Secondary Mirror Collimation

[ee001]

Am I right in understanding that it is the smaller 'tilt' screws that secure the secondary mirror in place? It seems the centre screw simply moves the mirror up and down. So as long as the three tilt screws are tight there will not be a risk of the secondary mirror falling off?

For obvious reasons it is important that I get this right before starting.

The scope is a skywatcher dobsonian.

Thanks

[dweller25]

Nooooooooo.

The small tilt screws DO NOT secure the secondary.

If you unscrew the centre screw enough the secondary will fall off - possibly onto the mirror.

[Doc]

Always work on your secondary mirror with your scope in the horizontal position so if anything does fall off it drops no where near your primary mirror.

As dweller has already said the centre bolt is the one that secures the secondary the three bolts encircling it only adjust the secondary.

[kniclander]

you may need to loosen the centre screw to adjust the position of the secondary but as Doc says, do it i the horizontal position and hold the secondary while you do it. If you havent read it, read Asrtro-Baby's collimation guide which I think set many of us on our way.

[ee001]

I just read astro-baby's collimation guide and watched a video as well. From the video I got the impression that turning the centre screw moves the secondary up and down which would mean tightening it would change the mirror position as well. However, from your comments this is my understanding: to adjust the secondary mirror position along the tube I loosen the centre screw, move the mirror up or down as necessary and finally whilst holding the mirror in the correct position tighen the centre screw to make sure it doesn't fall. Is that the way to do it?

[mdstuart]

i loosened the three small screws first then used the centre screw to broadly get the mirror under the eyepiece tube but holding mirror holder to make sure mirror did not fall out and then lined up mirror accuratly using 3 small screws. very fiddly. you must use a chesire collimator to reduce risk of swearing.

[Astro_Baby]

Hi,

Okies the centre screw adjusts the mirrors up/down orinetation (ie closer to or further away from the primary). The centre screw ALSO holds the secondary into its holder. Loosen it too much and it will fall off and go down the tube - thats a situation which is absolutely not good.

First off always work with the tube close to horizontal so if anything does fall off it wont go down the tube and into the primary mirror.

Now the centre screw and the three tilt screws always work kind of together. You'll not be able to adjust the centre without loosening the tilt screws but you can adjust the tilt without changing the centre screw. There is however a leetle problem which is as you adjust the tilt you can introduce rotation into the secondary mirror requiring you to loosen the centre screw to get the rotation error out.

Heres what I do to cheat a little - get yourself some cotton gloves from Boots the chemists before you do this.

1/ Get a sight tube into the focuser ( or your cheshire ).

2/ Loosen all of the tilt screws completely. The mirror will now flop around a bit - thats ok.

3/ Now hold the secondary while wearing your cute little cotton gloves (you can look just like Jackie O here ).

4/ Look into the focuser and using your cotton gloved hands twist the secondary around and see if its centre and can be made to look completely circular. If not adjust the centre screw up/down to get the mirror so while holding it (without touching its surface - the gloves are to sop any sweat running onto the mirror surface) wiggle the mirror around manually to get it perfectly centred under the focuser and circular.

5/ The hard part - now hold the mirror, look through the focuser while the mirror is in a good position. Keep the hand holding the mirror steady and now tighten ONE of the tilt screws so the mirror flops less and now holds its position one the single screw. If you have a helper get them to lightly do up that tilt screw while you peer into the focuser and keep the secondary true by hand.

Now lightly do the other two screws up while continuining to look through the focuser/sight tube to keep the mirror true. Do the tilt screws up very, very lightly at this point. Too much torque on them will start to introduce rotation error. If you do this lightly enough you can still twist the secondary around while the tilt screws are relatively loose.

By constant trial and error you will eventually get the mirror centred and perfectly round.

Some points to note;

1/ LIGHT TOUCH all the way. Dont force anything. If its sticks or doesnt want to shift theres a reason - so find out whats holding it up and dont be tempted to use brute force

2/ PATIENCE IS A VIRTUE - secondary collimation is nearly always a mare of a job and sometimes it just doesnt go right even for me. Sooooo take your time and if things dont appear to be working out go have a cup of tea, watch the telly, play with the kids etc and come back to it.

3/ YOUR NOT CALIBRATING MOUNT PALOMAR. Remember that most commercial scopes are not the perfect scientific instruments of a professional laboratory. There may always be a slight imperfection in them. To try and eliminate all possible small errors is the route to madness. If its looks more or less ok it probably is - a star test will tell you.

4/ DONT collimate if you dont have to. Once a secondary is done it shouldnt need any more twiddling unless theres a disaster.