Posted 11 January 2013 - 06:46 PM
If this picture is AFTER collimation, you're still way out.
And it's a sign your laser is way out of collimation itself.
Here are my typical laser collimator instructions:
SIMPLE INSTRUCTIONS ON COLLIMATING WITH A LASER
1. Primary mirror must have some form of centermark. A “donut” ring is the most common, but other shapes will work. It is recommended that the centermark have an opening in the center, i.e. not be solid.
2. Stand back and eyeball the outline of the secondary mirror as seen through the focuser. The secondary mirror should be adjusted up and down the tube and rotationally so that it appears round and concentric with the inside of the focuser. High precision is not necessary—close is good enough.
3. Insert the laser and tighten the setscrew. Turn the laser on. The beam should hit the center of the primary mirror. Adjust the secondary collimation screws to achieve this.
4. Modify a Barlow lens so there is a paper cover over the bottom of the Barlow. A cardboard plug will work fine. Make sure there is a hole of 1/8” to 3/16” punched in the center of the cardboard so the laser light can get out. Insert the laser in the Barlow and secure it with the screw. Insert the laser + Barlow in the focuser, secure it, and turn it on. Note: Howie Glatter offers a laser with a white surface barlow attachable to the bottom of the laser which serves the same function and is a lot more convenient.
5. The return beam of the barlowed laser will be a diffuse red glow containing the shadow of the primary mirror’s centermark. What you need to do is adjust the Primary mirror to center that shadow on the bottom of the Barlow (now you understand why the cardboard is there), making the shadow surround the small central hole in the cardboard plug in the bottom of the Barlow. This is called the Barlowed Laser Collimation protocol, and it is the MOST ACCURATE way to use a laser to collimate a primary mirror. The return beam of a simple beam laser is simply not accurate enough to achieve collimation of a Newtonian scope. The bottom of the Barlow may be difficult to see if it is very deep in the focuser, so use a small hand mirror opposite the focuser so you can see the red glow and the mark shadow on the bottom of the Barlow.
6. Howie Glatter makes a Barlow with a white face that inserts in the bottom of the focuser (on the inside of the scope). It’s called a BLUG. This Barlow’s bottom can be seen without a hand mirror. But the white cardboard on the bottom of a barlow works just as well.
Why can’t the return beam of a single beam laser be used to collimate the primary?
Because in order for the return beam to be accurate enough to collimate the primary, you have to have the out beam hit the center of the primary to better than 0.01”, which is a difficult level of precision to achieve when looking down the tube from the front to see where the laser beam hits the primary mirror. Plus, most laser beams are larger than that where they hit the primary mirror.
Howie Glatter lasers are expensive. Can’t I use a cheaper laser?
Yes, you can. But the cheap lasers in the market have 2 problems: 1) they come out of the box miscollimated themselves, meaning they cannot be used to align the optics because their beams are tilted relative to the line of the barrel, and 2) Their beams do not end in a point but often in a small rectangle or oval which has enough size to it that estimating the point where it hits can be a problem. FarPoint and Glatter lasers have small beam apertures that result in tiny pinpoints where they hit the mirror, and they are both internally collimated to better than 15 arc-seconds, making them more than adequately precise to align optics. You can use less expensive lasers to collimate with, but only after you have spent quite a bit of time to collimate the laser first. Even after collimation, their beam patterns are somewhat imprecise, being large dashes instead of small points. Inexpensive lasers offer no advantage for collimation over a low-cost collimation tool like the common sight tube + cheshire collimation Eyepiece (which might work better).
Do I have to Barlow the laser to check the primary in the middle of the night?
No. Once you have achieved good collimation with the barlowed laser technique, you can check the primary’s alignment in the middle of the night with the simple return of the simple beam to its source.
Just remember this will not reveal tiny errors, just large ones.