Saturday, October 09, 2010

 

SCT Collimation the New-fangled Way


Hardly seems possible it’s been eleven frackin’ years since I started my SCT-User Yahoogroup. I’ve had a lot of fun with it, and it’s still quite active, though time seems to be passing the good, old Yahoogroups by in favor of glitzier venues. I will admit SCT-User has occasionally been a lot of work, but I knew that going in. Knowing that it would be real work, why did your lazy old Unk start SCT-User in the first place?

Two reasons. First of all, the mother SCT list, the ancestor of all SCT mailing lists, CelestronUser, was dying. I’m not sure why it was giving up the ghost just as Internet astronomy was taking off. Maybe because it was a Major Domo list—referring to the mailing list software running on a university server. It was far from friendly for either users or administrators, and with the new eGroups web-based mailing list system (which would eventually become Yahoogroups) being much more friendly and full-featured, it was no surprise text-based Internet discussion was gravitating to this More Better Gooder.

With CelestronUser going down the virtual tubes, I thought it appropriate to start a new list on eGroups. I was gonna name it Celestron User, just like its antecedent, but just I was about to click the “go” button, I hesitated. Why not appeal to all SCT users, not just Celestron fanatics? Thus was SCT-User born, and long may she wave.

The other reason? I was gobsmacked that in 1999, nearly thirty years after the popularly priced Schmidt Cassegrain hit the streets in the form of the Orange Tube C8, plenty of SCT owners still weren’t able to correctly collimate their telescopes. Some didn’t collimate at all. Worse, most of ‘em didn’t understand why proper collimation is so important with these scopes. Many of these people attributed the poor performance of their telescopes to BAD OLD SCT OPTICS instead of pilot error, their failure to properly collimate or collimate at all. SCT User’s prime directive in the early days was getting the word out on the hows and whys of collimation.

I think we got the job done, spreading the word first to online SCT Users, and then out to our clubs. What did we preach? That collimation ain’t hard. That it’s, in fact, easier to do on an SCT than on a Newtonian (the SCT has only one user adjustable element; the Newt has two). Even more than how easy it is, we tried to impress on users how important it is. Given the SCT’s 5x magnifying secondary mirror, that’s not surprising. The tiniest alignment errors are magnified. Good collimation makes the difference between excellent optical performance and poor optical performance; especially on the planets.

Coincident with our crusade, a small SCT collimation cottage industry sprang up. Most significantly, a man named Bob started selling a collimation aid for Schmidt Cassegrains. Bob’s idea was an humble one for a simple product, but one that was sorely needed. The only puzzle is why it took till the 90s for somebody to begin selling it. What “it”?

Why, KNOBS, Bob’s Knobs. Nobody ever much liked using Allen wrenches to collimate SCT secondaries. Bob’s Knobs provided a much simpler no-tools path to collimation, and when Celestron in their “wisdom”—NOT—changed to Phillips screws for secondary adjustment, the Knobs really took off. I mean, who wants to bring the sharp tip of a screwdriver anywhere near their beautiful corrector? Especially in the dark.

Yep, Bob’s Knobs saved the day. They are simple, yeah, just knurled knob-headed screws that replace Celestron Phillips screws or Meade Allen screws, but they make collimation, if not always fun, at least easy and safe. Bob wasn’t the only person thinking about SCT collimation accessories, however. Some folks were staring at the elephant in the front parlor: why couldn’t SCT users collimate with a laser just like Newtonian users were now doing?

Why would we want to do such a thing anyway? We were now happily tweaking our telescopes to perfection with Bob’s Knobs, after all. Because we were jealous of what the Newt troops could do with their handy-dandy laser collimators. Not only did they, unlike we, not need a star, real or artificial, for collimation, they could make their adjustments in the daytime. In the cotton-picking dining room if’n they wanted to. Dang.

This was during the years of the go-go astronomy marketplace about a decade back, so it’s not surprising several firms picked up on SCT owners’ laser lust. The most famous of these outfits was Kendrick Astro Instruments, who were well known and respected as the folks who commercialized (if not invented) the dew heaters we all use today. When they came out with an SCT laser, I was excited. Given the company’s reputation, surely it would be just what we were hoping for. Unfortunately, doing an SCT laser collimator was a tall order, and Jim Kendrick and company came up just a little short in this old boy’s opinion.

