonian rocker box, the ability to rotate the ota/focuser into optimum position, and small range in travel of the eyepiece in altitude, these should not be underrated.



Honestly, I don't get it. All the work, expense and effort, the compromised stability of the mount, all to avoid an adjustable astro-chair that, will even with these shorter tubes, will make viewing more comfortable??

I bought a Starbound chair about 12 years ago. Of all the accessories and junk I own, it's the only one that gets used each and every night, regardless of scope I am using. I would sooner part with my good eyepieces than the Starbound chair.

The one thing that Phillip might include in his writeup is that some believe that keeping the mirror further from the ground is beneficial at higher magnifications, less affected by the ground heat.

Jon



Yep, you're not getting it. Not if you are convoluting the purpose of this type of set up into an excuse for not buying a $150 observing chair. Though such an observing chair would obviously work for multiple scopes, observing chairs were not the subject of this thread. Nor were longer ota's.

The non dobsonian alt-az mount capable of stability for a 20# 8" f/5 newt will be spiffy for most smaller scopes. Making it a great addition to the gear, so one does not need individual mounts for every ota. Particularly if it is adjustable in height, which allows for even more ota's to work.

#20 Phillip Creed

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Posted 30 July 2013 - 07:49 AM

One other advantage of having a short tube Newt is collimation. My personal experience is that tubes hold collimation better than trusses, but having a short(ish) tube 8" Newt means I can adjust the primary's collimation knobs and look through the eyepiece at the same time, all while seated at a comfortable height.

I'm working to have a CN article ready to be submitted sometime in August. The set-up has some intriguing pros and cons that I quite frankly hadn't thought of prior to acquiring the 8" scope. This set-up works for me, but there's aspects of it that others should be aware of before embarking on mounting a similar-sized Newt onto an alt-az tripod setup. I initially thought of writing just a review-style CN article for the 8" f/4.9, the green laser pointer, the mount and the tripod. I decided in the end to steer the article more towards the "journey" vs. the "destination", as I found the process of optimizing the set-up to one's circumstances more intriguing than its acquisition, assembly and performance.

I'll briefly say this about observing chairs--I did need to modify my existing observing "chair" (actually a Gibraltar drum throne) to give it more range. It worked fine with my short-tube achros since the eyepiece height was relatively low, but not with a reflector. So I had to tweak it, and definitely for the better.

I'll still do a cursory appraisal of the equipment. Slight spoiler--the 8" f/4.9 OTA has been favorably reviewed for its optical performance by others, and I concur in that assessment. It's a good bang-for-the-buck.

Clear Skies,
Phil






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