September 24, 2017

Mirror Collimation; Some Tips & Tricks.

This week, we here at ye ole Astroguyz are going to delve into an oft avoided but crucial technique that will allow you to get the most out of your shinny new (or old!) reflecting telescope; the fine art of mirror collimation. Sure, nearly every owner’s manual gives you a how to, but I’m going to share some neat tricks learned in the field through years of mistakes and experience. Ready? Let’s collimate!

Any type of optical system that uses mirrors will occasionally require a re-tweaking of its alignment. Refractors neatly avoid this procedure, but of course, the manufacture of large, canon-sized apertures are prohibitively cumbersome and expensive. A majority of reflectors and virtually all commercial scopes come in two types; Newtonians and Schmitt-Cassegrain Telescopes (SCTs). The beauty of an SCT is that there is only one adjustment that is necessary (or even possible); that of the secondary mirror. Newtonians will more frequently require this maintenance procedure, and if your scope has a truss tube assembly that must be constructed in the field, expect to re-collimate virtually every time you use the scope!

So, how do I know if I need collimation? How good is good enough? Here are some quick checks;

- Center a bright star (about +2 to +3 magnitude) and about 100x and throw it out of focus. Do you get a nice concentric donut, or is it pinched to one side? Does it look the same on both sides of the focus?

- Center a bright planet such as Jupiter or Saturn at high power, as high as seeing will allow. Does it focus tack sharp across the field, or only focus on one half at a time?

- Under steady skies, does a bright star present an Airy disk?

If the answer to any of these is no, your mirrors probably need tweaking a bit. Probably 90% of your observing “happiness” quotient is dictated by light pollution (or hopefully, lack there of) and atmospheric seeing; proper collimation can help you grab that last 10%. For planetary astrophotography or splitting challenging close double stars, it’s absolutely necessary.

That being said, a slight warning is due. Do not let your obsession with perfect collimation interfere with your enjoyment of the night sky! Like polar alignment, there is always a tiny room for error. If you stare at any star test image long enough, your mind can convenience you that any telescope is out of alignment. Like medicine, the first rule of telescope maintenance should be to “do no harm” … constantly going for that “perfect mirror alignment” can lead to an hours-long session of trying to bring back the nearly perfect initial alignment of mirrors that are now badly out of whack. And all the while, those pristine skies are wheeling overhead…

Now, down to the nitty- gritty. First, the Newtonian telescope. I like to do initial alignment in the daylight, and then fine align on a test star in the dark. I’ve read some manuals that tell you to focus on some daytime wonder such as a flashlight or the chrome on a bumper down the street, but in practice, I’ve never had much luck with these. Nothing really beats a star for as an infinitely small pin point light source.

During initial daytime calibration, I like to rough align the mirrors and then do final tweaking under twilight skies. Then you’re ready for some serious deep sky action once astronomical twilight rolls around. A minted Astroguyz pattern for quick collimation is this; Rough tune primary, rough tune secondary, then fine tune primary, fine tune secondary.

First, the heart of the telescope, the primary mirror. Most have two sets of three screws located to the aft; locking screws and adjustment screws. Large scopes will typically come with a tiny black circle imprinted on the reflective center; this circle is for alignment with a laser collimator. However, let’s assume you are poor, like Astroguyz, and insist on doing everything cheaply or from scrap. To adjust the primary tilt, first assure that all of the clamps are off. Again, I like to do this in the daytime, looking straight down the focuser sans eyepiece; a set of homemade cross hairs can also help. I’ve used dental floss or a webcam setup, which we’ll get to in a moment.

It’s worth also noting that only tiny movements are necessary… even a knob turn of 1/10 can drastically change the mirror alignment. Remember, our goal is to do no harm! It is also extremely handy if you can recruit a second pair of hands; that way, you, with the experienced eye with the $1,000+ plus dollar investment, can watch which way the mirror moves as the other person turns the knobs. Movement can be counter intuitive; most of the time, we can see which way the mirror needs to go, but it may take a bit of experimentation to figure out how to get there. Many times, it’s a combination of one-knob-looser, two-knobs-tighter. And don’t forget, we’re coming back to the primary again; we only need to be close on rough alignment.

