Sunday, July 12, 2009

 

Zen and the Art of Telescope Collimation.


Unfortunately, for more than a few amateur astronomers, collimation isn't actually an art at all; it’s blood, sweat, and bitter tears. But it doesn't have to be that way. Despite what you may have heard on Cloudy Nights or some other Internet amateur astronomy hangout, collimation just ain’t that difficult. It darned sure is important, though.

If you want good images, especially images of the planets, having the telescope in good collimation, having its optical elements correctly aligned, is critical. If you use a fast (let’s say f/6 or faster) Newtonian, improper collimation will make anything look just horrible—unless you enjoy seeing a field full of “comets” rather than pinpoint stars. SCT owners? Don’t feel left out. Bang-on collimation is equally—maybe even more—important for the good performance of your telescope, and the same goes for the owners of other catadioptric designs. Heck, e’en refractors need to be in proper optical alignment if they are to deliver the goods.

Newtonians

Yeah, most telescopes need collimation, but usually not as often as Newtonians do. For many Newt owners, collimation’s a way of life. Most often asked novice question in this regard? “How often will I have to mess with collimation?” Answer? "That depends." On the telescope’s design. Solid tube Dobsonians, non-truss  tube style Dobs, especially those with simple push-pull bolt mirror cells, can go a fairly long time without collimation even if you drive over some bumpy roads on the way to the Bugtussle A.S. dark site. Got a truss tube scope that has to be disassembled and reassembled every time you transport it? Expect to at least tweak the adjustments every time.

Second most common Newtonian collimation question? Do I need a laser? Before we start tweakin’, let’s settle that. Do you need to spend a hundred (or two hundred or three hundred) bucks on a laser collimator to get the scope in perfect optical alignment? No, you do not. One of the ubiquitous Cheshire/sight-tube crosshair collimators available from Orion, Celestron, and others for less than fifty bucks is more than good enough. One of these simple tools will allow you to get even a fast telescope precisely collimated.

Which doesn't mean lasers don’t have advantages. If you are new to Newtonians, you may find it easier to collimate by observing the position of a red laser dot than by trying to sort out confusing reflections, crosshairs, and peepholes. One other advantage of the laser is that if you arrive at the club site late and it’s getting dark, you will find one easier to use than a Cheshire. Oh, you can still use the latter by shining a flashlight into the Cheshire and down the tube, but it won’t be real fun.

What is this “Cheshire/sight-tube” I speak of? A laughably simple tool, a mere metal tube. On one end is a set of relatively thick metal crosshairs. At the other end is a peephole for you to look through. This peephole peeps through an angled silver (or white) surface that is illuminated by a large hole in the side of the Cheshire. There are variations on a theme, of course. Some folks prefer a separate Cheshire and a separate crosshair sight-tube, but the combination model I favor works well and is what I recommend.

Above all, don’t be scared. Some novices have the idea Newtonian collimation is a difficult and involved process that only gurus can master. The reason for this belief is often that Ms. Novice has been reading too many collimation threads on Cloudy Nights. Let a newbie spend too much time reading that stuff (and taking it too seriously), and she or he will wind up like the novice with a  new Dobsonian I encountered on the field of a star party where I was doing one of my collimation clinics.

All the conflicting collimation methods and theories and philosophies (and arguments) she had read about had helped her do one thing: get her scope as far out of adjustment as it’s possible to get a telescope. She was on the field sitting next to her new baby in total despair, having given up the idea of getting its collimation anywhere close enough to allow her to observe anything on what had the all the signs of being a perfect evening at a beautiful dark site in the West Virginia mountains.

Instead of immediately laying into her scope, I did as I usually do and told her to forget all the stuff she’d heard about Barlows and lasers and auto-collimators, and just think about what collimation really is. Beyond the fact that it’s getting your secondary and primary in line with each other and the primary more or less pointed straight out the tube, what collimation is is a very simple process involving three things:

1. Center the secondary mirror under the focuser.
2. Center the primary mirror in the secondary mirror.
3. Center the focuser in the primary mirror.

