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15-16" UC Dob: Summerian vs Obsession

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#26 sixela

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Posted 19 September 2021 - 04:13 PM

Two Canopus (one with optical finder and shroud) zero bungee cords. The cords are indeed delivered with the scope but they’re an insurance policy, to be used in emergencies and not for long term use.

 

If you ever constantly need them because you change the UTA weight significantly after the scope is delivered, in due course you’re better off sticking a removable counterweight right of the handle of the mirror box. Whether that’s even necessary usually depends on the f/ratio.

 

RAGBR-52-sumerians2.jpg


Edited by sixela, 19 September 2021 - 04:45 PM.

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#27 sixela

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Posted 19 September 2021 - 04:58 PM

I can give links to the consequences of the fall of the secondary on the primary mirrors

I can also tell you that my own scope (which I built — you’re not the only ATMer in these parts) I have had exactly zero accidents in 13 years. I had a secondary recoated recently and none of those silicone dots I had to cut were about to fail, and I would have been really surprised to see all three fail (even one would still hold the secondary, I tested just for fun).

 

Using a shroud for the secondary has some less desirable side effects if you do it as on the Obsession UC: it is usually impossible to mount the secondary as close as possible to the spider vanes, which increases the torque on the hub and makes the scope less compact and heavier (that pyramidal spider on the UC sticks out quite a bit).

 

Not everyone has to pick their poison in exactly the same way you decide to do it…


Edited by sixela, 19 September 2021 - 05:03 PM.

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#28 a__l

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Posted 19 September 2021 - 08:26 PM

If you ever constantly need them because you change the UTA weight significantly after the scope is delivered, in due course you’re better off sticking a removable counterweight right of the handle of the mirror box. Whether that’s even necessary usually depends on the f/ratio.

 

Why make an ultralight design, then make it heavy? Or is it not an ultralight design? Then it will not be much different in weight from Obsession UC.

 

Not everyone has to pick their poison in exactly the same way you decide to do it…

Those what the user writes about the imbalance with 0.7 kg N26 (without shroud, I understand his photo post # 25) or Mr. Lockwood's opinion about the glue for secondary is poison?

I think that this information should be perceived objectively and without emotion.

 

From my experience.
Show your video post # 25, the telescope is tilted 15-20 degrees below it, don't forget to add the P2+N31 to the focuser (in your photo, the focuser is empty). I think there will be imbalance on it or the mirror box has additional weight.
The picture looks unnatural.


Edited by a__l, 20 September 2021 - 02:22 AM.


#29 GeneT

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Posted 20 September 2021 - 12:12 AM

Looking for an ultracompact scope with a single ring upper is going down the wrong path, in my experience.

This is very portable, and a lot better design:

https://www.waiteres...ade-telescopes/

Assembling an 18":

https://www.youtube....h?v=JEkLScsZ1yU

Assembly of a 20" f/3:

https://www.youtube....h?v=vn1OWJVPxw0

I have posted several times before how much I liked my 20 inch Classic Obsession. In my opinion, it is excellent value for the money. Doing A B comparisions, I did not care much for my 18 inch Obsession Ultra Compact. I did not like the upper assembly nor lower assembly and mirror box, nor the folded side bearings which I was concerned that would loosen over time creating a wobble, or have to be replaced. There was no 360 degree baffle in the upper assembly and I had stray light problems, unless viewing in dark skies away from street lights and so on. I wish I had known about Waite telescope products. I believe I would have gone that route, or with one of the European vendors of Ultra Lites. However, dealing with European vendors from the U. S. probably would have been problematic. I have posted before that I had collimation problems  with my Obsession UC. That is one area I would have drilled down on when considering a Waite telescope. Waite telescopes seem to be better engineered from top to bottom than the Obsession UC's. However, to be fair, I have never viewed through a Waite telescope nor put one through its paces. However, please note--neither did I put an Obsession UC through the paces before I bought one. I learned an important lesson here. Before buying a telescope, 'try before you buy.' I lucked out with my Obsession Classic, but was disappointed with my Ultra Compact. If ever going down the Ultra Lite road again, such as a Waite telescope, I would first try one under the stars. I would want to be confident that it holds collimation, that it can stay in balance when using a variety of eyepieces, that it moves smoothly through the viewing arch, that the materials with which the telescope are made of are of highest quality, and I would really drill down on the quality of the optics--the primary and secondary mirrors. I am sold on the 'thin' mirrors because they would not only reach ambient temperature faster, they would hold figure better as ambient temperatures dropped. This is a problem in desert areas where it can reach 100 degrees during the day, but cool down quickly to 40 or 50 degrees late in the evening. 

