Dampening on gs dobs

[slice of heaven]

Whats the dampening like on everyones gs dobs?

[ballaratdragons (Ken)]

Dampening? Alt or Az?

On my GS 12" Deluxe the Alt needs both springs attached and is still too light. The action is very smooth though.

Az still has centre bolt problems. It loosens even with the correct Nylock nut PLUS a spring washer. I'm going to take out the roller bearings and replace them with teflon pads.

Why, what problem are you having?

[janoskiss (Steve H)]

No noticeable vibrations unless you look for them. When you move the dob mount, it settles very quickly (in much less than a second). The spider is the only bit of the scope that exhibits significant sustained vibrations. But I only found that out by plucking the vanes. I suppose in strongish winds that might be a problem... (But it can never be as bad as a cheap EQ mount.)

Edit: sorry I thought you were still shopping around for a scope; i meant that when I handle the dob it behaves like i said above.

[slice of heaven]

No problems with alt or az,with no springs .Moves sweet as.

The base is flimsy,the dampening time and wobble are shocking.

[janoskiss (Steve H)]

Pleeeaz erxplain.. ?

[slice of heaven]

B.D.
Chuck the bearings and go for the teflon pads ,just add a new surface to the bottom of the top base... formica ...LP...Im a cheapskate I used an LP. The bolt doesn't need to be tight at all. It just has to be there with NO tension. Mine moves easily & smoothly with one hand even up at near verticle.

[slice of heaven]

Hi Steve
My old dob's base is rock solid (double 6ply on the 3 upright sides + angled braces) so I'm comparing it to that. Dampening times & wobble is zilch,zero,nothing. The gs base is too flimsy.

[ballaratdragons (Ken)]

My GS base is rock solid. Not flimsy, wobbly or shaky.

It also has 2 thin metal plates with the roller bearings in between them. I intend to use one of them on the underside of the top base plate.

I get no wobbles, and when i stop at an object there is no backlash or slide. The only problem is that the springs are not applying enough pressure and it can move in alt too easy, which is contrary to other dob owners.

At least when I remove the roller bearings I will not have to do the centre nut up at all! It will rely on friction instead of bearing pressure. What size is yours?

Steve, I have never experienced vibrating spider vanes. Have you tightened them?

[janoskiss (Steve H)]

Is it milk jug washers you'd be needing? Check this out!

[janoskiss (Steve H)]

Originally posted by ballaratdragons
Steve, I have never experienced vibrating spider vanes. Have you tightened them?

You have to pluck them like a string. On my scope they'll vibrate for several seconds. They vibrate faster than I can see the motion, so I see it as a blurring of the image in the EP.

[ausastronomer (John Bambury)]

[QUOTE]Originally posted by janoskiss
The spider is the only bit of the scope that exhibits significant sustained vibrations. But I only found that out by plucking the vanes. I suppose in strongish winds that might be a problem.

Steve,

You can fix that problem. What you do is put 2 stainless steel 1/4" washers under each of the 4 thumb screws that tighten the spider vanes. When you tighten the thumbscrews the washers help to take up a little freeplay in the spider and allow you to load it up tighter agianst the tube stiffening the spider vanes in the process. Another advantage of doing this is that it subsequently allows you to have a little more lateral movement respect to centering the spider in the tube.

Clear Skies
John Bambury

[iceman (Mike)]

heh i'm not surprised it shakes if you pluck the spider veins

I don't have wobble/dampening problems with my base, but still wish it was smoother to move in the az direction sometimes.

[Starkler (Geoff)]

If your base has a wobble one possibility is that you have one too many milk bottle washers on your centre bolt.

Some people go and distribute teflon pads at various points on their base but the ideal is to only have them at the outer circumference for reasons of stability, and then adjust the friction by packing the right number of milk bottle washers at the centre bolt.

[slice of heaven]

Mike Maybe I've been spoilt with the old dob's base. Or maybe it's the new base.The base on this 12 has a triangular bottom baseplate with three feet. Anybody else with this setup? The top baseplate is a 65cm circle. I Checked out your scope Mike and yours are both round and no feet. Whats the diameter of yours? Your sliders are placed at the outer limits .The roller bearing setup has the rollers 150 mm from the centre.I chucked these in favour of the LP and teflon setup which is still 150mm from centre. Your wider slider placement is a better choice for these chipboard bases.The chipboard is 20mm thick. Is this the same size on the 8's and 10's. The 12 needs more than this to make it rigid.
Maybe I'm nit picking but I like to be able to pan the sky and when I stop the scope I like the view in the ep to stop with it.