That’s all there is to it, and once I showed her how to use her Cheshire, she was flat out amazed at what a non-intellectual and unskilled activity it is. Collimation is so easy even a caveman—or your Uncle Rod—can do it. Oh, if you are new to all this, the “secondary” mirror is the little diagonally tilted mirror under the focuser. The “primary” mirror is the bigun at the bottom of the tube and is  what you paid all that money for.

One more preliminary: make sure the telescope is ready to go. Being ready means there is a mark dead in the center of the primary mirror to serve as a reference for our adjustments. Luckily, almost all modern Newtonians, even inexpensive Chinese Dobsonians, have this mark, usually a paper reinforcer, installed at the factory. If there ain’t one, you need to apply one. Don’t get all worried about that, Mr. Novice. The paper reinforcer will not affect images at all. The big mirror’s center is in the shadow of the secondary, and is not used.

Get yourself a sheet of paper not likely to scratch that pretty mirror, maybe some tissue-type wrapping paper. Use a compass to draw a circle on it the size of the primary mirror. Cut it out and fold it into quarters. Unfolding it will leave crosshair-like creases; the intersection of the creases will define the center of the mirror. Put a small hole there. Now, being careful, remove the mirror from the OTA. Don’t touch the surface, and for god’s sake don’t drop it. Lay the paper circle on the surface of the primary, making sure it and mirror are perfectly aligned, and using a marker or soft pencil, make a small mark through the little hole. Remove the tissue and apply a paper reinforcer (I like the self-adhesive kind) centered on the mark.

Once mirror is back in tube, we can get going with Step One, ensuring the secondary mirror is centered under the eyepiece tube. Insert the collimation tool and have a look. For now, don’t worry about all the funky looking reflections in the primary. Focus your attention on the surface of the secondary. Is it centered under the crosshairs? If it is off “vertically,” if it is rotated, you’ll have to rotate the secondary holder until the middle of that little mirror is smack under the crosshairs. If it looks off horizontally, move the secondary holder back and forth. Either is fairly easy to do with most designs.

Usually, the secondary holder is mounted to a threaded rod secured to the spider vanes (secondary support) with a pair of nuts. If you were off vertically, loosen one of the nuts and rotate the secondary holder until the mirror is centered under the eyepiece and tighten the nut back down. If the mirror needs to move horizontally, loosen the appropriate nut and move the secondary forward or back in the telescope tube until it is lined up. horizontally

Note that secondary mirrors don’t come from the factory with marks at their centers, and most of us don’t mark ‘em. You’ll just have to eyeball the middle, but eyeballing is good enough. Also note that the crosshairs on the end of the Cheshire will look out of focus to your eye. It won’t take but a little use, though, and you’ll easily be able to see them clearly enough. Centering the secondary is normally not something that will need to be repeated often.

After the secondary is centered under the focuser the hard work is done, and we are on to Step Two, adjusting the tilt of the secondary to center the reflection of the primary in it. Looking into the collimation tool, take a gander at the reflection of the primary mirror in the secondary. Do you see the paper reinforcer you went to such pains to paste onto the mirror? Good. Now, take a look the crosshairs. Are they centered on the paper reinforcer? No? Adjust the tilt of the secondary till they are.

How do you do that? It depends on the secondary mirror holder. If you are lucky, the holder will have three small knobs for adjustment. If you are less lucky, it will have three (or sometimes four) Allen screws. Adjust these until the crosshairs are centered on the paper reinforcer. Most secondary adjustment screws work in push-pull fashion. If a screw snugs-up before you’ve turned it far enough, you can loosen the opposite screw(s) to keep tighten the original one more. It is always better to adjust by tightening rather than loosening screws, as that will ensure everything is snug and the hard won collimation will stay good for a long time. The cardinal rule for collimating, however? Always move secondary (and primary) adjusters by small amounts.