 

Anyone own a Waite telescope? Many of us might find it interesting to get a review of one--likes and dislikes, pros and cons, with the bottom line--are you happy or not happy with the telescope? 


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#30 sixela

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Posted 20 September 2021 - 03:02 AM



Why make an ultralight design, then make it heavy? 

The Canopus doesn't make that choice. But if someone replaces a Rigel Quickfinder with a 60mm refractor as a finder, then yes, sometimes you'd have to adjust it, and my point is that if you do that then relying on a bungee cord on the long term is a bad idea. On the other hand, the bungee cord does mean that if you change the balance considerably you do have a 'spare wheel' to observe without having to rebalance the scope. It's not ideal but at least it allows you to observe while you figure out a better solution.

 

By the way, on my personal scope (not a Canopus) I did make the decision to make a naturally unbalanced scope that requires counterweights so that I could make my Alt bearings smaller. The reason is simple: you can make removable counterweights, which allows you to make the mirror box easier to carry. Yes, I need another trip to my box-of-things to get two (1kg and 0.5kg) diving weights with a strip of DualLock glued to them (usually when I go to fetch a heavy eyepiece), but that was a 'sacrifice' I was willing to make the bearings smaller without having to make the mirror box itself heavier.

 

 

Then it will not be much different in weight from Obsession UC

I have quoted some weight differences above. I assume you can read, so there really is no point in me repeating myself.

 

 

 

From my experience.

With due respect, you have zero experience observing with a 18" Canopus, while I just spent almost a week observing every night with my scope 3 meter from a friend's 18" Canopus (with shroud), frequently exchanging scopes, and looking at B33 using my personal 31T5 and 21E in that Canopus while B33 was still fairly low (although we did the exercise again when it was higher, at 20° altitude). Perhaps you think Googling odds and bits counts as 'experience' and trumps personal experience, but I don't.

 

 

 

Then another question, is this an ultra-compact telescope?

An Alkaid definitely is an ultra-compact telescope, but it makes compromises for it. On the other hand, even in a typical European car, it goes into a small corner of the boot (aka trunk) and you can still put all the luggage for your family if you go on a holiday (some of it on top of the Alkaid) and you can carry it up steps in one single trip from the car. Those are its raison d'être for the larger ones.

 

A Canopus? It depends on your definition. It does prefer larger bearings that you can unscrews and transport separately (or flat on the mirror box) to foldable bearings. If that makes it no-Ultra-Compact in your definition, so be it -- it's up to the original poster to decide what he wants and how compact it needs to be (for me personally, the ease of transport and weight of the heaviest component is my main worry).

 

Compared to a 16" Alkaid, a 16" Canopus is definitely not as compact -- except other designs also very like Strock scopes (anyone French will know these), nothing is. But the Canopus is more stable and is less of a puzzle to assemble (and to store as a box again after observing) than an Alkaid.

 

16" Alkaid (minus trusses):

post-256238-0-56894700-1497007772.jpg

 

18" Canopus (minus trusses) -- note the mirror box recesses into the rocker if you remove the Alt bearings:

post-6880-0-98978500-1479937514.jpg

 

 

18" Obsession UC:

1.jpg

 

My impression seeing all of these (but I didn't measure it) was that the Canopus stack looks taller but really isn't that much taller since nothing is sticking out. In this configuration you do have to add/remove both Alt bearings (which are smaller than on the UC because of the ground/rocker ring design that allows the mirror box to go lower than the rocker), but I prefer that over the foldable bearings of the UC.

 

Mr. Lockwood's opinion about the glue for secondary is poison?

You seem unfamiliar with the idiom 'pick your poison'. It's not invective directed at you (no, everyone is not out to get you), it's simply an idiom used when you have to make compromises and make a choice between several unpleasant choices (in this matter, when designing a UTA).


