Laser collimation errors

[blusky]

Hi all, I noticed the laser collimator that I have is not perfectly aligned. When I rotate it in the 1.25" socket the laser dot makes a tight ellipse on the primary, just about the size of the doughnut. So, my secondary is now aligned to keep the centre of the ellipse in the centre of the doughnut.

Now, aligning the primary is more difficult as the dot seems to be making a strange shape of the 45 degree white target piece. The dot never seems to go outside of the 3rd circle, but I don't get that crisp view of stars and planets I used to have once the scope cools down for an hour or so.

Any advice or quick tests for the proper collimation (smth about a star out of focus? What exactly to expect?)

How bad should the view be given the laser dot is about 2 circles away from the centre, if this can be described?

Thanks.

[angusb1]

If you put the laser in your barlow it will make it irrelevant that the laser is not aligned:

Rear View Barlowed Laser Method

[malc-c]

I use a Hotech which is self centering and doesn't appear to suffer the same collimation / fitting errors associated with other laser colimators.

[acey]

Hi all, I noticed the laser collimator that I have is not perfectly aligned. When I rotate it in the 1.25" socket the laser dot makes a tight ellipse on the primary, just about the size of the doughnut.

Might be an effect of the focuser ("slop" or not perfectly square to tube). Try racking the focuser in and out, or even just give the laser a nudge, to test the degree of error introduced by the focuser.

Could also be misalignment of the laser: wedge it in a stable holder (e.g. open book), shine it on a wall, and gently rotate to see if the spot stays fixed.

In any case if you're getting a tight ellipse then there can't be much error.

Any advice or quick tests for the proper collimation (smth about a star out of focus? What exactly to expect?)

Star test: high power eyepiece, any star that's not too bright, in stable atmospheric conditions. Out-of-focus star should be a donut surrounded by closely separated rings, hole in donut should be exactly centred. When you go through focus, the inside and outside (intrafocal and extrafocal) images should look the same. If they don't then there are lots of reasons that have nothing to do with collimation (astigmatism, coma etc).

A useful extra tool would be a cheshire. See for example this collimation guide:

Starizona's Telescope Basics

[umadog]

The first pass of the laser (focuser to primary) measures the focuser axial alignment error. If you're not using a paracorr, this step is pretty tolerant to misalignment of the laser. The return beam, however, measures primary axial alignment error and is NOT tolerant to misalignment. For this reason, using the barlowed laser approach is always to be recommended. Alignment tolerances vary with focal ratio and primary diameter and can be found here: http://www.catseyecollimation.com/Newtonian%20Axial%20Tolerances.pdf

For reference, the first pass of the laser is doing the same thing as sight-tube and the second pass (return beam) is doing the same thing as a Cheshire (or a "collimation cap" with a silvered surface, which basically is a Cheshire). The main advantage of the laser, IMHO, is that you don't have to keep running back and forth to look through the focuser. This makes it much quicker to use. Both procedures can be equally accurate if done correctly. Some people find a sight-tube hard to use so the laser is a bonus there. An auto-collimator is twice as accurate as these procedures as its reflections contain up to four passes of the tube, whereas the Cheshire and laser produce only two passes.

A sight-tube (which you find usually on Cheshire/sight-tube combination tools) is useful for rounding the secondary as viewed from the focuser.

[acey]

As a matter of interest, I was collimating my dob yesterday in the usual way - laser for secondary, cheshire for primary. Rotating the laser, I got the sort of ellipse the OP describes.

I have a 2" adapter and a 1.25" twist-lock eyepiece holder. I loosened the 2" adapter and kept the laser fixed in the twist-lock while slowly rotating the adapter. I still got an ellipse, showing it to be innacuracy in the focuser that was the cause.

As Umadog says, error arising in this way is small and not very consequential - which is why I'm happy to use the laser for secondary adjustment. But for primary and overall check it's always the cheshire, which is the simplest and most accurate way. Once the cheshire showed perfect collimation, the laser return spot was not central: the effect of error introduced by the focuser. I always trust the cheshire, not the laser.