We are now two-thirds of the way home. How about that? Is the anxiety beginning to evaporate like morning dew? All that remains is to adjust the primary. Now, we’ll be using the actual Cheshire part of the Cheshire instead of just the crosshairs. Please note that the crosshairs are not shown in the image at left--you don't need 'em anymore. What looks like crosshairs in the picture is the secondary support, the spider. OK, let's get that darned primary mirror done!

Turn the Cheshire in the focuser until there is enough light entering the hole in the tool’s side to illuminate the angled interior surface (don’t point it right at Mr. Sun, of course); you’ll know when the light is sufficient because the reflection of this surface in the primary (as you look through the peephole in the Chesihire) will turn from black to silver (or white). It will light up.

In the middle of the reflection of the Cheshire’s inner surface you will see a round black dot. That is the reflection of the peephole you are looking through. I tried to depict the situation as well as I could in the drawing above (click on it for a bigger version). Your final task is to center that black dot in the paper reinforcer ring. To do that, use the three adjustment knobs or bolts on the primary mirror cell (some imported scopes actually have six bolts, three for adjusting, and three for locking the mirror down--see your instruction manual). 

As with the secondary, but even moreso, move each adjustment bolt  by small amounts. This is where newbies usually get in trouble. They get way out and start “chasin’ the donut,” that dadgummed paper reinforcer. If that happens, stop, take a deep breath, and think about which way you need to go. Experimentally turn each bolt both ways until you find one that moves the black dot closer to the donut. When it’s as close as it can get using that adjuster (assuming you are not on the money), move on to the next one and turn it a little bit to see if it helps. Try both directions. If that doesn't help, move on to the next adjuster.

In the beginning, it helps to have somebody looking through the Cheshire while you tweak. It can be a godsend to have someone, even somebody not overly experienced in collimation, to tell you, “No, dummy, the other way. Good, now try another one.” Above all, do not get frustrated and turn any adjuster a large amount in a fit of pique (as Unk has been known to do occasionally).

The only common complication when it comes to primary adjustment is that, as above, some scopes, mostly imported Chinese telescopes, have three pairs of bolts on the primary cell. One set is the adjustment bolts, and one set is the locking bolts. Check the manual to see which is which, loosen the locks, and start collimating with the adjustment bolts. One thing I don’t like about this arrangement is that you usually have to collimate twice. On most designs, tightening a locking bolt will change collimation a little, so you’ll have to be careful to tighten all the locks sequentially by small and similar amounts, and keep checking the ol’ Cheshire. If a lock moves collimation, you will have to adjust another lock bolt to bring it back. Yeah, it's like adjusting the primary twice, darnit.

That is all there is to it. There ain’t no more to Newt collimation. Do the above, and your baby will be ready to face any adventure that might befall the telescope tribe. Oh, there’s one more check you can make. When it gets dark, turn the scope to Polaris, insert a medium-high power eyepiece, and defocus the star just slightly until you can see diffraction rings, until the North Star looks like a little bullseye (as in the picture below). Does it look like the bullseye on the right, or is it like the one on the left—squished? If it is squished, adjust the primary by very small amounts until the rings are as concentric as you can make them. Every time you adjust the primary, the star will move off center of course; always re-center it before adjusting further.

Catadioptrics

Schmidt Cassegrains

Newtonian adjustment, as we’ve seen, is just three lousy little steps. Not hard or bad at all. If you are an SCT owner, collimation gets even easier. You have only one measly step. You really do have to do it, though, since good collimation is even more critical for good SCT performance than it is with a fast Newtonian. “Why is that Unk Rod, huh, whyizthat?” Because an f/10 SCT uses a 5x magnifying secondary mirror to up the focal ratio of the scope’s fast f/2 primary mirror. That magnifying factor tends to exaggerate small errors in collimation.

OK, it’s important. How do you get started? The first thing to do is put away the collimation tools. Step away from that Cheshire. Because of the mechanical design of commercial SCTs, a Cheshire will almost always guarantee a badly mis-collimated telescope. OK. A laser then? Same-same. You will find laser collimators designed for use with SCTs, but they don't work well or at all (with the exception of the Hotech Advanced CT collimator, which might be a bit much for a novice to handle). With a Schmidt CAT, you need to do the adjustment the old fashioned way.

