Collimation techniques and tips

 

Kendrick 2" laser collimator for checking/aligning the secondary mirror (and focuser, during scope construction).  I've used three different laser collimators, and the Kendrick is hands down the best.  It has built in shims that can be used to ensure a snug fit in the barrel of the focuser (a necessity, since virtually anything done to move the laser causes a appreciable shift in the location of the laser spot on the primary mirror!)  It has a beveled face that allows you to see the return laser beam after it has bounced off the primary and secondary again.  (I don't use this feature, preferring the more fine-grained techniques mentioned below.)  It even has easy adjustment screws in case the collimator gets drops and loses alignment.  (http://www.kendrickastro.com/astro/laser.html)

 

"EZ Collimator" laser.  I have owned both the 1.25" and 2" versions of this product.  Its initial claim to fame was that it was among the first to contain an internal bevel that let the user stay at the bottom of a large closed tube telescope-- where the collimation adjustment screws are-- and still see the laser spot up at the focuser.  Nonetheless, construction quality is average, the units eat batteries quickly, and because it has six screws vice three, making any corrections to beam alignment is correspondingly harder.

 

Catseye Modified Cheshire for aligning the primary mirror to the secondary.  I "discovered" Jim Fly's ingenious modified Cheshire system shortly after acquiring Frankenscope in 2001, and have been an enthusiastic user ever since.  Pre-dating the "barlowed laser" scheme, it still rivals it in terms of ability to accurately align the primary to the secondary.  In essence, you align a reflective triangle-- on the primary-- within a hollow circle reflected from the back of the eyepiece itself.  The human eye is quite good at aligning geometric shapes like this, and can be quite precise when it comes to ensuring that the three vertices of the triangle intersect the circle at comparable points.  (Contrast this to the inability of the human eye to attain similar accuracy in placing a laser beam point onto the primary!)  It comes with an acetate template that can be used to position a reflective triangle precisely in the center of your primary mirror.  It comes in multiple sizes and variants;  I own the 2" Big Cat" version. (http://www.catseyecollimation.com/)

 

Tectron 3 piece tool set:  the "grand daddy" of collimation tools, I was never particularly impressed with it.  I found making my own peephole eyepiece (tool #1) out of a 35mm film canister to be nearly as easy.  Tool #2 (crosshair eyepiece) worked OK, but I was bothered by the coarseness of the crosshair wire and fixed length of the tube (which sometimes too long for use in a short focal ratio or large secondary mirrored scope.)  Tool #3-- the 1.25" autocollimator-- worked, but frankly was tough because of the low reflectivity of the mirror.

 

Catseye "Infinity II" autocollimator:  This is another Jim Fly product, and represents the autocollimator concept "done right".  It has a highly reflective first surface mirror (important when you are trying to see 4 "echoed" reflections), and perhaps more importantly, comes in a 2" format that lets you much more readily see alignment "near misses" that the 1.25" Tectron format wouldn't find.  The autocollimator is an excellent way to set or check the alignment of the focuser to the secondary mirror, especially in verifying that the mirror is not "skewed" but rather faces the focuser at a "dead on" 180 degree angle.  While the conventional wisdom is that you make adjustments to correct any misalignment the autocollimator shows by twisting the central hub of the spider or using the screws on the secondary mirror, it is in fact easier to use the shim screws on your focuser base plate-- if it has them.  This reduces the likelihood of needing to make a subsequent tweak of the primary mirror's collimation, as would be a near certainty if you adjusted the secondary instead of the focuser.  Autocollimation works at 4 times the focal length of your telescope, double that of a cheshire or barlowed laser, and is a great way to spot mis-alignment of the any element of the optical chain.

