Why is the laser and cheshire collimation differant?

[pat]

I've centered the secondary under the focuser. And the primary mirror and clips are showing in the secondary. Then I collimate the primary with the cheshire eyepiece. So it all looks collimated.

To further check the collimation I put in a laser (no port). The beam is on the donut ring (not centered). The beam on the ring does not move when the laser is rotated in the focuser tube. When racked in and out (where my eyepieces are used) the beam stays in the exact spot.

What might explain this? And what is the best way to fix it? Pat

[Damo636]

It all sounds good to me Pat. The fact that the laser is staying in the same spot when rotated and racked in and out tells you that the laser itself is well collimated and your focuser is nice and square. A minor tweak of the secondary will have the beam hitting the centre of the doughnut.

[Tinker1947]

Check the laser using a piece of wood with some nail/screws set to make 2 "V"'s then aim the laser at a wall some distance away, turn the laser in the V-Groove the red dot should stay in the same place, assumes your laser is a tube type.

[blinky]

I had the same issue last night, collimated with a colli cap and cheshire and all looked spot on. Stuck in my laser (which I know is good) and collimation was a cm or so off!

Mind.... when I did the laser collimation then re-checked with the laser it still looked good so maybe a cheshire is not as accurate as a laser?

[Damo636]

I find the laser the most accurate way to align the secondary. I find it easier than trying to focus the Cheshire crosshair but i always use the Cheshire to final check the primary.

[Bart]

I would trust the Cheshire more than the laser. I have a laser and find it good to get near the sweet spot. I'm happier when the Cheshire says I'm there.

Bart

[acey]

I've centered the secondary under the focuser. And the primary mirror and clips are showing in the secondary. Then I collimate the primary with the cheshire eyepiece. So it all looks collimated.

To further check the collimation I put in a laser (no port). The beam is on the donut ring (not centered). The beam on the ring does not move when the laser is rotated in the focuser tube. When racked in and out (where my eyepieces are used) the beam stays in the exact spot.

What might explain this? And what is the best way to fix it? Pat

The centre ring on the mirror may not be exactly centred. But if that's the case then it doesn't matter: the cheshire and laser are both telling you the scope is perfectly collimated.

[pat]

Never thought about that. Thanks, Pat