Actually, there is one collimation tool you use with an SCT. The good news? It is free: Polaris. Yes, you can collimate an SCT using an artificial star of some kind, but Polaris is easy and available every clear night in the Northern Hemishpere (if you are south of the Equator, just use any star of about magnitude 2).

I said collimation of an SCT only involves one step, and that is technically correct. The only user-adjustable element in Meade and (modern) Celestron SCTs is the secondary mirror. And that is all that needs to be adjusted for perfect collimation and performance. We can, however, break the process of doing that into four steps:

1. In the Ballpark
2. Rough
3. Fine
4. In-focus

In the ballpark. Most of you will be able to skip this step. The only reason to do “in the ballpark” is if your scope’s collimation is so far off that stars look like comet-blobs or worse, and it’s impossible to get it “in” using Polaris. I’ve seen novices get their CATs in this condition once in a while. Luckily, picking up the pieces is easy. Adjust the scope’s aim until you are looking straight down the tube from about two meters away. Observe the series of reflections of primary, secondary, and baffle tube. Is it all concentric? Does everything line up, or is the reflection of the secondary in the primary tilted off to the side? If it is, adjust collimation until it all looks centered. How?

You’ll find three Allen or Phillips screws on the secondary holder. Adjust these just like you did the screws on the Newtonian’s secondary: tighten by small amounts, and, only loosen opposite numbers to continue in the proper direction. Please note, y’all, that some older Celestrons have orange plastic covers over their secondary adjustment screws. Pop the cover off by gently prying with a screwdriver and put the thing away in a drawer somewhere. A few newer Meades may have three pairs of allen screws; see the manual for details as to which is which and what for. Caveats? The only one is that  you must never fully loosen all three screws at once. Do so, and the secondary mirror will go KER-PLUNK on the primary.

Rough. You are in the ballpark. Now comes rough collimation. Point the scope at Polaris and center the star carefully. Insert a medium-high power eyepiece. Defocus until the star looks like a Krispy Kreme, until you have a big white donut with a dark center. Is that dark center, the shadow of the secondary, in the middle or is it off to the side (before deciding that, double-check that the donut is as centered in the eyepiece as you can get it)?

If it is off to the side, adjust the collimation screws until the donut hole is centered. Which screw should you turn? At this stage of the game, I just pick one and tweak it a smidge. If that doesn’t move things in the proper direction, I switch to another. When the donut looks good, move on to the meat of the matter, fine collimation. Yeah, I know some folks stop with the donut, but that is rarely good enough for exacting tasks like high power planetary observing. Besides, if you are going to go to the trouble of doin’ the job, you might as well do it right.

Fine Collimation. Re-focus Polaris until it is almost sharp. Until it looks like the series of concentric bullseye rings. Once again, make sure it is as precisely centered in the field as you can get it. The rings are not concentric? They are squished on one side? You’ve got more collimating to do, muchacho.

What’s that? The bullseye is jumping around and boiling so much you can’t tell if the rings are concentric or not? If you think it’s the seeing, you can either wait a while and see if it settles down, or throw in the towel for the night. If the stars ain’t twinkling madly, the problem may be cooldown. If you just brung Miss Scope outside, give her a while to acclimate. How much power should you use for this stage? I like about 150x or so.

Time to finish up. If the bullseye is squished, you’ll adjust the secondary screws until it is unsquished. Which screw do you move? You could pick one like you did before, but this is more exacting work than rough collimation, and it can help to have a system. Lots of folks use lots of different methods to determine “which one.” Me? I like to keep things simple, so I use a slightly more complex variation of my low tech “pick one” strategy.

Look at the bullseye. Which side is squished? Move the star until that side is against the edge of the eyepiece field. Now, turn the collimation screws by small amounts until you find the one that moves the star toward the center (again, moving a collimation screw will move the star in the field). Use that screw (or screws, if necessary) to move the star back to the middle of the field. Keep moving the star to the field edge and re-centering it with the screws that until the rings form a perfect bullseye.

