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15-16" UC Dob: Summerian vs Obsession

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#51 a__l

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Posted 24 September 2021 - 06:48 PM

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

 

 

The cord cannot be elastic over the entire temperature range in which the telescope can be used.

Other nuances. UTA without Telrad weighs 2.5 kg with two plywood rings for 16". My UTA for 14.5" (plywood) weighs 4.5 kg.
Even taking into account the steel secondary holder and the fact that my UTA is higher, the weight is clearly low and there are big doubts about stability when using heavy eyepieces with two plywood rings.
If the spider is thin, the rings will deform over time (from the pull of the spider). If the spider is very thick (to keep its shape), this is also not very good.
The diameter of the aluminum tubes is 18 mm and the weight of the mirror box is 17.5 kg (without battery). The tubes are clearly insufficient in diameter.
There are doubts with vibrations, in the Italian review this issue is not fully disclosed.



#52 Jon Isaacs

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Posted 24 September 2021 - 07:29 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

 

Don:

 

This is a basic 2-D mechanical engineering problem in statics.

 

It can be shown that if the scope is balanced top to bottom and front to back with half the weight of the heaviest eyepiece, then if the frictional force is equal to half the weight of the heaviest eyepiece, the scope will not move with any eyepiece and will not require more than half the weight of the heaviest eyepiece to track.

 

That was why I pointed out that my 22 inch would slowly rise if the 16 ounce Paracorr 2 were removed, it is balanced with no eyepiece as well as with the 31mm Nagler and 21mm Ethos, from the horizon to the zenith but with the added 16 ounces from removing the Paracorr, it will shift.  This is how it can be.  My 16 inch is this way, my 13.1 inch is this way, my 12.5 inch is this way. They require about a pound max to track. 

 

Your description of requiring more frictional force than the weight of the eyepiece to balance indicates to me that the scope is not balanced front to back, just top to bottom.  

 

4755777-Balancing a DOB.jpg
 
I think this is what Sixela is saying but in different words. 
 
Jon

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#53 a__l

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Posted 25 September 2021 - 02:19 AM

 

It can be shown that if the scope is balanced top to bottom and front to back with half the weight of the heaviest eyepiece, then if the frictional force is equal to half the weight of the heaviest eyepiece, the scope will not move with any eyepiece and will not require more than half the weight of the heaviest eyepiece to track.

 

This is mistake or at least has ambiguity.

You should have approximately the same friction force as your heaviest eyepiece/2.
For example, if your eyepiece weighs 1 kg (Nagler 31) then the friction force should be equivalent 0.5 kg and the top scope is balanced without eyepiece weight minus 0.5 кг. Then by attaching this eyepiece or removing this eyepiece, your telescope will not move on its own.

Those other words. You find the balance point without an eyepiece. Next, reduce the weight of the UTA by 0.5 kg (increasing the torque on the bottom is equivalent to 0.5 kg for UTA).

Without an eyepiece, the frictional force opposes the movement of the telescope with a minus sign, with an eyepiece with a plus sign.


Edited by a__l, 25 September 2021 - 03:23 AM.


#54 a__l

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Posted 25 September 2021 - 09:09 PM

If the spider is thin, the rings will deform over time (from the pull of the spider). If the spider is very thick (to keep its shape), this is also not very good.

I carefully looked at the photo, this is a thick spider. A red arrow indicates an unstable structural element.
The second photo is my thin spider (not yet painted). This attachment allows you to stretch a thin spider (carbon rings resist deformation well with low weight) and take full advantage of such a spider.

Attached Thumbnails

  • Canopus_.jpg
  • Заделка_.jpg


#55 sixela

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Posted 26 September 2021 - 01:12 PM

The spider on the Canopus is indeed fairly thick, but still under tension (it’s made slightly too short and acquires its tension when it’s screwed onto the UTA.)

I do like the mechanical arrangement on a__l’s spider more, no question about that (I have something similar on small raised blocks protruding from the UTA on my personal scope).

The Canopus spider definitely is inferior mechanically (though the UC spider is even worse in my book) but it still seems to do its job, probably helped by the fact the secondary is close to the hub (which more or less forces the use of silicone points rather than a shroud, but we’ve discussed that ad nauseam already).

Edited by sixela, 26 September 2021 - 01:28 PM.


#56 a__l

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Posted 26 September 2021 - 06:54 PM

The spider on the Canopus is indeed fairly thick, but still under tension (it’s made slightly too short and acquires its tension when it’s screwed onto the UTA.)
 

