Balancing a Dob

[tims]

:question: When people talk about balancing a Dob (any size), what are they referring to?
Is it that you can put the Dob at any angle and it will stay there without swinging back to it's centre of gravity?

If so, how does it work when you have different eye-pieces, barlows etc installed as I would have thought that would upset the balance of the scope or is a couple of kilo's not going to make any difference?

Thanks

[ZeroID]

Yup, you're dang right, it changes with EPS, cameras, anything. Hence the dec bearings surfaces are critical. It is a matter of 'stickion'. Enough friction allow some weight differences but slippery enough to not inhibit easy movement of the scope to the next target. Always going to be a compromise. I am using Box strapping, the plastic ribbon strapping you find around warehouse goods. Some people add springs or bungee cords to increase the pressure between the surfaces.
Most of my EPs are reasonably close and once I have added the nights accessories, finder, camer etc then I balance it with additional weight etc and we're away.

[tims]

Thanks

Slightly OT, but in your answer you mention bearing 'stickion'.

Is it then a compromise in having enough 'stickion' so the dob doesn't 'wander' off by itself with say changes in eyepieces etc and too much 'stickion' which might put strain on a go-to system like the servo-cat possibly causing premature failure?

Or are the standard teflon bearings used on a dob never that slippery to have this problem.

Cheers

[mental4astro]

Balancing a dob has more to do with where the centre of gravity, COG, of the scope than stickion, and the diameter of the altitude bearings has an effect too.

If the COG is very close to the geometric centre of the OTA, then the balance of the instrument will be very sensitive. If the bearing diameter is small too, this is even more so the case. Sure you can compensate for changes in balance by using some type of braking mechanism, but the trade off is the scope becomes harder to move. It can also add the complication of backlash. Many folks, instead of using a braking mechanism because of the constant change in moveabilty, use a large magnet that they then slide up and down the tube to compensate for the change in balance.

If the COG is really low, making the thing bum heavy, the amount of change in weight up at the top end is a lot less sensitive. Particularly if the bearings are really large in diameter.

I'll give you two examples with two of my scopes: My 8" dob uses a commercially made solid tube OTA. Its COG is pretty much dead on to the geometric centre. This makes it a real bugger to balance with any change in eyepiece. I use a magnet and a velcro strap to help control the balance issue. My 17.5" dob that I designed and built myself has a very low COG. The diameter of the altitude bearings is 80cm. I can use a tiny eyepiece and then switch to a huge 1kg piece of glass and she stays put! No braking mechanisms, and her movements are silky smooth.

Stickion is a term that was created to describe the relationship of the ideal movement in a scope. It should be easy to move and have no backlash. To get the scope to start moving it there should be just enough resistance so that once the resistance is overcome, it happens with the slightest "pop" and then the force needed to keep it moving should be very little. Stickion should be a constant force, not a changing one that will happen if there is a braking mechanism on the scope.

The two pics are of these two scopes of mine I've mentioned. The only way with the 8" I have of not altering its "stickion" properties is by shifting the magnet. Otherwise if I just rely on the braking mechanism, the constant changing of the friction properties of the scope would make it a trying task during the night. Yet because of the design of the 17.5", balance is never a problem.

Stickion also refers to the property of Teflon sliding over laminates. That it IS slippery running over highly textured laminate is what makes this combination the materials of choice. BUT, unless the designer of the scope doesn't understand both this property and the mechanical properties of a dob, then the resulting scope will have both balance and ease of movement problems. In fact, we even WAX our laminate bearing surfaces to make them even MORE slippery! But, unless your scope is finely tuned, you will always have an unrulely instrument. Even waxed, my 17.5" dob doesn't loose its balance regardless of the eyepieces I'm using, and the bearings are the Teflon/laminate combination. Off course there is a limit to this capacity of my big dob, but I've also got design elements built into it to deal with eyepieces larger than 1kg, ;).

[Keltik]

Hope I don't sound pedantic, but i believe the term is 'stiction' (http://en.wikipedia.org/wiki/Stiction).
Readers with a big shop-bought Dob like my LB 16" may find that the altitude bearings have a little bit too much stiction. These bearings are alloy/felt. I had a lot of annoyance with 'jumping' when trying to get the scope moving up or down, and at anything higher than a 15mm EP, the target would like as not leap out of the field. It was also difficult to position targets at the leading edge of the field so you could watch them as they transit the field.
Step forward Scotch Invisible Tape (http://www.officeworks.com.au/retail/products/Office-Supplies/Stationery/Adhesives/Sticky-Tape/Invisible-Tape/3M3105). This is a matte-finish tape my daughter uses to mount photos for her Uni work. It has a very silky finish. I taped over the felt pads on the altitude bearings, and there has been a marked improvement in the motion. It doesn't take half as much effort to get the scope going, and I have had no trouble with unwanted movement ( due to the scope being one heavy mutha anyway!) The tape is cheap too, at a few bucks a roll, and if it gets dirty or worn, you can peel it off and add a couple of new strips.

[mental4astro]

Hi Trevor,

I like the simplicity of the tape solution. Just watch that you don't get residue build up on the metal bearing. You are asking the tape to perform a task it wasn't designed or made for, so it may leave behind a build up of that very 'silky' stuff. All that it means is having to clean the bearing a little more often to keep the performance.

[Keltik]

The tape isn't actually coated with anything ( except adhesive on its 'business' side). It's matte in the same way as the thin little plastic bags they give you in the fruit and veg section at Coles or Woolies. I did have a bit of a think about placing the adhesive against the felt, in case it left a residue. But I figured the felt/alloy interface was essentially unusable, and anyway, replacing a couple of strips of felt would hardly bust the bank, even in these financial times.