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Older Vixen Newtonian sled focuser slipping, help!

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#1 Littlegreenman

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Posted 21 November 2012 - 02:55 AM

I recently grabbed a Celestron / Vixen 6" f/5 reflector with the sled focuser. The focuser slips. I removed it, and the best I can see is the gears look to be okay. I need to look closer in daylight to confirm.

There is tiny threaded locking pin that holds the knob on. I removed that but the knob is still on pretty tight.

If anyone has ideas to adjust this? Or ? Or who's selling a replacement?

Thanks,
LGM

#2 desertrefugee

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Posted 21 November 2012 - 08:40 AM

Yikes! I'm glad mine (knock knock) so far works well.

I've heard of a number of problems with that focuser and a friend in our local club whose unit has a focuser that's pretty much ruined (stripped).

I've never had mine apart, but if your gears are in good shape, it must be a meshing issue. I'll peek tonight and see if there's a "penetration" adjustment. If not, you might have to get creative. I suspect the slipping will make things worse and should be corrected.

#3 *skyguy*

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Posted 21 November 2012 - 12:11 PM

I used to own a Vixen 6" f/5 reflector with the sled focuser ... I regret selling it! I do have a Celestron 5.5" Comet Catcher that also uses the Vixen sled focuser. I also had a homemade 2" sled focuser on my 26" dob that was based on the Vixen design.

At the base of the focuser knob, there are two adjustment screws. You can use these to tighten the pinion gear against the track. Also, there are very tiny hex screws along both sides of the track. They can be used to center the "sled" in the track and to increase or decrease friction along the track.It takes a bit of "fiddling" around, but when properly adjusted the sled focuser can be a joy to use.

#4 EverlastingSky

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Posted 21 November 2012 - 10:42 PM

These sled focusers have a variety of adjustment screws to adjust in order to obtain good operation. The more one gets into this issue the more complex and quirky these sled focusers can become. I've been around and around with these things more than I'd like to admit.

Start by checking that the focus knob and the pinion gear are in contact. The focus knob assembly attaches to the cast aluminum sled wall with two small Philips head screws (at least in my 1990 version). So as to provide the ability for adjustment and to prevent binding, the mounting holes in the focus knob assembly are quite over sized. This results in the assembly slipping down by as much as 1.5mm - taking the pinion gear with it - causing the rack and pinion to become separated. Simply tightening the two screws is seldom enough. I believe that this mechanical separation, or partial separation, is the root cause of many a stripped and worn out rack and pinion on these telescopes.

I found that a piece of solid copper wire could be flattened and fitted underneath the focus knob assembly to permanently eliminate this problem from repeating (or starting at all). Focus knob assembly slides down resulting in rack and pinion separation.

A few more related pictures: View with focus knob assembly removed & View of SP-C6 sled focuser from inside the telescope itself.

And as skyguy already indicated, you need to check those other two small tension adjustment screws located in the sides of the cast aluminum sled holder.

#5 Littlegreenman

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Posted 22 November 2012 - 01:57 AM

Everlasting Sky said:

"The focus knob assembly attaches to the cast aluminum sled wall with two small Philips head screws (at least in my 1990 version). So as to provide the ability for adjustment and to prevent binding, the mounting holes in the focus knob assembly are quite over sized. This results in the assembly slipping down by as much as 1.5mm - taking the pinion gear with it - causing the rack and pinion to become separated. Simply tightening the two screws is seldom enough. I believe that this mechanical separation, or partial separation, is the root cause of many a stripped and worn out rack and pinion on these telescopes. "


Thanks for your posts. I've been busy since my first post. This device is subtle. I think Everlastingsky's problem is my problem. So far I have been unable to remove the focuser knob. It's either stuck or glued on, or there is another mechanical thing holding it on other that the one threaded pin I removed. The knob is is the way of getting the those two bolt heads. At least I will have some time off in the next 4 days to play with it.

LGM

#6 Littlegreenman

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Posted 25 November 2012 - 08:51 PM

Thanks to skyguy and EverlastingSky. The problem was the pinion gear not meshing, due I guess to shifting of the pinion gear. Loosening and re-adjusting the two bolts that hold the focus knob-pinion gear shaft assembly was in this enough to fix the problem. Here is a picture showing that where those two bolts on:

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#7 Littlegreenman

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Posted 25 November 2012 - 09:01 PM

While I was figuring things out, I took the sled focuser off the tube. I didn't need to do that to fix the problem I had, as was would later find out.