What made it hard to do an SCT laser? Why can’t you just use a pea-picking Newtonian model? Because the modern SCT has only one user adjustable element, the secondary mirror. For a standard laser collimator to work, ever’thing else—the primary mirror, the baffle tube, the rear port, the visual back—has to be precisely aligned and has to stay that way. Which isn’t often the case, but which doesn’t matter for visual collimation.

It really wouldn’t make any difference if our scopes still had primary mirror adjustments, anyway, since a standard laser inserted in the visual back only reflects off one element, the secondary. If the primary is slightly out of whack, the laser won’t “know” about it. Laser collimation in this fashion is a recipe for SCT MIScollimation.

So how did Kendrick get a laser to work with a Schmidt Cassegrain? By means of a clever idea. You’ve seen some of the modern collimators that allow you to observe the position of the return beam in a little window cut in the side of the laser, aincha? That’s what Kendrick did. Only instead of a bulls-eye target for the return beam, there was a grid. Adjust the secondary so the beam fell on the proper place on the grid, and you would be collimated.

The problem with that was knowing where on the grid that beam should go. The only way to find out was by collimating the telescope precisely the old fashioned way, on Polaris, first. You then inserted the laser, observed the spot on the grid where the beam hit, and marked that with a little sticker. When the scope went out of collimation, all you supposedly would have to do would be to adjust until the beam was on the sticker again.

A good and reasonable idea, one that sounds like it ort-ta work. Alas, it didn’t, not very well, anyhow. Given the mechanical exigencies of mass produced SCTs and their moving mirror focusing, returning the beam to the sticker rarely produced a dead-nuts-on (re)collimation. Yes, the Kendrick unit COULD be useful, as when you were faced with a sky full of sucker holes, your scope was badly out of collimation, and you wanted to see something in brief patches of clear sky. Under those conditions, the (no longer made as far as I can tell) Kendrick SCT Laser could produce an acceptable alignment.

But as amateur astronomers always do, we wanted more better gooder. We wanted to be able to do a precise collimation, and we didn’t want to have to tweak the scope in on Polaris first. That was a tough nut to crack, though, and nobody was able to for years. Not until word began to spread about a new SCT collimator from Hotech.

When I heard David Ho was marketing a new SCT laser, I was skeptical. Yeah, I knew the man’s reputation for quality, but we’d been down this road before hadn’t we? Still, I kept my ear to the ground, and soon divined that the Hotech Advanced CT (“Cassegrain Telescope,” I reckon) laser was entirely different from the Kendrick or any other laser collimator I’d heard tell of. I was told the unit was fully capable of producing perfect collimation all by itself. And I was hearing that not just from average fumble-fingered Joe Amateurs like yours truly, but from people whose opinion I respect, like imaging guru Craig Stark.

So, naturally, I was anxious to get my hot little hands on the CT for a review, and when David H. agreed to send me an evaluation unit, I was right excited. And maybe a little bit apprehensive. One other thing I was hearing through the grapevine was that using the CT required a far more complicated setup than just sticking a laser in the rear port. Would I be able to make the dang thing work?

When the collimator arrived, I was immediately impressed by Hotech’s attention to detail. This is a nice package, with the laser and its accessories contained in a pretty fabric case. In addition to the collimator itself, I found, among other stuff, a pair of eyepiece-like thingies, one 1.25-inch and one 2-inch. Turned out these are mirrors you place in the scope’s visual back in order to get the beam from the laser, which is pointed down the corrector end, propagating through the telescope’s optical system, hitting the primary mirror as well as the secondary.

How about the laser itself? It is a laser of a new type; actually it’s an enclosure equipped with multiple lasers. There are four of them mounted on the unit’s back. The whole shebang is powered by a single (included) CR123 battery.

What else? There was also some scary-looking stuff. Several finely ruled measuring tapes, a fabric strap with a metal buckle, and some funny looking plastic “tabs.” Looking at the tapes, I felt fear returning. In addition to being told I’d have to precisely square the laser with the front of the collimator, I had heard that would be particularly difficult with a Celestron SCT (all I own). Ah well, we’d cross that bridge, yadda, yadda, yadda.