Edited by sixela, 20 September 2021 - 05:12 AM.

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#31 osbourne one-nil

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Posted 20 September 2021 - 03:28 AM

This thread has caused me a bit of work this morning. If we can fall out when talking about something as wonderful as telescopes, then humanity is doomed. 

 

On that cheery note....


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#32 sixela

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Posted 20 September 2021 - 05:37 AM

I would want to be confident that it holds collimation, that it can stay in balance when using a variety of eyepieces, that it moves smoothly through the viewing arch, that the materials with which the telescope are made of are of highest quality, and I would really drill down on the quality of the optics--the primary and secondary mirrors.

In Europe in Germany, France and Poland (I did already mention Taurus Telescopes) there is an important class of telescopes neither as ultra compact as some of the scopes discussed in the original post nor as bulky as a 'classic' Obsession. In fact, they're more or less dominant in the market over here.

If you were in France you sound like someone who'd like a mirror plus telescope from Skyvision or a mirror from Terence Pelletier and a telescope from this guy: https://arp83.fr/?page_id=466 (who builds fairly compact but not Ultra-Compact scopes). Not sure how easy they are to deal with for people in the US, though (and for both Terence and arp83 the waiting lists are loooong).


Edited by sixela, 20 September 2021 - 07:05 AM.

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#33 Mauro Da Lio

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Posted 20 September 2021 - 10:22 AM

In the EU, I would also point these two other brands: 

 

http://nauris.de/ind...leskope/mirrage (carbon fiber)

http://www.doctelescope.com (metal and motorized)


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#34 Starman1

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Posted 20 September 2021 - 10:34 AM

In Europe in Germany, France and Poland (I did already mention Taurus Telescopes) there is an important class of telescopes neither as ultra compact as some of the scopes discussed in the original post nor as bulky as a 'classic' Obsession. In fact, they're more or less dominant in the market over here.

If you were in France you sound like someone who'd like a mirror plus telescope from Skyvision or a mirror from Terence Pelletier and a telescope from this guy: https://arp83.fr/?page_id=466 (who builds fairly compact but not Ultra-Compact scopes). Not sure how easy they are to deal with for people in the US, though (and for both Terence and arp83 the waiting lists are loooong).

That's unusual--an octagonal secondary mirror and a primary mirror with a central hole.

Conventional primary mirror collimation methods would have to be modified or just star collimation employed.



#35 sixela

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Posted 20 September 2021 - 10:41 AM

That's unusual--an octagonal secondary mirror and a primary mirror with a central hole.

He doesn't only make scopes that weird ;-). Don't know who provided the optics. Certainly not typical of what e.g. Terence Pelletier makes.

 

Here's a larger mirror from Terence (and Terence himself) in a structure which I think is also from ARP83:

cropped-eb682761f0a6efc8bd73b4181c2f21be


Edited by sixela, 20 September 2021 - 10:43 AM.

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#36 Dan_I

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Posted 20 September 2021 - 10:44 AM

That's unusual--an octagonal secondary mirror and a primary mirror with a central hole.

Conventional primary mirror collimation methods would have to be modified or just star collimation employed.

Almost certainly the owner of the scope provided his own optics to the maker, it is not a design feature of the telescope (these are custom-built instruments).

 

A more recent build from the same maker (without hole smile.gif )

 

https://www.webastro...-83fullum-r241/


Edited by Dan_I, 20 September 2021 - 10:50 AM.


#37 alexvh

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Posted 23 September 2021 - 07:10 PM

Ive owned Both the 16” alkaid and the 18’’ UC obsession.
If anyone wants my opinion I’m happy to share ….

#38 a__l

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Posted 23 September 2021 - 08:51 PM

Doing A B comparisions, I did not care much for my 18 inch Obsession Ultra Compact. I did not like the upper assembly nor lower assembly and mirror box, nor the folded side bearings which I was concerned that would loosen over time creating a wobble, or have to be replaced. 

GeneT, OP asks about 15-16"

Alt bearings for 15" UC are different from 18" so your message is incorrect....

https://www.youtube....h?v=GbwL116hvEg

It has optics from Terry O. The edge support on the steel slide has been updated here.

