Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Vintage orange C14 focus knob

  • Please log in to reply
8 replies to this topic

#1 Gert

Gert

    Mariner 2

  • *****
  • topic starter
  • Posts: 289
  • Joined: 15 Apr 2008

Posted 22 September 2018 - 10:08 PM

Hello All,

 

After checking all the bolts in the OTA rear cell for safe mirror clearance, I put the OTA on my mount to try it out.

 

The 'rough' motion of the focus know now became really noticeable and this will be the next item to clean up. This is an orange C14 from the age when the orange paint did not yet have the pebbled finish. Turning the focus know gives the feeling of corroded metal scraping within some other corroded metal. Need suggestions to fix it.

 

Below is a shot of the focus knob as seen from the outside of the rear cell.

  • Is there documentation to access the focus mechansim threaded bolt?
  • Is it possible to clean / regrease the focus rod mechanism without necessarily removing the corrector and primary?
  • What (if any) cleaning / lubricating agents / oils / grease / WD40 spray should be used in the process (i.e. considering gassing and condensation within the OTA etc.)

Thanks & Clear Skies,

Gert

 

TVS_C14_16ss.jpg

 

 



#2 davidmcgo

davidmcgo

    Mercury-Atlas

  • *****
  • Posts: 2796
  • Joined: 09 Oct 2004
  • Loc: San Diego, CA

Posted 22 September 2018 - 10:29 PM

You can get at the focusing mechanism from the rear without removing the optics.  Do this with the tube horizontal or pointing the corrector slightly upwards, not pointing down.  

 

There is a little set screw on the side of the focusing knob, loosen it but don’t let it all the way out.  The black knob will slide off.

 

The three button head screws on the black collar around the knob are removed next.  Now the ball bearing assembly and threaded rod can slide rear wards.  In the center of the threaded ro the should be a set screw and washer.  It nay be necessary to thread the bearing assembly up the rod to see the.  Some versions used a c clip instead.  Either case, remove them.  Noe unscrew the bearing assembly all the way off the threaded rod and slide thevrodvrearwards to expose as much length as possible.  Clean off the old grease, I find a little GooGone citrus gel on a cloth PecPad works fantastic. Repeat as needed and then wipe dry.

 

Clean the inner threads of the bronze focusing shaft inside the bearing races with a cotton swab or little roll of towel and the same solvent,

 

For the bearin housings, there are two nested together, Imusually use WD40 to soak in between and pentetrate a bit and wipe off the excess.

 

Then dab a little Superlube or Napa Silglyde on the end of the threaded rod and screw the bearing assembly back,on and run up to the rear cell of the tube, Leave it a little loose.  Reinstall the washer and setscrew or C clip on the end of the threaded rod.  Reinstall the bearing retainer with the three screws.  Don’t tighten each screw all the way initially, round robin them and try to keep the bearing centered.  Snug is good but not a lot of torque.

 

Now put the tube front cell down and run the focuser up and down and see if it binds, if so, shift things around a little until they are free moving.  Then gradually snug up the bearing retainer screws.  Reinstall the knob but don’t press it all the way up against the back of the bearing or it will grate and feel rough.

 

Dave



#3 J A VOLK

J A VOLK

    Viking 1

  • *****
  • Posts: 958
  • Joined: 24 May 2006
  • Loc: So. Cal.

Posted 22 September 2018 - 10:40 PM

first thing to check is that the knob is not bearing on the black collar. Loosen the knob setscrew(s), pull it out a bit so there is clearance, then retighten. If you still have the problem proceed as above.
  • NMBob likes this

#4 Gert

Gert

    Mariner 2

  • *****
  • topic starter
  • Posts: 289
  • Joined: 15 Apr 2008

Posted 22 September 2018 - 11:51 PM

Hello J A VOLK

first thing to check is that the knob is not bearing on the black collar. Loosen the knob setscrew(s), pull it out a bit so there is clearance, then retighten. If you still have the problem proceed as above.

That was a good idea. Just checked at the scope outside. The knob was clear vs. the collar. (No scraping) However visual inspection shows that the collar and knob (and the brass shaft inside it) are not concentric. I would estimate that an ideal air gap equally spaced around the knob would be 0.5mm. However in this specimen the gap is near 0 towards the visual back and about a full 1mm towards the outside. So the collar is definitely not centered. Not sure if it should be! :-)

 

I'll proceed with Dave's instructions on the bench tomorrow.

 

Just a OT FYI. I did a quick star Ronchi-test on Altair with a 200lpi grating. 5lines inside + outside focus are quite straight to the edge. (real-time subtracting seeing using eye-brain combination) Also did 0deg 45deg and 90deg rotation of the grating showing no discernible astigmatism. Quick guess is that the OTA is probably better than 1/2wave. Not bad for this old and this big.

