Quote:
Originally Posted by multiweb
Don't adjust the spider once you have aligned everything. It's supposed to be offset. That's because the diagonal must be slightly offset away from the focuser to intersect the light cone coming from the primary. It confuses a lot of people because it's the reflection of the secondary into the primary (third reflection). It's more apparent on a fast scope with a small aperture and a large secondary. The laser is good for the tilt of the secondary into the primary but always check visually with a drawtube or cheshire as it won't give you any info about the rotation of the secondary, unless you have a large DOB and you use a holographic laser (the one that projects a grid).
First offset the secondary away from the focuser by a distance which is calculated depending on your aperture and F ratio. Usually a couple of mm. Then don't touch the spider anymore.
1_ center the secondary under the focuser (center it both sideways and along the length of the tube).
2_ adjust the secondary tilt and rotation
3_ adjust the primary
Loop through 2 and 3 until you get closer and closer. Finish with a start test by modifying the tilt of the secondary with the 3 screws.
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Hi Marc,
I find some of what you've said about the offset hard to understand.
What is the difference between a drawtube and cheshire collimator?