Either, except that it is a barlow installed on teh end of a laser, not vice versa.
and to capitalize on the barlow feature, you need a white disk with a center hole taped to the back of the laser/barlow assembly. It projects back to the end of the assembly a shadow of the primary mirror center mark; you adjust the primary collimation bolts to center this shadow (a shadow donut, often)over the exact center of the laser/barlow assembly. It's possible to use a 1.25" laser and 1.25" barlow to jury rig it. If you have a 2" laser and 1.25" barlow might be harder, I guess. I really have never done it, I'm not sure. I bought a dedicated self-barlowed laser collimator, the Glatter laser by Howie Glatter in the States. Never used anything else, sorry.
You know, for now, forget the whole idea of a barlowed laser....you have a chesire and a straight laser, you do not need a barlowed laser at this point.
And I should have said 'centering the secondary under the focuser', rather than squaring the focuser...that latter term is older and sticks in my brain when we talk about making sure the focuser is moving straight in and out perpendicular to the tube side long axis while always pointed at the secondary. In truth it's rarely a problem in scopes like yours but worth reviewing after you've mastered mirror collimation...on a rainy night when you want to do something astronomical..
There are three basic elements to collimating a newt.
1) position the secondary to capture the entire light cone coming up from the primary and send it laterally to the focuser tube.
2) point the focuser tube axis at the center of the primary mirror axis.
3) point the primary mirror axis (i.e. the center mark) at the center of the focuser tube axis.
Everything we do in collimation can be boiled down to these three concepts, usually but not necessarily in that order.
2 and 3 are not the same thing.
Here's an analogy I came up with to understand this. Please don't think I'm a gun-crazed American. I'm not (gun-crazed at least). But the analogy works well: Your focuser is ONE gun barrel. The primary mirror is a SECOND gun barrel. You want to aim both gun barrels so a bullet from one will drill right down the barrel of the second. The secondary mirror is a steel plate that "bounces" bullets (the light cone) perfectly.
You can have one gun barrel (the focuser) aimed at the other barrel (the primary center mark) even though that OTHER gun (primary center mark) isn't aimed right at the first (the focuser). And vice versa. What you want, or rather what you NEED is each aimed exactly at the other's barrel for perfect collimation. That's called co-axial alignment: collimation aka "same line."
Your "sight" for your focuser tube gun barrel is that straight laser beam from your laser collimator. You point that at the primary, but you have to bounce it off the secondary to get it there. Now, the secondary can be a few cm too high or low or left or right, and you can still bounce your bullet off it in such a way as to aim it the way you want. But, as you can envision, if that secondary is off a lot, it won't catch a full shotgun spray of bullets (the full light cone). For it to catch the entire spray (of light from the primary), it needs to be positioned "under" the focuser in such a way. That is called centering the focuser. Non-centered and you are losing some light from the primary, that's called vignetting. When all is said and done, the bullets can still bounce perfectly - those that hit the secondary at least - but you are just losing some light needlessly. Your 10" starts acting like a 9.5" or less...
Once you've centered your secondary under the focuser, you start by aiming one of the guns. Really, in most cases, it doesn't matter which you aim first, but since one can't really adjust the focuser plate that well, we adjust the "tilt" of the secondary in order to "aim" the focuser barrel down to the center mark. Put the laser in and tilt secondary until it drills down on the center of primary.
Once the focuser is aimed (i.e., the secondary tilt screws adjusted so that the straight laser beam hits the primary center mark), you can move to aiming the other gun barrel (the primary) at the focuser. Imagine that gun barrel is the center hole in your primary mirror center donut. That alignment is done with the chesire, or the barlowed laser. In fact, you can use the NON-barlowed laser to do this also. You use the bounced beam of the laser off the center mark, it bounces back up to the focuser, and it should hit (fuse) with the exiting beam in the exact center of the laser unit. If you see the return beam hit the back of the (again, non-barlowed) laser OFF of center, you move the primary bolts until it does, thereby disappearing into the exiting beam path and not visible anymore. But, most of us use the Chesire or the barlowed laser device for aiming the primary, as it's easier to be accurate to the tolerances most new (fast) newts require.
After adjusting your primary bolts until you have the chesire dialed in, you might want to recheck the straight laser, as sometimes (not normally, but if the mirror cells is clunky) the secondary needs a little tweak to make sure it's center. If that was needed, you need to redo the primary again, after that.
Technically, you can do it ALL with a sight-tube alone, if your eye is good. It also helps if your f ratio is high, as you won't notice any deviations from perfection like you will with an f4 scope. For fast scopes, the different tools start to matter. Most of us collimation GEEKS (and I'm proud, darn it) use a sight-tube, a regular laser, and a chesire or Barlowed laser, and lastly a device called an autocollimator which is just a super fine tuning of the mirror alignments. But the basics (for you, for example) would be a sight-tube and a straight laser and a chesire. That'll get you real good, and you are two-thirds there.
So, you just need to fine tune the secondary until that hits center of center mark, then redo chesire, and perhaps do that cycle one more time to perfect it, since when you move one gun barrel, the other one might need re-aiming, too!
If you end up adjusting your secondary to center it under the focuser, you'll need to redo the whole shooting match (American gun analogy again!) but then you'll be good to go.
If you google and fine Jim Fly's Catseye sight, he has some good stuff. So does Nils Olof Carlin - google him.
Lastly, I'll make a pitch again for teh Yahoo group collimate_your_telescope, a great resource.
I see you are in Melbourne? If so, there are many veterans here (MPAS, ASV, etc) who could help out if you wanted a double check of your collimation.
Cheers,
Anti-Gun Scott
Quote:
Originally Posted by timelord
Thanx for the quick replys-- when you say barlowed laser do you mean a specific design of laser or a barlow with a laser collimator installed in the end of a barlow? I did assess the position of the return beam and it is not centred but not trusting the discrepancy between the two collimators have not yet adjusted the primary. Sight tube to square the foccusser hmm I.d better read a few more posts.
Alex.
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