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Equipment Discussions >> Eyepieces

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Ebbisham
super member


Reged: 10/02/10

Loc: Surrey, Uk
Re: Eyepieces of similar weight ... what's your range? new [Re: Jon Isaacs]
      #5956628 - 07/05/13 07:37 PM

The closest i could find is my Pentax XL 40 and my Leica ASPH 17.8-8.9 zoom.
The XL 40 is pleasant but surely not the best out there. Unfortunately everything with good reviews in the 30-40mm range seems to weight a ton so i might stick with what i have until i find a mount that tolerates different eyepieces weights better than my Ayo AOK and my Gibraltar.


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iluxo
sage


Reged: 09/23/08

Re: Eyepieces of similar weight ... what's your range? new [Re: karstenkoch]
      #5956763 - 07/05/13 09:29 PM

Rebalancing was one of the factors leading to my decision to buy a set of vixen LVW's - all roughly the same weight from 5 to 42mm and 2" barrels - no swapping adapters either. Weights here http://www.vixenoptics.com/acc/lvw_eyepieces.html

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JustaBoy
Post Laureate


Reged: 06/19/12

Re: Eyepieces of similar weight ... what's your range? new [Re: iluxo]
      #5956778 - 07/05/13 09:41 PM

Hi,

Those are not the actual eyepiece weights for the Vixen LVWs.

For the actual weights of each EP please see this site:

http://agenaastro.com/eyepieces.html?ca_ep_series_bucket=176&camptype=hom...

Vixen has a very poor home page and poor marketing to go along with it.

Such very fine eyepieces, and yet few have ever tried any of them:-(

Clear Skies!

-Chuck


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McUH
member


Reged: 05/29/13

Re: Eyepieces of similar weight ... what's your range? new [Re: Jon Isaacs]
      #5957771 - 07/06/13 03:53 PM

Quote:

Balance is about the mount...




I kind of disagree. Balance is about gravity. If you have well balanced system, adding 1kg anywhere outside centre of gravity will unbalance it. Longer the scope - bigger the imbalance (lever). Lighter the setup, the bigger the imbalance.

Of course you can fight it with stiff bearings (with slow motion controls), with heavy setups. But it is there.

Best solution is to observe from space, no gravity, no problem with balance, and as a bonus no fear of dropping heavy expensive equipment .


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Eyepieces of similar weight ... what's your range? new [Re: McUH]
      #5957919 - 07/06/13 05:32 PM

Quote:


I kind of disagree. Balance is about gravity. If you have well balanced system, adding 1kg anywhere outside centre of gravity will unbalance it. Longer the scope - bigger the imbalance (lever). Lighter the setup, the bigger the imbalance.




If imbalance is a force or moment, then the weight of the scope has does not directly enter into the equation, the imbalance force is proportional to the length of the lever.

How the scope reacts to the imbalance depends on a number of factors, in a Dob, the diameter of the altitude bearings and the friction are the relevant factors. The weight and coefficient of friction enter into the equation as they affect the friction...

I think it really does come down to balance but not the simple balance one thinks of, rather balancing the forces and the friction... low friction is easily achieved, but not desirable. Some friction is desirable and necessary.

Jon


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McUH
member


Reged: 05/29/13

Re: Eyepieces of similar weight ... what's your range? new [Re: Jon Isaacs]
      #5958066 - 07/06/13 07:20 PM

Quote:

Quote:


I kind of disagree. Balance is about gravity. If you have well balanced system, adding 1kg anywhere outside centre of gravity will unbalance it. Longer the scope - bigger the imbalance (lever). Lighter the setup, the bigger the imbalance.




If imbalance is a force or moment, then the weight of the scope has does not directly enter into the equation, the imbalance force is proportional to the length of the lever.

How the scope reacts to the imbalance depends on a number of factors, in a Dob, the diameter of the altitude bearings and the friction are the relevant factors. The weight and coefficient of friction enter into the equation as they affect the friction...

I think it really does come down to balance but not the simple balance one thinks of, rather balancing the forces and the friction... low friction is easily achieved, but not desirable. Some friction is desirable and necessary.

Jon




Yes, balance itself is just sum of forces, which will be mostly gravity, perhaps some wind too. Aside from balance alone, it is important how the resulting force affects the system (will it slide?). And here mount, weight etc. comes into account. Big friction can render the resulting force incapable of moving scope, but it also makes it harder to move manually (compromise). To move heavier object, you need bigger force, or more precisely, the same force will move heavier object slower. So heavy setup will be more resistant to unbalancing and more precise to track manually despite big friction. And so on.

To cut it short, I prefer lighter setup (for comfort) without too much friction (to make high power manual tracking easier), so for me eyepiece weight matters more.
And I need some good reason why not to spend big money on these 100-120° eyepieces, which are heavy .


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karstenkoch
professor emeritus


Reged: 04/21/12

Loc: GMT+9
Re: Eyepieces of similar weight ... what's your range? new [Re: McUH]
      #5958129 - 07/06/13 08:19 PM

Funny! I just used the heavier weight of the ES100 14mm to justify it's acquisition! Previously, my Meade UWA 24mm was the only thing I had above 500 grams. It was way up there all by itself at 825g. Didn't play well with all my other EPs. Recently, I saw the ES100 14 weighs in at 836g. Having been very interested in getting into the 100 degree experience, that was all it took. Now, I'll soon have two nicely complementary EPs that I can use without any rebalancing or tightening so I can get that smooth pan across the cosmos feeling! The 800g range will be my "wide" set. Got another set in the 400g range that is my long eye relief set. Clear skies,

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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Eyepieces of similar weight ... what's your range? new [Re: McUH]
      #5958207 - 07/06/13 09:53 PM

Quote:

And here mount, weight etc. comes into account. Big friction can render the resulting force incapable of moving scope, but it also makes it harder to move manually (compromise). To move heavier object, you need bigger force, or more precisely, the same force will move heavier object slower. So heavy setup will be more resistant to unbalancing and more precise to track manually despite big friction. And so on.




