More Collimation Questions

[Chillie]

My mate came around last night with his Cheshire(?) collimator to help me collimate my telescope. Between his collimator and my laser collimator, we were able to get it pretty close. But the dark circle (I think this is the secondary mirror) is slightly off center.

What my mate does to adjust his is to adjust the spider adjustment screws on the outside of the OTA. Is that method OK or is it a no-no?

I have watched http://www.andysshotglass.com/Collimating.html a few times to get the idea.

I don't have a collimation EP, just the laser collimator.

Sorry if what I've written above sounds confusing. I am trying to write from memory as what I saw.

[multiweb]

Don't adjust the spider once you have aligned everything. It's supposed to be offset. That's because the diagonal must be slightly offset away from the focuser to intersect the light cone coming from the primary. It confuses a lot of people because it's the reflection of the secondary into the primary (third reflection). It's more apparent on a fast scope with a small aperture and a large secondary. The laser is good for the tilt of the secondary into the primary but always check visually with a drawtube or cheshire as it won't give you any info about the rotation of the secondary, unless you have a large DOB and you use a holographic laser (the one that projects a grid).

First offset the secondary away from the focuser by a distance which is calculated depending on your aperture and F ratio. Usually a couple of mm. Then don't touch the spider anymore.

1_ center the secondary under the focuser (center it both sideways and along the length of the tube).
2_ adjust the secondary tilt and rotation
3_ adjust the primary

Loop through 2 and 3 until you get closer and closer. Finish with a start test by modifying the tilt of the secondary with the 3 screws.

[Blue Skies]

What Marc said. Don't touch the spider now it's been offset for you! Your mate had to do it so the laser collimator would work to its best potential. All you should need to adjust in the future is the tilt of the mirrors.

[Chillie]

Hi Marc,

I find some of what you've said about the offset hard to understand.

What is the difference between a drawtube and cheshire collimator?

[Chillie]

No! My mate adjusted the spider because he couldn't be bothered going to the trouble of adjusting the 3 screws to change the angle of the secondary on his scope. I did not let him adjust the spider on my scope

Until I lent him my laser collimator, he had not adjusted the screws for the secondary on his telescope.:shrug:

[RAJAH235]

Hi Henry,
Just a little more on 2ndary "Off Set".
> http://www.skyandtelescope.com/howto/diy/3306996.html
& Collimation in general,
> http://web.telia.com/~u41105032/kolli/kolli.html
L..

[Blue Skies]

ooookaaayyyy Thats a whole 'nuther kettle of fish!! :eyepop: I thought he was adjusting for offset, as Marc described. I wont muddy the issue further then.

I think what Marc calls a drawtube I call a sighttube, and if this is the case then its a long tube with a cross-wires at the end which you use to get the secondary in the right place before you use the cheshire.

I know its going to do your head in for a while but persevere, it will make sense eventually and seem straightforward and simple. And if you're going to continue with just a laser you will need to get to grips with the offest issue for best results.

[Chillie]

Thanks for the links. The first one is relatively easy to understand. The second will take a bit of time to work through. I'm still playing "catch up" with my forums.:whistle:


Thank you for that. Having several different hobbies doesn't help but I'll get there in the end.;)

[bmitchell82]

i use a combo of Laser and cheshire. it was only in recent times i figured the ceshire is a very powerful little tool.....

of course it can nail your secondary / primary alignment. but it can do more than that. when looking though the sight tube you can see a few things. the Primary spot, the Cheshire spot (Primary mirror alignment) then you can see the outer ring of the sight tube (cheshire) this will give the secondary rotation as if it isn't perpendicular to your cheshire you wont see a clean ring it will be like your looking at a cylinder skewed(you can see down the sides of the tube) with this in mind you can adjust.
As for the Secondary offset, this is kind of easy depending on the scope you have aperature wise. <8" put the primary mirror spot in the middle of the secondary. 8"> adjust it slightly to the lower part say 35 - 40% from the bottom. that is done by adjusting the spider.

With my experience with SW gear, don't expect it to be "aligned from factory" because it isn't!!! most of my screws where loose and they had tightened the secondary screws to the point they ate into the secondary stalk making the secondary alignment impossible.

the other thing is don't be affraid of pulling it down.! its not that hard. and theres really nothing that you can really stuff up... apart from dropping something on the primary!


1. "The angle of the dangle" getting the secondary to point to the middle of the primary.

2. the angle of the primary (aligning the primary to the secondary and hence the center of the Focuser draw tube.

3. Rotation of the secondary.

Like i say nowdays, look at what the light cone is doing once you figure this out then youll be on the road to success.!

[PeterM]

Hi Henry,
Going over old ground here, but perhaps useful.
One little trick often forgotten that will help you considerably is to remove some of the multiple reflections. It is easy with the new SW collapsibe dobs - just place the fitted plastic dust cap over the mirror box.
With a solid tube Newt/dob -
1. Carefully sticky tape a sheet of A4 white paper just behind the secondary - effectively blocking off the main mirror - do this with the tube on an angle so the paper can't fall down the tube. Do not allow the sticky tape or paper to touch the mirrors of course.
2. Place a sheet of white paper directly opposite the focuser on the tube wall on the other side of the secondary.
The above will remove the many confusing reflections and blackness around the secondary holder that blends with the black tube.
3. Now you can put your Cheshire/Site tube in and easily see where the secondary is sitting and make your adjustments to the secondary holder along the tube, and/or rotating it as needed (NOT the 3 screws on the back of the secondary) to ensure the outer edge of the secondary is circular with the bottom edge of the Cheshire/Sight tube in the focuser.
4. Remove the paper carefully and now make slight adjustments to each of the 3 screws on back the secondary - Looking into Cheshire/Site tube make it so that you can equally see the 3 clips holding the main mirror in its cell in the reflection in the secondary mirror.
5. Now looking into the Cheshire/ Site tube adjust the screws at the back of the main mirror to bring the dot on your primary under the black dot created by the Cheshire/Site tube.
5. Don't put all your faith in the laser collimator unless you have set up a v block, rotated the laser and ensured the beam is exactly centred through the full rotation in the v block on a wall several meters away.
6. As Brendan correctly notes the Cheshire/Site tube is a very powerful tool.
7. Read the link below "Myths about Collimation" by the best in the business.
http://web.telia.com/~u41105032/myths/myths.htm
"If you center the secondary as seen in a sight tube, the offset toward the primary will be accomplished automatically, and you may not even be aware of the fact that it is optically offset."

And the all time classic "Perspectives on collimation" - someone in an astro club nearby will no doubt have this.
http://www.catseyecollimation.com/perspectives.html
PeterM.

[Chillie]

Thank you all for your advice and suggestions.

I am putting the practical side of astronomy on hold for a while due to a shoulder injury. :(

[mswhin63]

Good luck with getting better, I understand how you feel. Hope its not bad. Maybe think about putting wheels on the 12" as I am looking into.