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Limits of Power & Field of View for Alt/Az Mounts

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#1 Joe Cepleur

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Posted 12 November 2013 - 09:15 AM

I am interested in buying or building an alt/az mount for its portability, as part of a truly great grab-and-go setup. I actually prefer using equatorial mounts, but I hate carrying them due to the heavy counterweights, and aligning them every few minutes upon moving them to dodge the trees gets old. If I were to buy a mount, it would likely be a Vixen Porta II. I have not chosen the scope yet, but an Orion 127mm Mak would be a good combination of weight and optical quality. So, I know I want to ditch the counterweights for this scope, yet I have almost no experience with good alt/az mounts!

What is the practical lower limit of true field of view, or the upper limit of magnification, or the highest practical focal length, or whatever is the best way to express it, that can be used with an alt/az mount? This could affect the choice of telescope, or at least my expectations of what I may be able to view with this setup.

The beauty of an equatorial mount is that once the object is centered, a simple turn of one knob tracks it easily; and, if the object is lost after a few moments of not turning the manual slow motion control, a few extra turns brings it back into view. Alt/az mounts require two hands for tracking and one must search again if the object is lost. Low power, wide field views drift slowly, so I know I could manage those, but past some point, it seems that high powered views with narrow fields would become more painful than they were worth.

I am not interested in a Dob mount, because I want a smaller, lighter telescope that stands on legs above the snow in winter, and never requires squatting low to the ground to reach the eyepiece.

#2 Cotts

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Posted 12 November 2013 - 09:30 AM

Joe, I would suggest that the limiting factor for your mount's high-power abilities and ease of use will be how smooth you can make the motions (if you build it yourself) or how much $ you spend on a commercial product.

I have personal experience with the Discmount DM6 which is about as expensive an alt-az you can get without motors. Its motions were so smooth that it was easily pushed along at 300x with my 6" Mak Newt. I ran out of short eyepieces before the mount had any problems. The DM6 would easily handle your C8, by the way.

There are other excellent alt-az mounts - I've heard good things about the Half-Hitch products and the Televue, for instance.

Dave

#3 Joe Cepleur

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Posted 12 November 2013 - 05:00 PM

Thanks, Dave;

Sounds like the old slogan, "You get what you pay for." I like the idea of a good alt/az for one of my C8s, because although I have not updated my signature yet, I actually have two. A C8 is not so heavy by itself; it's the mount that is a bear. I could leave one on the original fork mount, and have one on the alt/az as a big aperture grab-and-go, provided these mounts are lightweight enough to carry easily. Otherwise, may as well leave the C8 on its original forks.

So, if a good alt/az could track well, even at 300x, that still leaves the effort of brining a lost object back into the eyepiece. In the time required to swat a mosquito while viewing at that power, the object could be gone! How do you deal with that?

#4 core

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Posted 12 November 2013 - 05:15 PM

So, if a good alt/az could track well, even at 300x, that still leaves the effort of brining a lost object back into the eyepiece. In the time required to swat a mosquito while viewing at that power, the object could be gone! How do you deal with that?


With a 8mm Ethos in a C8, you get 254x magnification, .39° FOV, and a field transit time (time it takes a star to drift across the diameter of your AFOV) of 94 sec - so if you have your object in the middle, it'll take about 45sec to drift out of your FOV.

#5 Joe Cepleur

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Posted 12 November 2013 - 05:19 PM

Thanks for the referral, Dave. I found the Web sites for those mounts. They are pricey, yet a good value, considering any of those mounts could make astronomy far more of a pleasure for the rest of my life. Intriguing! Amazing to see that they are designed to resolve just the issues I am facing. One Web site even discusses the ease of moving the scope to dodge trees! It's wonderful just knowing that there are folks making things so perfect and beautiful.