Also in the shipping box was additional and, I believe, optional item, a tripod fine adjuster. You’ve no doubt seen these before: altitude-azimuth fine-tuners that attach to a photo tripod’s head. If you are contemplating the CT, I urge you to get one, as being able to make fine adjustments of the laser is extremely helpful, and can preserve your sanity and your hairline. A sturdy tripod is also desirable—the CT is mounted on a ¼-20 thread tripod for use. I used our Manfrotto, though I found an el cheapo Focal tripod (K-Mart) was at least useable. The fine adjuster is more important. The Hotech furnished adjuster is not exactly a premium item; the one I received was sticky in altitude at first, but it worked fine, which is all that matters.

What next? A look at the manual. If I can find fault anywhere with the CT system, it is with the manual. It is sufficient, OK, good enough, but could be better. Some of the pictures in this laser-printed sheaf of pages are too small and too dark. And some of the wording is a bit awkward or unfamiliar: “focuser” instead of “visual back,” “intercept” when “overlap” is meant, “tangent” when “perpendicular” would have been a better word choice, etc. However, the instructions are, again, good enough; especially when used in concert with the instructional videos available on Youtube. I have suggested to Mr. Ho that it would be nice to have the manual on a CD or DVD as an Acrobat file with larger color pictures. I also think it would be good for the videos to be on an enclosed disk.

Let me make sure I am clear about something rat-cheer muchachos. I know you are used to me being cavalier about and joking about instruction manuals. I’ve often told y’all that the first thing I do when I get a new piece of kit is throw the consarned instructions in the old rubbish bin. Not this time. Uh-uh. READ THE INSTRUCTIONS. Then read ‘em again—several times. Watch the videos. Then watch them again, several times. I guar-ron-tee that if you don’t, you will never, ever figure this thing out. And keep them instructions at your side your first time using the CT. You will need to look at them again.

“Hokay. Time to get started. Gimmee them dadgum instructions.” Which was when I hit the first landmine. At the get-go you are warned not to do the collimation on carpeted or wooden floors, only on a sturdy surface with no give. Waaalll…I got news: Chaos Manor South ain’t got nothing but wood floors, and I suspect many other folks, even those who don’t live in ancient Victorian manses, have the same. I could go outside, but one of the beauties of this rig, I thought, was that you could use it inside. When all was said and done I found I didn’t have any trouble collimating on a wood floor as long as I used our sturdiest tripod for the collimator.

The first step in the CT collimation procedure is getting the collimator properly square-on to the telescope. Doing that with a Meade is fairly easy. With the CT in mode 1 (it has several modes), which projects a crosshair pattern, you aim the scope and collimator at each other so three screws inside the tube near the mirror are illuminated. The trouble for a Celestron Man such as Unk is that Celestrons do not have three screws spaced at 90 degree intervals at the mirror cell end. I’d have to make my own marks via the measuring tapes and the “tabs.” Rut-roh.

Whether you’ve got a Meade or Celestron, before you can point the collimator to the screws or tabs, you first have to place the collimator the proper distance from the scope. That will depend on the scope focal length, but it’s less than one tube-length in front of the corrector for my C8. Start out about there, move the collimator and tripod forward or back until the crosshairs are their smallest, then move toward the scope till the crosshair pattern projected onto the collimator’s face extends to the first ring on the target.

Once you’ve achieved a good crosshair return on the CT’s face, you are approximately aligned as well as at the right distance. But you must be precisely aligned to produce a good collimation, and, as above, that’s a little more involved with a Celestron than a Meade.

Referring to the manual, I grabbed one of the lengths of measuring tape. That scary measuring tape. Actually, while it looked scary, it was surprisingly easy to use. No critical measurements are required. Wrap the tape around the rear cell and mark the overlap point on the tape with a pencil or whatever. Remove the tape, keeping it overlapped. Fold it twice so it’s one quarter its length, and mark a starting “0” position spot on the top of the telescope’s rear cell with a pencil or use some other convenient reference. My Celestron has a screw on the rear cell perfect for that. From there, use the folded tape to mark two “90 degree” positions on either side of the initial 0 mark.