There is a small (weak) plywood ring and this tension relaxes after 1-2 years use, because plywood does not hold its shape. In addition, the steel corner can lend itself to further bending. Curious, have you checked this with an autocollimator while moving the telescope from the zenith to the horizon on a fairly used telescope?


Edited by a__l, 26 September 2021 - 06:56 PM.


#57 sixela

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Posted 27 September 2021 - 07:16 AM

Curious, have you checked this with an autocollimator while moving the telescope from the zenith to the horizon on a fairly used telescope?

No, a laser collimator (but one which is fairly precise, a HG whose pinhole creates a fairly precise diffraction pattern that allows you to see very small differences). I usually use an autocollimator only on photo scopes, where getting rid of focal plane tilt is far more important (and even there, the weak point this usually exposes is consistency of registration in the focuser and focuser sag with large photo trains).

 

I haven't compared it to a recent Obsession UC but the one 18" UC that I did tested had a UTA end that would not hold collimation as well.

 

I suspect that you might be right that you'd need to retension the vanes after 1-2 years. Can't say: the Canopus I tested had just been through a checkup. I've heard of one user who indeed had to retension his spider but he had a curved spider that Sumerian had installed on request (IIRC, he no longer advises people to order these).


Edited by sixela, 27 September 2021 - 07:21 AM.


#58 a__l

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Posted 06 October 2021 - 05:08 PM

I have a picture of Michael setting up a 24" on his own --not with a winch or wheelbarrow handles but from pieces dragged from a car's boot (aka 'trunk') but that's really not what I'd personally call a one man job (the mirror box is less than 30kg but unwieldy).

 

This interested me. I currently have a 24" telescope (photo) with parameters 25 (mirror)+8(cell)+13(box)+11(UTA)+10(Al bearings from Obsession)+3(tubes) = 70 kg or 154 lbs.

Total the weight mirror box is 25+8+13=46 kg without bearings. 56 kg with bearings. Lifting 56 kg alone is almost impossible.

 

I am calculating an option for a lightweight (but durable telescope) that won't wobble in a light breeze.
I got 25(mirror)+5(cell from Al+Ti, almost ready, replacement for carbon is possible)+5(box carbon+Ti)+ ~6(UTA carbon+Ti+Telrad+light weight FT focuser+Moonlite aluminium slide+Nexus DSC+P2+Astrosystems heavy duty spider+secondary holder+5.5" thick quartz secondary)+4(bearings carbon+Al+Ti in the making) + ~ 1.5 (tubes carbon+Ti)....
Total the weight mirror box in real design is 25+5+5=35 kg without bearings. 39 kg with bearings. I didn't take plywood into account. This is not a material that can be used for lightweight structures large telescopes.

 

Ps. Many calculations are preliminary. Subject to change. The worst case scenario is that I will fail. It is difficult work on the first attempt to make such a telescope without mistakes.

Attached Thumbnails

  • 2_Scope_2.JPG

Edited by a__l, 06 October 2021 - 06:25 PM.


#59 a__l

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Posted 06 October 2021 - 05:49 PM

The numbers for thought.
Weight of birch plywood 0.7 g/cm3, aluminum 2.7 g/cm3, carbon 1.7 g/cm3
Carbon's Young's modulus is three times that of high-strength aluminum of roughly the same thickness and 10 times the plywood that can be used for an acceptable structure (for the same weight).
Think about the tension of the spider in the plywood rings, the use of glued secondary 5.5" weighing one and a half kg, etc. and you will understand what this is about.


Edited by a__l, 06 October 2021 - 10:46 PM.


#60 sixela

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Posted 07 October 2021 - 02:50 AM

The photos of the telescope aren’t public, but there is one public image of the mirror box:
https://www.facebook...0724874/?type=3

[the 27” cell sticks out at the back, you can just see it stick out on the page’s cover photo.]

The mirror is lighter than yours, though.

Edited by sixela, 07 October 2021 - 02:54 AM.


#61 BGazing

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Posted 07 October 2021 - 03:15 AM

I have a recent Sumerian Alkaid 10", bought for travelling. I noticed that, compared to previous versions there are improvements. It is positive that Michael does not stick on a static design and improves it continuouslly.

The Alkaid is the lightest and most compact design: the 10" is the largest cabin luggage for many airlines. Nonetheless the truss in itself is quite rigid and it does not loose collimation. On occasion I transport the scope fully assembled by car, and it is still collimated after the travel. Of course lightness have a price but it is only in terms of (acceptable) vibration damping over say 200x when mounted on the rocker. There is an equatorial table for high power views though. For deep sky, between 40x and 200x it is perfect.