If I bought such a telescope, I would invest a little more in it. Buy two carbon rings + 5mm carbon focuser plate and make my own spider. Would install ServoCat and Nexus Dsc.

Perhaps I would replace the FT focuser with the MoonLite I have upgraded.

About what I did with the last edition of my 18".

https://www.cloudyni...obs/?p=11253004
If I had a choice, this is definitely Obsession 15". I don't need a telescope with rubber cords ...

 

https://dobsoniani.f....it/?t=69997553

 

(use google translator from italian, the review is not over, the reasons are unknown)


Edited by a__l, 24 September 2021 - 12:01 AM.


#39 sixela

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Posted 24 September 2021 - 06:03 AM

I don't need a telescope with rubber cords

Once again: you can get a Canopus and not use rubber cords. If anyone has doubts, I'd suggest asking Sumerian and not a__l .

 

To quote who you quoted:

 

 

The balance

Simply perfect. In typical set-up, that is with ES 24mm and telrad mounted, it does not matter if at 10 degrees or 80 degrees the telescope is perfectly balanced. Without the elastic the tip is always light more than it should which results in a slow movement towards greater angles starting from the telescope at 0 degrees.

 

That suggests that the owner has tried without all accessories on the UTA that Sumerian designed the balance for (usually shroud, finders and a Paracorr in addition to the eyepieces). I expect at first light that would sort itself out.

 

You criticize the scope and use a reference to a review where the owner's first words about balance are "simply perfect" -- I assume that you expect we'll take your criticism in stride without actually reading it?

 

By now you seem to have both asserted that the Canopus needs a rubber band because the UTA is too heavy and that it needs one because the UTA is too light (all without having used one for a single instant). Which is it? 


Edited by sixela, 24 September 2021 - 07:06 AM.

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#40 Jon Isaacs

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Posted 24 September 2021 - 06:43 AM

Once again: you can get a Canopus and not use rubber cords. If anyone has doubts, I'd suggest asking Sumerian and not a__l .

 

To quote who you quoted:

 

 

 

That suggests that the owner has tried without all accessories on the UTA that Sumerian designed the balance for (usually shroud, finders and a Paracorr in addition to the eyepieces). I expect at first light that would sort itself out.

 

You criticize the scope and use a reference to a review where the owner's first words about balance are "simply perfect" -- I assume that you expect we'll take your criticism in stride without actually reading it?

 

By now you seem to have both asserted that the Canopus needs a rubber band because the UTA is to heavy and that it needs one because the UTA is too light (all without having used one for a single instant). Which is it? 

 

In my experience, balancing a Dob is just part of the setup.  One night I might be using a 50mm RACI finder, the next night, a 80mm RACI finder that weighs a kilogram more. That will require adding some weight to counter balance the heavier finder.  As long as the scope doesn't depend on springs or bungee cords to be balanced, then I am happy and I know that the scope will track nicely with my eyepieces that include the 21mm Ethos and the 31mm Nagler.

 

Jon



#41 Starman1

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Posted 24 September 2021 - 09:58 AM

It's really not a big deal.

My scope is well balanced with a Paracorr and heavy eyepiece until it is below about 40°.

Then, the top gets progressively heavier as the scope points lower.

A very light weight eyepiece doesn't have this problem, but, like many, my eyepieces vary in weight by well over a pound, and the scope cannot be

balanced for all the weights in the focuser.

So, I attach a small mini bungee cord between the mirror box and the rocker box that has no tension on it until the scope points below 45° and which slowly grows more taut

as the scope points lower.  It works very well to yield an equal force requirement for up and down as the scope points low--no diving.

I attach the little bungee when I assemble the scope and then forget about it.

It's not a serious issue and does not detract from the scope in any way.

 

So it is not a negative that any scope might require that.  The other answer is to have all your eyepieces be exactly the same weight and to have the 

COG be exactly on the altitude axis.  You could arrange that, but it would seriously limit your choices.



#42 Jon Isaacs

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Posted 24 September 2021 - 10:34 AM

So it is not a negative that any scope might require that.  The other answer is to have all your eyepieces be exactly the same weight and to have the

COG be exactly on the altitude axis.  You could arrange that, but it would seriously limit your choices.

