meade 12 inch lightbridge

[david booth]

any fixes as primary mirror goes out of colimation if bumped

[PCH]

Hi David,

erm ... recollimate it :D;)

Only teasing. It's easy enough to do, although easier if you have already installed bob's knobs at the primary and secondary.

Actually, you'll find you have to do this fairly often just in normal use. You'll soon get it down to about a minute or two !

For starters, you'll need some sort of collimation tool. Many here have preferences, but it will either be a laser collimator or a cheshire eyepiece. Each have their own pro's and cons, but I use a laser. However - you MUST be able to collimate the collimator too, so ask before you buy, or you might find you're using a tool that's inherently out of alignment to align your scope. Clearly the result won't be terrific.

Having got the necessary tools (from www.bintelshop.com.au or similar) either follow the instructions in your handbook, or have a look at this site
http://www.andysshotglass.com/Collimating.html

That should do it. You'll get heaps of help here David, so don't be afraid to ask. But this will sort out this particular problem.

Cheers :thumbsup:

[wavelandscott]

Several people have changed the collimation springs to "stiffer" ones...this should allow your scope to maintain collimation better.

There are some kits available to do this or a trip down to your local hardware strong with a sample could be the go...

[david booth]

thanks

[White Rabbit]

Yeah, I have the same problem with mine, and I have the bobs knobs kit including the bigger springs and it still goes out of collimation even when just moving it around looking at the sky.

As yet I havent found a cure for this yet so I'll keep a keen eye on this thread.

Sandy

[erick]

Try to see if something in the upper assembly is moving relative to the mirror cell? It might not be the primary shifting in its cell.

[richardda1st]

Hi David.

Eric may be right.:thumbsup:

I have the 10". Check/tighten the screws that fix the upper and lower cylinders to the cast metal rims. I felt some movement when lifting from the trusses on mine. There's 6 screws per rim, 4 rims. Top rim will not be the cause. My instructions say to carry the scope by the trusses, this may cause some movement. I carry mine by grabbing the large aluminium alt pivots.

Hope this helps.:D
Regards
Richard

[JethroB76]

how tight are your secondary spider vanes?

[Quark]

Hi Dave,

I also have a 12" LB and have made many mods in an attempt to solve this problem. I have heavier springs and Bobs Knobs.

I have found that the problem really is due to the poor design of this scope. Who in their right mind would build a truss with only three sets of struts. Four sets would have been the go. If you collimate your scope and push or pull the top end it will go out of collimation, this is due to several different deficiencies in the design.

If you look closely at the clamps that lock the struts to the bottom end of your scope you will notice that there is a round lug on the cast section of the clamp that comes into contact with the cast ring that the truss tubes locate in. This happens before the clamp has locked onto the ends of the truss tubes.

The holes in the truss tubes that the locking screw passes through are a fair bit larger in diameter than are the locating lugs that are meant to locate and lock the ends of the truss tubes.

As these lugs are to long and bear on the bottom ring before they clamp the tubes and because the holes in the tubes are larger than the diameter of the said lugs, there is scope for enough movement of the tubes to totally stuff up the collimation of the scope, just by grasping the top end of the scope and moving it around the sky.

People that don't use high magnification with these scopes, such as 6mm or 4mm eye pieces are blissfully unaware of this shift in collimation.

There is only so much that can be done to remedy this problem. I have found that by accurately measuring the length of these lugs and accurately measuring the width or depth of the struts, I was able to grind or file the lugs. To the extent that now when the clamps are screwed up tight, the clamps actually firmly clamp the struts. This has made a great difference to how well my LB maintains collimation, however if it had been fitted with four sets of truss tubes in the first place and if the holes in the tubes actually matched the diameter of the lugs on the clamps then the problem would not exist.

Well thats my rant for the day.
Regards
Trevor

[astronut]

I've never had any of the problems discussed here, but I must say that I'm rather anal at making sure that everything that should be tight, is.
I had the end rings powder coated matte black, almost immediately after purchase, so I never used the scope "as is " from the factory.
When powder coating an object, you are actually thickening its width with the application of the powder.
Maybe it was this process that eliminated one of the causes of collimation shift.
My 2 cents.:thumbsup:

[richardda1st]

Hi Trevor,:hi:

My clamps make a tight fit on to the struts, have you tried grinding the lugs down a touch. Sometimes when clamping, I need to position the lugs so they fall into the hole of the strut before tightening.
As per my previous post the screws that fix the rims to the tubes need attention, the clearance holes are much to large for the screws. A set of spring/star washers may help to stop loosening. Any movement here will affect collmination.

Hi John,
I also plan to powder coat the rims matt black.

Richard.

[PCH]

Can anybody tell me, does powder-coating the rims black really make a difference? I'd love to hear someone tell a before and after story re this, as I have a 12" LB but I won't get this done unless I know it makes a difference.

Cheers

[Robh]

I going to go against the train of thought here.
It is not unusual for any DOB to go out of alignment if bumped. The harder the bump the more the likelihood. If you pick up the scope and move it, I would expect to re-collimate it. It only takes a slight movement of the primary mirror.
The solution ... don't bump it, don't move it, or expect to re-collimate it.

Regards, Rob

[david booth]

thanks guys