Welcome, timelord.
Hmmm…I’ll go out on a limb here and ask, based on your descriptors below, whether you are using a barlowed laser collimator or not. You mention only the laser beam dot down to the center mark on primary, so it really sounds like you are not using a barlowed laser collimator. I’ll apologize, because it sounds like you were modestly misled by some of our collimation thread lingo. Due, no doubt, to our increasingly abbreviated vocabulary and tendency to condense descriptors. In this case – if I’m correct and if I’m not I will dummy slap myself as penance - I think you may have interpreted laser collimator for the somewhat different tool, the BARLOWED laser collimator. These are lasers with barlow lenses right in front of the laser. Many are commercially made mini-barlows that screw in to the end of the laser.
If you have a laser WITHOUT the barlow attachment, you are assessing the position of the secondary relative to the primary. Bouncing that laser beam off the secondary and making it hit the primary center mark. Now the angle of the secondary is aligned okay relative to the center of the mirror. Check.
A chesire assesss the collimation of the primary relative to your focuser tube. It’s not really precisely assessing secondary position, as strange as that sounds. It's possible for the primary to be aligned relative to the focuser well enough to make the chesire look "good", while simultaneously permitting the secondary to be off just enough to have the (NON-barlowed) laser beam miss the center mark on the primary.
A chesire and barlowed laser collimator should agree 100% if both are made and working correctly.
First, you have confirmed your laser is collimated. Good. That’s the part that bogs 90% of users down. Before you go to the chesire, put that laser back in, and adjust the SECONDARY mirror’s collimation screws (or better follow instructions pertinent to that particular secondary unit) until that center mark has the laser dot bull’s eyed. I don't know the mechanism of your secondary so I won't try to be too specific. But, once you have the laser hitting center, you are done with the laser.
Now, place the chesire in and zero it in.
Then, you are done. You are collimated.
[There are other steps one could take, but if you scope is f5 or slower, and your focuser is reasonably square to the tube, really, you are fine]
The only other thing you could do to maximize image quality is to center the secondary under the focuser tube using a sight-tube. That is also something that will not be assessed with the above tools. There are nuances – partial offset vs full offset of the secondary – but let us know if you want some references on-line for centering the secondary. It’s easy, but you need a sight-tube, and you can make one for $1 or buy a $100 one, the options are broad! But, to beat this dead horse, secondary position largely only affects vignetting of the light cone, not true image quality-affecting mirror alignments.
And if you are using a barlowed laser, I now dummy slap myself and will now say that I have no clue what's going on!
Cheers
Scott
Quote:
Originally Posted by timelord
Gooday to all--I,m fairly new to astronomy and have been sandbagging on the side reading as many threads as possible trying to clue myself in for the last 6 months or so.
I've just installed a crayford style focusser to my 10" Skywatcher Newt. and decided to collimate the optics,I used a cheshire collimator and finally got everything alligned as per instructions looked good as I took my time and was very carefull to get it as perfect as possible. I used the cheshire after reading many threads as to the supposed inaccuracy of the laser collimator which I also have. I then checked the collimation with the laser and found the laser dot to be just outside the centre spot on the primary. I have collimated the laser on precision engineers cast iron v blocks and it was spot on--(pardon the pun) so I am confident the laser isnt telling me lies but am also confident the cheshire was spot on also--which do I beleive? Also can anyone tell me why 1.250" eyepices and anything else that fits into a focusser are all accurate to a thousanth of an inch but everything that recieves them measures approx 1.256--6 thou clearance is way to sloppy! As a machinist I'm used to working with a lot closer fits than these. Could this be the reason why the discrepancy between the two collimators?--although I did rotate the laser through 3 90 deg segments and the beam did not walk around on the primary it stayed in the same position.
timelord.  |