Crosshairs: What should you do first?

[pat]

There are two differant versions I've read about centering the crosshaires of the cheshire. On a reflector.

Adjust the primary so that the center dot is lined up under the crosshair center.

Or adjust the secondary till the crosshairs bisect the center of the dot on the primary.

Does it make a difference which one you do first? Pat

[Tinker1947]

Center the cross hairs on the paper circle on the primary mirror first, then adjust the primary mirror.

[umadog]

You have a combination tool with a Cheshire and a sight-tube. A Cheshire doesn't have cross-hairs, it's the sight-tube portion of the tool that has cross hairs. A Cheshire is a tool for adjusting the primary and the cross hairs are for adjusting the tilt of the secondary.

You adjust the /secondary/ until the cross hairs match up with the primary spot. Then you adjust the primary until the primary spot matches up with the Cheshire pupil. Then you back to the cross-hairs and repeat, back and forth between two adjustments, until everything is lined up. It's best to start with the cross hairs but if you're itterating back and forth it doesn't matter too much. The only situation it does matter is for rounding the secondary in the focuser. Don't do that unless you have already adjusted secondary tilt with the cross hair step.

[pat]

Thanks for the clarification. If you move one mirror then you have to move the other. Back and forth as to said. I would set the secondary first. Showing the whole primary mirror reflection in it. And leave it like that. Then put the primary spot in the annulas of the Chishire. And left it like that. Thanks, Pat

[soggybongo]

This is the method i use.

Astro Babys Guide to Collimation

[umadog]

Thanks for the clarification. If you move one mirror then you have to move the other. Back and forth as to said. I would set the secondary first. Showing the whole primary mirror reflection in it. And leave it like that. Then put the primary spot in the annulas of the Chishire. And left it like that. Thanks, Pat

Remember that the first step is put the centre spot under the cross hairs. The purpose of this step is to adjust secondary tilt. If the secondary isn't correctly rounded in the focuser then you can't adjust the tilt by centering the primary in the secondary. Also, this isn't an accurate way of doing things. To round the secondary you first adjust secondary tilt by aligning the cross hairs and then center and round the secondary mirror in the sight tube.

If at the end of the whole procedure the whole primary isn't visible in the secondary then either the secondary is too small or it's not correctly positioned with respect to the focuser. You then need to adjust the spider, secondary position, or even the focuser. Check out this thread for more details: Telescope Reviews: Useful info about secondary mirror alignment

[soggybongo]

I watched this youtube vid

(there are nine parts to watch) to give me the knowhow along with my previous post (astro babys) hope this helps you mate.