GSO laser collimator - slit?

[Cluster]

Hi,

Today I received a GSO laser collimator for my GSO 6" reflector. I've turned it on and did a simple roll test (roll across the end of a desk) and it seems to be in a straight line. I'll do a V wedge test later. Turning the collimator in the focus tube shows a tiny deviation from dead center (about 1 or 2 mm on the primary mirror).

The main question is, the laser doesn't come to a point when shone on the wall. At about 2 metres away it's a slit about 4mm wide. On my telescope's primary mirror it's very difficult to tell if it's in the center of the mirror as the laser is so broad. I think I have achieved collimation of the primary and secondary as the return laser beam is bang on target in the collimator.

When I pointed the laser collimator through my GSO 3x ED barlow, collimation was a bit off but not too bad (in my opinion). Is the laser point (it was pretty large, about 2cm across) meant to be dead bang in the center of the primary mirror?

Unfortunately it's very windy outside so I can't do a star test tonight.

Do the three Allen key holes in the GSO collimator just move the beam, or do they adjust the focus of the beam itself?

GSO did not provide any instructions with the collimator. I think that's a bit slack.

Thanks,

[Night Owl]

Originally Posted by Cluster

Hi,

Today I received a GSO laser collimator for my GSO 6" reflector. I've turned it on and did a simple roll test (roll across the end of a desk) and it seems to be in a straight line. I'll do a V wedge test later. Turning the collimator in the focus tube shows a tiny deviation from dead center (about 1 or 2 mm on the primary mirror).

The main question is, the laser doesn't come to a point when shone on the wall. At about 2 metres away it's a slit about 4mm wide. On my telescope's primary mirror it's very difficult to tell if it's in the center of the mirror as the laser is so broad. I think I have achieved collimation of the primary and secondary as the return laser beam is bang on target in the collimator.

When I pointed the laser collimator through my GSO 3x ED barlow, collimation was a bit off but not too bad (in my opinion). Is the laser point (it was pretty large, about 2cm across) meant to be dead bang in the center of the primary mirror?

Unfortunately it's very windy outside so I can't do a star test tonight.

Do the three Allen key holes in the GSO collimator just move the beam, or do they adjust the focus of the beam itself?

GSO did not provide any instructions with the collimator. I think that's a bit slack.

Thanks,

I know what you mean. Many cheaper lazer modules I have used over the years in different projects have not been round beams, but elongated. You have to spend big dollars for a good module. A friend of mine has a top quality = $$$$ module mounted on a very high power rifle, and at 100m you could take the head off a mouse with it (not kidding).

Usually to adjust the focus of the beam you need to remove the brass lens housing, and adjust the beam focus. Don't know how much of a hack job this would be on the GSO Lazer Coll. I have no idea how bad yours is, but from what you say it isn't as good as some I've had, which would be 4mm at over 50m.

The deviation in the focus tube might just be run out in the focus tube? You might have to shim the focus tube / collimator / and then eye pieces up, to avoid going out of whack every time you drop a new piece in the hole?

You know what's bad, I've ordered one as well, for my 12" GSO Dobs, which was destroyed in transit by a fool who crushed it with a forklift!

Ah what the hell, it's only money I would otherwise spend on more crap I don't need....

[erick (Eric)]

Originally Posted by Cluster

Today I received a GSO laser collimator for my GSO 6" reflector. I've turned it on and did a simple roll test (roll across the end of a desk) and it seems to be in a straight line. I'll do a V wedge test later. Turning the collimator in the focus tube shows a tiny deviation from dead center (about 1 or 2 mm on the primary mirror).

The main question is, the laser doesn't come to a point when shone on the wall. At about 2 metres away it's a slit about 4mm wide. On my telescope's primary mirror it's very difficult to tell if it's in the center of the mirror as the laser is so broad. I think I have achieved collimation of the primary and secondary as the return laser beam is bang on target in the collimator.

Yes, my experience is that these cheap red lasers (pointers etc.) do generate an elongated spot rather than a round spot. 4mm at 2m sounds a bit on the larger size.

Originally Posted by Cluster

When I pointed the laser collimator through my GSO 3x ED barlow, collimation was a bit off but not too bad (in my opinion). Is the laser point (it was pretty large, about 2cm across) meant to be dead bang in the center of the primary mirror?

Did you mean you used a barlowed laser to check collimation? With a 3x barlow, i've seen quite a large elongated spot - several cm across - more than 2cm. "dead bang in the centre"? Well, no, in fact if you wobble the laser in the focusser, the diffuse spot will move all over the primary. But this is the object of using a barlowed laser. While the spot moves on the primary, the shadow reflection of the centre spot (paper ring donut or triangle or whatever you have stuck on your primary) back at the laser does not move.

Originally Posted by Cluster

Do the three Allen key holes in the GSO collimator just move the beam, or do they adjust the focus of the beam itself?

Not in my experience, they just adjust the direction of the beam BUT as you move it, the beam can begin to clip on the edge of the hole it is exiting so you may see the projected shape change due to that. But at that point, the beam is probably not aligned with the mechanical axis, anyway.

Eric

[Cluster]

Originally Posted by erick

Yes, my experience is that these cheap red lasers (pointers etc.) do generate an elongated spot rather than a round spot. 4mm at 2m sounds a bit on the larger size.

I initially thought the beam was just diverging due to poor laser collimation, but shining the laser onto a target 5cm away shows a slit instead of a point almost 4mm across. My $10 laser pointer/pen next to it produces a point less than 1mm across.

I visited a telescope shop in Adelaide today and asked about collimators. The sales staff didn't think it would be a problem as long as the laser is straight and it does bounce back off the two mirrors back into the collimator into a pretty small point. Aiming the laser at the donut on the mirror is a little more difficult when the beam is not a nice tight point however.

Originally Posted by erick

Did you mean you used a barlowed laser to check collimation? With a 3x barlow, i've seen quite a large elongated spot - several cm across - more than 2cm. "dead bang in the centre"? Well, no, in fact if you wobble the laser in the focusser, the diffuse spot will move all over the primary. But this is the object of using a barlowed laser. While the spot moves on the primary, the shadow reflection of the centre spot (paper ring donut or triangle or whatever you have stuck on your primary) back at the laser does not move.

Ah, ok, I wasn't sure what to look for. From memory, the paper ring donut did not move but I'll check it again tonight.

I asked about why looking down the focuser without an eyepiece, the secondary mirror isn't centered yet the laser collimator shows everything is fine. I was told that short focal length reflectors (like my F5) have some bias in the alignment towards the front and it'll never look text book perfect down the focuser despite being perfectly collimated. I've read several collimating tutorials and none mentioned this. GSO's horrible instruction manual shows everything centered as well.

Another thing I learned from my laser collimator is that collimation is dependent on which direction the rack & pinion focuser is moved. If the final movement is inwards it's fine, but if outwards, the laser point moves by about 2mm. The focuser is tightly screwed in.

I'm new to reflectors (and really telescopes in general). There's a lot to learn...