collimating nerves

[Karen]

p { margin-bottom: 0.21cm; } I am posting this here because I am a total newbie at collimating.

I have a Skywatcher 10inch Dobsonian reflector and I believe it needs collimating although I do not yet have a collimating device, cheshire or laser, just a collimating cap. When looking through the cap I can see four of the six primary mirror clips. The other two are only just out of view. That confuses me to start with because all the collimating guides including the one for my 'scope say look for three clips. So obviously that means getting all six clips visible in the secondary mirror although they are quite thin and barely visible at the edge of the mirror.

Also the primary mirror centre spot does not reflect back to align with the pinhole in the collimating cap. It is only slightly off to the side .

I presume this means both the secondary and primary mirror will need adjusting? I will be getting a cheshire before I do anything. So something to do on cloudy evenings.

[SplintUK]

Is your mirror centre spotted? If so I would get a laser and use that to sort out the secondary before I went anywhere near the primary. I then use a Cheshire to sort out the primary and check the accuracy.

Most common reflectors use 3 clips to hold the mirror, hence the guides talking about 3 clips, but yes you should be seeing all of the clips equally.

Let's face it, you're going to be collimating a lot for years to come, so having the right tools now is a good investment interns of money vs your time trying to get by without them.

The precision Cheshire at FLO seems to be causing a bit of a stir amongst affixianados. http://www.firstlightoptics.com/collimation/premium-cheshire-collimating-eyepiece.html

[NGC 1502]

I can see four of the six primary mirror clips.

I have owned a SW 10" and it was unusual in having six clips to hold the primary mirror. Most have three clips, which is why collimating instructions mention that. You need to adjust the secondary mirror to get all clips (however many there are) equidistant.

Having sorted the secondary, you can adjust the primary mirror using an out of focus star.

Regards, Ed.

[Tinker1947]

If you come back here when you have a Collimator, things can be sorted no problem..

[umadog]

You can do most of what you need with a collimation cap, even though it isn't the best tool for aligning the secondary. The cap is a Cheshire only and is designed to align the primary. Nonetheless, you can (at a pinch) use it to round the secondary in the focuser.

You have been referred to purchasing a Cheshire/sight-tube combination tool (e.g. Collimation Tools - Do It Yourself - SkyandTelescope.com). This allows you to adjust with accuracy both the primary and the secondary. It will help you to both round the secondary as viewed from the focuser and to adjust the tilt of the secondary. A laser will help with the latter but NOT the former. The Cheshire portion of the tool allows for primary adjustment, as does the laser. You're better off starting with the combination tool alone rather than the laser since you will need the combo tool anyway to round the secondary. Also, cheap lasers will create more problems than they solve.

Here is lots (probably too much) information: Rob Campbell's Home Page

Finally, I want to highlight that collimation is an iterative process. It is usually not sufficient to adjust the secondary, then adjust the primary, then declare yourself done. You need to go back and forth between the primary and secondary until both look right.

[Moonshane]

in a way the mirror clip instruction is often misunderstood. really it can be ignored if you have a Cheshire/sight tube with cross hairs. The following steps will get you good enough for visual use:

1) Check the spider vanes are approximately all the same length. If not, loosen one and tighten the other of two opposite nuts until they are. This ensures the secondary holder is central in the tube. They will probably be OK and need no adjustment.

2) You can pretty much assume your focuser is square to the tube and in the right position so don't worry about this stage.

3) Using your Cheshire or your collimation cap, check that the shiny part of the secondary is centred in the focuser tube. Again this is likely to be the case. If not then carefully adjust the length of the centre bolt in the secondary holder until it is. Then gently tightern the three secondary adjusters to take up any slack (You may need to loosen then if the secondary needs to be adjusted up the tube.

4) With nothing in the focuser (other than the 2-1.25" adapter) and with your eye a little removed from the focuser look in and you should see a nice round reflection of the secondary mirror. If not then turn the secondary mirror until it is.

5) Now insert the Cheshire in the focuser (usually all the way for me) and put a little bit of card wrapped into a tube around the shiny Cheshire part (the 45 degree silver slanted face with the hole). This simply makes the image for the next part easier to see. Now to align the secondary mirror, simply centre the donut of the primary with the cross hairs on the Cheshire using the three adjusters on the secondary holder.

6) Now the easy bit, take off the mask from the Cheshire and centre the black dot with the donut on the primary. You are now collimated well enough, but as suggested just have a double check on the secondary again and then the primary to make sure it all looks right.

As long as you hit nothing and drop nothing (always collimate with the tube at a shallow angle for this reason among others) then there's nothing you can do that cannot be adjusted back so don't worry.

It's really quick and easy once you get the hang of it.

Good luck.

[Karen]

Thanks for all the very helpful advice.

I have just been using the 'scope to look at jupiter which is very bright in the eyepiece and has a bright flare around it and very obvious diffraction spikes. I can just make out the cloud bands.

Is this due to the collimation being out?

[SplintUK]

The best way to check is to pick a single bright star (not a planet) and move the focus out. You should start to see a polo mint of light, with turns into concentric rings and the black dot right in the middle. If not, your collimation is out. Check out the very bottom of Asto_baby's guide here Astro Babys Guide to Collimation It shows in and out of collimation images as well as other issues...

[Moonshane]

The best way to check is to pick a single bright star (not a planet) and move the focus out. You should start to see a polo mint of light, with turns into concentric rings and the black dot right in the middle. If not, your collimation is out. Check out the very bottom of Asto_baby's guide here Astro Babys Guide to Collimation It shows in and out of collimation images as well as other issues...

star testing is fine as long as :

the star remains constantly in the exact centre of field - it will move when you adjust the primary too.

the seeing permits high magnification

Polaris is a good example of a star to use as it's bright but not too bright and it stays 'still'. when defocusing (you need to try in and out) only defocus by maybe quarter of a turn.

the thing to bear in mind with collimation is that there is some slack in the parameters except with very fast systems and that the mirror moves around a little anyway when observing so it won't stay smack on at all times anyway. some imagers collimate every time they reposition the scope.

if the view looks good then don't worry too much.