Collimating a 8"Dob??

[johnkirkpatrick]

How exactly do I get my mirror out to check the centre spot is centre?? on my last scope it was just 3 screws and it fell out, But not sure how and im a total numpty when it comes to DIY, I gona use the barlowed laser method as I have nothing else and dont trust the laser on its own, The Secondary mirror looks fine through the ep tube using the dust cap with a small hole through the middle, I can also see the three mirror clips but thats when I notice the side of the primary mirrior as well ( I think ) sorry no camera to show you.

The barlow laser as it stands shows the centre spot to be off by a few mm, but could this be my piece of card cut to cover the laser end with the small hold in the middle being not spot on??? Sorry guys dont know how else to explain it to you..

[Moonshane]

when I used the barlowed laser method, the shadow of the primary donut showed on the face of the collimator. try this first - it's big but does show as a dark circle. try moving the primary screws about until you can see it. this maybe varies between collimators and barlows but hopefully will work. the centre spot could be out but it's best to rule out yours first.

try the above and see if it works.

just though of another way you could make something if it doesn't. get some 30mm tube from a DIY store - long enough to go through the focuser and into the OTA. cut it square and then find something round that will fit in the end. drill a hole in the middle and glue it in and this should be quite accurate?

[Astro_Baby]

I'd suggest its far more likley your laser isnt collimated or the bit of card isnt accurate. Mirror centre spots can be off but I dont see many people finding them off on here.

If you are a numpty at DiY I would seriously suggest NOT messing with the mirror because the potential for harm is so great and the liklihood of the centre spot being off doesn't seem to me to be very high.

[John]

I agree with AB - much more likely that your laser is off or just that the tilt of your secondary isn't quite right yet.

[Tim]

Barlowing the laser eliminates the problem of it not producing a central beam.

Can you not collimate it as best you can with what you have, then test it on Polaris? A star test is always the acid one

Whereabouts are you? Maybe a local member can help??

Tim

[Tim]

erm, just re-read your post.

What is the idea of having a piece of card on the end of the laser?????

[johnkirkpatrick]

Newtonian collimation with lasers and using barlows in the Barlowed Laser Technique this is where I got the idea from ..

[John]

I see from your signature that you have a revelation laser collimator ?. I have one of those and have used the barlowed laser collimation approach. I found that the image of the mirror "doughnut" was projected onto the angled face of the collimator clearly enough not to need the card screen on the end of the collimator.

The barlow lens diffuses the laser into a patch of light rather than a spot and thats what hits the primary mirror. As long as the patch of light covers the central "dougnut" on the mirror then you can do accurate laser collimation - it does not have to be perfectly central.

My method came from this web page:

http://www.smartavtweaks.com/RVBL.html

Hope that helps.

[Moonshane]

ha ha

in other words what I said earlier!

[John]

ha ha

in other words what I said earlier!

Just re-enforcing your good advice Shane

[Moonshane]

I was all hurt and ignored

[coffee_prince]

Guys I need some advice

I think my secondary is aligned because the three clips are all in view in equal measure.

The problem is adjusting the primary mirror, I can't get it right, and I am using a home made colliecap at the minute, but even when I unfocus on a bright star I can see that the black hole is not centered in the circle of light but off centre.

Having to screw and then look, screw look takes a lot of time and I can't get it centered after a while of trying.

I am not sure over a laser of cheshire

Even then the views are still very nice, but could be better I expect once collimated better