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GSO Focuser Separation Anxiety


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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.




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It seems these 2mm grub screws are for keeping the focuser on. They must have become loose over the years.




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  • 2 months later...
3 hours ago, Stephen Waldee said:


I appreciate reading this report; I tend to repurpose things rather than buy new ones, so I had changed the focuser of my own ST-80 by attaching one from a Newtonian with brackets I made; but after almost ten years of using it that way -- the brackets were actually quite stable and the thing is still mechanically solid and perfectly aligned - I have been thinking more and more about finally replacing this kludge with the proper upgraded focuser.  I was in the comparison stage, just looking at ads and reading reviews and comments.  


I like NEGATIVE reviews far more than POSITIVE ones.  The positive reviews go like this all too offen, quote: "I just got the XYZ twerpomatic protochromatic orthomat yesterday and LOVE it!".  Then there is an encomium to the nice anodizing and the shiny gleam.  The negative reviews say, "my first good impressions were at first negated when the external lens and spacer fell out into the baffle tube of my C-14 the moment I pointed the OTA off horizontal; while extricating them my secondary was ruined--thanks XYZ Ltd. for your shoddy workmanship and cracked potmetal!"


Sometimes, of course, the, um, 'little shortcomings' may be corrected by a do-it-yourselfer.  Looks like the only solution for such a focuser is to eliminate the rotation function, probably the very thing that convinces one to buy it.  That sort of thing has happened to me all too many times with "innovative" products with special functions that most others in their general category lack: the "distinctive feature" is the bottleneck!


So I am crossing this off my list of candidates!


Steve & Regina, Ivins UT



Now I've fiddled with the grub screws a bit the rotation has been relatively problem free. 




I had no idea that the grubs had gradually loosened over a period of several years. When they loosen too much the rotating part can separate from the V-ring on the flange. This won't happen if the grubs are kept tight enough to stay connected but loose enough to allow adequate rotation. 




It isn't the most elegant design for a rotating focuser but the 86mm version has no other quirks or problems as far as I know. There are a few variants made by GSO, mine is a heavy duty model. However, the 96mm version has some gearing problems that don't seem to affect the smaller version. All in all I'd recommend the 86mm focuser. Although I'd also recommend keeping an eye on the grub screws. 

Edited by Nightspore
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