FOCUSER TIPS

I will cover the basic XT10 2" rack and pinion focuser first.  This should help new owners of XT's and Guen Sheng dobs get the most out of the stock focuser. 

First of all, you cannot break the focuser unless you get really, really mad and bash it with a hammer.  You can take it off the scope, take all the screws off and put it back on without breaking it. 

 

But, you can knock it out of alignment if you mess with the collimating screws.  If you really want to mess with these, read the owner's manual where they recommend not to.  That advise is germane.  If you still want to mess with them, then read my squaring the focuser section on how to get the focuser square again.

 

The first thing you should do with the focuser is take it apart and remove all the concrete glue that the manufacturer calls grease.  This is the sticky stuff inside that makes that horrible crinkling noise when you move the focuser.  How do we take it apart?

 

1.  Remove the 4 screw that hold the pinion gear cover plate on.  They are the 4 screws in the middle of the first picture:

You will see the metal retaining clip thingy.  It goes back in a certain way as shown in the picture.  Now lift out the pinion gear (the thing with the focus knobs on it) and pull the draw tube out of the top.  You may see a piece of plastic fall out of the focuser.  It's shown here:

That piece of plastic sits on two tiny allen head screws and the focus lock knob.  These allen head screws are what we will use later to adjust the slop out of the focuser.

 

Now, use some goof off, windex or some such cleaner (no acetone, it'll eat the paint) to get all that Chinese glue out of there.  Clean everything off.  No lubrication and clean is better than the stock "grease".   Now, choose your grease.  The best to use is white lithium grease found in the checkout line by the tool section in Lowe's.  Actually, any home store will sell it.  I have also used spray silicon, WD-40, hair clipper grease and motor oil.  Just be careful not to gob it on so thick that it will drip onto the optics.  Don't spray it on the focuser, if it's a spray lubricant.  Spray it on a paper towel and then apply it to the focuser parts.

 

Now you can put everything back together, being careful to get the pinion spring thingy on correctly.  The plastic part that may or may have fallen out has two indents in the back that fit the little allen screws.  Make sure you match them up.  Finally, be sure the pinion gear is centered when you put the metal cover back on.  This can get pinched under the metal cover.

 

Now it's time to adjust this puppy so it's smooth and predictable.  There are 2 places that you can adjust the focuser, neither of which is mentioned in the manual.  The most important is the two allen screws shown in this picture:

They are .5mm, if you're looking for an allen key for them.  These adjust that plastic piece inside the focuser.  The best way to adjust them is to slowly rack the draw tube up and down while tightening each screw until you start feeling resistance in the draw tube motion.  You can fine tune this by by doing each screw a little at a time.  Too tight, and the focuser will be too hard to move up and down.  Too loose, and it will wobble freely.

Another adjustment that is not often mentioned is the 4 screws that hold the plate over the pinion gear.  These wouldn't seem to have any effect on focuser slop, but tighten them up too much or make them very loose and you will introduce issues.  I keep them tight enough to firmly seat the pinion gear in the rack gear.  You can tell if it's too loose by pulling out on one of the focus knobs.  If it has a lot of play and is easy to wiggle, you may want to tighten the 4 screws up a bit.

I use my laser collimator to check how much focuser slop there is.  Put the laser in, turn it on, and rack the focuser in and out.  Note any motion of the laser spot on the primary.  I have found that there is a magic place where the tension is just enough to keep the slop down.  If you go too tight, the spot will move even more as the draw tube tilts in the focuser body.  The best I could ever get was about 1/8" of laser spot travel on the primary. 

That's it.  Your focuser is now a working machine. 

