GSO Focuser Separation Anxiety

[Zeta Reticulan]

I’ve been trying to retire my two ST80’s for several years now. I have better quality short tube refractors. The first to be modified with an aftermarket GSO focuser (as TS Optics) was the Orion ST80. This not only improved the focusing abilities it also enabled 2” accessories to be used. Moreover, the focuser could additionally be rotated although I didn’t normally utilise this option. Apparently Guan Sheng Optical are not particularly enthusiastic about the feature either:

 

These focusers are rotatable, i.e., you can turn the entire focuser body/diagonal/eyepiece combination into a better observing position by loosening or tightening a large silver thumbscrew on the top of the focuser. However, please note that this is a relatively weak feature of this otherwise excellent product. We inspect every single focuser and tweak/lube it as much as we can, but the rotation is often not smooth over the entire 360-degrees and may bind in a few spots, requiring additional effort to rotate it. This is probably a minor annoyance since most people will not use this feature very often.

~ Guan Sheng Optical

 

Which doesn’t exactly inspire confidence. Admittedly I had some problems with the similar aftermarket GSO focuser on my ST102. This eventually led to its replacement with a different make of focuser. When the conditions are not optimal, and there is no Moon, the ST80 is good for occasional rich field observing sessions. It is robust, uncomplicated, and comparatively light to carry. To further simplify things I usually only take a couple of eyepieces out with it at most. The predominant eyepiece being a 30mm GSO SuperView. Which is agreeably lightweight in the diagonal and gives about five and a quarter arc minutes of true field.

One cold January night I decided to take the Sky-Watcher ST80 out for a quick grab and go session. Due to a physical disability I often remain seated while orienting the OTA to view a specific target. There are usually no problems with rotating the focusers on my other scopes. As I was rotating the GSO focuser it stuck. When these focusers rotate a single thumbscrew loosens the entire focuser assembly.

Allowing it to detach slightly from the V-ring/flange plate which is secured into the OTA itself by three grub screws.

As a consequence, the more the single screw is loosened, the focuser becomes slightly more detached from the plate. The OTA was pointed up at an altitude not far from the zenith. The focuser rotation was quite stubborn and I increasingly loosened the screw. So much so that to my astonishment the entire focuser, diagonal, reflex sight and 30mm SuperView came off in my hand!

I wasn’t totally sure what had happened. I did have an idea that nothing had broken or sheared off though.

I took the now bisected parts of the SW ST80 back to the house and replaced it with the Orion ST80. I set the Orion scope up the same way and attempted the same procedure as before. Sure enough the focuser separated from that too! Undeterred I took that inside as well and finished the session with my 72ED DS Pro.

Later I got to analyse what went wrong. Well, apart from having a screw (too) loose, something that seems to happen to me a lot. The entire focuser is basically attached to the OTA base plate V-ring with three equidistant grub screws. Over a period of time these screws can loosen. If they loosen enough the entire focuser can separate!

Although instead of rectifying this at the time I resorted to pushing the focuser more square before attempting to rotate. I gradually loosened the thumbscrew. Luckily nothing dropped onto the ground or was damaged. I feel more confident now I understand the ins and outs of the focuser mechanism, so to speak. The ST80’s might not get retired quite yet after all.

 

 

 

 

 

Edited August 23, 2022 by Zeta Reticulan

[DaveL59]

I assume the 3 grub screws also locate into the groove so as to keep the focuser part attached?

Wonder if there's any form of threadlock or wax to keep the grub screws in place?

You could perhaps try a drop of black sealing wax into each grub screw hole once you have them set to the right place which may limit their ability to turn. Some of the miniature japanese binos used that idea to hold the prism screws and maybe give a better all-back finish to the body.

Alternatively there is low strength threadlock (Sealey do one) that might be OK for these, tho I can't confirm that as I've yet to use it on any grub screws myself. What size/type of grub screws are used?

[Zeta Reticulan]

1 hour ago, DaveL59 said:

I assume the 3 grub screws also locate into the groove so as to keep the focuser part attached?

Wonder if there's any form of threadlock or wax to keep the grub screws in place?

You could perhaps try a drop of black sealing wax into each grub screw hole once you have them set to the right place which may limit their ability to turn. Some of the miniature japanese binos used that idea to hold the prism screws and maybe give a better all-back finish to the body.

Alternatively there is low strength threadlock (Sealey do one) that might be OK for these, tho I can't confirm that as I've yet to use it on any grub screws myself. What size/type of grub screws are used?

Yeah, they stop the V-plate from detaching. They're 2mm with an Allen head. There is some grease on the flange. TBH it's basically an inelegant design. I just keep an eye on the grubs now. You can tell from the amount of free play whether they need tightening a little. 

Edited August 22, 2022 by Zeta Reticulan