 

"No hands" collimation light:  my 24" Dob Brutus Main Page is just too large in size for my traditional approach of holding a red LED light in front of the aperture with one hand while I adjust the secondary mirror screws with the other.  (I can marginally make this work with the Cat's Eye, but absolutely cannot manage the ergonomics with the Autocollimator).  Therefore, I took some scrap ATM materials (pine, velcro, and a wing nut/bolt) and made a bracket that would securely mount (via velcro and a retaining 'ledge') on the front of the Dob across its Optical Tube.  This bracket had an adjustable velcro-faced pivot that could have a bright LED light (my Astrolite) velcroed to it (with the wrist strap run through the bracket for extra insurance.)  This allows the light to be set to whatever position is needed to fully illuminate the view through either the Cat's Eye or the Autocollimator.  As an extra 'plus', leaving it in place while both tools are used insures a constant light source placement and should reduce the (already probably minimal) collimation error stemming from different light source locations.   This is one of those gizmos I made on speculation, and which proved its worth the first night out!

The bracket affixed to Brutus.  The bracket and its simple parts.

Here are a few collimation-related tips and techniques:

 

use "tool-less" hardware on your secondary mirror if possible.  Anything you can do to make it easier to adjust this increases the likelihood that you'll do it, plus collimation wrenches can impart some torque to the position of the secondary holder while you use them, and this can throw your adjustment off from the "true" position.  While I have always bought (from Astrosystems) or built for myself "tool-less" four point thumbscrew adjusters, I broke with this on Natasha Main Page in putting on Protostar hardware.  Protostar uses a standard sized allen wrench (ball-headed, so the angle of insertion isn't crucial), which is a minor pain... but this is more than made up for by the 'clutch' system in the Protostar secondary hub which means that you can adjust a single collimation screw without having to take up or release pressure on the others (it features a three, not four point system).  This is a real convenience.

 

put wrist bands and restraining cords on any hardware used in collimation.  I once had an allen wrench land on a primary mirror.  That is NOT a sound you want to hear!

 

use oversized collimation knobs on the primary mirror. Simple geometry shows that, as you make the diameter of a circle larger, you increase the circumference proportionally.  Since this circumference is also the "turning radius" for your collimation adjustments, the larger the radius, the smaller the adjustments that can be made.  This is especially handy if you are using a system like the Catseye that can show you comparatively minor mis-adjustments.  Natasha and Brutus both have knobs ~ 5 times larger than the Dob standard (appliance levelers), and frequently I end up turning one "a gnat's whisker or so" as I near completion of my  collimation routine.

 

in the case of a primary cell where gravity instead of springs holds the mirror in place, if you turn a bolt counter-clockwise during collimation, point the scope to near zenith to "settle" the mirror against the pads before your finalize your collimation settings.  (This avoids you doing all of this finicky final collimation, only to have the mirror settle minutely on its sling or pads.  Obviously this is a non-issue if you have your mirror glued to the cell.)  The converse of this, of course, it to turn your collimation screws in a clockwise (tighter) direction if possible.

 

If you are star-testing, use a crosshair eyepiece to ensure that the star image is in the dead center of the field of view before you make your collimation adjustments.  It is difficult to judge centering visually with high accuracy, and even being a mere 5-10% off the actual center can cause the star-test to give a distorted image with fast optics.  I prefer a double-reticle crosshair eyepiece over a single crosshair to insure maximum accuracy of centering.  (Something like the "EZ Collimator" eyepiece with concentric rings works well, too.)

 

DSC Star alignment.  While not a collimation issue, one technique for placing the target star in the exact center of the field of view when performing alignment of Digital Setting Circles at the start of an observing session is similar to the star-testing technique above.  A crosshair eyepiece-- single or (ideally) double reticle produces the highest degree of accuracy when doing two-star DSC alignment, and can pay dividends throughout the observing session.  Simply "eyeballing" the alignment with a medium powered eyepiece is not likely to produce results that are as precise.  One "tip" though, is to use a medium powered eyepiece, and then once the target is as centered as possible, slip in a high powered eyepiece to tweak the alignment.  In my 24" f/4, the Meade 14 UWA (174X, and my default medium powered eyepiece) yields a half-degree FOV that can be used to roughly center the star, then the Nagler 3-6mm zoom can be used. Slipping the Nagler in (406X at 6mm) and without changing the focus, the defocused star image almost fills the field of view, so centering it is relatively easy and yields DSC alignment nearly as precise as that produced by crosshair eyepieces.