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The Easy Way to Align Your Telescope's Optics: Indoor Artificial Star Collimation
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The Easy Way to Align Your Telescope's Optics:
Indoor Artificial Star Collimation
By Derek Wallentinsen
Keeping your telescope collimated is a task, a chore, money out of pocket or some combination of all three for most scope owners. Many avoid collimation for just these reasons.
Star testing is one way of doing it that's relatively simple. Just get your scope, a clear night and some knowledge of what a diffraction disk looks like for a collimated scope, right? Right!
Except it's not always clear. Even if it is, maybe the seeing is not that great and the stars you want to use are distorted so you cannot possibly fine-tune your scope under those conditions. Or the night's too good and you “forget” to do it, procrastinating because your urge to observe is too strong.
How about doing it inside under controlled conditions where it’s cozy and convenient? There is a way. You can use an artificial star to collimate your telescope. It's not only controlled, cozy and convenient, it's also cheap. Hubble Optics came out with its “5-star Artificial Star” a few years back. I got mine for $15 on the introductory offer, did some math and worked out a method to successfully align my optics in my apartment without too much fuss. First, I'll show how I do it and then I'll cover the theory, math and details behind the technique.
Indoor Artificial Star Collimation
1) The setup
The basic setup is very simple and consists of three parts or components (Fig. 1) - the telescope to be collimated, a spherical mirror and an artificial star. For my example, the telescope is a Celestron C5+, a Schmidt-Cassegrain telescope (SCT) optical tube. When collimating indoors, you don't have to use a tracking mount. Stability is the major requirement and some aiming capability is convenient. A sturdy tripod fulfills these needs and I chose the Astro-Tech Voyager to use with the C5+ (Fig. 2).
|Fig. 1: The basic setup consists of a telescope, spherical mirror and an artificial star. Here the components are in configurations A and B. (They are spaced for illustrative purposes only. In a real setup the components would be farther apart.)|
|Fig. 2: The Celestron C5+ optical tube is mounted on an Astro-Tech Voyager tripod. This is a stable combination with fine pointing capabilities that works well for collimation.|
The spherical mirror is the second optical part of the configuration. It is a convex mirror. As explained in more detail below, it's used to shrink or reduce the apparent size of the artificial star. I like discarded secondaries from SCT scopes. Usually I choose those from 8” or 11” scopes. Here it is from a broken 11” SCT. (Fig. 3)
|Fig. 3: A secondary from a broken 11” SCT optical tube serves as the spherical mirror for collimation.|
Last is the artificial star light source. The Hubble 5-Star flashlight has several bright blue-white LEDs and a steel mask with five precision pinholes ranging from 50 microns to 250 microns diameter (Fig. 4). In front of the light is a black rectangular piece that is basically a refrigerator magnet. It’s placed on the light when testing begins to block light from all but the one pinhole you actually use as the artificial star (Fig. 5).
|Fig. 4: The Hubble 5-Star flashlight energized and showing light from all 5 pinholes that serve as artificial stars.||Fig. 5: The Hubble 5-Star flashlight with pinhole cap in place. The cap has the mask with precision pinholes and blocking magnet covering all but one pinhole.|
Next you need to find a place with at least 25 feet of clear line of sight. Here are pictures of a setup in my apartment, which was “New York City style” with a long hallway (Fig. 6). At the bottom of the picture is the scope (shown here by an out-of-focus view of the eyepiece), which is pointing down the hallway to a folding table at the end. The table supports an equipment case, the spherical mirror and the Hubble flashlight (Fig. 7).
|Fig. 6: A typical test setup for indoor collimation. The telescope is at the bottom of the picture, separated by a long hallway from the artificial star and spherical mirror components at the top. (As explained in the article, this is an example of configuration A.)||Fig. 7: The table supports an equipment case, the spherical mirror and the Hubble flashlight.|
I’ve used two configurations that I call A and B. Configuration A (Figs. 1 and 6) is featured here. The scope and mount are placed about 25 feet from the small tabletop containing the flashlight and convex mirror which are within 12-36 inches of each other. The alternative configuration B (Fig.1) places the scope and the light parallel and physically either side by side or close to one another and pointing towards the mirror located about 25 feet away. While configuration B gives the smallest apparent size for the pinhole, either setup can give a suitably small image of it to get a diffraction pattern for collimation. With configuration A, the artificial star is brighter and easier to work with in the field of view of the scope.