[umadog]

If the laser isn't collimated then you'd expect to see the beam move in both the scenarios you mention: both rotating laser and rotating the adapter+laser. Unless I'm missing something?

[acey]

My point is that I kept the laser fixed in my hand and rotated the adapter round it: this showed that the 2" adapter introduced some alignment error.

Just for fun I tried barlowing the laser last night, after collimating with the cheshire, and got a return shadow nicely centred on the diagonal screen of the laser. The diverged laser disc was nevertheless off-centre when viewed directly on my primary. Again, I think it shows some initial misalignment introduced by the focuser, but the cheshire gave me good collimation and star tests were fine.

What this makes me realise is that in the past I've wasted a lot of time by starting with the secondary and centring the spot on the primary: this has just had the effect of making me put it out of collmation at the start of every session, then recollimating the primary to match. I think what I'll do in future is begin by checking the primary with the cheshire, getting that aligned, and only retouching the secondary if needed (i.e. cross-hairs not aligning etc) - then back to the primary to finish.

Moral of the story, as ever, is not to place too much faith in lasers.

[umadog]

Yeah, it could indeed be the adaptor. 2" collimation tools are always better if you can get them.

[Voliton]

When I got my first Laser collimator three years ago I found it was out of collimation. After some heated discussion the tech-support at Orion admitted that as many as 50% of the lasers shipped were out of collimation. They would be happy to replace if I returned the original and paid for all shipping charges ( $24.00 total ). Of course there was a good chance the new one would be bad also.

I immediatly tore off the "do not remove) tag and found 3 tiny collimation screws for the laser tube. Not sure about my focusers I installed a focuser vertically in a hardwood stand and using a 0.00 +/- .01 digital verified horizontal accuracy of the focusers in all directions. I then collimated the laser with a target on the floor ( 4' distance until there was no movement during full revolution of the laser ). I've now checked some 15 different focusers so far and have yet to find appreciable surface defect causing laser error. I have found many focusers errors in installation - not perpendicular or not centered on the secondary mirror or secondary not centered in tube.

These errors can be difficult to detect with normal collimation. In fact you can get what appears to be good collimation with a fair amount of focuser error - until a star test, and consequent bad language interval.

With a new Newt I drill a .5mm hole directly opposite the focuser and, with the secondary removed, use a sight tube to shim the focuser perpendicular. I then center spot the secondary (water based) and center the secondary. The spot washes off easily with distilled water stream from a squeeze bottle.

I center spotted the primary with a tiny dot of luminescent white nail polish. When the laser hits this spot it flares so I view and adjust off axis behind the secondary. Primary colimation by an auto-colimator .

Star test tweaks using a 1/4" ccd camera complete the job.

Later recollimations can usually be done simply by star test - unless you bump into a door frame or drive 25 miles over washboard dirt roads.

Regards, Gary

[Merlin66]

Gary,

The only thing I disagree with is that the spot on the secondary should NOT be in the physical centre of the mirror. Depending on the focal ratio of the scope the secondary will have some "off-set" to bring it in line with the reflected cone of light from the primary, and the centre line of the focuser. The mark on the secondary should be positioned at this off-set point.

[Voliton]

H Merlin

Absolutely right. Forgot to mention that offset is automatically introduced when centering the secondary with a crosshair cheshire ( sight tube ) Offsetting Your Secondary Mirror - Do It Yourself - SkyandTelescope.com

There is much debate as to the need for deliberate offsets. Secondaries are usually large enough to produce a fully illuminated field without offset FAQ about Collimating a Newtonian telescope

Regards, Gary

[David Smith]

The lack of clear guidance on how to collimate my potential new scope was one of the major factors in my choice of scope, and I very nearly bought a small refractor instead. I am however very glad that I did not as the views through my 250mm f5 are spectacular.

