Dampening on gs dobs

[slice of heaven]

Whats the dampening like on everyones gs dobs?

[ballaratdragons]

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]

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]

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]

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]

Is it milk jug washers you'd be needing? :) Check this (http://www.iceinspace.com/index.php?basemods) out!

[janoskiss]

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]

[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]

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

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

[Starkler]

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]

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]

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]

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

[ausastronomer]

Geoff,

I checked and I used 4mm thick teflon pads for the shims on the altitude hubs and 3mm thick teflon for the hubs to ride on. You will notice the shims are about 20mm long, it is only the end 3mm or so of the shim that does any work the extra length is necessary to provide 2 screw mounting holes.

[ausastronomer]

Can someone please tell me how to attach the pictures. I tried to attach 1 but it wouldn't work. I resized it to 54kb but still wouldnt work.

CS-John B

[janoskiss]

Sorry, don't have an answer, just acknowledging the problem. Tried to attach an image using the "Attach file" entry below the message edit and it didn't work. Lets me pick the file ok but nothing appears in the post. The "IMG" button for images on a webserver works fine though.

[Starkler]

John it works if you hit the "post reply" button and attach from there. For some reason its not working from the quick reply option.

[ausastronomer]

Picture 1 attached

[ausastronomer]

Picture 2 attached

[ausastronomer]

Picture 3 attached

[slice of heaven]

Thanks Aus
We'll check that tomorrow

[janoskiss]

Correction to my earlier posts... Tonight with high magnification viewing of Jupiter (http://www.iceinspace.com/forum/showthread.php?s=&postid=14038) I noticed (with an 8" GSO Dob) a lot of vibration. I didn't investigate because I was too busy trying to catch all the action. Normally I mostly look at deep space stuff at lower magnifications (40-130).

[ballaratdragons]

Sorry to change the subject but the photos above reminded me to show you how I turned my spring set-up upside-down so the springs stay on the base. You can just see the cloth tab at the top of the spring.

[iceman]

I can attach files with the "quick reply" browse button below, no problems?

Anyway it looks like he got it working. Good idea too John i'll have to try that out on mine.

[ving]

i might do the same john... looks practical :)

[ballaratdragons]

WHAT! No comments on my spring conversion!!

I have done a ring around and cannot find anyone in this area selling Teflon. Where should I be trying? What type of shops?

[ving]

that should be spring inversion, ken. :P

I am sure you have memntioned it before, but why turn the spring upside down? :confuse3:

[slice of heaven]

BD
Mitre10 etc sell magic sliders.Their self adhesive and come in a variety of shapes and sizes.
For strips of teflon in short lengths I dont know .I buy it in 10m rolls because I use it for work. It comes in different grades(soft-hard) and surfaces(smooth dimpled rippled).I source mine from an engineering firm but the magic sliders should be fine.

Slice

[ballaratdragons]

Mentioned it, but couldn't show it until now coz I got a photo of it.

1. Springs stay on base, not flopping around on the OTA.

2. No need of the huge ugly bolts.

3. No busted knuckes if the strap breaks coz I lift it on by placing my thumb on the spindle and lift the tab with my index finger. (very easy)

4. Looks tidy.

[ballaratdragons]

Thanks slice, I'll go check out Mitre 10 and others.
Probably best to get 'Hard' and 'smooth' for this application.

[ving]

you have mentioned it a couple of times now :P

might do it. costs nothing and makes sense :)

[ausastronomer]

Forget telescope stores etc, most won't have it and those that do will rip you off. Use the Yellow Pages and contact "Specialist Plastics Supppliers" Virgin Teflon is sold by them as "PTFE". You will probably find only 1 out of 10 carries it but patience and time on the phone will pay off. You need to get 3mm thick or close enough to 3mm. It will cost you about $20 to $30 for a couple of strips about a foot long. I got them to cut me a strip 19mm wide and a strip 30mm wide, then I just had to chop the pieces off to length with my drop saw.

Clear Skies
John B

[ballaratdragons]

Thanks John. There is only one Industrial Plastics Supplier in Ballarat so I will go annoy them.

All you need David are 2 more of the bolts used at the bottom and chuck the big ones. Put the new ones in the top.

Make sure the plastic spacers step down as they come out. Place a washer on the outer end on the bottom spacer so the spring won't fall off. I placed a diagram in here somewhere a long time ago but now I can't find it so I have done another one:

EDIT: If you are one of those people in the habit of only using one spring you will not have to purchase any new bolts. Just use the bottom one from the side you dont use.

[Starkler]

Ken could you let us know the bolt size and thread just for reference purposes thanks?

[ballaratdragons]

Don't know, I'm not much of an engineer. The original assembly manual calls them 'Philips Head Screw Black'.

The thread length is 38mm. The thread itself I don't know, but the O.D. of the thread is 6mm. It is the bolt in the bottom spring mount. I took one of mine to the Bolts and Fasteners shop and they matched it for me.

I just went and compared it to other bolts I have and it is the same thread as a standard Gal Coach Bolt of the same diameter.

I've never been good with AF and Whitworth etc.

[ausastronomer]

They are standard 6mm metric.

