Sorry Collimation again - Cheshire vs Laser

[Bart]

Sorry all. I recently received a 10" Dob and already had a laser ready. First night, popped in the laser and didnt need too much adjustment of secondary and primary. Fine. Star test was pretty good but not exactly there. As my arms are not 1.5m long to reach the primary, I was happy to put up with it. Views of Jupiter were excellent. HOWEVER, it bugged me that it was slightly off and discussions here on lasers being uncollimated made me think. Last night I was all set up and clouds rolled in so decided to use my cheshire to check it. I was messing around for some time to be honest, but appeared ok finally. Popped in the laser to see what it was like and i was WAY off from the centre circle on the primary.

I immediately thought the laser was off, knocked some nails into a piece of wood, sat the laser on it... and it was perfect. Looks like I made a bags of the cheshire collimation. Its the secondary, so I'll have to have another go. I could see the 3 clips of the primary but i suspect now that at 12.45am my eyes were boggled and must try again. Shame, cos I thought I had the collimation thing pretty well sorted.

The clouds here are terrible so I'm guessing I'll have a month or so to get it right.

Bart

[umadog]

The star test is very sensitive, so it could be slightly off and you won't notice it at the eyepiece. One reason for this is that your 10" is going to be seeing limited on virtually every night, so if the collimation is off by a little bit you won't notice it.

So your focuser axial error was off by a bit. That error has fairly high tollerence. How far off was it? Are you sure you used the Cheshire correctly when you performed that aspect of the collimation? I ask because "seeing the three clips" isn't really what you're going for. It's having a round secondary and having the cross hairs hit the mirror centre spot. You also want to see the clips, but if the other two things are right then the clips will automatically sort themselves out. You want to focus on the spot.

Did you try rotating the laser in the focuser and exploring if there's slop with the sight-tube?

[Bart]

Thanks Umadog, thats interesting. I probably didnt centre the secondary correctly using paper to remove confusing images. But your comment about centering the crosshairs on the secondary is news to me. I centre the secondary using the cap and only insert the cheshire when I think I have the secondary in place, but now that you say it, it makes sense. I'd say there is a little slop in the focusser as well. I'll have another go tonight.

Regards,

Bart

[wiseman]

do you have bobs knobs?, i found they made collimating with a laser a one man job ( admit

dly mines a bit shorter than yours)

[umadog]

Ok...

This is what the tools are for:

A Cheshire is used to adjust the tilt of the primary. A collimation cap with a reflective surface is a Cheshire. The Cheshire/sight-tube combo tool contains a Cheshire, cross-hairs used for adjusting the focuser axial alignment, and the tube itself which is used to round the secondary as viewed from the focuser.

This is how you do it:

The order in which you do the steps matters.

1. Use the cross hairs and adjust the secondary so that the primary centre mark is centred in the cross hairs. This is the focuser axial alignment.

2. No use the sight-tube as a guide to adjust the secondary so that it appears round and is in the centre of the sight-tube. This is the white paper part.

3. Go back to step 1 and check that primary spot is still centred. Repeat 1 and 2 until both are satisfied.

4. Now use the Cheshire portion of the tool to adjust the primary. This is primary axial alignment.

5. Go back and check that 1 is still correct. Repeat until all look good.

These steps are described here: John Reed Home Page Collimation - Cheshire

The purpose of adjusting the secondary is to ensure that you get light-cone going down the middle of the focuser so your field is correctly illuminated. Unless you're using a coma-correct this isn't a critical adjustment. The purpose of adjusting the primary is to ensure that the focal plane is not tilted. If the focal plane is tilted you will not be able to get the whole field in focus at the same time. This blurs the image. You can see these errors graphically here: Telescope Reviews: Concise thread about autocollimators+improvements

A laser can achieve the same thing as your combo tool. Adjusting the secondary to point the laser at the centre spot is the same as using the sight-tube cross-hairs. Bouncing the beam back to the focuser is the same as using the Cheshire. Note, however, that bouncing the beam back is not recommended. It's more accurate to barlow the laser and project an image of the centre spot onto a target located at the focuser. The reason this is more accurate is because it doesn't require the laser to be collimated. Of course, the laser has to be collimated to adjust the secondary, but this alignment has far greater error tolerance and the beam is only travelling half the distance (it goes up and down the tube for adjusting the primary if you don't barlow).

I second Bob's Knobs. Those set-screws that most Dobs come with are total bleep.

[Bart]

thanks again Umadog , appreciate the effort . I had another go centering the secondary and used the laser for the primary. Popped in the Cheshire and it looked spot on. We are under permanent cloud cover at the moment so can't check against a star. Your link looks really good as well. Cheers again.

Bart