Collimating frustration

[barrie greenwood]

Hi can anyone give me the answer to this issue

I use a Cheshire collimating tool and get it as good as I can with that as far as I can tell the secondary mirror is spot on then I put my laser tool in that has been collimated it's self on v blocks in a vice

But this laser never hits the center donut ever if I tweak the secondary mirror to get the dot in the center it's way off in the Cheshire tool

Now I know you will probably say if it's right in the Cheshire then leave it but I need to know why it's this way

Any help would be welcome

Regards Baz

Photo taken from the Cheshire

[Timebandit]

Hello. My personal opinion of lasers are sometimes they are more trouble than they are worth and you may have to collimation of these also numerous times. I use a collimation cap and star test.

If the laser is out you are just go around in circles. Use the Cheshire or cap to collimation, then do a star test to confirm. If the star test is fine then you have your answer to the best way for you to collimation 

I hope this helps☺

[spike95609]

I agree, despite never having used a laser before. If your secondary is properly in line with your focuser, and with a collimation cap you can see all the primary mirror clips spaced equally around its edge, then that's as near as you'll ever need to get perfect collimation. And if by looking through the Cheshire you have the dot inside the donut on the primary mirror, then that too is as close as you need get it. If you have all of those things done then what it means is that your focuser is properly in line with the secondary, the secondary is looking directly at the primary, and the primary is looking directly back at it. If the laser is telling you something different then I'd be inclined to trust your own eyes and assume there's something awry with the laser.

[Ricochet]

Is that photo through your Cheshire or did you swap over to a collimation cap to take the photo? When people say to use a Cheshire they mean a combination Cheshire and sight tube, which has a metal cross hair at the bottom of the tube that you have to line up with the doughnut to align the secondary mirror. In your photo I don't see a cross hair, just what I think is the reflection of your secondary spider. If what you have is literally a Cheshire, a collimation cap with an angled reflector, and it does not have a cross hair then it is only for aligning the primary and it is unsurprising that you are having difficulty using it to align the secondary.

If, however, it does have a cross hair and I'm just not seeing it then either one of your tools is off or there is so much slop in the focuser that the difference in weight is changing the angle at which the tools point at the secondary.

[MarkyD]

I used a laser collimator and barlow as the final step to align the primary on my 200D-PS.  It worked very well.  The start tests came out perfect

[Charic]

Baz..........What ever method or tools you use, the first thing is to get the secondary mirror looking perfectly circular and concentric to the walls of the focuser tube, ie equal space all around.
Next, check to see if you can see the three primary mirror clips whilst looking through the focuser tube, with or without any tool?
If then your using a laser, that is collimated, ensure its a snug fit in the focuser tube, now adjust using the three adjustment bolts in the secondary holder to get the laser beam centralized on to the primary mirror centre spot.
Once achieved, now adjust the primary mirror adjusters to get the laser spot back into the hole on the face of the laser tool.

That is the generalized procedure, but study and re-study which ever guide you prefer to understand whats going on. I like the astro-babe guide.

[barrie greenwood]

Arrgh!!

OK it was a Cheshire but I put into my pocket and forgot shoved my hand in and broke the cross hsirs?

I've just spent 2hrs trying to get the secondary mirror lined up in the tube and now I've set the primary mirror and that now lines up what is the easiest method to get your secondary square under the focus tube and by that I mean down the screw that moves it up and down the tube because It can never have been correct even when it was new it seems a yard out  this is how it looks now

I've taken these with the phone upto the Cheshire so it's probably not square as it is really I'm hoping an experienced eye can see my mistake

Baz ?

[Charic]

....difficult to  assess from  your photos?

Check this video, to see if it helps in any way....... ( https://www.youtube.com/watch?v=YAVGcGEBmCE )

 

[barrie greenwood]

OK I've just fixed my Cheshire this is what I have

[Louis D]

I only use my laser to align the secondary directly into the center of the primary.  I rely on a Rigel Aline to align the primary.  I only use the sight tube to align the secondary under the focuser.  Three tools for three different alignments.

Even with a non-aligned secondary, you can align your primary to reflect the laser beam back to the center where it originated, so I never rely on them for that alignment.  Under those conditions, you can see that the laser misses the center of the primary.

A big problem with lasers that I've found is focuser tube 2" to 1.25" adapter sag, even when fully tightened.  You really need to use them in 2" only mode (assuming a 2" focuser).  Even a tiny bit of sag will throw off the laser, but have basically no effect on the Aline, thus my preference for it.

Lasers are great for gross adjustment on the primary when using a long focal length truss dob by yourself.  While standing at the primary mirror end, you can adjust the collimation knobs until the laser returns to the same spot it left on the secondary's face.  Some of the newer lasers have the angled cutout on the side to achieve a similar result when used at a distance.

[barrie greenwood]

Yeah I've now managed to sort it the laser is a pain if the mirror is slightly out it really makes matters worse however I've now just come back in from doing a star test and all seems well showing the concentric rings on the airy disc all well defined and separated used a combination of both laser and Cheshire to finally get it right..... Not how I planned to spend Fri eve

Anyway thanks guys for all the help it is appreciated

Many thanks Baz

[Dave In Vermont]

Congratulations! The first time is always a monster - now it gets easier.

I always use both - just to see them agree, as they always do in my experience.

Enjoy!

Dave

[barrie greenwood]

Monster is right it's that secondary mirror and I made matters worse by moving it a good bit in the first place without really knowing what I was doing. But after the headache yesterday I now know what's correct and should know what I'm looking at from here on

Astrobabys guide is one of the best out there really explains it well so thanks again guys for the advice I can now look upward at the sky instead of down a Cheshire ?

Regards Baz

[Charic]

Not only have you got the collimation right, and your happy with the Star test ( the ultimate test ) more importantly here,  you have discovered and learnt more about your telescope than you knew before, and the process gets easier the more  you understand it, and quicker the more you practice it........well done.