FOCUS LEVER

This is an incredibly easy mod that makes focusing at high powers a snap.  I made this mod when I was spending a lot of time above 200x with the binoviewers in the focuser.  That was a lot of weight to move around precisely.  Focusing with the stock knob induced a lot of vibration and was difficult to control.  So I added this little focus arm that essentially turns my focus knob into one with an 8" radius.  Here are some pics:

The arm is made of 1/8" x 1/2" aluminum. Steel was too heavy. I had to bend it a bit to clear the binoviewers. This had the added benefit of clearing the finder as well. The wingnut is used to activate or deactivate the arm. I can use the focuser normally without tightening up the arm and then when I am viewing at high power, I just tighten the wingnut and focus on an 8 inch knob instead of a 1.5" knob. The reason I did this was to allow a fine focus option at high powers. With the focuser tight enough to eliminate the play, I was getting a lot of tube motion when I was trying to nail the focus. This should eliminate this.

 

I also made a second focus arm that is a lot easier to use, but requires some searching to make it work.

The gold part is just a cap from a can of spray paint that I found in the garage.  I cut the bottom half of it off to make it less deep.  It was the perfect diameter to fit the focus knobs on the crayford focuser.  I tried making a focus arm like this for the stock XT10 focuser, but was unable to procure a cap that fit the smaller XT10 focus knobs.  I was luckier with the crayford focuser knobs. 

 

I used the same aluminum arm and just bolted that to the cap.  The black that you see around the edge was electrical tape that I used to stiffen up the thin plastic.  I used just enough electrical tape to make the fit snug.  If I put too much tape on, it was too hard to remove the focus arm.  Too little tape, and the arm just slipped.

 

This solution is much easier in operation than the wingnut version.  But, it requires some luck in finding the right cap.

 

SQUARING THE FOCUSER

 

This is actually part of collimation, but I wanted to include a little trick that I found works very well with the XT10 and 4 vane spider.  This will not work on a 3 vane spider telescope. 

 

Making sure the focuser is square should only be required if you have futzed with the factory focuser collimation (you know those focuser collimation screws you weren't supposed to touch) or maybe you're just curious like I was.  Well, in all honesty, I messed with the focuser collimation screws.  You see, my secondary was not centered under the focuser, and I had run out of screw length to move it any further down the OTA, so I tilted the focuser to center it up.  Big mistake.  The focuser must be square with the OTA and should never be adjusted in order to center the secondary.

 

So, here's a quick way to check it.  First, you have to remove the secondary.  Be sure to do this with the OTA horizontal so you don't drop anything on the primary.

 

Now put your laser collimator in the focuser.

 

Now, go get your handy metal square that has the nifty sliding ruler part on it.  Here's a picture of what we're going to do:

 

 

Set the short part of the square flat against the front of the OTA and slide the square down until the metal ruler part is resting on the spider vane.  Not the round threaded part of the spider holder, but the spider vane itself.  You will have to tilt the ruler a little to get past the round threaded part.   The laser spot should be right on the bottom edge of the metal ruler.  If it's not, then something is very wrong with the focuser adjustment, or this method won't work for your XT10.  Now slide the ruler in or out until the laser spot is right on the bottom corner of the ruler.  Look closely at the left hand picture above and you will see the red dot representing the laser spot.  Put the square on the other side of the OTA as shown in the right hand picture.  Again, you will have to tilt the metal ruler part to rest it on the spider vane.  If the focuser is square, the laser spot will be right ton the top corner of the ruler.  If it isn't, adjust the focuser until it is.

 

Three assumptions must be made in order for this to work:

 

1.  The front of the OTA is square with the sides of the OTA

2.  The attach points for the secondary spider vanes are exactly opposite each other

3.  The laser is square with the focuser.

 

All 3 of these assumptions seem reasonable enough to me, so I think this is a quick and easy way to square the focuser.

 

There are other, more complicated ways to square up the focuser but I'm not sure I completely understand how they work.  So, you can ask the question on the Skyquest group or do some research on the web for more info.

CRAYFORD FOCUSER

If you still don't like the stock focuser, you can visit my William Optics Crayford focuser install page for a relatively inexpensive alternative.