How to collimate primary mirror without centre spot

[bendiddley]

I've just acquired a 130P and it has no centre spot and wondering how I'm going to collimate the primary. Is there some trick or technique to get some sort of accuracy? I know how to collimate with a spot as I also have a 130PDS and used to own a 200P, I just can't see how I'm going to get the same level of accuracy.

[Littleguy80]

Simplest answer is to centre spot it yourself! I recently did this on my SkyWatcher 130M. Should be the same process for your scope. I documented my experience and all the vary references on this thread. Assuming your scope is the same as mine, you can remove the 3 adjusting screws and 3 locking screws from the back of the primary cell to get the mirror out. This is much easier than taking the 4 four screws out around the outside. Everything else you need to know should be on the thread below. If not, ask away

 

[Oddsocks]

Remove the mirror and apply your own centre spot, you can use an expensive manufactured spot like the CatsEye from FLO or use an inexpensive self-stick plastic ring binder reinforcement ring that you find in the stationary dept at W H Smith etc.

Use a sheet of clear copy acetate or tracing paper to put over the mirror and trace over the outline of the mirror with a wipe away non-permanent marker to gauge it's size, remove the acetate from the mirror.

Next, measure the mirror diameter size accurately from the marking on the acetate and wipe the marking off the acetate, then use a compass to mark out accurately the outside dimension of the mirror on to the acetate using a permanent marker.

Next, measure the diameter of the hole reinforcement ring, adjust the compass to that diameter and using the same centre hole in the acetate mark out the diameter of the ring reinforcement with the permanent marker.

Next, apply a tiny piece of Blutack or piece of unused postage stamp folded back on itself and moistened to make it sticky to the front, non-sticky side of the hole reinforcement ring and attach the ring exactly to the centre marking on the acetate so that the sticky side of the ring is facing up.

Now, turn the acetate over, keep it taught and carefully hold it above the mirror so that the outer marking corresponds with the outside of the mirror then lower the centre spot into place, press firmly to make the spot stick and gently peel away the acetate to leave the ring fixed in place.

You now have a geometrically accurate centre spotted mirror, if you go wrong then peel the ring off and try again.

Should take you no more than an hour from start to finish.....(famous last words!!!)

[bendiddley]

Thanks Guys. Oddsocks another fabulously detailed explanation, appreciate that. Ok so looks like adding the centre spot is what's needed for an accurate collimation. If I just wanted a rough collimation though without adding a centre spot, what's the deal here, anyway I can do this?

[Charic]

The 'P' in 130P suggests the mirror is paraboloid, which is better than just a spherical mirror, and a centre spot may aid in your accuracy when setting up.....the other designation for 'P' is Pyrex!
However, my old Celestron did not have a 'P' mirror and subsequently no spot?
I stripped, cleaned, spotted and rebuilt the scope, and it was no better than doing the collimation, sight alone using the guide supplied in the manual!

If its a Skywatcher 130P then page 22 is the start, and basically, using a modified dust cap or 35mm film cap method, sight on a star then check and adjust if required.
This keeps the task simple but efficient. There's no despair about touching/removing the mirror in-case of damage.

Unless the spot has detached, and  actually requires  a new one, thats your choice. I have seen them with spots, not sure if their factory fitted or user.

One thing for sure, you'll learn pretty quick (or not) how these scopes are assembled and how they work, if you go ahead and spot the mirror. 
When I spotted the Celestron, I used one of those paper ring binder strengthener's. Looked the part, but I did not experience any improvement with the spherical mirror.

[Charic]

26 minutes ago, Littleguy80 said:

This is much easier than taking the 4 four screws out around the outside.

I find it easiest to remove the whole cell ( Skyliner 200P ) by removing  the screws around the circumference!
Its possible to take my mirror out for dusting, and re-fit without changing the collimation.
 

[Oddsocks]

21 minutes ago, bendiddley said:

If I just wanted a rough collimation though without adding a centre spot, what's the deal here, anyway I can do this?

Before centre spotting of telescope mirrors was common primaries were collimated using a basic sight tube (which is just a Cheshire without the cross hairs) and final collimation was done with a star test, in those days though the optics were generally slower, typically f8 or slower and were more forgiving of mis-collimation, fast optics need greater accuracy which is when centre spotting became more important.

[Littleguy80]

4 minutes ago, Charic said:

I find it easiest to remove the whole cell ( Skyliner 200P ) by removing  the screws around the circumference!
Its possible to take my mirror out for dusting, and re-fit without changing the collimation.
 