#6 astrophile

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Posted 13 November 2013 - 01:49 AM

Joe,

If you're looking at the premium mounts seriously--my experience with a Nova Hitch leads me to recommend one of those as an excellent option. With the slo-mo controls I have easily tracked at 390x with mine, and could have gone higher no problem. According to the website there are a couple 'Cassegrain' versions still available at intro pricing, and it appears digital trainable tracking is part of the package.

Good luck whatever you end up with!

#7 mark8888

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Posted 13 November 2013 - 09:39 AM

So, if a good alt/az could track well, even at 300x, that still leaves the effort of brining a lost object back into the eyepiece. In the time required to swat a mosquito while viewing at that power, the object could be gone! How do you deal with that?


It's really not an issue in practice. If you lose the object, you just move the scope a little in the direction the object was moving, and you find it again. It becomes quick and instinctual. If you do actually lose the object, a quick glance in a guidescope and a move of the scope takes a couple of seconds. Anyway, I've been using an DM-6 for years with various refractors and that's the experience. You just move the scope with the object, including at high powers. Very smooth, natural and easy.

#8 Joe Cepleur

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Posted 13 November 2013 - 11:35 AM

With a 8mm Ethos in a C8, you get 254x magnification, .39° FOV, and a field transit time (time it takes a star to drift across the diameter of your AFOV) of 94 sec - so if you have your object in the middle, it'll take about 45sec to drift out of your FOV.


This is interesting because I know so little about modern eyepieces. I've been building my outfit on a modest budget, and have been happy with a set of Plossls and a few eyepieces that were fancy in their day: an orthoscopic, a Brandon. What is the deal with Ethos, and similar, modern wide fields? Do they just play with the image to create that "porthole" with a wide apparent field, but the same old true field; or, for a given focal length, do they reveal a wider true field than classic eyepieces? If the true field were wider, transit time would be longer, so there would be much less tendency to lose the object to momentary distractions.

If you're looking at the premium mounts seriously--my experience with a Nova Hitch leads me to recommend one of those as an excellent option. With the slo-mo controls I have easily tracked at 390x with mine, and could have gone higher no problem. According to the website there are a couple 'Cassegrain' versions still available at intro pricing, and it appears digital trainable tracking is part of the package.


Also intriguing! To be honest, when I began this thread, I had no idea the premium mounts existed. I simply hoped to cobble together a simple, alt/az, grab-and-go rig. Now I'd say, offhand, that such a fine mount is surely on my "someday" list, even if, for now, the budget remains more modest. The idea of having a C8 on a lightweight, easily moved mount is just too attractive to ever forget! Sounds like the perfect package for the all-around visual observer.

#9 astrophile

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Posted 13 November 2013 - 11:06 PM

What is the deal with Ethos, and similar, modern wide fields? Do they just play with the image to create that "porthole" with a wide apparent field, but the same old true field; or, for a given focal length, do they reveal a wider true field than classic eyepieces? If the true field were wider, transit time would be longer, so there would be much less tendency to lose the object to momentary distractions.


Both the apparent and true fields of view are significantly wider. Approximate TFOV = AFOV/magnification. So yes, you do get significantly bigger field and longer transit times.

#10 mark8888

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Posted 14 November 2013 - 02:08 AM

If the true field were wider, transit time would be longer, so there would be much less tendency to lose the object to momentary distractions.


Indeed, it makes a huge difference. Using a Brandon or a Nagler/Delos at high power is a totally different experience. Much much easier with the Nagler or Delos.

#11 mattz

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Posted 14 November 2013 - 02:50 AM

If you want to quickly build a mount that is much more stable than a Porta II, you can try:
Posted Image



#12 Derek Wong

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Posted 14 November 2013 - 03:24 AM

If you're looking at the premium mounts seriously--my experience with a Nova Hitch leads me to recommend one of those as an excellent option. With the slo-mo controls I have easily tracked at 390x with mine, and could have gone higher no problem. According to the website there are a couple 'Cassegrain' versions still available at intro pricing, and it appears digital trainable tracking is part of the package.


