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How exact does the collimation need to be?


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I am collimating my 10" dobsonian flextube with a laser collimator, but in not really sure how exact i need to bee. I can easily center the laser on the middle dot on the main mirror, and then center it on the "bulls-eye" on the collimator. But there is enough "slopp" in the mechanism/telescope/collimator that the collimation changes slightly every time i put the collimator in the eyepiece holder. It also changes slightly when i change elivation, probably due to too much play in the eyepiece holder. Slightest push on the collimato changes the laser spot. It also changes slightly if i rotate it.

The laser is still within the smalles circle, but is it enough?

How much off can the laser be on the bulle-eye, before the view is not longer usefull? 

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Edited by Aston
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As exact as you can without spending too much time worrying about it, trust me on this on, from a recovered collimation addict. once it looks as close to centre as you can make it, trust your work and have a look

through the scope. If the views are great then great! collimation can be all consuming and, even worse, can lead to to make unnecessary adjustments which will open a collimation black hole.

Sorry to rant but, i've been there before.

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5 minutes ago, Aston said:

there is enough "slopp" in the mechanism/telescope/collimator that the collimation changes slightly every time i put the collimator in the eyepiece holder

Hi Aston,

you nailed right there! Indeed the play in focusers (and 1.25" - 2" adapters) will always mess up the alignment. For this I only use the laser to get the secondary right (for my SW 300PDS that is) and then a Cheshire to see if the primary is fine as well (although I always start with a coarse check with the laser). When using the laser I always rotate it in the focuser, while keeping it pressed against it as even a well collimated laser may still produce a small circle when turning it around in the focuser (this may depend on make). I also test it in various focuser-positions as the focuser too may not always point in the same direction is my experience. In the end I aim at an average collimation. Recently I borrowed a Catseye 2-pupil Infinity XLKP Auto-collimation eyepiece from a friend and I have to say that collimating your scope with that is much more reliable. Please note that the altitude of the scope may also slightly affect collimation (it is noticeable in my 300PDS).

I have no idea how well you should collimate to not have a significant degrading effect of it, but perhaps others may know.

Nicolàs

 

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Your collimation based on the photos is fine, probably better than mine is a lot of the time. I agree with @Sunshine don’t stress about it too much.

As a side note, the slop you refer too is common in cheaper lasers. There are others such as the HoTech that use a compression ring to keep the collimator secure and central in the focuser 

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Having tried with a laser and then a Cheshire collimator my conclusion is there's a risk of relying on a laser that is out of collimation itself . I now realise my eyes are better than the laser.

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Unless you are using a self centreing adapter or a Hotech laser  that self centres and locks into place then you will always be chasing your tail as relying on the normal locking thumbscrew will push the laser off centre....

Visually collimation is a lot more forgiving, but for imaging it helps to have it as near perfect as possible

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Lasers should only be used as a guide. They aren't accurate enough the achieve perfect collimation.

Once you are close, you can get collimation perfect with a star test.

Collimation, even slightly out, will reduce contrast and smear fine detail. Anyone seeing Mars as a featureless disc at the moment needs to check their collimation :wink2:

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I will add that if you have a Barlow lens then the laser Barlow trick can work very well as a collimation aid/check and is quick and easy for no extra money - there's lots of guides if you do a search on SGL.

But the comments on focuser stability etc will always be true. I found one of the best upgrades to my 200PDS was to replace the focuser with a Baader Steeltrack Diamond - the improved rigidity and stability made collimation much more robust.  If you're routinely using extension tubes etc then those will also play a role - the TeleVue extension tubes I've found to be very good in this regard but there's other good ones out there which will be a big improvement over the included ones, and none will break the bank (even the TV ones are only ~£50 iirc)

There'll always be something, though, so get it close enough that you're happy with the views and enjoy would be my strong advice!

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