Quote:
Originally Posted by Merlin66
John,
I can only quote the most knowledgeable expert I know on collimation, Jason D.
Spotting the secondary mirror was never the norm and it should not be the norm. Following the collimation steps using a cheshire eyepiece will get the job done without the need to spot the secondary mirror.
Jason
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Quote:
Originally Posted by multiweb
+1 for JasonD. I can't understand the need to spot the secondary either. There are two many variables involved in positioning the secondary: twist, offset, centering under the focuser and on the tube axis. Also a mark on the secondary would interfere with the various reflections of the primary center spot.  |
Hi Mark/Ken,
At no point in time did I say you used the center spot on the secondary mirror to collimate the telescope. I just said a center spotted secondary was more common than you think, particularly with the builders of premium truss tube dobsonians. Further, at no point in time did I suggest anyone collimate a telescope in this manner.
In fact the center spot on the secondary mirror is used by "some" of the builders of premium truss tube dobsonians to correctly position the spider vanes and secondary holder in relation to the focuser, prior to the telescope ever being assembled for the first time. ie. When you have the Upper Cage Assembly (UCA) sitting on a bench and the primary mirror and mirror box not even connected. It's good to use this method because it allows you to make the first cut on the length of the truss poles a lot more accurately than just guessing where the secondary is going to finish up. You can use a sight tube, but many telescope builders prefer to use a center spotted secondary, as opposed to a sight tube. A Cheshire under these circumstances is pretty useless as you are missing a few reflections. Using a laser and the center spotted secondary method allows you to properly square and position the focuser on the UCA prior to positioning the secondary and secondary holder.
Once this is done it is highly unlikely you will ever use the center spot on the secondary mirror, ever again. You certainly don't use it to collimate the telescope when it is assembled.
When you subsequently use a Cheshire to collimate the telescope, or a laser, or a barlowed laser for that matter, you don't even see the center spot on the secondary mirror because of its position in relation to the focal plane.
While Jason D is very knowledgeable on collimation, there are plenty of people that know just as much about collimating a telescope as Jason D. Some of them were accurately collimating telescopes long before Jason D was born. The difference is they don't spend hours of their spare time writing about it and posting on the internet. A couple of them in fact are members of this forum.
Cheers,
John B