2" filter with pinhole for laser collimating with barlow?

[mark3d]

So I use my 2" barlow for laser collimating my dobsonian, and previously used a disc of paper with a hole that I made myself to focus the beam as it leaves the emitter.

I just realised the barlow has a thread for a 2" filter, and I figure I must be able to do better than the paper disc.

Is there such a thing as a 2" filter with a pinhole.. or is this something someone (without many tools or technical experience) can make themselves?

[mark3d]

I figure I'll buy a cheap 2" filter and stick a paper disc to that. Hopefully it ends up being somewhat professional.

[gb_astro]

Mark, what's the point of directing a barlowed laser through a pin hole?

gb.

[mark3d]

Often the lasers are not a perfect circle and the inaccuracy gets magnified when using the barlow so that you can end up with an elongated circle (in my case more like a rectangle) of light hitting the primary mirror.

I guess it's better to refine the beam shape within the collimator itself (http://www.iceinspace.com.au/forum/showthread.php?t=75601), which may or may not preclude the need to do it again in the barlow, but this works for me!

[gb_astro]

So when aligning the secondary without the barlow attached you find the laser spot too large/irregular for accurate placement?

gb.

[mark3d]

Yeah, on the primary mostly. Previously I've just aimed it at the middle of the non-circular shape and it's been OK. Just trying to get a bit more serious with it.

[Steffen]

By putting the laser through a Barlow the beam becomes divergent and only serves to illuminate a largish portion of the primary, in particular its central region with the donut sticker. The bright patch on the primary is reflected back to the focuser, where the image of the (non-reflective) donut is then centred around the pinhole via primary collimation adjustments. The collimation (or lack thereof) of the laser gun is of no importance at all, that's the advantage of this method.

Of course, the laser would have to be well collimated for step 1 of the process, which is adjusting the secondary so that a laser spot is centred on the primary...

Cheers
Steffen.