 

Thanks & Clear Skies,

Gert



#5 J A VOLK

J A VOLK

    Viking 1

  • *****
  • Posts: 958
  • Joined: 24 May 2006
  • Loc: So. Cal.

Posted 22 September 2018 - 11:59 PM

the star test is more sensitive then the ronchi, but that is a good sign. Generally scopes from this era (smooth orange) are better then later and modern scopes, with a smoother figure. Even so-so SCTs show good ronchi lines.

Edited by J A VOLK, 23 September 2018 - 12:00 AM.


#6 Skywatchr

Skywatchr

    Mercury-Atlas

  • *****
  • Posts: 2777
  • Joined: 03 Jun 2006
  • Loc: North-Central Pa.

Posted 23 September 2018 - 08:49 AM

I've seen the focuser bearings rust and become rough.  However they didn't produce the sound you describe.  If the focuser is not the issue, then it is the mirror tube slide which will necessitate disassembly of the optics to fix.  If there is no corrosion on the slide tube, then just a re-grease will cure it.  I have used the SuperLube with great success and it won't "separate" over time like conventional lubes do and cause an "oil bleed" to creep all over things.  I am sure there are other greases available now that will work as good, or better.  Mobile One red synthetic grease is nice too, but is a little thin for this application.  It's awesome for gears though where you don't want a thicker lube.


  • B 26354 likes this

#7 Gert

Gert

    Mariner 2

  • *****
  • topic starter
  • Posts: 289
  • Joined: 15 Apr 2008

Posted 23 September 2018 - 07:03 PM

Hi All,

 

@Dave : thanks for the good description. I was able to take the focus mechanism apart and made some attempts improving the functionality.

 

  • Cleaned the threaded rod from old debris.
  • Cleaned the focus knob threads as well
  • Re-greased and re-assembled.

One thing I was unable to do was to get the two stacked ball bearings pulled off the focus knob (forgot to take a photo of the issue)

 

Once all is back together again the knob moves much 'lighter'.

There is still some 'knurlyness' / 'bumpiness' in the motion, which I attribute to the two stacked ball bearings. Are those bearings they used in their LA facility in those days taken from their kids' skateboards? They look like it. :-(

 

Are there instructions to replace those bearings?

 

Thanks & Clear Skies,

Gert


Edited by Gert, 23 September 2018 - 07:29 PM.

  • Skywatchr likes this

#8 davidmcgo

davidmcgo

    Mercury-Atlas

  • *****
  • Posts: 2796
  • Joined: 09 Oct 2004
  • Loc: San Diego, CA

Posted 24 September 2018 - 03:53 PM

I have never tried to get the stacked bearings apart, didn’t want to mangle anything trying.

 

Dave



#9 Skywatchr

Skywatchr

    Mercury-Atlas

  • *****
  • Posts: 2777
  • Joined: 03 Jun 2006
  • Loc: North-Central Pa.

Posted 25 September 2018 - 05:55 AM

Hi All,

 

@Dave : thanks for the good description. I was able to take the focus mechanism apart and made some attempts improving the functionality.

 

  • Cleaned the threaded rod from old debris.
  • Cleaned the focus knob threads as well
  • Re-greased and re-assembled.

One thing I was unable to do was to get the two stacked ball bearings pulled off the focus knob (forgot to take a photo of the issue)

 

Once all is back together again the knob moves much 'lighter'.

There is still some 'knurlyness' / 'bumpiness' in the motion, which I attribute to the two stacked ball bearings. Are those bearings they used in their LA facility in those days taken from their kids' skateboards? They look like it. :-(

 

Are there instructions to replace those bearings?

 

Thanks & Clear Skies,

Gert

Since you've done all that, it would seem the issue is in the ball bearings.  If/when you get them out, there will be a number on each bearing.  That is the number you will look for to find replacements.  It is *not* a "Celestron" part number, it is a "standard" number for that particular size of bearing.  They come in 3 flavors. Unshielded- meaning the balls are open and require periodic cleaning and/or greasing, and are open to get water and dirt in them.  Shielded- meaning they have metal shields over the balls as added protection, but will still allow moisture and debris in over time and cannot be easily re-greased or cleaned. Sealed- means they actually have rubber seals and will never need cleaned or greased over their lifetime. However the seals add a small amount of "resistance".  But you want that in your focuser.  Replace them with sealed bearings and you'll never have to worry about them again.

I haven't had a Celestron focuser apart in years, but this small thread may help. https://www.cloudyni...ut-c14-focuser/




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics