Collimation showing 2 laser dots on Primary.

[Space Cowboy]

Decided to collimate my 2nd replacement Skyliner flextube 250 Dob (first 2 were delivered damaged). Put laser in focuser and looked down OTA but there are 2 laser dots showing on the Primary. Does this mean the Secondary needs turning (by loosening the centre screw)?

[brantuk]

Hmmm.... sounds odd - you usually get two dots on the secondary when the primary is off - not heard of it the other way round, sounds like the laser might be duff. You should get a single beam only with a single dot unless its reflected from somewhere making a second.

[Space Cowboy]

The previous scope had only one laser dot showing on the primary so I don't think its a faulty laser. Hang on though am I looking at the secondary mirror reflecting in the Primary?

[Jason D]

Turn on the laser and point at a far away wall (no scope is used in this experiment). How many dots do you see on the wall? Avoid rooms with wall mirrors.

Jason

[Space Cowboy]

Just the one dot.

[brantuk]

So the laser beam (mounted in the focuser) should bounce off the secondary at 90 degrees straight down onto the primary showing a single dot.

If the primary is tilted out of collimation it will bounce back up onto the secondary at an angle showing two dots on the secondary. Adjust the primary till the beam is reflected straight back up to a single dot on the secondary then you'll be very close.

Simply then look at the target and adjust the primary till the dot dissapears down the central spot on the target and you're collimated.

[sixela]

I think you're simply looking from the front and seeing the forward beam and also --in the reflection of the secondary-- where the return beam hits the laser collimator again in the focuser (there's actually sometimes a third and fourth spot to be seen indirectly as well through the primary, and that is where the forward and return beams hit the secondary).

Here's an image from my Dutch collimation wiki:

You can see the spot on the primary ( directly), then (through the primary and after ignoring two small dashes) the spot where both forward and return beams hit the secondary, and finally, through the primary and then the secondary, where the return beam hits the laser collimator again (note the dark doughnut, the silhouette of the centre spot in the return of the diffraction pattern around the central beam on the laser collimator's face, used in barlowed laser collimation. It's actually a better reference than the return beam itself!)

Obviously it's the spot made by the forward beam you're interested in when setting the tilt of the secondary. The return beam's spot on the laser collimator is something you can try to centre on the laser collimator by adjusting the primary's tilt, but that's really only good for rough collimation of the primary's tilt. Use either a Collimation cap, a Cheshire or the barlowed laser collimation protocol to set the primary's tilt precisely.

[Space Cowboy]

Cheers guys I shall take another look. Thought I'd better ask on here before making the wrong adjustments and making things worse.

[SirFoo]

Hi Space Cowboy,

I also collimated ny newtionian for the first time last evening using a laser. It is a bit nerve racking but i think its better. Did you notice a real difference to your views? It's hard to tell!

[Space Cowboy]

I've never collimated with anything else as my other scope is a Mak so this is completely new. The idea of getting a laser was for convenience as the flex-tube Dob needs tweaking every time its used.

Had another look today and it looks ok just the one dot.....maybe I was looking at a funny angle before. The true test will be when I view a star.