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dobsonian altitude bearings

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#26 jmillsbss

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Posted 22 October 2020 - 08:18 AM

This is bouncy...

 

gallery_217007_5817_31131.jpg

 

This is not bouncy, this is stiff and smooth...

 

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Same trunnions in both photos, unfinished and finished.

Structure (such as braces) make all the difference.

 

gallery_217007_5817_87674.jpg

Now, THAT'S  SEXY!!!  I WANT 2!!!


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#27 jmillsbss

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Posted 22 October 2020 - 08:24 AM

Disadvantages to standing:

  • you get tired faster so can't last as many hours.  I've noticed "standers" usually close up shop really early compared to seated observers.
  • you are less stable standing so your head moves back and forth and in and out relative to the eyepiece a lot more.  Most eyepieces are more of a problem to use.
  • the eyepiece will often be at positions where you have to bend over, hurting the back.
  • you won't spend as much time on each object, so you'll see less.

Now, if a ladder is used on a big scope, you can hook an arm over the ladder, or lean against it, so it's not the same issues as just standing.

But standing?  Just doesn't make sense compared to a chair.

You'd be proud to know I got a chair!  Jon Isaacs has been on me about a chair for years!!!


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#28 Eddgie

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Posted 22 October 2020 - 09:05 AM

 

It's just that the people at ES understand the advantage of a larger altitude trunnion.  Instead of increasing the friction with a spring, they use a larger trunnion.

I am not so sure that even this is the reason.  I think that they just do this because there was no engineering involved and the part is a common part to the other ES truss dobs and the Costco dobs.   

 

It is just a set of tube rings and the same bearings as used elsewhere.  Probably more of an economic decision. Cheaper to use common parts and no need to design a new and specific bearing system. Otherwise, they would have had to invest in new molds for the altitude bearing and some kind of altitude friction system.   

 

So, my money is on simple and cheap to make using existing components already on the shelf. Now I think the bigger bearings are a good idea too, but this is a low margin business, and decisions made on margin probably count more than decisions made on design benefit. 

 

But this is just all of us thinking out loud. 

 

Just business I think. 


Edited by Eddgie, 22 October 2020 - 09:30 AM.


#29 Starman1

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Posted 22 October 2020 - 05:14 PM

I am not so sure that even this is the reason.  I think that they just do this because there was no engineering involved and the part is a common part to the other ES truss dobs and the Costco dobs.   

 

It is just a set of tube rings and the same bearings as used elsewhere.  Probably more of an economic decision. Cheaper to use common parts and no need to design a new and specific bearing system. Otherwise, they would have had to invest in new molds for the altitude bearing and some kind of altitude friction system.   

 

So, my money is on simple and cheap to make using existing components already on the shelf. Now I think the bigger bearings are a good idea too, but this is a low margin business, and decisions made on margin probably count more than decisions made on design benefit. 

 

But this is just all of us thinking out loud. 

 

Just business I think. 

The trunnions on the First Light Series are different than the ones used on the truss tube dobs, so obviously a different manufacturing for them.  Not a shared part.


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#30 Asbytec

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Posted 22 October 2020 - 05:20 PM

Here is a great mod. https://astrogoods.com/bearings.shtml

 

I got tired of my previous mass produced Dob because the bearings we way too smooth for their own good. I guess we all like silky smooth movement, but some designs come with a price. I first realized I had a problem when I was changing eyepieces and heard a soft thump in the night as the OTA rotated to (beyond) the zenith on it's own.

 

Balance was a tricky trade off between friction and stiction, even with counterweights. I guess it's difficult to add push to and go to to larger trunnions, but my personal view is all Dobs really need larger trunnions.

 

I bought the ES Firstlight mostly because of it's bearings. It turns out to be pretty well built, as well. More sturdy. My previous Dob also suffered some secondary mirror droop with changes in altitude. I could watch the laser dot move in line with gravity, even with the spider vanes so tight I could play dueling banjos on them. 

 

Contrary to any connotation "Firstlight" might offer, it's not a beginner's scope. It's built well enough most seasoned observers might actually enjoy it. I do, and no more soft thumps in the night or drooping secondary mirrors. What a relief. Now I can spend more time using it instead of finding a balance solution that might or might not work.

