laser collimator central in eypiece holder why?

[tms48]

I am looking to get a laser collimator and It seems that the ideal is to have the collimator central in the eyepiece holder. and Hotech make a great play on this.

When collimating I line up the secondary with a cheshire using the lock screws, so it will not be exactly central, this also applies when using eyepieces.

Because of this would you get better collimation when the laser is inserted in the holder in the same position as the cheshire and eyepieces. The holder itself does not centralise accesories used, so can someone please explain the advantage of centralisng the laser collimator.

[Bizibilder]

If you collimate off centre by using the lock screws you will be perfectly OK as long as all your other attachments (eyepieces etc) are off centre by the same amount and in the same direction. However, in the real world, most things are manufactured to tolerences and therefore nothing will be "perfect" - unless you are very lucky! This is why "posh" focussers do have a mechanism that holds whatever you are using centrally. Ordinary "lock screw" types don't.

So - carry on and collimate with the lock screws - you won't be far out. However for the best results don't allow any of the focusser components to rotate and use any attachments "the same way round" - if you see what I mean.

[acey]

With a Cheshire it's not critical that you're exactly centred in the focuser - with a laser it is. With the Cheshire in place and the scope collimated, try pushing gently sideways on the Cheshire to move it a tiny bit out of alignment - you'll see that the reflected spot doesn't move noticeably. Do that with a laser and the return beam will go way off. This is why it's very easy to get bad collimation with a laser, and why many people (myself included) prefer to use a Cheshire. I find a laser handy for aligning the secondary - I use a cheap Antares model. If I nudge the laser then the spot goes out of the centre ring on the primary - but that's OK, there's room for some error when aligning the secondary.

[Damo636]

If you are going to use a laser to collimate I would recommend one of these. Orion 1.25" Self-Centering Adapter :: Collimators & Bahtinov focussing masks :: Altair Astro. I have the Antares version which is exactly the same. Much better than stock thumbscrew adapter.

[acey]

I also use the Antares adapter - only because I got fed up turning thumbscrews when changing eyepieces. The adapter is far more convenient - as long as you're using 1.25" eyepieces in a 2" focuser.

But with a laser inserted in the adapter, it's still the case that if I give it a nudge or turn it, the spot moves.

[Damo636]

I also use the Antares adapter - only because I got fed up turning thumbscrews when changing eyepieces. The adapter is far more convenient - as long as you're using 1.25" eyepieces in a 2" focuser.

But with a laser inserted in the adapter, it's still the case that if I give it a nudge or turn it, the spot moves.

I have to confess that i don't use a laser as i don't think i could trust one 100%. It sounds like your laser may be out of collimation. The beam should stay more or less on the same fixed point when rotated in the adapter.

[malc-c]

For an experiment I placed my Hotech in the holder and then screwed the holder onto the adapter / focus ring. The dot remained central through out

However I can see the argument regarding the slight off axis you get when using thumb screws, provided the user applied the same force to each eyepiece or attachment every time and that the postion of the thumbscrews did not change orientaion.

Personally I think we all get too hung up on this colimation business !

[Damo636]

I totally agree Malcolm. No such thing as perfection. If its close its good enough imo.