Collimation advice

Kencollins

Hi again,

I bought a scope yeasterday (5" 650mm reflector), which I haven't had a chance to try out yet.

When I bought it (from Plug!) he said it had never been collimated, but he never noticed anything wrong with the image either.

Since the weather hasnt been great, I decided I will try to collimate it tomorrow evening and see what happens. I don't have a laser, so it will be an eyeball job. I looked at some guides on the interweb, and have gotten this so far:

1. Use a film canister with a hole in it, no eyepiece, bright room

2. Secondary mirror first. Align it so you can see the three "clips" that hold on the spider legs, so you can see an equal amount of each one.

3. Main mirror. Adjust the tilt so the hole in the film canister is centred with the mark on the main mirror, as seen through the film canister collimater.

4. Refine. Find a star, defocus slightly, and check that the halos are round. If they are oval or worse, refine the tilt on the primary mirror in the direction of the oval until it becomes round.

Would this be a reasonable summary of what needs doing? Some of the guides are mega long, and I dont want to make it worse if I dont need to!

Ken

peterako

Hi Ken,

My first piece of advice.......

Don't!

Or at least not yet.

As someone who also likes to tinker I can, hopefully, save you a bit of pain.

I would advise that you wait till you have a good night with good seeing conditions and then carry out a star test.

If you THEN need to collimate I would refer you to Astro Baby's Great Collimation Guide.

Clear skies,
Peter

Kencollins

Hi Peter,

Thanks for the reply! I must try harder to resist the urge to take everything apart!

Now, when will we get those clear skys!

Popoutman

Try to look through these tutorials to try to get an idea of what's going on, and what order to move things in.
http://web.telia.com/~u41105032/kolli/kolli.html - theory till halfway down.
Here's a good one:
http://www.schlatter.org/Dad/Astronomy/collimate.htm

You can get near-perfect collimation without using a laser - and for certain types of collimation errors a laser will not help you get good collimation. Having said that, a laser that is well made and aligned will make it easier to get the primary mirror aligned. But the secondary mirror alignment and centring cannot be done with only a laser.

If you have to collimate:

1: Make sure the focuser tube is orthogonal to the tube - makes sure that the axis of the eyepiece and focuser pass through the centreline of the tube. Most ordinary reflectors won't have any adjustment for this so you'll just have to trust that the manufacturer did a good job.

2: Make sure that the secondary mirror holder is centralised in the tube when looking from the front. Again, sometimes this is just not possible to adjust. If you can adjust, try to ensure that there is an even gap (measure with a cloth tape) between the centre bolt and the inside edge of the end of the telescope tube.

3: Using the adjustments on the secondary mirror holder, and using the sight tube you've made with the film canister, try to get the edge of the secondary mirror concentric with the inside edge of the focuser tube. Most scopes can only move the secondary up and down along the centreline of the main tube.

4: Using the tilt and rotation adjustments on the secondary mirror holder, make the primary mirror concentric with the edge of the secondary mirror. If there's a centre spot on the mirror even better - this becomes a bit easier - centralise this mark in the secondary mirror.

5: Tippex or paint white the inside of the film canister cap so that you can see it reflected in the primary mirror. Using the primary mirror adjustments, center this reflection of the film canister cap in line with the centre of the primary mirror.

Viola - collimated well enough to do.

If you really want to get a good collimation, get a Cheshire/Sight tube combination tool - makes the job so much easier.

Kencollins

Hey all, I'm back again!

It being Christmas, the wife has released the telescope she impounded to give me as a Christmas present!

I got a laser collimator too, and gave it a try last night. The first big problem was the collimator itself. When I rolled it on a flat surface the beam was drawing circles on the wall. Put it into the focused and turned it, and it was drawing inch wide circles on the primary mirror.

So, an hour later I managed to collimate the collimator. Not easy seeing as the adjustment screws were siliconed in.

So I think I have it done, all the dots match up, and the beam is returning to the centre of the collimator.

The only issue now is the focus tube reflection in the secondary mirror. It doesn't look perfectly centred (the reflection of my eye!), and the reflection has a black circular outline. It can't be changed on the focus end, and I'm afraid to mess with the secondary mirrors position in the tube.

I'm looking down the scope now (130 mm 650mm focal length, with a 4mm plossal eyepiece. I can make out the individual pebble dashes and their colors on a neighbors house 200m away, through my kitchen window.

Would that indicate the collimation is reasonable, and I need not worry about the focuser alignment?


Thanks for reading,

Ken

Popoutman

Some of the diagrams the links that I suggested above show that in a well collimated Newt, the reflection of your eye should be concentric with:

1. The outside edge of the secondary mirror, a few inches from your eye.
2. The edge of the primary mirror.
3. The reflection of the focuser tube in the primary mirror.

The outline of the secondary (the one that is in line with the attached spider) is not supposed to be concentric - this is deliberate in short f/ratio scopes.

Try to see a reflection of a light on something small and round like a torch pointing at a ballbearing or a glint of sunlight on a power line insulator. If collimation is good, you should see no flaring to the side as you rack through the point of best focus.

As it is, if you are getting satisfactory views at ~160x that it's hopefully good enough.

Enjoy the views, and I hope that it gives you plenty of pleasure as you look and learn!

peterako

Star test Ken (when the sky clears)

As long as the rigs on the test are reasonable concentric you should be fine

160x will be about the limit of your scope...but that's still a lot of magnification!

Happy Christmas,
Peter

Kencollins

Hey all,

Thanks again for all the replies! It's great to get advice from experienced people.

I did a star test on vega tonight while it was still high, under 160x. I was pretty happy with the results. The skies were a bit turbulent here, but the rings were all perfectly circular.

Jupiter looked great tonight, the bands were very dark, and all the circular patches (can't remember the proper name, I remember it being something obvious though!) we're really obvious. Venus was just too low in the west to get anything other than turbulence.

I'm still shocked how badly the laser collimator was out of alignment. It came from a well known telescope shop, and wasn't too cheap either. If I hadn't copped it I would have assumed the scope was rubbish and probably given up!

I just can't wait for a proper clear night now, it been ages since i have seen a truly clear night.

Popoutman

The laser colliamation devices can often get misaligned in postage, through no fault of the seller.

Good to hear that you're enjoying the scope and that it's giving good views - hope you have clear and steady skies and the time to get under them!

The dark things on Jupiter - probably "barges".