The 16" size might be preferable in the Canopus version though.

 

----

 

PS I am an engineer and I do not see any problem with crumpled tubes as long as they work a trusses (as it should be). Crumpled has no effect on push-pull capacity (they would have effect for flexure but the structure is a truss).

I concur with this. Up until 200x is my comfort zone for nudging. If I take Paracorr out balance is a bit better. Bungees do a good job until about 30 degrees alt if Paracorr is in, below that I have to add Celestron battery over the mirror box handle in order to balance it.

It is much smoother in motion than any commercial dobs I handled. Never had a chance to compare it to any premium dobs, I am sure they are smoother (at the expense of weight, of course). 


Edited by BGazing, 07 October 2021 - 03:20 AM.


#62 sixela

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Posted 07 October 2021 - 04:37 AM

BTW, you're also talking about the Alkaid, right, which is not supposed to be the subject of the thread?



#63 BGazing

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Posted 07 October 2021 - 04:54 AM

BTW, you're also talking about the Alkaid, right, which is not supposed to be the subject of the thread?

I believe there was John's question about the bungee cords, both designs share it. 



#64 a__l

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Posted 09 October 2021 - 07:00 AM

The photos of the telescope aren’t public, but there is one public image of the mirror box:
https://www.facebook...0724874/?type=3

[the 27” cell sticks out at the back, you can just see it stick out on the page’s cover photo.]

The mirror is lighter than yours, though.

Does it have contact with the mirror? I would not save so much on size (and weight).

 

I decided to replace the bars (titanium) and triangles (aluminum) with carbon. This will ease my cell construction (18 points).

 

In the photo 1 it looks like the cell is made of plywood ...

 

I like the MoonLite 1.25" clamps better(photo 2). But in order to match them with the metric carbon tube I have to work on a lathe. 

Attached Thumbnails

  • 22in (1).jpg
  • Нижние.jpg

Edited by a__l, 09 October 2021 - 07:55 AM.


#65 sixela

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Posted 12 January 2022 - 04:52 AM

I believe there was John's question about the bungee cords, both designs share it. 

As said before: both kinds of scopes have it, but on the Alkaid you will need the bungee cords, whereas you normally won't on the Canopus (except if you change the UTA weight with respect to the design criteria).

 

My 508mm Canopus was designed with 4kg of counterweight in mind on the mirror box (I'd rather have them removable than in the mirror box, which is heavy enough as is) to keep the bearings slightly smaller while retaining the capacity to observe on the horizon (and that has to do with the size of a Tesla Model 3's trunk). Most of this "counterweight" is a battery for fans, dew heaters, a dew controller, fan controllers and some voltage converters. With that setup I don't need the bungee cord at all even when fully loading the focuser (with a Paracorr and 31T5) and pointing at 0°.

On the other hand, the bungee cord does come in handy when I forget my battery, which makes the mirror box lighter (in use). So as a "spare tyre" it's still handy, even though my engineering heart bleeds whenever I --rarely-- use it ;-).


Edited by sixela, 12 January 2022 - 04:53 AM.


#66 GeneT

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Posted 14 January 2022 - 01:33 AM

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

This is an attractive telescope. Note that the secondary rests within the UTA and therefore has a 360 degree baffle. Note that the secondary on the Obsession UC rests above the UTA which is only a ring, with a tongue like baffle on the opposite side of the secondary. Lastly, notice the rocker box on these telescopes which holds the mirror and mirror cell. To me they seem to be far better designed than their Obsession UC counterparts.  



#67 sixela

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Posted 14 January 2022 - 05:05 AM

The flex-rocker has been redesigned on the larger models, BTW: instead of one wall with holes it's now supported by an inner and an outer wall, and it's now SUPER stiff (and slightly heavier, but since it's not really the heaviest element it's largely irrelevant).

Why the guy on the right doesn't use his light shield is a mystery to me, though. Except if there was too much wind.

Canopus.JPG


Edited by sixela, 14 January 2022 - 05:12 AM.

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#68 GeneT

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Posted 14 January 2022 - 08:22 PM

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

Fire away.



#69 sixela

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Posted 15 January 2022 - 05:57 AM

The Alkaid really has different design criteria, to be fair. I think a Canopus occupies a much more similar niche.

Having compared a 18” Canopus and 16” Alkaid side by side: unless I wanted a small box that fits in a corner of the trunk, I’d rather have a Canopus; it moves more smoothly, settles down a lot faster after you move it, and is faster to assemble.

But yeah, fire away.

Edited by sixela, 15 January 2022 - 06:00 AM.



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