Even more collimation problems

[vinny]

Below are pics of my laser collimator in two different scopes

the first is my 8" dob (out of collimation to show laser spot) as you can see it is a good small laser footprint on the target screen

the second is the same collimator on my 4.5" newt all i seem to get is a large smudge of light on the target screen (i have tried different laser power settings but they all show the same effect)

any ideas on what can be causing this ?????

by the way the viewing of stars and planets through the 4.5" is crisp so i dont think it is two far out.

I have also tried it with the focuser wound in and would out and all points between

[beamish]

Vinny, I'll be blunt- don't trust the collimation of the laser unless you've shown it to be collimated ,it'll screw with your noodle !! Been there- sold the laser.

I now use a good colly cap, sight tube and cheshire !

[Starflyer]

Been there- sold the laser.!

Me too Karlo

[John]

Is your 4.5" newt one of those catadioptic ones ? - either with a flat piece of glass at the front of the scope where the spider would normally be and / or with an additional corrector lens at the bottom of the eyepiece drawtube ?.

I'm just wondering because I'm not sure that a laser collimator will work with cat-newts.

John

John

[vinny]

no just a straight forward mewt (celestron astromaster 114)

[Doc]

I remember reading somewhere that the newt secondary mirror is a convex one so the return beam doesn't meet correctly and hence the spot is rather large and un precise.

[John]

no just a straight forward mewt (celestron astromaster 114)

If that is the one that has a focal length of 1000mm (F/9) then I think it either uses a correcting lens screwed or pushed into the lower end of the focusser (the end nearest the diagonal) or a convex diagonal mirror as Doc says. They do this to squeeze the longish focal length into a shortish tube.

Either way will affect the way a laser collimator beam is focussed (or not) when it gets back to the target on the collimator.

I'm with the others - I've given up with laser collimators and use a simple cheshire eyepiece for my 12" dobsonian.

John

[vinny]

just checked and you are right there is a lens buried at the end of the focuser tube

thanks everybody for all your help and comments

[Whippy]

What you may be better off doing is star testing to get your collimation right. Have a look at this: A Star Test Primer .

Tony..