The secondary mirror is attached to the sled focuser, and focus is achieved by moving the mirror back and forth in relation to the primary mirror. If you do need to remove the focuser you will find the secondary mirror holder is too large to fit through the focuser hole. The secondary mirror holder is attached to the secondary shaft by one bolt in the center. There are three other bolts for collimation. Simply unscrew the center bolt while hanging onto the secondary mirror holder to remove it. Putting it back on is easy. The three collimation bolts/screws line up with indents on the secondary mirror holder, so the secondary mirror will be aimed in the right direction. All it needs is fine collimation. Hopefully. Here is a pic of that, with an anemic hand.
Thanks again for the help!
LGM

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#8 droid

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Posted 07 December 2012 - 06:39 PM

Thanks to skyguy and EverlastingSky. The problem was the pinion gear not meshing, due I guess to shifting of the pinion gear. Loosening and re-adjusting the two bolts that hold the focus knob-pinion gear shaft assembly was in this enough to fix the problem. Here is a picture showing that where those two bolts on:



Not to hijack the thread or nothing, but the screw your pointing at, is whats missing my Comet catcher sled forcuser. Any idea where I might the correct screw?

#9 RAK on Tour

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Posted 11 December 2012 - 11:02 AM

Littlegreenman, The focuser knob that is reluctant to come off is not held on with just the set screw - it SCREWS ON, TOO! The focus knob on my SP-C6 had about 1/4 inch play in and out (towards and away from the sled focuser). I removed the set screw and tried to remove the knob. Like you - no go! I tried pretty hard to pull the knob off, too. Then I held the focuser in place and found that I could screw the knob closer to remove the play in the shaft. I have more work to do on this because I can see the the ball bearings in the focuser have slipped out of the cages that locate them. Those ball bearings haven't fallen out of the focuser, but they're not properly located, either. What I have now is a focuser that is very smooth for part of it's travel, but it jams as the focuser moves toward the main mirror. I'll be carefully loosening the various adjustment screws until I can roll the bearings back into their proper location. The loose knob is fixed; just a little more work to go!

#10 Littlegreenman

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Posted 11 December 2012 - 11:17 AM

Thanks to skyguy and EverlastingSky. The problem was the pinion gear not meshing, due I guess to shifting of the pinion gear. Loosening and re-adjusting the two bolts that hold the focus knob-pinion gear shaft assembly was in this enough to fix the problem. Here is a picture showing that where those two bolts on:



Not to hijack the thread or nothing, but the screw your pointing at, is whats missing my Comet catcher sled forcuser. Any idea where I might the correct screw?


I'm blessed by having a couple of halfway decent hardware stores near my (nothing compared to what was available 15 or more years ago though). If it's Japanese made is probably metric, just gotta find out the right size. Which I don't know.

LGM

#11 Littlegreenman

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Posted 11 December 2012 - 11:19 AM

RAK on Tour,

Everlasting Sky clued me in to the same thing in a PM. The knob comes unscrews off after you take the set screw off.

I got mine working right now...so I'm not messing with it.
LGM

#12 EverlastingSky

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Posted 11 December 2012 - 02:43 PM

The focuser screw in question is indeed a 2mm metric. (I don't know anything about thread pitch though.) Thread length is 5mm. Due to the tapped thread holes going right through the cast aluminum focuser body, it appears that one could get away with using screws that are much longer, if necessary. I keep meaning to buy some "real" 2mm stainless machine screws with hexagonal socket heads, instead of these feeble Philips head ones Vixen used. For example, the ones shown here: The Largest Fastener Hardware Store on Ebay!

(Note for those interested or curious: I modified the focus knob by stretching a piece of rubber over it. The rubber tube is actually a roller from an old broken printer - the part that was used in the tray to grip the sheets of paper and move them along. This roller turned out to be awesome for the task because it is made of a very stretchy "sticky" rubber and has random style texturing. It dramatically improves the accuracy and fine tuning of the focus. I then filled in the extra tube length at the end with a hard rubber roller from elsewhere in the printer.)

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