 

 

There is another answer. 

 

 The large diameter altitude bearings are used because they require more force to move. 

 

With a given weight eyepiece, if the telescope is balanced both top to bottom and front to back, then it will be balanced from the horizon to the zenith. If a heavier or lighter eyepiece is used, it will no longer be balanced.

 

3912615-Balancing a DOB.jpg

 

This is where the large diameter bearings come in, they provide friction which keeps the telescope from shifting. If the amount of force required to move the scope is at least half as much as the weight of your heaviest eyepiece, then the scope will be balanced with all ones eyepieces at all elevations from the horizon to the zenith. The force required to raise the scope and to lower the scope will be somewhat different but this is mostly a factor at lower elevations.

 

My heaviest eyepieces weigh about 2 lbs so slightly more than 1 lb tracking force does the job. I find that with my scopes, they just seem to be about that.   But again, careful initial balance is critical.

 

Jon


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#43 sixela

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Posted 24 September 2021 - 11:41 AM

I agree -- a telescope ideally should not require a bungee cord and stay put both when you have your heaviest eyepiece installed and when you remove it. On my own scope if I don't get that I'm very dissatisfied, and on the one I designed I didn't even put a bungee cord on it at all. Plus, a telescope moves a lot better when it's almost perfectly balanced.

 

Even on my 10" Alkaid I've counterweighted it to achieve this (despite its design which forces bearings that fit in the box under your airline seat.) Granted, I don't put a Paracorr plus a 1kg eyepiece on the 9kg scope either (the largest field eyepiece I use is a 24mm ES 68°). I told Sumerian that I would not use a Paracorr and that he could use that to optimise the box dimensions (the Alkaid was not yet a series production scope).

 

But getting the weight on the exact location needed can be tricky.

 

I consider a bungee cord like one of those diminutive spare tires you find on some cars: it gets you home but you should really not be driving with one all the time. 


Edited by sixela, 24 September 2021 - 11:45 AM.

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#44 Starman1

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Posted 24 September 2021 - 11:42 AM

Yes, but if your eyepieces differ in weight by 30 ounces and you don't want the force to move the scope to be excessive,

then you have a choice what weight in the focuser will balance when the scope points to 10° altitude.

 

Balancing the scope for a point half-way between heaviest and lightest eyepieces might work.  I haven't had luck with that.

Some inexpensive scopes add friction to keep the scope from moving.

 

But if my 12.5" scope is balanced for a 2.3lb eyepiece at 10°, then removing the eyepiece to change it heads the scope up.

That occurs with a min bungee cord just as it would with a counterweight.

 

The trunnions are 19-3/4" in diameter.  Making them larger could work if the psi on the bearings were properly calculated to not

yield a too-much-force-to-move arrangement.  A stiff movement requiring a lot of force is worse than having to balance the scope somehow.

There really is no answer other than to have all eyepieces be the same weight, or have the scope move very stiffly if you don't want the scope to move

with or without eyepiece at all altitudes.

 

Your example simply doesn't work when installing or removing a 2.3lb eyepiece.  The scope cannot be balanced both with and without eyepiece.

At least, it can't below about a 40° altitude.

 

The larger the scope, the lower the percentage of the overall scope weight that is represented by the eyepiece and the less the eyepiece affects the balance, regardless

of where the scope is pointed.

The smaller the scope, the more change in weight by percentage occurs when a heavy eyepiece is added or subtracted.

An easy answer is a sliding counterweight, but I accomplish the same thing with a mini bungee cord.  Both are variable counterweights, in a sense.

The goal is to have upward and downward forces equal so the scope doesn't dive.

You can easily accomplish that for one weight in the focuser, but not a lot of different weights, alas.


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#45 sixela

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Posted 24 September 2021 - 11:47 AM

you don't want the force to move the scope to be excessive,

Personally? If your friction isn't large enough to handle removing your heaviest eyepiece then I think you don't have enough friction. As long as the static and dynamic friction is the same and both axes move with the same amount of force applied I don't really mind if there is some friction. Now, sometimes "not enough friction" is unavoidable due to design restrictions (e.g. if you must have a travel scope in a box with the bearings in a box and a fairly low rocker) but for a scope like a Canopus or Obsession UC that shouldn't matter.

