What goes in this threaded hole?

[chicharrero]

I bought this used Celestron Travelscope 70 for under a tenner. Very happy with it. OTA only in great condition. Will take it on holidays when the boot is rammed full of toddler equipment. But what is the brass threaded hole in the top of the focuser for (centre of attached image)? I'm guessing perhaps it locks the focuser? If so, can I get some sort of knob that fits in it? What size thread would it be? Any help on this relatively trivial question appreciated...

[John]

There should be a thubscrew in there to lock the focuser if needed. You can see it in this picture of the scope:

 

 

[barkis]

That thumbscrew looks as though it bears down directly onto the focusing tube, so perhaps
it ought to have a Teflon or Nylon tipped end to prevent marking the tube.
It just seems to be so, I'm not certain about it. John would know better than I,
he is much more knowledgeable about Refractors

[LukeSkywatcher]

I have the same scope, and as John says..........there should be a thumbscrew there to lock down the focus tube.

[Dave In Vermont]

If the focuser has a brass-ring for the (missing) bolt to engage, you needn't worry about a Teflon® - tipped bolt to engage the focus-lock. Any bolt that fits will do fine. Take a peek into the tube on the focuser. See if it has a brass compression-ring in there below the hole for the bolt to fit.

Have fun,

Dave

[Bizibilder]

You can also see two socket head allen screws in line along the raised bit of the focusser housing.  This implies that there may well be a gib strip (strip of metal) housed within a groove under the raised part.  if so then a suitable screw in the brass lined hole will contact the gib strip and allow you to lock the focusser - you wont have to worry about marking the focusser tube as the screw will never touch it.  The two allen screws are used to set the focusser rack in correct mesh with the pinion on the focusser knob.

However - the brass insert is serrated on the outside which implies that it has been pressed into the focusser housing (which is most likely made of plastic or maybe light alloy if this method of assembly has been used) to hold it in place.  If you were to tighten the locking screw too much you would simply jack the insert back out of its position!  So take care and only lock with light pressure.

[carastro]

Wow, you got a bargain, a scope for a tenner.  Not sure where to buy a suitable replacement locking knob, but might be worth looking on Astroboot, or asking Steve at ENS Optical.  

Astroboot seems to have a section for bolts and also a contact tab where you can ask them.  

http://www.astroboot.co.uk/AstroBoot/bolts-screws-and-knobs-for-telescopes.html

Steve has often helped me out with various bits and bobs and seems to know what fits what.

http://www.ensoptical.co.uk/

However, if your focuser doesn't slip and you're only doing visual as opposed to astroimaging where a lot more weight is placed on the focuser you might be able to cope without one.

Carole 

[chicharrero]

Thank you everybody for your helpful replies!