I think the issue here is friction, to move the scope, you are overcoming the friction of the bearings, the "weight" or more properly the moment of inertia, is probably a small factor. This is similar to pushing a car. On flat ground, the reason a car is hard to push is not it's weight but rather the frictional forces that result from the weight.

The weight increases the friction of the bearings in a Dob design but the bearings could be designed so there would be essentially zero force required to move the scope. Decreasing the diameter of the bearing reduces the force required to move the scope. The fact that premium Dobsonians all use very large diameter bearings should be an indication that small forces are not desirable because those large bearings increase the force required to move the scope.

Myself, I set my scopes up so they balance with all my eyepieces as well as with no eyepiece, that means 0 lbs to 2.3lbs. The motion probably requires more force than some might like but force is not a major issue, it's only a couple of pounds and in any event I am able to track at magnifications in the 500x-800x range with what I find to be acceptable precision.

Jon


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dennilfloss
sage


Reged: 01/06/13

Loc: Ottawa, Canada
Re: Eyepieces of similar weight ... what's your range? new [Re: Jon Isaacs]
      #5958399 - 07/07/13 01:56 AM

I only have two eyepieces: the 24-8mm zoom listed at 370g and the 36mm listed at 415g. Hopefully I won't need to rebalance until I add the 2" Luminos barlow (312g).

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McUH
member


Reged: 05/29/13

Re: Eyepieces of similar weight ... what's your range? new [Re: Jon Isaacs]
      #5958701 - 07/07/13 10:43 AM

Quote:

Quote:

And here mount, weight etc. comes into account. Big friction can render the resulting force incapable of moving scope, but it also makes it harder to move manually (compromise). To move heavier object, you need bigger force, or more precisely, the same force will move heavier object slower. So heavy setup will be more resistant to unbalancing and more precise to track manually despite big friction. And so on.




I think the issue here is friction, to move the scope, you are overcoming the friction of the bearings, the "weight" or more properly the moment of inertia, is probably a small factor. This is similar to pushing a car. On flat ground, the reason a car is hard to push is not it's weight but rather the frictional forces that result from the weight.




Long time since I had any physics class, but I think weight (more precisely mass) plays considerable role, and not only in increasing friction. True, with zero friction you can move car, truck, even a train applying small force. But how fast it moves (how much unbalance you perceive) depends on mass. That is why Earth does not move much when I jump and apply force to it .

To be resistant to 0-1kg difference, it must be resistant also to your force to move it. You need to apply bigger push, which is more unbalancing on light system if you push too much. But all in all I think we are talking about the same thing.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Eyepieces of similar weight ... what's your range? new [Re: McUH]
      #5958923 - 07/07/13 01:06 PM

Quote:


Long time since I had any physics class, but I think weight (more precisely mass) plays considerable role, and not only in increasing friction. True, with zero friction you can move car, truck, even a train applying small force. But how fast it moves (how much unbalance you perceive) depends on mass.




It has also been a while since I have had a physics class but in my job as a research engineer, I deal with those things I learned in physics on a daily basis.

In this situation, based on some simple moment of inertia calculations for a typical Dob, it looks to me like the forces required to move the scope at tracking speeds is almost entirely used to overcome the friction of the bearings and essentially none of it goes into accelerating the mass of the scope.

I think that is desirable, with the effort going into friction and not momentum, it means that when the observer stops pushing, the scope stops moving. As evidenced by the large bearings on the fancy scopes, friction is actually the observer's friend, it's a question of how much...

Jon


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McUH
member


Reged: 05/29/13

Re: Eyepieces of similar weight ... what's your range? new [Re: Jon Isaacs]
      #5959314 - 07/07/13 05:32 PM

Quote:

In this situation, based on some simple moment of inertia calculations for a typical Dob, it looks to me like the forces required to move the scope at tracking speeds is almost entirely used to overcome the friction of the bearings and essentially none of it goes into accelerating the mass of the scope.




Yes I understand. For example let's say you need force of 1N to overcome friction, then only the part above this will be used for moving scope. My argument is, that if you need to apply 1N+something, your "push precision" will be lower than if you need to apply only lower force (say 0.5N). It will be individual and maybe not issue with these values. But try to move a cabinet and it will be very hard to push just "little bit more than necessary" :-).
Yes, it is necessary to have it smooth, otherwise you can't control it.


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Eyepieces of similar weight ... what's your range? new [Re: McUH]
      #5960112 - 07/08/13 07:40 AM

Quote:

My argument is, that if you need to apply 1N+something, your "push precision" will be lower than if you need to apply only lower force (say 0.5N).




It is worth recognizing the forces needed to move the scope are in the range of the weight of the eyepiece. If the scope is to stay put with the eyepiece both in and out of the focuser, that means the friction must deal with a differential on that order. So the forces are in the range of a pound or two. It is also worth recognizing that the azimuth force is almost always greater than the altitude force and that balancing the two is part of the key to a smooth scope.

Add in the fact that the big scopes with the big bearings are the ones with the best action, I think friction and somewhat higher forces are the observer's friend and the key is freedom from "sticktion" rather than an absence of friction.

Jon


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