Collimation confusion

[Stu Ward]

So I have this GSO Laser Collimator and Bobs knobs on the secondary of my 8" Dob

Collimation appears to be good, although i can never quite get the laser beam into the middle of the donut on the primary.

Stars appear to be sharp moon and planetary seems good.

But what bugs me is that if i look at a star totally out of focus the shadow of the secondary cast on the blurry star is nowhere near the middle ( as basic collimation methods say it should be )

So is my dob collimated properly or not ?

Stu

[bmitchell82 (Brendan)]

Question 1. have you collimated the laser?
Question 2. when you check for the concentric shadows is your star is it in the dead center of the FOV if not this stuffs the whole ball game!

Nobody could tell you for certain if your collimation is good or not without actually seeing it first hand.

Hope that helps for a start.

[mozzie (Peter)]

hi stu are you useing the screws to adjust your primary mirror put the laser beam into centre of donut,once it centre then you collimate secondary then the mirrors are aligned

[Stu Ward]

Originally Posted by bmitchell82

Question 1. have you collimated the laser?
Question 2. when you check for the concentric shadows is your star is it in the dead center of the FOV if not this stuffs the whole ball game!

Nobody could tell you for certain if your collimation is good or not without actually seeing it first hand.

Hope that helps for a start.

Q1 I put the collimator in a V block and it appears to static when i rotate

Q2 I must admit no it was not dead centre, more centre bottom left lol

[Stu Ward]

Originally Posted by mozzie

hi stu are you useing the screws to adjust your primary mirror put the laser beam into centre of donut,once it centre then you collimate secondary then the mirrors are aligned

I actually collimated it by doing the seconday first, then the primary, then rechecking the seconday

Stu

[bmitchell82 (Brendan)]

That is normal practice so you are right, i think it will be that your star that you where checking wasn't in the middle of the FOV. its a simple fact that although the defocused star does infact fill 80-90% of the FOV its not centered itself.

Just so you can see what im saying go in a dark room with a torch.

Move the torch around your hand you will see the shadow of your hand moving with the torch. Even though the star is stationary your tube moves so the star comes in on different angles to your primary hence throwing the shadow to a different side.

[mozzie (Peter)]

sorry stu yes i worded it wrong had the kids screaming in my ears!!!!!! when im aligning secondary i always put laser beam into centre of donut then primary into centre of laser in focuser!!your saying secondary doesnt go into donut once my secondary was a fraction loose and had to nip up screw before collimating this might be happening to yours

[Jason D (Jason)]

Originally Posted by Stu Ward

although i can never quite get the laser beam into the middle of the donut on the primary.

Why? Are some of your Bob's knobs getting too tight which prevents you from moving the beam further to the center?

But what bugs me is that if i look at a star totally out of focus the shadow of the secondary cast on the blurry star is nowhere near the middle ( as basic collimation methods say it should be )

It sounds you are defoucsing too much. In this case, the secondary shadow will indeed look shifted especially for fast scopes. To evaluate collimation, you are suppose to defocus by a small amount at high magnification -- enough to see only few rings .

Jason

[Stu Ward]

As it was so cloudy last night i spent some maintenance time on my scope.
Making the swivel motion more fluid and a good look at the collimation

After loosening the secondaries and then re-adjusting i got the beam nicely in the middle ( thanks for the tip )

And when looking down the focusser without an EP in the view looked remarkably like your pic Jason D with the inner circle of reflection (my eye ) just offcentre within the black disc.

Is this right ?

Thanks

Stu

[Jason D (Jason)]

yes, that is normal