Also intriguing! To be honest, when I began this thread, I had no idea the premium mounts existed. I simply hoped to cobble together a simple, alt/az, grab-and-go rig. Now I'd say, offhand, that such a fine mount is surely on my "someday" list, even if, for now, the budget remains more modest. The idea of having a C8 on a lightweight, easily moved mount is just too attractive to ever forget! Sounds like the perfect package for the all-around visual observer.


If you look at the website for Nova Hitch http://www.halfhitch.../Nova Hitch.htm and look at the third picture, it shows a Cass scope on a Nova with the prototype tracking system that is beta testing right now. One thing about these mounts is that Charles Riddel says he won't do another run for over a year. My Nova is in the shop waiting for the tracking system which should be ready in 2-3 weeks. The Nova weighs 17#, add a few for tracking and you are at a much lower weight than a GEM.

There are apparently plans for a cheaper, simpler Nova sized mount, but there is no delivery time set and no info on the website about those.

Derek

#13 Joe Cepleur

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Posted 14 November 2013 - 09:24 AM

If you want to quickly build a mount that is much more stable than a Porta II, you can try:
Posted Image


Perfect! Are there plans available, or could you please comment on the bearings? What is inside those pipes so that everything turns smoothly?

Nova-Hitch looks also perfect, except, now, for the expense -- an item for my Someday List! I can truly see how a super Alt/Az would be better than a GEM for grab-and-go, big-fine-scope astronomy. Thanks!

The home-built version meets my budget and mentality. I'd be happy with most of the essential function at a fraction of the cost, but I bet the joy of the smooth action of those finely-machined mounts would make them seem essential, should I ever become accustomed to one. Honestly, right now, I'm still trying to learn the skies, and have many Messier objects to go. I'm happy when I can find one without charts! I want to learn the skies like a human Go-To, something I've seen among members of my club. I am currently less worried about studying obejcts in detail than in simply finding them, so I could be quite happy with simpler gear for now. (Good thing, given the budget!)

#14 BuffaloTri

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Posted 14 November 2013 - 09:58 AM

A C8 works beautifully on a DM4 mount. And I haven't had any trouble tracking at 200x (nudge, observe, nudge). My seeing conditions haven't supported anything higher than that. I don't know if I would consider it a grab & go setup. It is too bulky to get out the door in one piece. Typically I'll bring the mount & tripod out in one trip, and then the telescope, and finally eyepieces and a chart. I can pick the entire setup and carry it around the backyard, but I rarely do so.

I used to use a 127mm Mak with a DSV-M mount on a photo tripod. That was a grab and go setup. I could pick the whole thing up with one hand and carry it out the door. There was some wind up on azimuth axis, but I think it was caused by the extension on the tripod. I used this setup for travel or quick sessions in the winter.

#15 Joe Cepleur

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Posted 14 November 2013 - 12:20 PM

I used to use a 127mm Mak with a DSV-M mount on a photo tripod. That was a grab and go setup. I could pick the whole thing up with one hand and carry it out the door. There was some wind up on azimuth axis, but I think it was caused by the extension on the tripod. I used this setup for travel or quick sessions in the winter.


That's exactly what I want now. I have been considering buying an Orion 127 Mak, and putting it on a Porta II mount. That would be a good balance between reach and weight, while reducing setup time to essentially zero. When I have time for the C8, I'd be more than happy for now with its original mount. Another option for now, if I had the shop, time, and energy to build it, would be to mount my 4" Criterion Dynascope reflector on that sturdy-looking homemade mount. I could also swap the original, steel legs off the Dynascope's tripod and replace them with aluminum. That's sacriledge, I know, but it would cut the weight, and could not make it shakier!

Interesting that you've been happy with a C8 on a DM-4. Reading the specifications earlier, I noted that, officially, such use was discouraged, yet appeared to fall within weight limits.

