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There is a point where there is too little friction.

For example, you want to swap out our heavy 31mm Nagler for a 13mm Nagler. If there is no friction, but the scope is perfectly balanced for the 31mm Nagler, it will slew to the zenith when the 31mm is removed. Not good. So, what you want is just enough friction so that the scope remains balanced from your lightest EP through your heaviest EP.

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My understanding is that for a fixed coefficient of friction, the friction--both static and dynamic--is approximately independent of the contact area.

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My understanding is that for a fixed coefficient of friction, the friction--both static and dynamic--is approximately independent of the contact area.

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Yes that's how it looks in an equation using the coefficient of friction. But in reality the surfaces are bumpy and grab at those bumps so surface area does matter. Same reason wider tires get better friction on dry pavement.

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Actually, the tire case is much more complicated.

Now three comments from me:

1. Where bumps are few and far between, a wider contact area makes it more likely that you will always be in contact with a bump, thereby making for a more consistent friction without slick areas. This probably applies to the case of tires, where a wider tire makes it more likely that some part of the tire is in contact with something not slipper, but not the case of teflon on, say, ebony star, because the bumps on ebony star are uniform on the scale of the area of the teflon pads.

2. Yes, a larger surface area will encounter more bumps. But because the pressure will be lower, it will be proportionately easier to surmount these bumps. Since the pressure is inversely proportional to the surface area, the two effects should cancel out. Easy thought experiment: imagine a two-wheeled wheelbarrow being pushed up a pair of two exactly parallel slopes, with each wheel of each wheelbarrow being on a different slope. Assuming the wheel axles are frictionless, so the only thing we're dealing with is the force of having to lift the wheelbarrow to the top of the hill, the amount of force needed is equal to that of pushing a one-wheeled wheelbarrow of the same weight up one of the two slopes. (But the two-wheeled wheelbarrow will be easier to push in a more consistent manner: it is less likely to get stuck in the mud, it is less likely that two wheels will simultaneously hit a slick than that one would, etc.)

3. In the case of sticky surfaces, as the linked article notes, things are completely different. Obviously it's a lot harder to dislodge something glued over a larger surface area. But there is very little stickiness between teflon and just about any other surface, so this is irrelevant to this case, but it may be relevant to the tires. (Suggested experiment: Put a large piece of teflon on something and lift it up. It's easy! On the other hand, rubber does actually stick--you have to peel it sometimes.)

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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer

Edited by arpruss (11/04/09 12:50 PM)

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For the alt. bearings, two pads are used for each bearing, ideally spaced around 65-75 degrees from each other

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What happens when you have less than 65 deg? What's the absolute smallest angle?

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Ed Jones

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For the alt. bearings, two pads are used for each bearing, ideally spaced around 65-75 degrees from each other

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What happens when you have less than 65 deg? What's the absolute smallest angle?

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The alt bearings on my travel scope are about 30 degrees apart to maintain the very low profile of the rocker. (I am planning to re-do this with about 45 degrees.) Instead of a counterweight I use springs, and it works fine for all my eyepieces, which range from about 2 oz to 14 oz (which is not so big a range), down to about 20 degrees altitude. Below that, the heaviest eyepiece is too much and I need to put something on top of the mirror box (like a bolt case with another eyepiece) as a counterweight. The other problem with the close spacing is that it is easy to knock the scope off its bearings.

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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer

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But I view those compromises as necessities in order to get the friction (and stiction) as low as I wanted them.

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Your solution only works when all your eyepieces are nearly the same weight. When they vary from 6 to 36 ounces (as mine do), the friction necessary to hold the scope in place would make it nearly immovable.

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I go from nothing to Paracorr plus 26T5 without the scope budging (at the horizon and zenith). I can still move it very easily with one hand, and the static and dynamic friction on both axes are almost identical, making movement very intuitive.

No, it's not "nearly immovable" at all, and those grapes aren't sour. I can track easily at 800x. Not that I have to (as the scope is on a platform).

Yes, my bearings are large, and so are my support pads. And the scope isn't made *too* light, just light enough to be transported easily. Making it heavier than would be possible if lack of weight were the only design criterion helps prevent the COG moving too much when the eyepiece train is removed.

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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)

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And that we should be able to do, unless our eyepieces use gravitational lensing and contain miniature black holes. :-)

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I think Sixela is right: it should be possible.

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Not only is it possible, I have proof by existence, about 1 metre from me.

Note that I don't mind if Don prefers lower frictions (and lives with the consequences). I couldn't live with a scope tuned the way he tunes his, because I change eyepieces *a lot* and I don't want my scope to weather vane in the lightest of breezes, but if he can, it's a free world.

I can actually make my scope a lot more frictionless (by just applying some SailKote). I don't feel I need to, as I can nudge it very precisely even at high powers and without stiction.

The only issue I have with what he wrote was the possible implication that one could (mis?)read (but he may not have meant) that it is a necessity to select too little friction to balance your scope across the eyepiece selection to get low stiction (which is certainly desirable), and his statement that a scope that stays put when you remove an eyepiece train is necessarily nearly immovable (which, as you correctly point out, is only true if eyepieces are twice as immovable).

If anyone wants to repeat the experience:

-mirror box to UTA combo (including trusses, shroud, Telrad, etc. etc.), with half the heaviest eyepiece train: 31.5kg.
-Teflon pads are 24mm wide on 35mm long, with a 15mm diameter hole for the screw to hold the pads. That's a contact surface of 670mm2 per pad or 6.7cm2.
-They're at +-33° on a bearing with radius 350mm.

In total, that's 1.175 kg/cm2 or 16.7 lbs/inch2. Pretty close to the K&B recommendation.

This is the scope:

http://www.cloudynights.com/ubbthreads/attachments/3425019-DSC_0089.JPG

And the Alt bearing and side support looks like this:

http://www.astroforum.nl/download/file.php?id=21298

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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)

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There is a point where there is too little friction.

For example, you want to swap out our heavy 31mm Nagler for a 13mm Nagler. If there is no friction, but the scope is perfectly balanced for the 31mm Nagler, it will slew to the zenith when the 31mm is removed. Not good. So, what you want is just enough friction so that the scope remains balanced from your lightest EP through your heaviest EP.

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To each his own. In my opinion, if the friction is high enough the scope does not move when a heavy eyepiece is removed from the focuser, then the friction is WAY too high. I set my own so that a 2-4 ounce weight difference in the UTA weight causes noticeable imbalance. Yes, it means I have to balance for every eyepiece at every angle. Yes, it means I had to install a "brake" to hold the scope in place while I change eyepieces. But I view those compromises as necessities in order to get the friction (and stiction) as low as I wanted them.
Your solution only works when all your eyepieces are nearly the same weight. When they vary from 6 to 36 ounces (as mine do), the friction necessary to hold the scope in place would make it nearly immovable.

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I actually built and used my scope the way you describe for 8 years, then a couple of years ago I added just a touch of friction back into the scope and I like it better then way it is now. I still have to maintain a hand on the upper ring assembly while swapping EPs in order not to loose where I was pointing. Alternately I have a 90 degree redlight flashlight that I can place on the front ring and it will add just enough weight to hold the scope.

Tracking Neptune at 571X is not a problem.

The trick is to make the friction uterly smooth.

{As an aside: my 3 most used EPs all weight about the same:: 31mm Nagler T5, 20mm Nagler T2, 13mm Nagler T1}

Edited by MitchAlsup (11/04/09 08:36 PM)

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