Quote:
Originally Posted by Cluster
The GSO laser is so easy to use for both the secondary and primary that honestly I don't know why anyone would use something else.
Anyway, that's my experience :-)
|
Well the GSO laser might be very easy to use but unfortunately life isn't that easy
You cannot properly collimate a telescope from scratch, with just a laser only. The laser cannot help with setting the secondary distance from the primary mirror. You can only do this with a sight tube or if you're good, eyeball it. Unfortunately things get worse, if the secondary/primary distance is not correct it is still possible to adjust the tilt of both mirrors and get the laser striking back perfectly upon itself, thus making the telescope appear properly collimated, when in fact it is not. Whilst the laser would show the telescope as collimated in this situation, a sight tube or star test on both sides of focus would show it to not be collimated. A laser is only useful to adjust the "TILT" of both the primary and secondary mirrors once the secondary is properly positioned in terms of its distance from the primary and in its rotation.
Once you have the telescope properly collimated a laser does an excellent job of adjusting the tilt of the mirrors. The barlowed laser method is only useful for adjusting the tilt of the primary mirror.
The best test as to whether your scope is properly collimated, is to check it with a bright star. I always do this several times during the night as the mirror cools, or shifts slightly with sling movement. A defocused star image on either side of focus contains an enormous amount of information about the scopes collimation and its optical quality. I don't need to go into all that here as Dick Suiter wrote an entire book on it. In very very simple terms follow whats bellow:
Use an eyepiece giving 10X to 15X per inch of aperture, if seeing will allow, less power if not.
Defocus the same distance both sides of focus until you get a diffraction pattern showing "rings". The diffraction rings should appear "PERFECTLY CIRCULAR" and not egg shaped. The secondary shadow should be perfectly centered in the diffraction rings on both sides of focus. If the secondary shadow position in the diffraction pattern appears to change from one side of focus to the other, the secondary position is incorrect in its distance from the primary. In other words, it is not properly aligned with the focuser.
Cheers
John B