It seems there is no right way to collimate and all of the methods are flawed in some way. Personally I use a laser and collimate based on the guide that came with it. I suspect I am not getting the best from my scope but it appears that I could use any 1 of a number of different methods and someone would still tell me I could do better!

When someone comes up with the ultimate collimation tool / method, be sure to let me know (that's why I read this stuff).

All I know is that the images I see are satisfactory for my purposes.

I suspect I am probably not the only one to question purchasing a dob / newt with this in mind and a lack of clear, uniform guidance from the experienced amongst us will not help.

Steps down from soap box - sorry, find this quite frustrating.

David

[acey]

Worth bearing in mind that people have been happily using Newtonian reflectors for more than 300 years - collimation tools are a lot younger. William Herschel tested all his mirrors by star test alone; Lord Rosse did much the same with his 72" Leviathan (though he did make use of a watch dial as "artifical star"). When sight-tubes were introduced on large observatory reflectors in the 19th century it was noted that the method revealed flexure of the scope - but that didn't stop people using their scope. Good enough is better than unattainable perfection.

[Voliton]

Hi Dave,

Collimation of a Newt is really not difficult once competed the first time. With commercial scopes straight from the manufacturer many of the difficult parts are done so you are just checking to make sure they didn't screw up.

As is often the case there are a number of methods that work ang good results. Newts are rather forgiving until you go below 4 f ratio. There arre three pure visual astronomers, oldtimers, in my club that haven't collimated their 16, 18 and 24" Dobes in over a year - they are finishing their Hershall lists. They regularly haul those beasts 50 to 200 miles over dirt roads to darkl sites.

As examples - Lasers/cheshire/auto-collimator are fast and accurate for part of the collimation, but only if set up properly, Cheshire/ sight tube/auto-collimator are a little slower but just as accurate.

All amatuer astronomy is an exercise in compromize. weight and cost are major factors. Even optical desigh is a compromize. Parabolic Mirrors have coma, Refractors have chromatic abberation, SCTs have verry long focal lengths, a lot of glass and are relativey heavy R/Cs have astigmatism, Adaptive optics are hugely expensive.

Newts and Dobs are more difficult to control as a GEM and just plain big. SCTs and refractors are more popular because they are compact and easier to mount.

The only test that matters is a star test at 6mm ep. If you can get a defocused airy disk with perfectly concentric rings you are getting the best your scope can give without question. The only way you can get the experience needed is to try and then practice. Even experiment occasionally. That little trick with nail polish I posted earlier originated when I wanted to get a mirror mounted which was not centerspotted. I didn't have a binder ring so I tried something thay sounded good in theory but worked even better in practice.

Regards, Gary

Regards, Gary

[umadog]

There is clear guidance on how to collimate and there are ways to do it which are not flawed. Here are some links: Rob Campbell's Home Page

Precisely placing the focuser in the tube is not very important. You want the secondary rounded and centered as viewed from the focuser. A sight-tube will help with this. It doesn't matter if the focuser is pointing exactly towards the opposite side of the tube. http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=reflectors&Number=3033065

[blusky]

Gary, many thanks, what a fantastic answer.

Indeed, I have tried the Barlow collimation method and was a bit disappointed that a cheapish laser created a shapeless blob rather than a rounded cloud we need.

Anyway, what I found doing instead and was really happy with the outcome was collimating the laser collimator itself! Apparently it's got 3 screws, 2 of which are used in a similar fashion as those on the secondary. Now the dot hardly moves when rotating the collimator in the socket, and this is repeatable.

Glad it was an easy and intuitive solution. Now I know the laser collimator under the hood

[wrinkly]

Dion from "The Shed", claims that there is a "Collimation Disease". Tried to avoid it but found myself lining up the Mak yesterday.

HELP!