 

Your example simply doesn't work when installing or removing a 2.3lb eyepiece.

On a really light 10" f/5, you have a point (which for me is one reason not to use a 2.3lbs eyepiece on it, though I prefer not to lug these around when I travel anyway). But on a 16" f/4.5, my scope really proves that's not true. It stays put with a 31T5 or 21E in the focuser at zenith and at the horizon, and in both positions I can remove the eyepiece and it won't budge. And that's the size of scope this thread should be about, although I guess most people have forgotten it by now.

 

An easy answer is a sliding counterweight, but I accomplish the same thing with a mini bungee cord.  Both are variable counterweights, in a sense.

Correct, but the movements are smoother and you have less vibrations with the sliding counterweight. On one scope I've seen someone replace a Rigel Quickfinder with an 80mm (!) finder relying only on two beefier bungee cords to compensate for the difference, and I did nudge that owner to get a diving weight to stick to his mirror box (it's even colour matched). It made a world of difference...


Edited by sixela, 24 September 2021 - 11:59 AM.

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#46 Jon Isaacs

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Posted 24 September 2021 - 12:22 PM

The trunnions are 19-3/4" in diameter.  Making them larger could work if the psi on the bearings were properly calculated to not

yield a too-much-force-to-move arrangement.  A stiff movement requiring a lot of force is worse than having to balance the scope somehow.

There really is no answer other than to have all eyepieces be the same weight, or have the scope move very stiffly if you don't want the scope to move

with or without eyepiece at all altitudes.

 

 

~One pound max to move the scope is not stiff. I find it just about right. I do a lot of doubles with my 10 inch and 13.1 inch, that means I'll be tracking near the zenith at somewhere over 800x.. The altitude friction is near ideal.  If there's not enough friction, then the scope's momentum comes into play and motion can be prone to overshoot. I think one wants a somewhat over damped system. 

 

I pay a lot of attention to balancing the scope top-bottom AND front-back. If the scope doesn't stay balanced over the entire range, it's likely that the front-back balance is off. It's not as critical as the top-bottom balance because the lever arm is shorter but it's a factor that needs to be addressed since the effect of both axes is additive or subtractive depending on the elevation angle.

 

As an experiment, I rested my elbow on a raised pad and placed my closed hand on a scale and tried to relax my muscles. That was 800-900 grams, or about 2 lbs. My entire arm is obviously much heavier. Experimenting with the scale, a one pound force seems surprising light. 

 

That's what works for me. I have my 22 inch balanced so it stays in place with all my eyepieces but if I remove the Paracorr, it will slowly climb.  Even with a heavy scope, light, balanced motions are possible.

 

Jon



#47 Starman1

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Posted 24 September 2021 - 01:44 PM

Fantastic, but your experience and Alexis' experience contradicts my own.

 

On my 12.5", I have no trouble with a 2.3 lb eyepiece, in or out of the focuser, above 45°.

Below 40°, I do--the scope tends to dive.  It slides down slowly.

For years, I just added a counterweight on the mirror box when the scope pointed very low.

That worked at 25-40°, but the scope still took a dive below that unless I added a second counterweight.

 

I needed a sliding counterweight, which is what I installed on my first 12.5" scope with 12" trunnions.

It moved too easily and required EXACT balance not to rise or fall, even with the sliding counterweight, which had to be adjusted for every eyepiece and every 20° of altitude change.

When changing eyepieces, I hung a counterweight on the UTA to hold the scope in place.

 

On my current scope, I can attain perfect balance at 10° with a 2.3lb eyepiece, but the instant the eyepiece is removed,

the scope starts rising.  There is plenty of stiction in the altitude bearing, but it is physically impossible to balance the scope at all angles with that much weight in the focuser

without having the scope result in too much force being required to move the scope.  The eyepiece is 20% as heavy as the entire UTA.

The force to move the scope would have to exceed the weight of the eyepiece to stay in place when the eyepiece is removed or installed.

OR, the scope's balance point must be directly over the altitude axis at all altitudes of pointing, with or without the weight in the focuser.

