Checking collimation of a refractor

[steve_bham]

Is it easy to check the collimation of a refractor?

I have the SW 120 mm and it doesn't have an adjustable front lens.

Views are pin sharp but want to learn more etc.

[Peter Drew]

Steve. A refractor without an adjustable cell will have been collimated by the manufacturer, refractors tend to be robust so this situation should be maintained. Checking collimation is best done on a star. If you have access to a laser collimating tool it's a good idea to check the alignment of the focuser. With the laser fitted in the eyepiece holder the beam should exit through the centre of the objective. Similarly, it's worth checking the diagonal as these are sometimes not properly collimated, just place it in the scope and try the laser in it and see if there is a difference.

[steve_bham]

Thanks, I've read about adjusting the focuser. Which will be helpful when I save my pennies for a moonlite!

[John]

I use a laser collimtor plus a cheshire eyepiece to check the collimation on refractors I've owned.

The laser collimator is used to check that the central axis of the focuser drawtube is in line with the optical axis of the objective lens - the laser spot appears in the centre of the objective lens as it exits the lens if all is aligned OK. If it's off to one side or another then the focuser needs to be tilted a bit.

To check that the objective lens is correctly adjusted you leave the objective lens capped and put the cheshire in the focuser (no diagonal). With a light shining on the angled face of the cheshire, looking through the cheshire you will either see a single small circle of light reflected back up the tube from the objective lens which means the collimation is OK or you will see two small circles of light either touching or separated a bit which means the collimation is off and the lens needs to be tilted slightly. There used to be a nice web page explaining all this but it seems to have shut down now.

Sorry if this repeats some of what Peter has already said !

[steve_bham]

I use a laser collimtor plus a cheshire eyepiece to check the collimation on refractors I've owned.

The laser collimator is used to check that the central axis of the focuser drawtube is in line with the optical axis of the objective lens - the laser spot appears in the centre of the objective lens as it exits the lens if all is aligned OK. If it's off to one side or another then the focuser needs to be tilted a bit.

To check that the objective lens is correctly adjusted you leave the objective lens capped and put the cheshire in the focuser (no diagonal). With a light shining on the angled face of the cheshire, looking through the cheshire you will either see a single small circle of light reflected back up the tube from the objective lens which means the collimation is OK or you will see two small circles of light either touching or separated a bit which means the collimation is off and the lens needs to be tilted slightly. There used to be a nice web page explaining all this but it seems to have shut down now.

Sorry if this repeats some of what Peter has already said !

Thanks for that, can you recommend a laser and cheshire eyepiece that could be used on a frac?

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[John]

Thanks for that, can you recommend a laser and cheshire eyepiece that could be used on a frac?

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I use a cheap plastic short cheshire and a Revelation laser collimator. Both low cost items and I did need to collimate the laser before it was any use but they both do the job well now.