Dumb question newtonian collimation

[Ian Robinson]

Is it necessary to use a centre spot on the primary when laser collimating ?

Not experienced in newtonian OTA assembly and initial collimation , my thoughts are that if the laser retraces it's own path perfectly back to laser emitter (which is in the focusor) then that's sufficient proof that the secondary and primary are properly aligned , centred and collimated.
Else the laser will not retrace it's own path ??!!??

Thoughts anyone ....

[Starkler (Geoff)]

Originally Posted by Ian Robinson

Is it necessary to use a centre spot on the primary when laser collimating ?

Yes it is.

Else the laser will not retrace it's own path ??!!??

Thoughts anyone ....

The laser could strike anywhere on the mirrors surface and the mirror could be tilted to make it retrace its own path. Only one orientation is correct and that is when the laser strikes the exact centre.

[Astro78]

I can see scatter in the laser return beam if not centred correctly on the primary. For me the beam comes back pin point about 2-3mm above the centre mark, this despite being certain it's marked bang on. Believe this is slight astigmatism in the mirror.

[bmitchell82 (Brendan)]

As others have said, the beam can be made to return along the same path with the primary mirror. this does not mean that the Secondary is perpendicular with the primary, for that you need a accurate spot in the middle of the mirror.

your collmination should be

- align your focusing tube to be perpendicular to the secondary,
- align your secondary to the center of the primary (this should happen when you do step one but make sure it is)
- align your primary to the center of your focuser.

[multiweb (Marc)]

As Brendan rightly pointed out a laser alone will not work. Nothing will replace a good cheshire or reticulated draw tube for initital alignment. The hard bit in newt is to get the secondary aligned. Then it becomes an iterative procedure of moving the primary, then the secondary, then primary, again and again as you're closing in towards the sweet spot.

[Ian Robinson]

I've an old AstroSystems combined Chesire/Sighttube that I'll use to check centring of the secondary and primary, and my AstroSystems Laserbeam to fine collimate once I've the spot in place.

OK .... wasn't sure ..... I'm now convinced I need a centre spot on the primary so I've ordered direct from Catseye some triangular spots with 5/32" hole (recommended by Mr Fry for laser collimation) and a placement template. (the small triangle with 5/32" hole ::: http://www.catseyecollimation.com/spotsbig.jpg is what Mr Fry recommends for lasercollimation using my LaserBeam collimator.}

I'll have to partly disassemble to OTA to place the spot , might as well give it another clean, noticed some finger marks on the edge when I was collimating secondary by looking in from the top of the tube (don't know how I managed to touch the surface , much have been when I was centring the cell in the tube).