That's what I did with the 130M but really struggled to get it back on as it has nuts on the inside and I couldn't find a way to hold the nut while I screwed it in. I ended up removing the screws mentioned above to take the centre section of the cell out. I could then reattach the outer section before putting the centre part with the mirror back in. So based on that, it seems to be easier to just remove the centre part of the primary cell. Scope will obviously need collimating after that.

[Charic]

2 minutes ago, Oddsocks said:

.......... final collimation of the secondary was done with a star test..............

This indicates to me that your collimating the main mirror first, then finally the secondary!!!!

[Oddsocks]

1 minute ago, Charic said:

This indicates to me that your collimating the main mirror first, then finally the secondary!!!!

You are right I mistakenly added an extra term, I have edited the original post to remove the reference.

[Charic]

2 minutes ago, Littleguy80 said:

nuts on the inside

Yes that would cause an issue, but strange?
Of the only two scopes I've owned, the screws pass through the tube into the edge of the mirror cell, no captive nuts, nuts go on bolts!

[Charic]

1 minute ago, Oddsocks said:

....... I have edited the original post to remove the reference.

I'm not your teacher, just helping out, someone else would have been behind me?

We've all done it, and will continue too........

[Littleguy80]

6 minutes ago, Charic said:

Yes that would cause an issue, but strange?

Definitely odd. I found a post from someone who'd encountered the same issue and came up with the method I mentioned 

[spillage]

When I did mine I took the mirror out placed upside down on some plain paper and traced around the edge. Cut the circle out and folded into 1/4 and made a small hole in the middle. Placed this over the mirror and used the hole to make the center with marker.

Cheap ring binder sticker job done.

I would say that your best bet would be to do your collimation outside using a star for best results.

[bendiddley]

35 minutes ago, spillage said:

When I did mine I took the mirror out placed upside down on some plain paper and traced around the edge. Cut the circle out and folded into 1/4 and made a small hole in the middle. Placed this over the mirror and used the hole to make the center with marker.

Cheap ring binder sticker job done.

I would say that your best bet would be to do your collimation outside using a star for best results.

So did you mark a spot on the mirror through the centre hole in the folded paper and then put the ring binder sticker over the top?

[spillage]

Yes that is exactly what I did. If I think I just used one of the kids felt tip pens and then wiped it off afterwards. I thought about using a tipex pen and just leaving this as a single spot but didnt have it to hand at the time. Not sure how tipex would deal with condensation.

[bendiddley]

Just now, spillage said:

Yes that is exactly what I did. If I think I just used one of the kids felt tip pens and then wiped it off afterwards. I thought about using a tipex pen and just leaving this as a single spot but didnt have it to hand at the time. Not sure how tipex would deal with condensation.

What did you use to wipe it off? Is it not a great idea to touch the surface with something?

[spillage]

I used a lens pen. Im a bit gun hoe at times and took the line of...its such a small area its not going to effect the scope.

The same reason having a sticker in the middle of the mirror will not effect it.

[bendiddley]

1 minute ago, spillage said:

I used a lens pen. Im a bit gun hoe at times and took the line of...its such a small area its not going to effect the scope.

The same reason having a sticker in the middle of the mirror will not effect it.

Ah I see. Cool thanks.

[brantuk]

If you don't center spot it you can still collimate. Do the secondary mirror first. Then estimate the center using a Hotech laser collimator. Then "feel around" for where the dead center is - you'll know once you hit it cos you'll get a nice thin red cross, perfectly symetric, reflected off the primary mirror. Takes a little time to master and you'll need bobs knobs - but it is doable once mastered. You can refine using the "barlowed laser" technique.

[bendiddley]

1 minute ago, brantuk said:

If you don't center spot it you can still collimate. Do the secondary mirror first. Then estimate the center using a Hotech laser collimator. Then "feel around" for where the dead center is - you'll know once you hit it cos you'll get a nice thin red cross, perfectly symetric, reflected off the primary mirror. Takes a little time to master and you'll need bobs knobs - but it is doable once mastered.

Cheers. Unfortunately I don't have a laser collimator only a cheshire. Think I'm going to go down the spotting route.

[iapa]

On 11/05/2017 at 15:25, bendiddley said:

Cheers. Unfortunately I don't have a laser collimator only a cheshire. Think I'm going to go down the spotting route.

This may help?

Collimate a Telescope without a Cheshire

ooopps

Uses a laser later on.

There is a youtube video as I watched it last week which does it without a laser, collimation cap and sight tubes.