Celestron collimation 114 EQ

[drewk1]

Hi all

I acquired a second hand celestron 114 EQ and the collimation is badly out.

It looks like the previous owner has fiddled and given up!

I understand how and what to do but have one area I'm uncertain with.

Does the secondary mirror have to be offset to the centre line axis of the focuser. i.e towards the primary mirror?

If so how do I calculate the offset?

I can set this accurately if it's required as I have access to some high accuracy measuring equipment.

Any help of tips would be appreciated.

Cheers

[nicnac]

Astro Babys Guide to Collimation

i found this very helpful

[yeti monster]

you won't need to apply any offset to a 114mm newt secondary, just ensure that it is central and aimed at the primary, then collimate the primary. Robert will be your mothers brother....

[drewk1]

Thanks for the replies

I'll carry on and see how I get on...

[Aussie Observer]

Is your scope the short tube 114? If so then you need to remove the lens from the focuser before you start collimating. Then it has to be replaced for the finishing touches.

[assasincz]

I have a 114 too, only Skywatcher (both Synta I reckon), so in therms of optics, the two should be similar. I have a few tips:

I am not sure if your telescope's primary mirror has a centre mark as default. Mine didn't so I took a time to mark a geometrical centre with a square of black tape 1x1mm. Fiddled with it for ages trying to figure out the centre without touching the surface. Eventually I got it right, even by measurments taken from a digital image! As it turned out, it was all pretty much in vain. But I'll get to that later.

When I got down to actually collimating the scope, I found a secondary mirror to be slightly more down the tube than it should have been. Getting it right took a lot of time, but was worth it, I guess.

Then I have made a collimation cap with white inner surface for better contrast.

It was a pain to align the centre mark on my primary with the hole in the cap, lot of messing aroung with a light torch, but I got it right eventually.

And here is the thing. Occasonally with cheaper optics, the gomertical centre of the primary is not identical with the, I'd say, optical centre - The mirror's prime focus isn't necessarily perpendicular to the geometrical centre, If you know what I mean - it may be offset. And I found this to be the case with my scope as well.

A couple of nights ago, I have attempted to do a star collimation as the seeing conditions were very stable. After doing a bit of study, I decided to give it a go. Vega seemed fine to use.

I got the star out of focus in 150x magnification, the difraction rings were nicely distinct but they weren't perceftly symmetrical.

The issue is, every time you fiddle with the setting screws, the star moves out of centre of view and it has to be dead centre in the eyepiece to do it right.

After I got the difraction rigs perfectly bullseyed, I have tightened the locking screws gently but firmly to make sure the difraction rings remained the same. Sounds complicated but I found it to be the easies and quickes way of collimating I have tried so far. But I guess that is because all of the rough alignment I made prior to star collimation.

So now, the rings were a perfect bullseye and got the scope back to focus - the crispness of the image exceeded everything I have ever seen with my scope before. The stars were not even slightly smeared as before, they were perfect.

As it eventually turned out, the marked centre of the primary was off by about 5-7mm from the focal centre. It doesn't sound like much, but I have to say, the difference was huge.

It was more of a polavar but it was well worth it because now I know my scope is collimated spot-on and the image is really as sharp as it ever could be!!! And that just feels good!!

[drewk1]

Interesting post and has given me something to think about.

I think I'm pretty close but waiting for a clear night to try a star test.

I did have a quick look at Jupiter and it's vastly improved from when I acquired it but I'm sure it's not quite right!

Thanks for all the input much appreciated.