Collimation rant

[RogerTheDodger]

This is beginning to get me down. I think I have read every tutorial there is and it's still eluding me... I set up the secondary with a collimation cap, put in the laser (and yes that is collimated...) and by the time I get that in the center of the doughnut then the secondary is wrong in the collicap...back and forth back and forth...

A star test *seems* to be ok with a nice even doughnut when defocussed, am I trying too hard/expecting too much perfection?

[Mikea]

If the star test is good and you can focus down to a nice sharp image, then I wouldn't worry! Have you tried a Cheshire instead of the laser?

[Freff]

Because of the possible slop in the focuser, there may be an inconsistency in your results. Hence the back and forth syndrome. If you have a good star test then I'd be happy with that.

[RogerTheDodger]

I think you may be right about the star test. I may also give a cheshire a go but wouldn't that also be susceptable to slop in the focusser?

[Freff]

Anything you use will be suscepable to slop. All you can do is get it the best you can then do a star test to confirm.

[astromerlin]

Understanding and sorting the collimation on my OTA caused my bald patch i'm sure.

Do the colli-cap thing to get your secondary spot on. Then use the barlowed laser method.

http://www.cameraconcepts.com/barlow...ollimation.pdf

Using this method to fine tune the primary I have found is the best way to see it is exactly right, being a bit of a perfectionist myself I understand how you are feeling, I was "at it" for weeks but once done my planetary images improved immensely.

[Jason D]

This is beginning to get me down. I think I have read every tutorial there is and it's still eluding me... I set up the secondary with a collimation cap, put in the laser (and yes that is collimated...) and by the time I get that in the center of the doughnut then the secondary is wrong in the collicap...back and forth back and forth...

A star test *seems* to be ok with a nice even doughnut when defocussed, am I trying too hard/expecting too much perfection?

Centering/rounding the secondary mirror under the focuser versus completing alignment with a laser are somewhat independent alignments. Check the attached animation. Each frame will pass the laser alignment test but as you can see only one frame includes an optimally positioned secondary mirror under the focuser. To achieve that setup shown in the optimal frame, you might have to adjust your secondary mirror central bolt and/or adjust the spider vanes thumb knobs.

The star test only tells you how well your primary mirror is aligned which also happen to be the most important alignment.

Jason

[star_chaser]

sorry to jump in here but having collimated (or tried to) i tried the star test

but i did not get the rings around the out of focus star but the doughnut effect ? . but said doughnut was in the center of the star is the scope collimated or do i need to see rings around the (out of focus)star

sorry again for the hijack and sorry i cannot help dodger but im lost as well

star

[Pete Presland]

when the whole collimation was really annoting me a couple of years ago, i was given some very good advice on here.

you are looking through millions of miles of space and atmosphere, so as long as your star test looks good you are not going to be far out.

go out and enjoy, i am sure its close enough

[pook]

I've been through this myself. I was using a home made colli cap to centre and set the secondary, then a barlowed laser to set the primary, and I was having teh same problem. When the laser read set, the secondary appeared to be offset in the drawtube.

I received a proper cheshire eyepiece today, and reset the secondary with that.. then set the primary with a barlowed laser, and now it appears perfect.

Lasers are useless at setting the secondary, and a home made collicap doesn't allow a narrow enough border around the secondary to accurately adjust rotation to set it to a perfect circle. The cheshire gives a secondary view that has a really, really thin border around the secondary (due to long drawtube extention) and this allows for very accurate rotation alignment of the secondary.

It will be your secondary rotation out. you can line up the secondary and primary with a barlowed laser, but that's no guarantee that the secondary is lined up with the focus draw tube.

Get a decent cheshire to set secondary, and then use a barlowed laser.

[Si W]

i have found a dead certain way to centre a collimation tool into my eyepiece holder without wobble or being knocked off centre when tightening up the eyepiece holder, I have rapped a continues piece of thin tap around my collimation tools just small enough to be a snug fit and to keep it dead centre, also done it with my EP's as well. Works a treat.

[kniclander]

when the whole collimation was really annoting me a couple of years ago, i was given some very good advice on here.

you are looking through millions of miles of space and atmosphere, so as long as your star test looks good you are not going to be far out.

go out and enjoy, i am sure its close enough

well said that person . also, with the OP's F6 scope, collimation does not have to be perfect for optimal viewing.

[RogerTheDodger]

Thanks for the comments all, and Pook has given the best reason as to what is good about a Cheshire so I'll probably give that a try too

[umadog]

It's true what is said: don't get overly concerned about collimation. If you're happy with the views and the star test is good then you should just go with it. Collimation doesn't have to be perfect, it just has to be within a certain tolerance window. This window is smaller for smaller focal ratios.

[malc-c]

One thing to note is that often the action of securing a laser collimator in the focuser can cause the laser to deviate. I use a Hotech collimator that has a self centering fitting that removes this problem altogether.

Like others have said, don't get too hung up on this. Provided its close and you can get sharp images then you need not worry too much