I don't believe that's possible, though I have tried with counterweights in strategic places.

 

I have used some large scopes where the force to move them exceeded 3-5 lbs and I hated the movement in those scopes--sticky and with overshoot when moved.

My friend's 32" is in that category, as almost every 25" Obsession I've ever used.

I think I understand the reason why so many large scope users install a ServoCat.

 

Your 22" rises when a 1.1 lb weight is removed from the focuser.

Imagine a significantly lighter UTA and scope.  How does removing a 2.3 lb eyepiece from the focuser allow the scope to stay in place?

 

The obvious answer is a larger scope, of course.lol.gif



#48 sixela

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Posted 24 September 2021 - 02:04 PM

Fantastic, but your experience and Alexis' experience contradicts my own.

They're simply not the same scopes.

 

That, or you've failed to put the COG exactly on the Alt rotation axis (i.e. in two dimensions -- it's even worse because you really want the COG to be close enough to the middle too, because a large difference in the weights suported by the two bearings can also play tricks on you, but there's more tolerance there). My main counterwieght is on the mirror box in the line that connects the middle of the Alt axis and the focuser. I've got a smaller one that I can stick at different places to adjust it all a bit.

 

Mind you, in those cases you can't get perfect balance with counterweights, I do agree (unlike some others who shall remain nameless) that using a cord is no big deal in practice°. That, or you just have a removeable counterweight that you add to observe close to the horizon and remove when you observe close to the zenith -- whatever works. But you should then still strive to make as little use of it as possible.

 

 

 

I have used some large scopes where the force to move them exceeded 3-5 lbs and I hated the movement in those scopes--sticky and with overshoot when moved.

 

'Sticky' and 'overshoot' means that the static friction is more than the dynamic friction. That's different from the force you need to apply to make it move, which can be fairly large without too many ill-effects as long as the force to keep it moving is not a lot smaller (obviously 5lbs is too much even for me, but 3lbs? I really don't mind that much.)

--
°on a scope I designed it's a BIG DEAL -- I consider it a shameful personal design failure, and something to be rooted out. But it's not something that will stop me from observing.


Edited by sixela, 24 September 2021 - 02:16 PM.


#49 Starman1

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Posted 24 September 2021 - 02:22 PM

They're simply not the same scopes.

 

That, or you've failed to put the COG exactly on the Alt rotation axis (i.e. in two dimensions).

 

Mind you, in those cases you can't get perfect balance with counterweights, I do agree (unlike some others who shall remain nameless) that using a cord is no big deal. But you should then still strive to make as little use of it as possible.

 

 

 

 

'Sticky' and 'overshoot' means that the static friction is more than the dynamic friction. That's different from the force you need to apply to make it move, which can be fairly large without too many ill-effects (obviously 5lbs is too much even for me, but 3lbs? I really don't mind that much.)

Ebony Star on PTFE (teflon) has a higher static friction than kinetic friction.

Several years ago (2003?) I read an article on PFA (in the place of PTFE) as a bearing material.  The static friction and kinetic friction were said to be a lot closer to the same.

So I found a source for the material and changed my PTFE pads to PFA.

The article was right--the static friction and kinetic friction were nearly equal--no overshoot.

But, the friction was easily 3X as high as PTFE.

I couldn't put up with the high force necessary.

 

A few years later, I tried replacing the Ebony Star strips with FRP.  FRP has a somewhat higher kinetic friction than Ebony star.

In addition to the noticeably bumpier movement, it too yielded a too-high kinetic friction for my taste.

 

So I went back to Ebony Star on PTFE, despite the high start up friction.  I tried Jon's suggestion and used bar soap to coat the Ebony Star to reduce the Startup friction, and it did without appreciably changing the kinetic friction.

It seemed like the ideal solution to making startup friction and kinetic friction closer to the same without adding the extra drag of PFA.

 

If it sounds like I've experimented, I have, including dynamically balancing the scope so the same heavy eyepiece was balanced from 10° to 90°.

That is, until the eyepiece is removed.  Or an eyepiece of 1/3 the weight placed in the focuser.

