Cheshire Collimator advice

[GeoffW1 (Geoff)]

Originally Posted by ausastronomer

Use it as a means to save time once you know how to collimate a telescope, but don't leapfrog the steps in the learning curve along the way.

Cheers,
John B

Hi,

Very well put, and I am obliged to agree completely.

I guess I must put away the laser every so often and see how I go with the Cheshire tool. Hmm, hard times ahead

Cheers

[Karlsson]

Originally Posted by ausastronomer

...if the secondary/primary distance is not correct it is still possible to adjust the tilt of both mirrors and get the laser striking back perfectly upon itself, thus making the telescope appear properly collimated, when in fact it is not.

I actually put this to the test a few months ago - I deliberately pulled my secondary out by about 4 mm and tweaked it out of the optical centre - the laser result could still be made perfect while the scope was plainly unusable.

Like John I have always relied on sight tube and star test - also because a star test reveals much more than collimation errors - it's the ECG of Newts so to speak (pinched optics, anyone?)

A few months ago I added a laser to the routine since it allows me to follow primary mirror adjustments real-time, rather than having to move from back to front - my kids all resigned as Vice-President of Collimation a long time ago... But the star test still has the last word... make that a veto.

So while a laser collimator may be convenient, do not be fooled by the false sense of accuracy the word 'laser' may convey - there is no such thing as a free collimated lunch...

Each to his own - some tools are indeed easier to use than others... and I took vernier calipers to the shop when I bought my sight tube, too. If you learn how to judge the star test it does not have to be expensive - or tricky, for that matter...

[Cluster]

Originally Posted by ghsmith45

Because a cheshire is more accurate and a barlowed laser (google this) is even better. A straight laser relies on the laser spot hitting the centre spot of the mirror exactly.

I will try the barlowed laser method and see how it goes. I haven't adjusted the distance between the primary and secondary simply because the secondary center screw is very tight on my 6" GSO and didn't want to budge.

There's a variety of opinions on collimation methods so when I have time I'll work through them and see what works for me.

[ngcles]

Hi BluBird & All,

Particularly agree with the comments by John B above.

FWIW, nowadays I've gone back to a good ol' combination sight-tube and cheshire. I used a laser for 2 years but twice the batteries let me down when I was alone in the dark somewhere remote. No batteries needed for the cheshire! In addition, It took me hours to collimate the laser itself and I was never totally confident it held its own collimation perfectly week-to- week, month-to-month. Cheshire itself never needs collimating!

Get one with the long sight-tube for the reasons stated above.

Make sure the fit in the focuser is very "snug" with as little side-to-side wobble as possible.

Are some dobs more sensitive than others? You bet! The smaller the f/stop number, the more twitchy it gets. An f/7 can tolerate slight miscollimation much, much better (ie less noticeable) than f/4.5 (or even f/4 etc).

Using a bright star as a double-check is also good advice (above by John B). Make sure you _also_ examine the image on either side of, but very close to focus for eccentric extra-focal rings etc.

Collimation is like riding a bike -- soon you won't even think about it while you are doing it and it will take only a minute or so to be spot on!

Best,

Les D
Contributing Editor
AS&T

[BlueBird]

Hi all,
rang Bintel to see about availabilty of the Orion tool and wound up having one delivered on thursday. stock just in.
Read John tweed`s web site and read instructions. 2 days later still trying to get it right.
I`m hoping that after the secondary is in the right place, I wont have to touch it again.
Of the 2 scopes at my place, one a 200mm diameter, 800mmfl the secondary is too large for the tool to see the outer edge. had to resort to putting a block dot in its centre to aid alignment. The spider is unadjustable so offset could be a problem.
The second is a cheap 76mm and its focuser tube doesn`t allow for complete insertion of the tool, althought its spider is very adjustable.

Maybe a shorter tool should be considered for scopes with specific mechanics.

One observation you may confirm, the crosshairs are out of focus when looking at the secondary. Is this expected?

thanks Marty.

[ausastronomer (John Bambury)]

Originally Posted by BlueBird

Hi all,

I`m hoping that after the secondary is in the right place, I wont have to touch it again.

Hi Marty,

The secondary usually requires less frequent adjustment than the primary, but you should still always check the secondary at the start of each observing session. Unlike the primary, it will not move during the session, unless it is not properly supported.

Originally Posted by BlueBird

Of the 2 scopes at my place, one a 200mm diameter, 800mmfl the secondary is too large for the tool to see the outer edge. had to resort to putting a block dot in its centre to aid alignment.

Slide the collimating tool partly out of the focuser until you see the edges of the secondary, then tighten the eyepiece locking screw or collar.

Originally Posted by BlueBird

Maybe a shorter tool should be considered for scopes with specific mechanics.

You don't need this. The Orion tool is better than the short ones. Whilst it is designed for "about" an F5 telescope, it will work fine with your F4 telescope by drawing the collimating device out of the focuser just enough to see the edges of the secondary mirror.

Originally Posted by BlueBird

One observation you may confirm, the crosshairs are out of focus when looking at the secondary. Is this expected?

Yes. When using the crosshairs you need to move your head back from the collimating device and look "at it", rather than "through it". Move your head back from the hole until the crosshairs go into sharp focus. This may be a good inch or even more. When you get it right and everything properly lined up (scope properly collimated and your head in the right place) you will be able to see the reflection of your own eyes pupil centred under the crosshairs of the collimating tool.

It aint hard, it takes a bit of getting used to. Be rest assured the tool you have will do the job very well, once you learn what you are doing. Patience and practice.

Cheers,
John B