A word to all of the would-be gorillas out there; optical maintenance is a gentle art! Do not tighten the screws to 1,000+ foot pounds just because you’re Lou Ferrigno and you can; we don’t want risk warping anything. Gently snug (a technical term!) is fine. Also, the setting locking screws can occasionally tweak the mirror; that’s why we always finish with the secondary, to compensate.

Speaking of which, now on to the secondary. Most have three tilt adjustment screws under a plastic cap. A fourth screw in the center adjusts the rotation angle of the entire mirror. Do images look a little pinched or squished? This simple screw could be the sole culprit.

Now, another word of warning is in order. At this point you will be going at the front end of your telescope with the stuff of nightmares; a pointed screw driver. If the scope has a corrective plate in the front, it is imperative that you do not drive the screwdriver into it. (Another reason we do initial adjustments in the daytime!)† If you have the open tube Newtonian variety, its double-dog imperative to perform this procedure with the optical tube mounted horizontal, so the screwdriver cannot be dropped inside onto the terrified primary mirror! Don’t say Astroguyz didn’t warn you…

The alignment of the secondary is pretty similar to the primary; turn and observe, in tiny increments. Do not loosen anything too far, or you’ll hear the entire secondary fixture come off, and you’ll have a whole new set of problems.† The right tools for the job are a set of jewelers’ screwdrivers; I always keep these handy in the field.

Now you’ve rough aligned both mirrors. I would then return to the primary, and re-tweak, if it needs it. Now we are ready for fine alignment.

At this point, we will move ahead to collimation of the SCT, as it’s absolutely the same as final tweaking of a Newtonian. Again, the only adjustment necessary is those three set screws under the secondary cap, and all other warnings and precautions apply. Most SCTs only rarely need this done, another plus to ownership. I find that once a year is enough for my trusty star party 8″.

So, we’ve tweaked and nudged everything; it’s down to a final star test. Just what are we looking for?

I find a webcam laptop setup immensely handy for this phase. I use K3CCD Tools, which allows me to overlay a virtual reticle and judge seeing. Also, I can work from the front, i.e., turn the screws while watching the test star in the screen. The shots look something like a series of rings.

Of course, this setup isn’t compulsory, and I usually find myself making the final judgment call the old fashioned way, eyeball-to-eyepiece. An eyepiece equipped with its own crosshairs will work, as well.

So, is that doughnut REALLY a perfect circle? Probably not. Fatigue, cruddy seeing, and just plain second guessing can all vex the astronomical soul. Barring the aforementioned cross-hairs, lasers, and virtual reticle(s), there is another, simple way to judge the “roundy-ness” of your out-of-focus test star; simply turn your drive motor off. That is, if you have one! Now, time the drift from the edge of the out-of-focus test star, to the center, to the other edge. Do this a few times for a better estimate. Of course, this method has some draw backs. Shoddy seeing will thwart your efforts, as stars will look like you’re viewing them from the bottom of a swimming pool.† It also only measures your potential offset across a 180 degree axis. The perpendicular axis can be tested by timing adjustments in this direction via noting the travel distance with fine tuners, either manual or electronic.

In closing, collimation is an easy optical house keeping chore that’ll greatly enhance your enjoyment of your instrument. Feel free to be frustrated at first, and always remember that even great telescopes had initial teething pains . At least your scope is not a Nasmyth focus, equipped with tertiary mirrors! Do give those mirrors a check from time to time; your scope will love you for it!

Trackbacks

  1. [...] Collimation, or the near-perfect alignment of optics, is key to the splitting close binaries, and also serves as a good test of a telescope and the stability of the atmosphere. A well-collimated scope will display stars with sharp round Airy disks, looking like luminescent circular ripples in a pond. We call the lower boundary to splitting double stars the Dawes Limit, and on most nights, atmospheric seeing will limit this to about an arc second. [...]

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