Dob that tilts

[Gef_Dintsonian]

While I am here maybe I should put in a question about a persistent problem to see whether people have some suggestions.

My Dob 'falls' when tilted below about 75 degrees from perpendicular; and 'rises' when tilted above 80 degrees (or maybe it is 78 and 79 degrees, as it seems to do one or the other no matter what position it is in).

I had a particularly exciting afternoon at a school showing the grade 3s the moon. As well as contending with the 'leaners' ("try not to touch the telescope"x100), I had to check that we had a moon preferably ours, since it was daylight) in the field of view between every child (just less than 50 kids).

I could shift the lugs that the springs attach to down a bit, but would like to know about how much before drilling more holes. I'm not sure what else I could do. I have thought about marine (small) turnbuckles for adjustability. I have tried water bottles to counter balance, but added a lot a weight without seeming to help (especially with swapping between chunky 25mm EP and superlight 9mm).

Any help would be appreciated.


...Geoff

[[1ponders] (Paul)]

and to the site Gef. Which Dob are you using?

[[1ponders] (Paul)]

Ah I just found your other post. An 8" Bintel Dob.

[Gargoyle_Steve (Steve)]

Have you got the "pressure" springs in place at the sides of the tube assembly?? Though to be honest even with the springs removed unless you have done something else to change the point of balance these dobs are usually very stable and well balanced, and shouldn't start tilting down on their own until they are something much closer to horizontal, like 10 degress maybe. Until I started modifying mine (ie changing the mass / balance point) it never tilted upwards at all.

You could check the 2 "screws" on each side that project inwards through the bearing units and into the scope tube itself. These bolts are held tight by a nut on the inside surface of the scope tube, ie you have a flat faced nut trying to bind against a surface with an 8" curve. One of the bolts on my scope (a 10" model, same manufacturer) had a nut that would always loosen and it would allow the scope to tilt a little. I added a star washer inside the tube to give the nut a means to bite against the curved tube. No more problems.

[Gef_Dintsonian]

I have pressure springs in place; and no loose nuts. I have had magnets suggested, but would prefer a lighter weight option.

...Geoff

[wavelandscott (Scott)]

Altough I have not experienced this problem with my 8 in Dob...I'd suggest that a magnet(s) might be the go...They can be repositioned as needed up and down the scope to balance. I've heard some folks wrap their magnets in cloth (like an old sock) to prevent "scratches".

A couple of other thoughts (never tried but just occurred to me so take these as untried brain storms)...

Get new tighter springs...When you tighten the springs are you connect the spring or the cloth loop? (just checking)

Put a small piece of velcro beside the teflon bits to add some friction (I know it might be hearsay but it will help reduce slips)...

Good Luck!

[ving (David)]

hmm... 2 springs should hold just about anything...

are you conecting the spring to the base or the material loop (if you know what i mean)... theres a material loop at the end of the spring, this is to help you pull the spring down to hook onto the bolt. some people hook teh material loop on instead.

hope this make sense.

[janoskiss (Steve H)]

I find that by placing a counterweight on the underside of the scope tends to balance the focuser, EP and finder(s) on the other side. I don't need to use the springs at all on my 12" but I do use them on the 8". I use them with strong keyrings on the ends to reduce tension.

[Gef_Dintsonian]

I know what you mean about the fabric loops - no, I have the springs on the lower lugs, not the fabric.

I wish I needed less spring, but no, I seem to need more.

Where do you all get these magnets, and how much weight should be enough?

...Geoff

[rmcpb (Rob)]

Odd that you get so much movement with the 8". I didn't have many problems with mine before I started my major mods except for a slightly sticky alt motion. The opposite to your problem.

Try the magnet solution, I worked for me as I never used the springs and preferred to have the sope balanced.

[ving (David)]

i have a 8" dob too. it doesnt start diving till it gets below 20 deg with electric focuser and 2 inch 30mm eyepiece.... your case sounds wierd.
you have me puzzled.

[janoskiss (Steve H)]

An alternative to the magnets is to use velcro on counterweights and OTA. I use both but velcro is more reliable, no chance of weight sliding/falling off. Just buy good quality velcro stick-on strips that stick and stay stuck.

[astro_nutt]

Hi Geoff..here's a thought..
Try cleaning the teflon pads with metho to ensure the're grease-free and dry.
Also clean the part of the hub which contacts the teflon pads..some peole use a spray polish to clean their scopes and some of this winds up on the hubs and teflon pads..try cleaning them with metho and see how it goes..as for springs...try a Bunnings store for something a bit stronger.
Cheers!!

[Gef_Dintsonian]

Status report:

1. When I pulled it all apart again I noticed that I was wrong about the loose nuts - not much movement, but possibly enough to allow vibration. Thank-you Gargoyle. I have tightened them now.

2. I went to clean the teflon pads and the round thing that runs along them, and found that one of the staples holding the teflon pads on was poking out, enough to scratch the 'the round thing that runs along them'. So I have bashed it in gently with a hammer and punch.

3. I cleaned the relevant surfaces with metho, and the cloth was reasonably clean but certainly not spotless.

There now appears to be no movement, but until I can get it outside with the 9mm eyepiece pointed at a galaxy far, far away I will not know the outcome of these efforts.

Thank-you everyone for your assistance. If all of this fails, I still have weights, velcro alongside the teflon and moving the spring thingys down a bit.


...Geoff