[Jason D]

Below is a copy of a post I wrote on another forum a while ago

Jason

==============

Based on my experience over the past few years helping others with collimation problems, here is a summary of what many of the new Newtonian reflector users had struggle with:

1- Facing collimation fear. The secondary mirror structure has 4 spider vanes thumb knobs, 1 central bolt, and 3 set screws. The focuser has 3 alignment set screws. The primary mirror has 3 collimation knobs and 3 locking screws. Too many knobs and too many screws to deal with. The only way to conquer collimation fear is knowledge and practice, practice, and more practice. It is like learning how to drive. Knowledge is important but nothing can replace learning driving behind the wheel – not even simulators. The same goes for collimation.

2- Dealing with mechanical frustration especially with the secondary mirror. It takes practice and potentially some mechanical modification to control the secondary mirror.

3- Not understanding the theory of collimation and the importance of different alignments. In a different forum, someone went out of his way to square the focuser and use a camera to center the secondary mirror under the focuser with high precision. But the same person did not pay as much attention to the primary mirror focal plane shift/tilt alignments. It is important to under the collimation theory at high-level to concentrate on the important alignments.

4- Not understanding how to use collimation tools. Again, in a different forum someone was getting frustrated because his laser did not agree with his collimation cap. It turns out that after collimating his scope with the laser, he used the collimation cap to assess the secondary position under the focuser. He did not know that a typical unbarlowed laser is not the proper tool nor it is intended to optimally position the secondary mirror under the focuser. That is the job for the sight-tube or holographic laser. In addition, he was not using the collimation cap properly since he used it to assess the secondary position rather than the primary alignment.

5- Not owning the proper set. There are 3 collimation alignments that we need tools for: Positioning the secondary optimally under the focuser, eliminating focal tilt errors, and eliminating focal shift errors. Each of the 3 alignments need a specialize tool. Some of the combo tools can handle several of these alignments. Some own several collimation tools targeting the same alignment without owning tools for the remaining alignments.

6- Not knowing how to perform star collimation correctly. Many defocus the star too much and end up attempting to correct the secondary offset as opposed to the primary alignment.

7- Relying on and trusting incorrect manuals. For example, many scope manuals show simplified diagrams of collimation where all circles are shown concentric. The off-center secondary mirror silhouette tends to confuse and frustrate new Newtonian owners.

Jason

[umadog]

Thanks for your post, Jason. I wonder if you can elaborate on point six? Why does de-focusing too much result in one correcting secondary offset?

[Jason D]

When someone defocuses too much to the point where the spider vanes diffraction appears clearly then the secondary shadow location with respect to the outer star diffraction ring is a function of the secondary offset. Say you have a perfectly collimated fast scope with a large secondary mirror which translates to a large secondary offset. When you defocus too much then the secondary shadow will not be centered. That will give the illusion that the scope is no well-collimated when in fact it is. However, if you defocus by a small amount – enough to show around 6 star diffraction rings at high mag – then the secondary shadow will be centered. In general, optics evaluation and collimation should be carried out at high mag with small defocus (spider vanes diffraction is not visible).

Defocusing too much helps in evaluating seeing and thermal stability condition.

Jason

[umadog]

Ah, ok... I didn't know that. Will the effect you describe occur irrespective of how offset is achieved (i.e. displacing secondary away from focuser or tilting the mirrors)? I adjust the secondary with a sight-tube and don't recall seeing an obviously non-centred shadow at f/4.

[Jason D]

The offset of the secondary away from the focuser has nothing to do with what I described. It is the offset towards the primary mirror will be visible. I know this does not sound intuitive.

In addition, the amount of shift depends on the offset versus the secondary/primary sizes. For example, if the amount of offset is 3mm and the primary mirror has an aperture of 300mm then the amount of secondary shadow shift will be 1% with respect to the outer star ring diameter – not too visible. But if the aperture happened to be 100mm then a 3% shift would be visible.

Here is a summary of my point: In general, when the star is defocused too much then you are looking at mechanical alignment. If is defocused by a small amount at high mag then you are looking at optical alignment.

[umadog]

Ah, I see. I've noticed that before but hadn't really appreciated what I was seeing. As ever, thanks for your insight.