...or you experience the 'squinted collimation' David07 is describing in his post http://www.cloudynig...read/?p=9224176
His method with a cheap 58 mm cardboard disc works like a charm:
"In my experience, the collimation of the GSO RCs, that is the design where the focuser extension tube is attached to the primary mirror support, is extraordinarily difficult. Any method based on using a laser or Cheshire inserted into the focuser is immediately compromised because of the lack of a fixed reference in the telescope design. There are unknown alignment errors between the primary mirror and it’s holder and the mirror holder and the focuser. Moreover, moving the primary mirror, when collimating the scope, also changes the pointing direction of the focuser. I spent 18 months fiddling about with my RC8 until I read the Austria Teleskop blog and that gave me the idea for the collimation method I now use.
I began by taking the scope to an optician friend with an optical bench and artificial star. The first discovery was that after I had spent weeks of adjusting the secondary to primary separation to get the specified focal length of 1624mm, a Ronchi grating test quickly revealed that the scope was overcorrected: the mirrors were too far apart. We adjusted the mirror separation until we got a properly corrected image. The focal length was 1660mm, as it still is. So problem number one, the actual focal length might not be as advertised.
To collimate the scope:
With the scope on a bench, I removed the focuser and extension tubes. I then measured, with a calliper, the inside diameter of the central hole in the primary mirror holder. I cut a piece a styrene about 2mm thick to exactly fit the hole and drilled a 1mm hole in its centre. Now gently fit the styrene disc into the central hole of the primary mirror holder.
Next, mark the position of the central screw holding the secondary mirror. A little dab of white acrylic paint, for example. Also, mark the side of the secondary boss and mirror holder. This will enable you to replace the secondary exactly in the correct place and not alter the scope focal length.
Carefully unscrew the secondary mirror central screw, holding the secondary mirror. Count the number of turns to release the screw and note the number. Don’t touch the three collimation screws.
Place a light behind the telescope and looking from the front of the scope, sight the hole in the centre of the styrene disc though the centre of the hole that the secondary securing screw came from. Now adjust the primary mirror so that the reflections of the secondary support vanes coincide with the support vanes themselves. You should see a tiny spot of light in the centre of the secondary screw hole and the reflections of the support vanes will be hidden behind the vanes themselves.
The primary mirror is now aligned with the secondary holder.
Replace the secondary mirror. Count the number of turns and align the paint marks.
Next, reach into the scope tube and unscrew the baffle tube. Let it rest on the inside of the tube.
Now bring the lamp to the front of the tube and set it to shine onto the styrene disc.
Go go to the back of the scope and look through the small hole on the centre of the styrene disc. You should see the central alignment ring on the secondary. Adjust the secondary until the reflection of the hole in the centre of the styrene disc is centred in the secondary alignment ring.
The secondary is now pointing at the centre of the primary mirror.
Replace the baffle tube. Replace the focuser etc. Sell your laser collimator.
Check the collimation on a star in the centre of the field of view. It should be really close.
On a night of good seeing I get HFD readings of 1.6 to 1.7 arc seconds at focus. Previously I couldn’t get below 2.5 arc seconds. I don’t use a tilt plate and I’m pleased with the images I get.
Hope this helps,