I’ll say this one more time: tighten screws to collimate. Only when a screw is snug should you loosen its opposite number(s). Usually you will then be able to turn the original screw some more. Doing that will ensure your scope stays in collimation—for months, or maybe even years. When I hear that an SCT owner is having trouble keeping their scope in collimation, not having at least one collimation screw snug is usually the problem.

I mentioned four steps, though. If you have very good seeing you can kick things up a notch with in-focus collimation. Center Polaris and bump the power up, to 300x or more, high enough so that when the star is in perfect focus you can see its Airy disk and first diffraction ring. Take your time and examine that first diffraction ring closely. Is it unbroken around the Airy disk? Just like the one on the right in the picture? Or is it broken? If it ain’t complete around the disk, your job is to move the collimation screws by tiny amounts until it is. Obviously, you’ll need a light touch and a steady sky to do this, but if you’ve a yen to look for Saturn's Encke Gap/Minima at 500x with your C8 (I’ve done that successfully), you need to get collimation as dead-on as you can, and in-focus adjustment is how to do that.

I said “no” to lasers and Cheshires for SCTs. Are there any accessories that help with SCT collimation in any meaningful way? Some new SCT owners replace their telescopes' allen or phillips screws with knob-headed bolts, either from the hardware store or from a commercial vendor, Bob’s Knobs. Do you need to do this? My verdict is "not necessary." If you collimate correctly, it will be a long time before you have to do so again, so the original screws are more than good enough. Some novices worry about having a screwdriver in close proximity to the corrector. This is not a big problem--I've yet to hear of someone scratching their corrector during collimation. Just take your time and take care.

What has caused problems for more than a few novices? The process of replacing the telescope's original collimation screws with knob-headed ones. Bob's Knobs come with excellent instructions, but some newbies have gotten into trouble during the procedure, nevertheless.

Schmidt Newtonians

A Schmidt Newtonian is like a Newtonian, but also like an SCT. What one is is a Newtonian without a parabolic mirror. The SNT uses a spherical primary, which would normally leave Saturn or anything else looking like a custard pie due to spherical aberration. To banish that, the SNT uses a corrector plate (lens) just like an SCT. Collimation? You adjust a Schmidt Newtonian just like you do a Newtonian. There is one small fly in the ointment: if the secondary has become rotated on one of Meade’s LXD 55/75 SNTs (the only ones out there in significant numbers today), the corrector lens to which it is attached may have to be loosened and rotated slightly to center the secondary under the eyepiece.

Maksutov Cassegrains

There are two possible collimation realities here: easy like an SCT, or potentially difficult. There are two MCT breeds, you see, Gregory Maksutovs and Rumak Maksutovs. Gregorys (or is that Gregories?) do not have a separate secondary mirror; instead it is an aluminized spot on the inside surface of the corrector. These scopes can be collimated, but it is sometimes not easy. The Meade ETXes, for example, require the disassembly of the rear cell to access pairs of push pull bolts that allow the primary to be adjusted. Not fun. The Synta-made Gregory MCTs Orion (and Skywatcher and others) sell, on the other hand, have exposed primary collimation bolts on the rear cell. To collimate a Gregory MCT, you proceed as you would with an SCT, just adjusting primary rather than secondary.

How about the Rumaks, which include the Intes-Micro MCTs? They have separate secondaries in holders just like SCTs, and are collimated the exact same way they are. If I had my choice, I'd take a Rumak. While the Gregory scopes can be collimated, it's sometimes more difficult than it would seem for several reasons. If you want "perfect," it can be, as telescope guru Roland Christen has said, a job for a trained technician and an optical bench.

Maksutov Newtonians

You don’t see many MNTs these day, despite the fact that they can be excellent telescopes. It's likely because they tend to be heavy for their apertures and a bit expensive compared to other designs. If you do glom onto one, you’ll find they are worked the same way as an SNT; you collimate ‘em  like a Newtonian.

