Primary mirror centre spot

[IckleImp]

Hi all

Quick question.

I have centre spotted my AM130EQ with a ring binder reinforcing ring as suggested on this forum using the thread and blutack method to achieve the centre point. Can any one tell me how accurate the position needs to be?. I think mine is within less than 1 mm, is this accurate enough?.

Many thanks in advance

[NGC 1502]

There's so much info around on this, some of it conflicting, but for what it's worth, here's my take on your question.

What I do is to get the centre spot as close as I can to the primary mirror centre, then use it to collimate the primary as near as I can in daylight, having first made sure the secondary is sorted. It's important to sort the secondary mirror first, because if that's off, then you'll can can fiddle around with the primary forever and get nowhere...........

Then at night, fine tune the primary on a defocused star in the centre of the field of view at medium to high power.

It works for me, had a run of clear nights recently, and at times, when the atmosphere has steadied, I've seen loads of excellent detail on Jupiter.

As I said - works for me  but I'm not dogmatic about it, Ed.

[NGC 1502]

Tried to edit my post, but for some reason it wouldn't work......

What I tried to add was - if having collimated using a defocused star, the collimation just using the centre spot in daylight looks a bit off, I ignore it and believe the out of focus star check.

So if your centre spot isn't exact centre, not to worry, because it's only a guide to get collimation close, before fine tuning with a star.

Hope that makes sence, and I'm happy to hear those with differing ideas

Regards, Ed.

[Dave In Vermont]

I got a center-spot as close as I could, but it's a little bit off. But my Orion LaserMate Deluxe II (which is perfectly collimated) land on it everytime. And the target on the bull's-eye rings show the primary is perfectly aligned. My Cheshire sighting-tube agrees with the laser. And so does a star-test. I'm a weirdo who actually enjoys collimating a telescope.

So, in light (koff!) of this, all is well even if my center-spotting wasn't exact.

Carry On,

Dave

[Jason D]

But my Orion LaserMate Deluxe II (which is perfectly collimated) land on it everytime. And the target on the bull's-eye rings show the primary is perfectly aligned.

Dave

Dave, that does not mean much. You can move your center spot by few inches off-center and still get the laser to hit the spot with the returning laser beam retracing its path to the source. The same applies to barlowed laser. Good collimation starts with a well-positioned central spot.

Jason

[Jason D]

About star collimation, it takes knowledge and skills to get it right. For example, can someone tell me if the following image of a defocused star corresponds to a "reasonably"  (not necessarily perfect) collimated reflector?

Jason

[faulksy]

its out of colimation, rings are good, mirror clips are good but spider vanes are out

[Dave In Vermont]

I already knew where the center-spot should be. The problem I had with was that the glue on their backside was defective, and would start sliding down the mirror. As is, I took drastic measures involving a heavy object and letting it dry overnight. In the furure, I will use a permanent marker. Anyways, the center-spot on my F/4 is halfway on the target, and halfway off.

It's not as though that it's off to a large degree. Hence I reiterate - being a little off should not eccect anything - as long as it's not fully off somewhere.

Regards the phote of the mirror, it appears to have a problem with the 3:00 - position. It resembles a pinched mirror, though I'm nowhere near an expert in analizing optics from a photo/image.

Clear & Sharp Skies,

Dave

[IckleImp]

Still confused, it appears that it is important for it to be accurately positioned but not as I may be able to do a final adjustment using a de-focused star?

[NGC 1502]

Hi again.  There's loads of info out there on collimation, but the link below is good, from one of Sky & Telescope's regular contributors -

http://www.garyseronik.com/?q=node/169

Hope it helps, Ed.

[Jason D]

In reference to my last post, here is the rest of the story. 

The photos correspond to a "horribly" collimated scope yet the left photo seems to show a reasonably collimated scope. The problem is that the star is defocused way too much. When it is defocused by a small amount then the horrible collimation becomes obvious. 

The point is that star collimation requires some knowledge and skills to execute correctly.

Jason

[NGC 1502]

Agree Jason,  I've seen people trying to collimate with the defocused star almost filling the field of view........

So the instruction "use a defocused star" could be better phrased "use a slightly defocused star" and it needs to be central in the field of view, another vital point.

Regards, Ed.

[IckleImp]

Thank you  NGC 1502 for the link. I actually found it very informative and a bit more user friendly than Astrobabys guide, which I found confusing. I have reset the centrespot and am happy that it is within 0.25mm. I am now awaiting a cheshire from FLO to finish the job.

I wonder how accurately positioned the centrespot is in the factory? .

Many thanks

[NGC 1502]

 

I wonder how accurately positioned the centrespot is in the factory? .

Good question, I think the answer probably is that the accuracy can vary.   So I do think it's best not to take a factory fitted centre spot as being "spot on"  (groan)

All the best with the cheshire, Ed.

[Astro_Baby]

Ohh thats nasty Jason - I would have said collimation looked good but then I would have been assuming I was looking at a magnified pic of a slightly defocused star instead of a massively defocused star....naughty boy putting that up to confuse me

Its a good point though that you need a high power eyepiece, good seeing and a very gentle tweak to the focusr.  I havenet been out in so long due to health, weather, work, family, more bad weather, more bad health that I have almost forgotten how to do anything anymore

[Jason D]

Hello Mel,

It is always good to hear from you. Hope you will feel better soon.

It was a tricky post to convey a point which simply states: To use star collimation successfully you need to know what you are doing

Jason

[Dave In Vermont]

Hi Mel(?),

I'm sorry to hear that your health is not running on all 8. I do hope you're aware that your guide to collimation has been extremely well received! Bravo! And I personally love your website, too.

Take your time and get well. The stars will still be there.

Best Wishes,

Dave