Dob balance advice needed

[Rod]

Hi everyone

I am rebuilding a 6 inch f5. The OTA is complete and a picture is attached.

The tube currently fits another mount I have so I have been trying to get it to balance so I can work out dimensions for the rocker box. At the moment the tube will only balance around 20 to 40 degrees. When let go from horizontal the tube moves to 20 degrees, when let go from vertical it drops to around 40.

I should note the altitude bearings are not centred across the width of the tube. They sit about an inch above the centre point. I did this because the crescent shaped bearings didn't have enough wood to attach to if the bearings are perfectly centred.

The focuser is a a standard GSO 2 inch focuser. I do have heavy eyepieces ES 100 degree.

Can anyone suggest a counterweight strategy that might work here?

Should I make new larger alt bearings and centre them properly?

Thanks

Rod

[Kunama]

Sounds like it raises as it is tail heavy but drops from the vertical due to the fulcrum not being in line with the centerline. When the tube is pushed to the vertical there is more weight on one side of the horizontal axis (fulcrum) thus it drops but won't hold horizontal due to the the fulcrum being too high up on the tube.

The reason it has this 20-40degree range is due to the friction of the bearings, if you had actual roller bearings with near zero friction, the tube would always end up in the same orientation whether released from horizontal or vertical....

Edit: if you want to balance with weights, place the tube horizontal and move it in the tube holders till it balances. Then place the tube in vertical and add counterweights equidistant above and below the level of the pivot point till it balances vertically.

I would remake the bearings and place them correctly.....
Not sure if this all makes sense....

I like the look of the tube, nicely done!

[Rod]

Thanks for the informative reply Matt

That's very helpful and yes makes sense. I'll cut new thicker alt bearings and centre them. I'm a little worried the heavier focuser and eyepiece might still throw things out but I guess if it does I can follow your counterweight suggestions.

Rod

[mental4astro (Alexander)]

The COG of your OTA is more involved for an OTA as yours. So designing a mount, and the alt bearings, cannot follow the same designs as seen on big, open tube dobs with heavy rear ends.

You mentioned you use big, heavy eyepieces with this OTA. The position of the focuser means you cannot use the long-axis centre line in the COG. Especially with heavy eyepieces, the long-axis balance centre line is pushed towards the focuser. Between a 6mm plossl and a 1kg eyepiece, the long-axis COG can be shifted by as much as 20mm towards the focuser in such a small scope.

Of course, I do not know how you determined the COG, so I can only say that with a short tube OTA, eyepieces need to be used in determining the "true" COG. And as eyepieces can vary greatly in weight, this variation also has a significant result in the COG between the primary mirror and the focuser - the position of the COG can vary by as much as 60mm between a tiny 6mm plossl and a 1kg eyepiece with such a short OTA. You would be best served to work out the average position of the COG for both axis.

This highly fluid COG then influences the design of the alt bearing. Like I mentioned earlier, a big dob alt bearing design will not be anywhere effective here. A large diameter bearing is an advantage, and for a more effective balance solution, without resorting to awkward, action-changing, clamps or tensioners, introducing more lateral squeeze to the trunnions (the OTA alt bearing component) is a great asset. This lateral squeeze increases the usable range of eyepiece weights, and reduces the need for the occasional counterweight use. Introducing more lateral squeeze does increase the effort to move the scope in altitude, but this increase is not so great as to off-set the improvement in the balance situation. Keeping the bearings clean improves the quality of the action. Also, the quality of the action is always the same, even when using a counterweight, unlike with a clamp or tensioner that always varies the quality of the action, and can have a detrimental effect on the bearings and dissatisfaction with the scope.

Short tube Newtonians make for fantastic rich field scopes. But they have their own set of mount requirements that set them apart from their big open tube cousins. Get the mount and the position of the bearings right, as Matt mentioned, and you've got the makings of a very satisfying scope,

Mental.

[Rod]

Hi Alexander

Thanks for the clarification. I clearly didn't think through the balance issues. I've made small scopes before but used small focuses and light eyepieces. These scopes were easy to balance.

I've made some new altitude bearings. They are the same diameter but closer to a full circle so there is plenty of space to move them up and down to test balance. I should be able to try them out tomorrow.

I was confused by the term 'lateral squeeze'? I once added Teflon to the inside of the rocker on a 4 inch scope. This squeezed the tube slightly, adding friction and making balance easier. Is that what you mean?

Rod

[mental4astro (Alexander)]

The Teflon pads of the alt. bearings on a big, bum heavy dob are placed anywhere between 40 to 60 degrees apart. This is fine with a BIG, BUM HEAVY DOB... You have plenty of mechanical leverage advantage - you can swap between a tiny 6mm plossl and a 1kg EP and experience no balance issues. But, as aperture decreases, you lose the leverage advantage. The COG begins to match the geometric centre, and such a close placement of the Teflon pads makes maintaining a balance impossible.

The lateral squeeze I mention is done by increasing the angular separation of the pads, say between 120 to 160 degrees. The trunnion is then not just sitting on top of the pads, but the pads also push in from the side - so you get a lateral squeeze!

This helps maintain balance for a much wider range of eyepiece weights.

A really short tube as yours, if you swap between small eyepieces and 1kg bruts, you will still need a counterweight with the big eyepieces. What you need to then figure out is a counterweight design that won't add further complications to the balance. In other words, consider where you are going to place the counterweight on the OTA. All of this I've had to consider with my 8" Kulali scopes. While the COG of Kulali is still below the geometric centre, I can swap between a wee 6mm plossl and a 750g EP, and balance is not a problem. I only need a counterweight to help with heavier EPs or when using a coma corrector with the 750g EPs. No clamps or brakes or clutches, and the same smooth action all the time,

[Rod]

Thanks Alexander

I'll give the wider placement of the Teflon a try. I just tested the new alt bearings and things are looking MUCH better. I get balance through virtually the whole rotation, just a bit off near vertical. But the Teflon on the other mount is mounted quite close. Hopefully adding some 'squeeze' will sort the remaining imbalance out in the final design. Otherwise I'll have to think through where to allow the addition of some wight.

Thanks for th help!

Rod