#16 BuffaloTri

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Posted 14 November 2013 - 01:17 PM

The C8 weighs the same as my 4" refractor, but it's 1/3 the length. It hardly even registers on the DM4. I'd say the only issue is that the guide handle is required with the C8. It's too short to accurately move at high power without the guide handle. And if you're using the guide handle, then you need the mount extension. Otherwise the handle will bump the tripod legs.

#17 mattz

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Posted 14 November 2013 - 03:22 PM

Sorry, no plans. I just grabbed some wood and started cutting.

The Porta II comes apart pretty easily. I used the gears and mounting plate. I've since swapped out the plate with an ADM.

I wanted everything stacked over the center of the tripod with the shortest moment-arms I could make.

This is very stable now. The Porta II as-is was unusable with a C6-for me.

Wood is joined with dowels and glue. Wood is cut so the dowels are under compression force, not a twist force.

I can try to get more pics if that would help.

Matt

#18 Joe Cepleur

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Posted 14 November 2013 - 05:21 PM

The Porta II comes apart pretty easily. I used the gears and mounting plate. I've since swapped out the plate with an ADM.


You cannibalized a Porta II for parts?! What is an "ADM?"

Wood is joined with dowels and glue. Wood is cut so the dowels are under compression force, not a twist force.


I don't follow that, unless you mean only that you pounded the dowels straight in; you did not turn them, as though screwing them in. The point is that this keeps everything together more tightly?

I can try to get more pics if that would help.


Such a great mount! Posting more pics would be a real favor, thanks, and not just to me. I'm sure other would build something similar. It looks robust. No shakes there!

#19 mattz

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Posted 15 November 2013 - 01:28 AM

ADM is a manufacturer of plates, rings, and the like. The astrofield is blessed with many fine people/companies making excellent stuff. ADM is among them.

Since this was wood, I need two right angles. If the dowels joining the wood are horizontal, they will be under a twisting force when a scope is attached. If the dowels are vertical, they will be under compression force.

I'll put a couple pics up over the weekend.

This mod should work for a 127. A 127 may be a bit much for a straight Porta II. My C6 (150) on a Porta II would shake enough when focusing that it was very hard to focus. Then I'd bump the EP with my head and the shaking would start again. This is nice now with the mods.

That said, the Porta II seemed to work well with an 80mm refractor. The Porta II is a real nice mount as is. It is well designed, looks great, and is well put together for a very reasonable price. I only ran into problems because I pushed it. The mods just help extend the range of its capacity.

#20 mattz

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Posted 15 November 2013 - 01:33 AM

You should look at TONGKW's posts as well. He has several great ideas on modifications.

#21 Joe Cepleur

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Posted 15 November 2013 - 10:07 AM

Sorry, Mattz;

I completely missed the point until after reading your last post! I see now that your mount is indeed a beefed-up, re-worked Porta II. Best of the commercial and homebuilt worlds! You've kept the pivots, and replaced wiggly aluminum spars with heavy-duty wood. Very cool! Odd as it may not sound, I thought at first that you have made pivoting bases out of sewer pipe. Seriously. I've seen those sorts of things before.

Your version has a different geometry from the original. Both have the same geometry in azimuth, but they differ in altitude. In the Porta II, the scope pivots in altitude such that it remains balanced. In your version, when pointing up, the weight becomes increasingly borne over the back of the tripod, instead of along a shared axis with the azimuth's pivot. (That can't be clear. I mean that the mass of the scope is not concentric with the point of pivot in altitude.) Since you report it works well, that suggests those pivots from the Porta II are tremendously strong.

#22 mattz

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Posted 16 November 2013 - 12:04 AM

You are correct about the alt. It was a trade off we had to make. The distances are short, so it's not a big deal. I usually position the scope on the mount so it is balanced when it is at a 45 degree angle. I essentially move the scope up an inch or two. But there is no problem due to this, even with a C6 at zenith.






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