Optimizing The Performance Of Your GSO Crayford Focuser
Agena Note: We inspect every single focuser before we ship it out to a customer. The performance is checked so you should not have to do this when you get a new focuser from us. However, with the passage of time and/or the load you place on your focuser, you will probably need to tweak your focuser a bit. At that time, the content of this article might be a very useful resource.
Pollux Chung has written an excellent article on GSO's dual-speed Crayford focusers. The following photos and description have been reproduced here with his kind permission. The same content can be viewed at his website at (www.polluxchung.com). We are, however, not responsible for any problems that might arise by the use of the information presented below.
If you are still using those Chinese-made telescopes you probably know their built-in rack-n-pinion focusers are driving you nuts. I sometimes call these things "toy from hell."
It's been a while since we have been seeing some crayford focusers from GSO (Guang Sheng Optical) in Taiwan. These focusers aren't quite in the same league as Feather Touch or Moonlite. However, they still offer really good performance without breaking your bank.
The problem with the GSO crayfords is that they sometimes do feel rough.
The good news is that there are ways to fix this little fella to make your scoping experience even more enjoyable.
This tutorial here covers the dual-speed GSO-based crayford. It may not apply 100% for other crayfords like those made by Long Perng since the structure (ways to break them apart and put together) of their focuser is slightly different. Scopes that use Long Perng mechanics are Astro-Tech (66 & 80mm refractors), Stellarvue (Nighthawk Next Generation & SV102ED), and the Skywatcher (Equinox Series).
Before you start doing the work, you need a few things:
- A wrench
- Allen keys
- Medium thick grease (Lithium or Graphite)
- A little bit of car polish wax, no really necessary but it's good to have
- And of course, a bit of patience ;-)
- Here's a typical GSO-made dual-speed crayford.
- Make sure you are not mixing up its tension and locking thumb screws.
- First, loosen up the TENSION thumb screw, or even take it off if you wish.
- Take the focus knob (at left) first, you need an allen key to do the job.
- Then pull the shaft out from the other side. After the shaft is pulled from the focuser the draw tube will come out very easily.
- The flat surface of the draw tube is where the focus shaft is touching. Generally this surface is flat and smooth enough (way better than Synta's crayford for sure)
- Many people who own a scope with Synta's crayford have to use a grinding stone to flatten and smoothen the "flat" curvy surface of its drawtube.
- The good news is, no grinding stone is needed for GSO crayfords since its flat surface is flat enough for doing a good job.
- You still can make it smoother by using some car polish wax (cutting wax is recommended). Apply a bit of wax and wipe it off once its dry. You should then feel it is very slippery. Yes, this is what you will want it to be.
- Now look at the inside of the focuser's main body. The 4 wheels (circled in red) hold the draw tube in place. Apply a small amount of grease on all of them.
- Now here comes the exciting part - the dual-speed knob!
- Again, use allen key to remove the fine focus knob first...
- ...and then the big knob.
- Here are the internal guts of the dual-speed knob. You see three steel balls sit inside the brass bracket.
- Here is the other side of the knob. The steel nut (which the red arrow is pointing towards) is very important, don't mess with it yet. This is one of the major reasons why some GSO crayfords are rough or tight.
- Look at the ball bearing side again. With all the GSO crayfords I have broken apart and looked at they generally don't have enough lubricant for the ball bearing to move around, this is the second reason why the focuser feels rough, so put some grease here but not too much. Then roll the brass bracket to make sure the grease is evenly applied.
Agena Note: This is the most important step by far. If your microfocuser knob rotates smoothly, we would recommend that you skip this step entirely. You won't damage anything, but you can make the performance worse if this is not done carefully. If the focusing action on your unit results in a "clicky" feel at the rate of 3 clicks per rotation (either in the coarse or fine focus knobs), then this is the step you need to perform to make things right again.
- The next thing to do is to loosen up the nut on the other side of the knob. Loosen it up VERY LITTLE (a fraction of a turn). Agena Note - Be VERY patient here. You need to turn the nut by 1/20th to 1/30th of a turn at a time. Even 1/8th of a turn would be too much.
- Be careful with this. If it's too loose the fine focus knob will not work or the brass bracket or even the little pin that holds the fine focus knob will come out. If too tight the knob will not turn at all.
- Now put the big knob back in. If you turn the knob you will find it very "clicky". Don't worry, this is normal. Once you put the small knob back, that clicky feel will be gone. I don't know why. Gravity? maybe...
- A friend of mine told me the fine focus knob on all Feather Touch focusers is made by heavy brass and this is the reason why the turn is so smooth. I can't really confirm this but I think it's interesting enough for me to mention it here.
- You can make a few turns on both the big and small knobs to see if they are to your liking or not. If the feel isn't right for you, you can adjust the tension by turning the nut on the other side.
- It may take a few tries to get to the feel you want.
- Again, make sure it's neither too tight nor too loose.
- Put the draw tube back in the focuser's body first...
- ...then the dual-speed knob and shaft
- This is a small part but don't miss it. Apply a bit of grease on that plastic tube thing (that sits below the other focus knob), then put it back to the shaft.
- Doing this minimizes the friction as much as possible.
- Put everything back together, and adjust the tension by turning the tension thumbscrew.
And that's it! Keep in mind that this will not turn the focuser into a Feather Touch, but at least it will make these GSO focusers perform the way they should.
You may not get it right at first but once you get the idea behind this you will later have a much better chance to get what you expect.