Once that was done, all that was required was that I wrap the supplied fabric strap around the rear cell, fasten its buckle, and slide the three provided plastic tabs under it at the three marked positions. It seemed a little Rube Goldbergish, but worked fine and was more than precise enough. In practice, I found I could tell when the collimator was properly aimed just by eyeballing the three laser spots on the edges of the C8’s mirror. The tabs helped get everything lined up, though, and if you are collimating a Celestron, you should definitely use them. In fact, even if you have a Meade I think it might be a good idea to use the tabs; I believe they’ll speed up the process of aiming.

Getting C8 and CT positioned with the three laser beam crosshairs on the tabs (in the case of a Meade that would be “on the screws”) and the crosshair reflection on the right spot on the collimator target took some doing, crying, and whining the first time out. The problem is that you’ve got so much freedom of adjustment when you combine all the possible movements of collimator and scope that it gets confusing. One trick I did discover was that having a tripod with a tilt adjustment as well as altitude and azimuth saved me from doing a lot of leveling of scope and collimator.

How do you keep out of trouble and resist the urge to throw the whole thing against the wall? FOLLOW THE INSTRUCTIONS! If you do, you’ll find the alignment process is a two step affair with no mystery to it. First you aim the laser at the tabs (or the screws), then you aim the telescope so the projected crosshairs are centered in the first target ring on the CT. You keep doing these things in turn, adjusting by small amounts till the tabs or screws are illuminated and the crosshairs are centered on the collimator.

Even with a fair idea of what I should be doing thanks to the manual and the videos, I easily spent a solid hour getting everything sussed the first time. Like anything else, though, what was difficult that first time was laughably easy the second. The second time, getting scope and laser lined-up took me all of 15 minutes.

Since, as SCT mavens will tell you, the Schmidt Cassegrain’s mirror can change position slightly depending on whether the tube is level or pointing at the sky, it might be a good idea to collimate with the laser elevated, so the tube can be at an up-angle. The first time, however, you will have your hands full getting the feel of things, and I urge you to get comfortable with the system with the CT and scope both level. Having the scope on an alt-azimuth type mount helps, too. A lot. While my C8, Celeste, normally rides on a German equatorial mount, I placed her on my Synta AZ-4 alt-az for collimation.

With scope and laser properly positioned near as I could tell, it was time to DO THE DEED, to collimate. Which turned out to be slightly anticlimacitic. Once you master the art of squaring CT and SCT, all you gotta do is go through the old “which screw do I tweak?” routine.

Before you can begin collimation, you must turn the rotary switch on the top of the collimator to one of its other modes. Mode One, which we’ve been using thus far, projects a crosshair only. Mode Two turns on the collimation lasers, and Mode Three provides a combination of collimation laser spots and crosshairs. I chose Mode Three so I could be sure the telescope was still properly aimed (I am quite famous for kicking tripod legs, day or night).

Only problem was that when I switched to Mode Three, I could only see one big laser spot. What the hey? I rechecked my alignment and everything was fine. I was badly puzzled. I could see three sharp little laser dots on the face of the CT if I adjusted things a certain way, but from the instructions and the videos I knew I should be seeing three fat spots. I fiddled and fiddled, but nothing helped. Was I done?

Thankfully, your silly old Uncle had the sense to read the instructions one more time. There was indeed an entry on “what to do if you can’t see the laser spots.” The first suggestion was to check the return mirror in the visual back. That seemed OK, though I had a rather difficult time getting the 1.25-inch model, which uses a compression ring system to hold it snug, tight enough. OK. What next?

The second troubleshooting suggestion was slightly confusingly worded, but the meaning was clear. The telescope focuser should be close to the in-focus position it would assume with an eyepiece inserted directly into the visual back. Hey! Wait a minute! The last time I used Celeste has been with the Stellacam II and the Meade f/3.3 reducer, which throws focus way off from visual. Maybe…

Sure enough…some turning of the ol’ focus control and three laser spots magically appeared on the target. With them visibile, the rest is easy. If they are not all on the same bulls-eye ring, twiddle the collimation adjustments till they are. Since Celeste’s were somewhat off, I did that very thing. The instructions caution you to check your scope on Polaris after your first time collimation, just to be sure the collimator (and you) are setup properly.

That very night, luckily, I’d scheduled a trip to our local club darksite. I admit to being just slightly apprehensive when I turned the scope to the North Star and defocused slightly. I sure didn’t feel like going through a full-blown collimation session. But I didn’t have to. The diffraction rings were nicely concentric with the star centered. I didn’t have the seeing to check the appearance of the first diffraction ring of an in-focus star, but I will.

