GSO Focuser Separation Anxiety

[Nightspore]

 

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 utilised. Moreover, the focuser could additionally be rotated although I didn’t normally use 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.

 

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.

 

 

On the 8th of January 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 is usually no problem with my other scopes equipped with rotating focusers. 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 flange plate which is secured into the OTA itself by three 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 with one thumbscrew. When the OTA was more or less upright the slightly detached focuser can hang at a canted angle causing it to stick.

 

 

Instead of rectifying this by 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.

 

[Nightspore]

[+Graham]

Focusers with a rotator normally have 2 additional nylon grub screws set at 120 degrees to the locking screw to prevent this from happening.

I would suggest you put a couple in if they are not there.

[philjay]

Some lesser focusers have the thumbscrew then two ordinary grub screws instead of nylon and these can bind if the thumbscrew is loosened too much. Sometimes these screws mark the track and makes for a rough rotation and binding. Nylon tipped is a better way. Its not the most elegant way of doing a rotating focuser

Edited January 17, 2022 by philjay

[Nightspore]

7 hours ago, Graham said:

Focusers with a rotator normally have 2 additional nylon grub screws set at 120 degrees to the locking screw to prevent this from happening.

I would suggest you put a couple in if they are not there.

 

I'm pretty sure there's no provision to actually do that. Unless I try and drill the holes myself. DIY isn't exactly my forte lol. GSO are pretty candid about it being a design flaw. The secret to rotating this focuser is to loosen it only enough to turn, and then to keep it square (uncanted) as you're rotating it. Not ideal, but it does the job. 

[Nightspore]

3 hours ago, philjay said:

Some lesser focusers have the thumbscrew then two ordinary grub screws instead of nylon and these can bind if the thumbscrew is loosened too much. Sometimes these screws mark the track and makes for a rough rotation and binding. Nylon tipped is a better way. Its not the most elegant way of doing a rotating focuser

 

Yeah, the GSO rotational mechanism could hardly be described as elegant lol. Their marketing spiel does try to gloss over this. Interestingly the 96mm GSO focuser for my ST102 had different problems.

 

 

I've no doubt this only had one screw holding it onto the flange plate. I had some odd friction resistance in the focus wheel itself sometimes, often when rotated. After some research I discovered this is a known issue with some GSO focusers. Although the problem isn't reported on the smaller 86mm flange models. I've talked to several people who have had the same problem with the 96mm focusers. Yet I've never known anyone who has had the friction issue on the smaller focusers. Astro gear is weird!

[philjay]

Looking at the pics on the gso website it would appear that all their focusers have 3 grub screws set equidistant (120 degree) around the focuser body to locate in the vee ring plus the lock screw half way between 2 of the grub screws. 

If you have taken the body off the vee ring then you have either loosened the grub screws or they are a, to slack or b, missing. Both would explain the tilting and binding.

 

Reassemble by carefully adjusting the grub screws till you can rotate the focuser without tilt or binding, then fit the lock screw

Edited January 17, 2022 by philjay

[philjay]

Oh, a lttle , smear of silicon grease on the vee ring track smooths rotation 

[Nightspore]

55 minutes ago, philjay said:

Looking at the pics on the gso website it would appear that all their focusers have 3 grub screws set equidistant (120 degree) around the focuser body to locate in the vee ring plus the lock screw half way between 2 of the grub screws. 

If you have taken the body off the vee ring then you have either loosened the grub screws or they are a, to slack or b, missing. Both would explain the tilting and binding.

 

Reassemble by carefully adjusting the grub screws till you can rotate the focuser without tilt or binding, then fit the lock screw

 

Yep, you're right. The grub screws are there. I should imagine they must have loosened over time. Thanks Phil. 

[Nightspore]

8 hours ago, Graham said:

Focusers with a rotator normally have 2 additional nylon grub screws set at 120 degrees to the locking screw to prevent this from happening.

I would suggest you put a couple in if they are not there.

 

Found them! I told you DIY wasn't a big thing with me lol. Cheers.

[philjay]

Great, don,t overtighten them, they are just guides in the runner vee ring, the lock screw does the locking

[Nightspore]

1 hour ago, philjay said:

Great, don,t overtighten them, they are just guides in the runner vee ring, the lock screw does the locking

 

I've been experimenting with the tightness. I think binding will still be an issue sometimes but at least the whole thing shouldn't come off in my hand now lol.

[+Graham]

Glad you got it sorted 

[Nightspore]

3 hours ago, Graham said:

Glad you got it sorted 

 

Thanks. 

 

 

I'd suspected these 2mm grubs earlier. But I came to the conclusion they were too small and were most probably placeholders. There were grub screws as placeholders for the finder shoes.

 

[philjay]

Yep, thems the ones. Tweak each one gently till it stops then back off a tad till it rotates but diesnt wobbke.

 

Tweak, Tad and wobbke are highky technical terms in the focuser world 🙂

[Nightspore]

49 minutes ago, philjay said:

Yep, thems the ones. Tweak each one gently till it stops then back off a tad till it rotates but diesnt wobbke.

 

Tweak, Tad and wobbke are highky technical terms in the focuser world 🙂

 

Thanks Phil. I'm getting tweaking anxiety now though lol.