Dobsonian Focus messed up

[common763]

I have a 10" Skywatcher dobsonian.  My son was playing with it when I wasnt around and the focus and lens area is messed up.  Basically the focus knobs when turned dont properly hold the lens and holder properly and it falls when elevated up, and sometimes I just turn the knobs and the same part doesnt move up or down.  Something is wrong.  Please help.  Thanks.

[cletrac1922]

 

33 minutes ago, common763 said:

I have a 10" Skywatcher dobsonian.  My son was playing with it when I wasnt around and the secondary mirror is messed up.  Basically the focus knobs when turned dont properly hold the mirror and it falls when elevated up, and sometimes I just turn the knobs and the mirror doesnt move up or down.  Something is wrong.  Please help.  Thanks.

 

Hi Buddy

I have same scope

Not sure what he was doing with secondary mirror, sounds like he has loosen the centre screw holding the secondary

These are an hex key screw

The 3 outside screws adjust the secondary mirror collimation

Tighten the centre screw to have secondary mirror secure

I use a laser collimator, to align my secondary, then primary mirror

Just had a re-read of your comments, and the secondary mirror does not move, when adjusting focusing knobs

Sure you not referring to eyepiece, rather than secondary mirror

Eyepiece is held in place by 2 small screw, adjacent to dust cap in your second pic

The secondary mirror is held in place by cross veins

Have seen, where club member, I belong to, have purchased a second hand dob, and for some reason the secondary mirror is off centre, and align using 4 adjusting screws protruding from the tube

Need more info

John

 

 

 

[common763]

sorry didnt explain properly...it isnt the secondary mirror but the knobs and where the lens goes.  That is loose and when I turn the knobs sometimes it doesnt move the lens, other times if elevated the part that holds the lens falls into place rather than decreasing with the knob rotation.

[Knighty2112]

Adjust the  silver knob underneath the focuser. If this is either too slack or too tight this will cause issue with achieving focus smoothly. 

[Carbon Brush]

Hello and welcome to SGL.

This sounds like a simple focus adjustment, as indicated by Knighty2112.

However, if you do get stuck, include your approximate location in your info.
Quite often the result is an offer from a local member to help 'on site'.

Hope this helps, David.

[common763]

are you referring to the screw that locks the focus in?  If so I played with that and if any give the focus wont move, but any looseness and the entire focuser just sinks down.  I am an amateur at this so apologies in advance if not explaining properly.  

[Moonshane]

it will be the one that's in the centre of the other four on the pic below. it will need a very small amount of tightening - just turn it a few degrees at a time until the focuser does what you need. if yours is not exactly the same then it will be the central one. the silver thumbscrew effectively locks the focus by applying even more (supposed to be temporary) pressure 

[Moonshane]

https://www.google.co.uk/search?q=skywatcher+crayford+focuser&safe=active&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjxtLPg07rcAhUpJ8AKHTXSCBcQ_AUICigB&biw=1745&bih=861&surl=1#imgrc=8InEsdLM-8ocPM:

shows the range of focusers which are all made on the same sort of theme.

[Moonshane]

ps don't panic. if it can be adjusted wrong, as long as you don't drop anything, it can be adjusted right,

[common763]

so I just looked.  the four corner ones have screws jutting out.  The center one is a hole and I cant tell if there is a smaller screw in there or if it fell out.  SHould that screw look close to the ones in the four corners or is it different?

[common763]

ok so there is no screw in the middle.  Could that be the culprit of it sliding everywhere?  Confused how that fell out.

[Geoff Lister]

Yes; there should be a grub screw in the centre hole. It presses on a block incorporating a Teflon pad, which in turn presses the focus-adjusting spindle on to the underside of the main draw-tube. Your symptoms indicate that it is indeed likely that the grub screw is missing.

Geoff 

[common763]

thanks so much for everything to all who posted.  Skywatcher is sending me a replacement screw.  Much appreciated.  It must have fallen out or my child pulled it out?  Who knows.  Thanks again.

[Moonshane]

It's a standard grub screw probably removed by your inquisitive lad assuming it worked before. No harm done. In the meantime try the silver thumb in the same hole. It might work. Just tighten by hand enough to hold it. May allow some scope use pending the delivery

[Knighty2112]

22 minutes ago, common763 said:

thanks so much for everything to all who posted.  Skywatcher is sending me a replacement screw.  Much appreciated.  It must have fallen out or my child pulled it out?  Who knows.  Thanks again.

In the famous words of Bart Simpson, you would only have got something along the lines of “I didn’t do it. Nobody saw me do it. You can’t prove anything!” from your son any how. Always got that from my son when he was growing up!  

[Geoff Lister]

11 hours ago, common763 said:

Skywatcher is sending me a replacement screw

It is still worth doing another check around the area where your son "didn't" dismantle the focuser. The other area to check, is that it did not fall down inside the lower section of the tube. An unattached screw can do nasty things to primary mirrors.

Geoff

[Louis D]

Better watch that son to make sure he doesn't disassemble something more critical in the future.  It sounds like he has a good grasp of tool usage because the outer screws require a screwdriver to loosen while the center one requires the proper Allen wrench/hex key.  Next time it might be the lug nuts on the car's wheels. ?