All the bolts and nuts on the GS dobs are standard metric sizes of 3mm,4mm,5mm and 6mm, I can't remember what the centrebolt on the groundoard was because I had to replace mine with a 1/2" UNC centre bolt to tie in with argo navis (long story) and I can't remember whether I had to drill the hole out. The only non standard bolts are the 3 collimation thumb screws on the bottom of the mirror cell which are 7mm metric which is a non standard metric size in Australia but used in Asia.

Clear Skies
John Bambury

[ballaratdragons]

Well, there you go! Thanks John.

They are 38mm X 6mm Metric domed head philips bolt.

[slice of heaven]

Fitted the extra teflon pads to the alt bearing and they removed all the sideways travel.....Thanks AUS.
. Also added some self adhesive foam strips to the alt bearing support to stiffen the alt movement. I used 5mmx12mm strip , this suits my (standard ) setup with no springs & barlow & ep fitted , test before you stick. Most benefit came from the 2 placed above the teflon strips. This has stopped the OT from dipping when near horizontal and over balancing when near vertical due to the height of the the finder. We have since replaced the finder with a telrad, their so much easier.
. With the dustcap fitted the OT will still nosedive so we've made a padded brace (prototype:just clamped) to prevent any damage and stop flexing of the 2 side supports. This has improved the dampening greatly and the real one should fix it totally when made.
. While at the cabinetmakers sourcing material I queried about Ebony star. It's a laminate made by Westernart , ebony star is a pattern , the texture is the real point of concern. The Touchstone texture is a 50 point dimpled surface with a gloss and feels like the right stuff. " Anybody know for sure?" It's roughly $90 a sheet 6'x4' but I can wait for an offcut for $15 (fitted) .Hopefully this would fix the sticky az movement problem. Mines 'ok' but Ive had to use 6 teflon pads,3 on the LP for slide & 3 on the outside for stability.


Slice

[ausastronomer]

[QUOTE]Originally posted by slice of heaven

Also added some self adhesive foam strips to the alt bearing support to stiffen the alt movement. I used 5mmx12mm strip , this suits my (standard ) setup with no springs & barlow & ep fitted , test before you stick.


Geoff,

This is a bandaid solution only IMO.

The proper or recommended way to adjust the altitude motion of your scope is to move the teflon pads that the altitude bearing rides on. The 1st thing to do is replace the supplied nylon pads with proper teflon pads. If you want to make the motion stiffer you move the pads further apart (I would try about 1/2 to 3/4 of an inch for starters) if you want to make the motion lighter you move them closer together.

PS You can re-use the nylon pads but they are stapled so you would need to re staple them when you reposition and thats hard to do without the staple staying proud of the surface. I used new teflon pads with 2 small 4 gauge c/sunk screws.

CS-John B

[slice of heaven]

I better get a bigger note book for all these facts and figures , a little bit of knowledge goes a long way ....Thanks Aus.
. Any idea about ebony star? I only have a sample piece ,it feels right.

What I'm after is a mount with a slick featherweight movement but stays put , and a perfectly stable view at the ep. As per the Parks. The kids have to be able to use it comfortably and enjoy it.
More like a family car with power steering rather than a truck without.
I'll definitely try repositioning the pads later on to fine tune it and I'll let you know the results. Then I can pull the 'bandaids' off. I've already thrown away the crutches supplied ( the springs ) had no intention of fitting them. In my experience with teflon, if teflon is mated to a flat surface and too much pressure applied it takes a greater force on startup to break the suction , which results in jerky operation' exactly what you dont want on a scope. Hence the long term use and recomendation of the dimpled laminate 'ebony star'. Their choice of materials on the az/alt is far from ideal. The az should have had the teflon riding on a surface similar to ebony star and a less slippery surface on the alt, or as you stated, better positioning of the pads. Minimal cost and effort to them for a better product. They should have done more R&D. Still ,it makes for a good project and keeps little hands busy.

Slice

[ausastronomer]

[QUOTE]Originally posted by slice of heaven
>[B]I better get a bigger note book for all these facts and >figures , a little bit of knowledge goes a long way ....Thanks Aus.
> Any idea about ebony star? I only have a sample piece ,it >feels right.

Yep, Ebony star is a specific pattern of Wilson Art in the USA. It has a dimpled finish and that is the critical thing. You need to get a laminate that has a matt dimpled finish. Any 1 of 100's from several different manufacturers work equally as well as ebony star. My scope actually uses a Wilson Art pattern called Mystique Night, works just as well, the surface texture is the critical factor not the colour. I suggest you go to a kitchen refit company and buy a scrap sheet of laminate with the same texture as ebony star. I have done that on a couple of occasions and they will sell it about $20 per 1/2 sheet. Which prolly does 2 scopes.

>What I'm after is a mount with a slick featherweight movement >but stays put , and a perfectly stable view at the ep. As per the >Parks. The kids have to be able to use it comfortably and enjoy >it. More like a family car with power steering rather than a truck >without.

You will have that after you do the teflon pad ebony star thingy.

>They should have done more R&D.

Couldn't agree more, but it all comes down to cost

>Still ,it makes for a good project and keeps little hands busy.

CS-John B

[slice of heaven]

Thanks Aus
I'll be back to the cabinetmakers tomorrow. I'll finger all their samples and see what comes up.

Hmmm...Ebony star....Mystique night...suitable names for materials to be used in telescopes.

Slice