Cassegrains

For the longest time, there not much need be said here. Non Schmidt and Maksutov Cassegrains were almost extinct in the amateur world. Those few worthies with Dall - Kirkhams and Richey Chretiens and Classical Cassegrains were almost always in the “advanced amateur” brigade, and knew how to collimate their telescopes. Today, that’s less the rule as inexpensive RCs fall into the hands of us rank and file amateurs. What do you do? Mostly you follow the directions for your specific telescope as outlined by the manufacturer in the manual.

What’s collimating these scopes like? I’ve done a Classical Cass or two and it ain’t no Classical Gas. The combination of an adjustable primary and an adjustable and strongly magnifying secondary mirror supported by a spider makes things, shall we say, "interesting." The inexpensive GSO Richey-Chretiens (sold under a variety of brand names) in particular can be a  pain. While they've been improved, their still somewhat deficient mechanical design can make collimation a hairline reducing and blood pressure increasing experience.

Refractors

Yeah, how about refractors? As you’ve no doubt been told by their fans, most will never need collimation. Most. Those that do? If it’s a pricy APO, the best bet is probably letting the maker do it. A Synta achromat with an adjustable cell? A Chehsire will make short work of that. Insert Cheshire and look in. Observe the reflection in the objective inner surface of the black dot (the peephole). Is there only one? Cool. If not, collimate until the two reflections of the peephole merge into one. Finish up with a diffraction ring collimation on Polaris.

If your refractor doesn't have an adjustable objective cell? Often there's enough play in the screws/holes that hold the focuser to the tube to allow you to collimate that way, by loosening those screws and tilting the focuser assembly one way or another--that's how I adjusted my old Short Tube 80. Such a collimation is not likely to be very precise, but you should be able to get close enough.

That does it. Now, you tell me, does that sound so bad? No. Is your scope in perfect collimation? Find out tonight (or this afternoon if you are not a CAT user). If it ain’t, make it so. I guarantee you will thank me the next time you get it out under the stars. If your babe has been in iffy collimation for a while, or maybe even forever, I guar-ron-tee it will be like havin’ a new scope. You may even decide your lemon is really a PEACH.

Comments:
Thanks for this post. When I got my first Dob, I anguished over collimation, reading post after post on the Internet. I finally bought a laser collimator and found it to be handy, but never understood or used my Cheshire. A few months ago I was out alone, had everything set up on a new scope and found the laser battery was gone--of course, I didn't have a spare 40 miles away from home and too far and too late to find one out in the boonies. The scope is a truss, so it needs readjustment and I wasn't able to get very good views that night, so the time and drive was wasted.

I WILL learn to use the Cheshire techinque!
 
Believe it or not, I have had my SCT since 1997 and I have never collimated it. Perhaps I should check it out...

Matthew Ota
 
Might not be a bad idea, Matthew...LOL

OTOH, if you don't take it on many road trips, I wouldn't be surprised if it's still in. DESPITE takin' my primary C8 on trips over plenty of rough country roads, I generally only have to collimate every YEAR or threee. ;-)
 
Rod, been waitin' for you to do this article. I'll refer newb's on Astronomy Forum to it along with Andy's video.

Thanks so much, and clear skies!

Bob
 
Re: collimation for Mak-Newt. Unca Rod says do it just like a Newt. Well, it's true some of the time and often not. The reason is that unlike a Newt, where the aperture stop is at the primary mirror, the aperture stop in a Mak-Newt is at the thick corrector. Thus you can't adjust the focuser tube, secondary and primary as you would in a Newt since not having the optical axis aligned with the tube axis is irrelevant (tubes are oversized and a reasonable misalignment there will not affect anything). For a Mak-Newt, this is a big no-no. Your optical axis *has* to be aligned with the tube axis - this is what makes collimating MN-66s hard. I learnt this the hard way collimating two Russian Mak-Newts.

TG
 
thanks unk that ruled in fine terms! can't wait to get call'matin i;m new but i'll be back! really liked your read on the history of the scts i found earlier aswell...cheers.
 
Thanks, you make things simple
 
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