5 replies to this topic

[Spacedude4040]

Fixing My GSO Cassegrain Focuser

 

I’m 99% sure that these focuser are used in all the line of GSO Cassegrain and generic scopes.
While attempting to collimate my New Classical Cassegrain I noticed that by spinning the focuser with the laser in it made a nice circle around my secondary. So I double checked my laser by spinning it within the focuser and found it was dead on. Plus I double checked it in a V block and still dead on.
Upon further investigation I concluded that the focuser tube wasn’t at 90 degrees to the focuser base. So I loosened the set screws that hold the focuser to the ring that mounts to the scope. Then I cut some shims and started moving them around between the base and focuser body with the scope pointing to the ground and spinning the focuser on top to see if I could get the focuser tube point at center without moving.
To my surprise they both ended up at the top of the focuser around the two set screws approximately at 120 degrees apart. So I moved them directly across from the set screws and tighten everything up and it solved most of the tilt problem.

 

 

So now this requires more investigation and after looking for a while I came to this conclusion.
I believe that GSO made that focuser with about .020 of a tilt at the base towards the rollers bearings. If you look inside the focuser there are two set screws on the top of the focuser that do nothing but may have been there to tilt the bearing block so the tube would line up on axis of the telescope. In my focuser the aluminum plate that holds the bearing is just short from reaching these two screws. They may have abandon this idea as it may have become too time consuming or a total failure to work as design. So we ended up with a tilted focuser. This adds a great bit more of difficulty to try to collimate these types of scopes. So they made tilt plate adapters available to correct. This is just my take on it.
What to do to correct this matter? I didn’t like the focuser tilt plates that they sell to fix this problem. Once adjusted you cannot spin your focuser around on the telescope back because you will only compound the tilt as you turn it. Plus the added cost $$$ and added steps in collimating the scope. Once you have the tilt plate aligned and move the primary you alignment is now off. Remember a fixed primary just means it doesn’t move during focusing and YES they have to be collimated! So they should be.

 

So on to fixing the problem.

 

Options to deal with this matter are:
-True the focuser to the tube.
-Buy a new focuser $$$.
-Shim the focuser ring the best you can. Hmmmmm?
-Purchase a tilt plate $$ and increase the white hairs on you head free of charge.
-Shim out the aluminum plate that the bearings are attached to inside the focuser, but that’s a lot of guess work and may stress the focuser body to crack or deform.

 

I chose to true up the focuser because I basically liked the focuser. So I made a jig so that I can now put the eyepiece end off the focuser tube in my lathe chuck and using the jig supporting the other end of the focuser tube. Now with the knobs and etc off I could spin the focuser on my lathe and true up the base to the focuser tube. Easy Pezy!

 

This solved the problem for me and I hope it helps others one way or another.

 

 

 

 

 

[waso29]

did you report your findings to your dealer about this defect? and what were their response?

[photoracer18]

Did you ever consider this was done intentionally to correct another problem in the scope? Just a guess but what if the flange on the back of the rear cell was out of spec slightly? One way to fix that would be to do an opposite adjustment in the focuser since you can't adjust the flange tilt if any. That would mean the focuser needs to stay in the same position and orientation.

One question, did you decide to check/redo the collimation because you found it off or did you just want to try to make it better?

You might try checking whether the focuser mount flange on the rear cell is square with the back of the rear cell itself. A couple of ways, one is use a bubble level on the rear cell and one on the flange upper surface (where you thread the focuser on) to see if they are square. Another way is using a dial caliper depth measuring part (end of the long scaled part) and see if the top of the flange shows the same depth to the back plate around the entire rim. I would not expect this to be off but similar things have happened before in Cassegrain scopes. I am guessing on some of this since you didn't bother to show a photo of the back of the OTA. I actually have one of those focusers in my parts bin from a GSO RC.

[waso29]

bump

[Spacedude4040]

did you report your findings to your dealer about this defect? and what were their response?

No I didn't as for me it was a easy fix and that's what I did. I really believe they know about this issue that's why they make the tilt plate

[Spacedude4040]

Did you ever consider this was done intentionally to correct another problem in the scope? Just a guess but what if the flange on the back of the rear cell was out of spec slightly? One way to fix that would be to do an opposite adjustment in the focuser since you can't adjust the flange tilt if any. That would mean the focuser needs to stay in the same position and orientation.

One question, did you decide to check/redo the collimation because you found it off or did you just want to try to make it better?

You might try checking whether the focuser mount flange on the rear cell is square with the back of the rear cell itself. A couple of ways, one is use a bubble level on the rear cell and one on the flange upper surface (where you thread the focuser on) to see if they are square. Another way is using a dial caliper depth measuring part (end of the long scaled part) and see if the top of the flange shows the same depth to the back plate around the entire rim. I would not expect this to be off but similar things have happened before in Cassegrain scopes. I am guessing on some of this since you didn't bother to show a photo of the back of the OTA. I actually have one of those focusers in my parts bin from a GSO RC.

They may have anticipated having problems with making the rear cell but I can tell you after I trued up the focuser it almost perfect. If it was designed to offset a problem they really should tell you. But from a manufacture stand point its best to deal with the problem then make a fix.