I could go back to hanging a counterweight on the UTA when changing eyepieces, but I have more or less solved the issue by not looking below 40° off the horizon, and using the variable "virtual counterweight"

when looking low.

 

One other option is simply a "brake" to hold the scope in place when switching eyepieces.

I put one on a previous scope and it was just OK.

I'll have to research that more.


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#50 George N

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Posted 24 September 2021 - 02:26 PM

There is another answer. 

 

 The large diameter altitude bearings are used because they require more force to move. 

 

With a given weight eyepiece, if the telescope is balanced both top to bottom and front to back, then it will be balanced from the horizon to the zenith. If a heavier or lighter eyepiece is used, it will no longer be balanced.

 

 

 

This is where the large diameter bearings come in, they provide friction which keeps the telescope from shifting. If the amount of force required to move the scope is at least half as much as the weight of your heaviest eyepiece, then the scope will be balanced with all ones eyepieces at all elevations from the horizon to the zenith. The force required to raise the scope and to lower the scope will be somewhat different but this is mostly a factor at lower elevations.

 

My heaviest eyepieces weigh about 2 lbs so slightly more than 1 lb tracking force does the job. I find that with my scopes, they just seem to be about that.   But again, careful initial balance is critical.

 

Jon

More force to move is ONE reason for a large altitude bearing. The other is - just like drive gears in GEMs - the larger the bearing - the less impacted GoTo pointing and tracking error is from mechanical errors. For a star-hop guy that might not seem like a big deal -- but most premium Dobs today are being ordered with some sort of GoTo or at least DSC system. One of the reasons over at New Moon for the move to ABS alt bearings is - complaints that slews result in an object being at the edge of the eyepiece field rather than the center. Testing showed - subcontractor error in making the Al bearings - also the new ABS bearings are not only more accurate, play better with StellarCat - they save some weight.

 

I should note that Jon's discussion of proper location for balance weights on a Newt tube was well covered in Sam Brown's "All About Telescopes" -- I remember referring to that book, my Physics 101 text book, and the trig functions on my new 'electronic' calculator when attaching balance weights to my 8-inch F/8 Newt ATM tube in 1972 to counter the 2-inch focuser and finder mass! With a Newt on a GEM getting the center of mass on the centerline of the tube is perhaps even more important than with a Dob - otherwise balance shifts as the mount is moved around the sky.

 

On my recently-sold Obsession 20 F/5 - one of my fav add-ons was the Markless counterweight system. It worked great - handling everything from a little Brandon 1.25" eyepiece to a Denk II bino-viewer loaded with two Ethos 13mm. Mine (unlike the picture on their website https://www.markless...t products.html) had a scale next to it so I could just shift the weights to the same position to balance the same item. ( weights were also nice to hold down my tent when putting it up in the wind!)

 

I recently had a chance (Stellafane 2021) to observe with a friend's 12.5-inch Teeter Journey Travelscope - airline version (some compromise to meet air transport weight requirements). My friend goes to Oz to observe about every other year - and ordered the TJ-Air for that purpose (his other Dobs include an Obsession 18 Classic and a SDM 22" F/3.8 -- but transport of that scope back 'home' to Oz-land was many thousands of dollars last time he checked). The very nice little 12.5" Teeter TJ-Air is a great scope - great views - very accurate slews with Nexus (Zambuto primary) -- but it does have a kinda clunky Y-shaped coil spring under the tube to keep balance. It worked fine - despite giving some loud "sprong" sounds with long moves. While a great scope - both my friend and I believe that a good misplaced kick would kill the thing - very well made, but delicate. Teeter makes it clear that it is intended for long-trip air transport - and the compromises have to be lived with to meet that goal. My friend used it at Stellafane and then another week in the Maine woods - no problems. (link: https://www.teeterst...ney-travelscope)

 

Oh... I can't provide any opinion about the Summerian vs Obsession UC -- I've never seen the former - and my only experience with the UC is walking up to someone else's and looking in the eyepiece -- to some very fine views. On the other hand - based on observing and using at least 6 -- my future in Dob-land is New Moon Telescopes Hybrid - I hope to have my 20" F/3.5 next month - with luck and no early snow storms (had our first in October last year).


Edited by George N, 24 September 2021 - 02:30 PM.

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