Has anyone a bottom cover for the 12" tube they want to throw away?
. Geoff

[slice of heaven]

Geoff
Yeah, the wider placement would be better. I have 5 teflon pads 3 on the outside and 1 each side of the centre bolt. The centre bolt is tightened to remove All the slop but not enough to add any friction...the bolt can still be turned by hand.

[Starkler (Geoff)]

Yes the pads either side of the centre bolt could be a problem.
You want the weight to be taken by the pads at the outermost part of the base rather than a have a potential pivot near the bolt.

Using pads you shouldnt have to tighten the bolt at all, its just there to serve as a pivot pin.

The beauty of using milk bottle washers around the centre bolt is that they are compressable to an extent, allowing fine tuning of the loading on the outer pads.

[ausastronomer (John Bambury)]

I agree with Geoff.

You don't need pads to support around the centre bolt.

Throw that stupid roller bearing thing over the fence.

You need 3 teflon pads placed 20% of the radius of the ground board in from the outside of the groundboard. Even though the groundboard is triangular measure the distance from the centre to the tip of the point. then come in 20%. You need to weigh the OTA and the base excluding the groundboard. You then get the total weight of this in pounds and divide it by 15 to get the total surface area of teflon needed for the 3 pads. I can tell you that the approximate weight is 70 pounds and if you can't weigh it use this weight. This gives 70/15 = 4.67. total pad area.

over 3 pads each pad needs a surface area of 1.55"

Therefore each of the 3 teflon pads needs to be 1.25" square. This gives a surface area of 1.56".

To do the job properly you should also cover the bottom of the rocker box with a suitable pimpled surface laminate like Wilson Art Ebony Star.

You don't need any support around the centre bolt and do not tighten it so as to exert downward presure between the bottom of the rocker box and the groundboard. It should be tightened so as to remove any slack but not exert pressure. The best way to get the tension correct on the centre bolt is to replace the supplied nut with a nylon lock nut on the centre bolt, I used a stainless steel 1 and replaced the centre bolt with a stainless steel bolt, centerbored to accept the encoder shaft for the azimuth encoder for argo navis. Not sure what size the bolt is on the 12".

Clear Skies
John B

[slice of heaven]

Hi AUS
Had plans of building another base even before this one arrived. I'll keep your little formula handy for this.
These chipboard bases are too flexy for a scope of this size regardless of az sliding setup. After my last post we dismantled the base and moved the teflon out to 50mm in from the outside and armoralled the bottom base for a temporary fix. So their now riding about 250mm from the centre. Improvement was there, but minimal. The flex is in the top base assembly. I'm only assuming that the 8's and 10's are 20mm chip as well .Which might be ok as their alt bearing support would be lower and their lighter, the 12 is 56cm(to the bearing centre) above the top of the baseplate. This base was not designed for total rigidity, but only to support the OT.
. My movement in alt and az is perfect . It's the wobble at the ep that is in question. I was wondering what everyone else is putting up with. Your statement on the centre bolt is spot on. There is no need to squash the assembly. The original bolt is too short anyway ,it's only 50mm long, the 2 baseplates & bearing assembly are 44mm thick, plus the washers, so the nut wont even screw down to the nylon lock. I used a 60mm bolt and nyloc and cut a new spacer tube(all stainless) to suit my setup . The supplied spacer tube is only 38mm long .

Slice

Too many double ups on the names (Mikes Tonys Johns Geoffs) Im sticking to handles.

[ausastronomer (John Bambury)]

Geoff,

Try shimming the sides of the base with teflon pads to take up the sideways movement with the altitude hubs. I actually had to do this with my 10" to get Argo Navis to align more accurately. I shimmed each side with 2 pads of 3mm teflon which improved the pointing accuracy of Argo Navis to the stage where it is now close to perfect. This flexes the sidepanels of the base out a little putting them under mild load and stops the OTA moving sideways in any direction. It also improved damping times immensely. I will go out and see if I can get a digicam pic and post it.

Clear Skies
John Bambury

[slice of heaven]

Thanks Aus
Ill check that area But I'll wait for your pics



Slice