 

Yes, the side boards could be a little taller especially with more weight up top, like a RACI finder and heavy eyepieces. It requires a little counter weight. Easy fix. Most commercial Dobs are a work in progress, anyway. But Firstlight requires only a few simple mods. The primary mirror springs hold the mirror's weight, too. It has locking screws, but they are not really needed. It's a good scope. 


Edited by Asbytec, 22 October 2020 - 05:41 PM.

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#31 Jim T

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Posted 22 October 2020 - 06:35 PM

Posted 10 July 2020 - 06:55 PM

jmillsbss, on 07 Jul 2020 - 12:14 PM, said:

    .  I find I enjoy my Dob more if I'm standing.

 

 

Despite the "disadvantages", so do I !!  I will add that for me this is only because the eyepiece height on my big scope is nearly at eye-level and viewing is very comfortable while I stand.  I am usually one of the last visual observers to leave from my group, but I confess that about 4 hours is all I need, both mentally and physically.  I am not quite the night owl that some are here, and I never cared for all-nighters.  I sit while I write in my log and when i consult my star charts.  That's plenty.  I need to get up and move around to avoid getting stiff!

 

To each his own.  Enjoy.


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#32 gwlee

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Posted 25 October 2020 - 04:28 PM

Here is a great mod. https://astrogoods.com/bearings.shtml

 

Have you tried the Astrogoods side bearings?



#33 Asbytec

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Posted 25 October 2020 - 04:44 PM

Have you tried the Astrogoods side bearings?


No. I wish I knew about them earlier when I was experiencing difficulty. I ran across them after I sold my Dob and again recently. But, I bet I would have considered it.

#34 Pezdragon

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Posted 25 October 2020 - 05:26 PM

I’m going to carry the flag for the old school Dob mounting and the smaller side bearings. 50 years of owning and 5 generations of rebuilding and refining while keeping the classic form has produced a scope that tracks superbly at 500x + using just my little finger. The bearings are 8” diameter Phenolic disks riding on Teflon impregnated woven nylon for a 10” scope. I have the balance tuned to allow a range from the heavy 26 t5 nag to no eyepiece. Huge side bearings hide a out of balance situation that uses increased friction with its inherent higher tracking force. I do admit they work well but don’t dismiss smaller, well executed ones.

 

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#35 Starman1

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Posted 25 October 2020 - 05:41 PM

If your scope moves that easily, the sled focuser would change the balance and heavy eyepieces would dive and light eyepieces would rise.

So either friction is higher than you say or you never observe below 50°.

I had a 12.5" that moved that easily, and it required a sliding counterweight to balance the scope and a brake to hold it in place when changing eyepieces.

 

Say, is that finder home-made or a commercial product?



#36 Pezdragon

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Posted 25 October 2020 - 05:54 PM

That a good question/point but the reality is the sled only has to move an inch or so to accommodate all my eyepieces and does not adversely affect the balance. It took years to find the magic combination of friction, materials, and design to make this happen. I do in fact observe almost to the horizon at times and the system holds fine but only by a ounce or less, it really is tuned to a hairs breath but is just a joy to use....this for the record is not a light scope though, weighing in at about 130 lbs all up...mass does have advantages though.



#37 Starman1

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Posted 25 October 2020 - 06:02 PM

Yes, high mass helps.  That 12.5" to which I referred was about 80 lbs total.

The smaller the % of overall weight represented by the eyepiece, the less sensitive the scope is to balance. 



#38 Pezdragon

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Posted 25 October 2020 - 06:12 PM

About the finder, the picture is a bit old but showed a Baader sky surfer 3. It now sports a sky surfer V, an excellent finder. Way back in the 1970’s I used a tail gunners reflex sight From a B-17 WW2 bomber mounted on the tube box!



#39 gwlee

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Posted 27 October 2020 - 11:51 AM

No. I wish I knew about them earlier when I was experiencing difficulty. I ran across them after I sold my Dob and again recently. But, I bet I would have considered it.

I have been thinking about ordering a pair of the AstroGoods altitude bearings to test on my XT8, but would like to inspect them first or at least talk to someone who has experience with them. 


Edited by gwlee, 27 October 2020 - 11:54 AM.

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