Yep, I’m not quite done yet. Look on this as a preliminary report. I plan to do a more in-depth one in my column, “Uncle Rod’s Cracker Barrel” in the next issue of Amateur Astronomy Magazine. In addition to seeing how close the CT can really get collimation, I want to see how much I can improve my setup time. Next time I’m going for “five minutes.” I will also try a different, older C8 OTA we have around here, one that ain’t been collimated in years and is no doubt substantially off.

“But what’s the quick-look verdict, Unk?” I hope to improve my setup time, but this unit will never be as quick to use as a Newtonian collimator. On the other hand, it’s likely to be significantly quicker than using an artificial star in the daytime. It works, and it does not require a lot of space. Hell, it hardly requires any space. It’s nicely made, and while the instructions/videos can be improved, they got me through my first time. Yes, there is a learning curve involved, but most worthwhile things have one. Expect to spend some time figuring out the CT, but when you get to that “OH YEAH!” moment, I suspect you are gonna be right happy with this thing, muchachos. Yeah, I know it ain't exactly cheap at $455.00. But for many of us that will be well worth it, and is less than some of us have got used to payin' for one more eyepiece. I give it one thumbs up, and, after another week or three of working with it, I hope to be able to give it two.

Next Time: We’ve had some nice, clear weather and your silly old Uncle is just TEARING UP the Herschels. Naturally you will hear all about it in the first of two H-Project reports next week.

Comments:
Woa there unk,

"Stellacam II and the Meade f/3.3 reducer"

???

Just had my theories punted their Rod, can you tell me why you use the 3.3 with the stellarcam in preferance to the 6.3 reducer.

all the best

Andy.
 
Hi Andy:

For most objects, the Stellacam, with its small chip, cries out for "more field" and performs much better with the 3.3.
 
Thanks mate, more sound info.

Andy.
 
Hi,

I have the CT as well for my C11 but i'm challenged with equatorial (EM400). I haven't been able to get it work precisily (even far off). It seems to work well on fork mounts. Any ideas / observations on this type of collimation in my configuration?

A frustrated user at this point :-)
 
i am very pleased with my $100 hotech 3-red 1-white green laser astronomy flashlight, but $455 for a collimation gadget struck me as just plain peculiar. with bob's knobs, manual collimation is quick and convenient; and i enjoy wrestling that little dot into the center of its rings. a small satisfaction of craftsmanship in an otherwise fully computerized and automated experience.
 
Hi Andy
I know you say its an expensive piece of kit and it works thats the ooint but if you just care to have a look at http://stargazerslounge.com/diy-astronomer/136406-colimator-project.html
And see what you think as at the moment this is my version of a collimator (look familiar?) But this stands me at less than £10 What do you think??

Another Andy
 
Bruce, I hear what you are saying and agree, but after my now infamous and unthinking moment of scope fetteling (see corrector screws post on yahoo sct/asgt/c8 groups)the Hotec would have saved a VAST amount of worry and grey hair after I screwed up big time.

Only problem is the cost but if you are planning to keep with sct's it does make sense [or even a group/club purchase]

cheers

Andy.
 
Thanks for a really good review. I was considering the CT but decided that the cost was a bit too much for me. This would be a great club or group purchase but the current 3/2012) price of $455 is pretty steep for someone just getting started.

I chose to use another HoTech laser http://www.hotechusa.com/category-s/22.htm in conjunction with an optical technique http://www.mira.org/ascc/pages/lectures/collim.htm and some bits from the old Kendrick method http://www.kendrickastro.com/astro/pdf/sct_collimation.pdf.

I can use the HoTech SCA laser on other scopes as well so it was a versatile tool. The "smack me in the face" realization occurred when reading "This ball of light is being projected onto the target by the primary mirror of the SCT and is following the precise optical path that your telescope is currently collimated on. This light is the extraneous scattered light that surrounds all laser beams. It is being bounced off the secondary mirror onto the primary mirror and out through the front of the telescope while the main beam of the laser is being directed back to the face of the collimator." in the Kendrick procedure.

Using a laser in the rear cell is very similar with one major difference being that the light is only making one trip thru the optical path.

Thanks for some really great info. I really enjoy your down to earth writing style.
 
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