Collimation - Lazer V's Eye

[John]

Following the recent thread on the best collimation tools, which concluded that the non-lazer devices are better, I thought I would try and collimate my 8 inch dob by eye rather than using the lazer collimator - and compare the results.

To ensure that my eye was centered in the drawtube I used an old 4mm eyepiece with the lenses removed (it's more useful now than when it had lenses !) and carefully adjusted the secondary and then the primary until I had, what looked to me, concentric circles with the spot on the primary smack in the centre.

This evening I had an opportunity to use the scope under the sky for about 40 minutes - so it did not really have enough cool down time, but despite this the star images at focus and inside and outside of it looked very nice - towards the end of the session I was getting a nice set of more or less concentric diffraction rings when I viewed Procyon. I could split Algieba quite easliy at 110x and Saturn viewed at the same power showed nice detail and 5 moons. My guess is that collimation is not perfect but quite OK for everyday use.

The above performance is similar to that which I got after collimating with the lazer collimator as well - BUT when I put the lazer collimator back in the scope following my collimation by eye, the laser spot on the primary was about 1.5 cm off centre . The lazer did appear to be re-entering the hole in the collimator though so I guess the scope is in reasonable collimation.

I just can't understand why the lazer misses the centre of the primary when I have collimated by eye, despite the images being satisfactory and yet when I collimate using the laser, and the laser spot is in the centre of the primary, and re-entering it's exit hole in the collimator, the scope also gives good images - I guess the difference between the 2 is the angle of the secondary because (as I understand it) it's that which determines when the laser spot falls on the primary.

Sorry for the longish ramble on this - I'm not too worried because the scope is performing well using either method (very well considering I practically used it as a grab and go this evening with very little cool down time). I'm just suprised that the lazer collimation and eye collimation don't tally - perhaps I shouldn't be

John

[jarbi]

John,

One possible reason could be that the laser collimator is not collimated. I mean, the leaser beam is not running in the optical angle of the collimator. You can check this by simply turning it in the focuser. In my case it was really necessary to collimate the laser beam, now it gives almost the same result as the collimating eyepiece. I am still waiting to receive my cheshire, that will be the ultimate comparison.

janos

[geppetto]

One possible reason could be that the laser collimator is not collimated

I would second that

Plus, with my lazer device, it's such a sloppy fit in the focuser, it could

never be very accurate.

Handy for a quick check in the dark though.

[John]

Thanks for the suggestions guys. I've checked the collimation of the laser itself - I can rotate the collimator around a fixed point and there is virtually no movement in the light beam projected on a sheet of paper 20 feet away - which I think indicates that it is pointing pretty well down the axis of the collimator.

The sloppy fit is probably an issue - though it's no worse than my eyepieces.

Maybe I'll swap it for a non-laser cheshire and see how that looks - any recommendations for one which is particularly good with newtonains ?.

John.

[Andrew*]

I've just successfully collimated my scope with the cheshire I just got from FLO. It's a very nice piece of kit and fairly easy to use. http://www.firstlightoptics.com/proddetail.php?prod=aecheshire (don't tell steve, but there's one of these in scopes n skies bargain zone)

The only tiny gripe I had about this cheshire (don't know about other ones) is that the FOV is so small that you have to move your eye in order to see the edge of the primary mirror.

Just an idea for that sloppy fit you're talking about - it shouldn't matter too much because the focuser should hold the collimator in exactly the same position as it holds eyepieces, which is the whole point.

Andrew

[The Warthog]

I would suggest the problem is with the secondary. It should be throwing the beam onto the centre of the primary, and you should make that adjustment first. Does the secondary need to be moved a little up or down the tube to get it in the correct position under the focuser?

OTOH, if it is giving good images, maybe you shouldn't fix it.

[John]

I would suggest the problem is with the secondary. It should be throwing the beam onto the centre of the primary, and you should make that adjustment first. Does the secondary need to be moved a little up or down the tube to get it in the correct position under the focuser?

OTOH, if it is giving good images, maybe you shouldn't fix it.

I wondered about the postion of the secondary as well WH - but your last point is foremost in my mind at the moment - I'm going to resist the tempation to fiddle too much while the images look nice.

Thanks for the reminder of what REALLY matters ...

John