To collimate, start by physically positioning all the components. The flashlight, OTA and mirror should all be about the same height. To produce a suitable diffraction pattern within the focusing capabilities of the scope, the separation between the two stations should be about 25 feet or greater. Darken the test area. Make sure it’s free of drafts and heat sources so as not to affect seeing. (Otherwise, you might as well go outside!) Turn on the flashlight. Position it so it shines towards the convex mirror. For configuration A, place it off to the side just enough so it doesn’t block the reflection to the scope. It should point directly at the center of the mirror, at an angle of 15 degrees or less. Your scope should point directly at the mirror as well.
Once the components are in place, start out with a low-power eyepiece to find the star, center it in the field of view (FOV) and focus it. An eyepiece with a focal length of 25mm is fine. An alt-az mount like the Voyager is easy to use in this regard to move right-left-up-down to get the mirror in the FOV. Once you find the mirror, focus on it and then find the tiny reflection of the pinhole in it. For collimation you can use the eyepiece either straight-through or with a good diagonal. I often use it straight through to eliminate the additional optical element.
Change eyepieces. I next use an 8mm (medium-high power) eyepiece which gives about 156x with a C5+. Once you put in the medium-power eyepiece, don’t fully focus the image. Instead bring it down to an out-of-focus donut (Fig. 10). The bright ring of the donut is the light from the pinhole while the donut hole is the silhouette of the secondary mirror and cell. It may look like the donut hole is off center. For a collimated scope, it should be centered. Now the work of actual collimation begins!
2) Coarse collimation: the concentric donut hole
At this point, go to your collimation screws. For the C5+ and similar SCTs, they will be in the center of the secondary mirror cell (Fig. 8). Especially on modern SCTs, the screws are often under a cosmetic cover. On my C5+, there is no cover and I replaced the factory screws with knobs (Fig. 9). The knobs make collimation tool-free, a great benefit.
|Fig. 8: SCT collimation screws are located on the back of the secondary mirror cell at the front of the scope.||Fig. 9: The C5+ has had the original Allen collimation screws replaced by thumbscrew knobs, Knobs are highly recommended for easy collimation.|
Coarse collimation will center the donut hole in the donut. First, check to be sure the image is still centered in the eyepiece. Now pick one of the screws. (Stick a finger in front of the scope and ease it into the light path until you see its shadow. Move the finger shadow until it’s in the same direction as the offset of the donut hole silhouette. That will help identify what screw to turn to shift the silhouette back towards the center.) Turn it ever so slightly, maybe 1/8th of a turn. With small SCTs like the C5+, you can do this while looking in the eyepiece, watching the scope vibrate a bit. The donut hole may become more centered or less centered as you turn a given screw. Also, you’ll notice that the FOV position of the donut will change. This happens because changing the tilt of the secondary by turning the screw also changes the direction of light from the primary. If the hole has moved further off-center, cancel your effort by turning the screw back the same amount. Then try turning it 1/8 turn the opposite direction.
Repeat this as needed with one or more screws. Bear in mind: easy does it!! Just slight turns are needed for normal collimation. You are not trying to remove the screws or drop the secondary into the tube. At this stage, the cumulative turning of one screw may amount to a half turn or so to get a good concentric donut hole (Fig. 11).
|Fig. 10: The defocused image of the star appears as a donut and an off-center donut hole.||Fig. 11: The concentric donut hole at the end of coarse collimation.|
3) Out-of-focus fine collimation
Once you have centered the donut hole, bring the star into focus. As you do this, note which way you are turning the focus knob. Clockwise is closer (towards inside) focus, counterclockwise is to (towards outside) infinity focus for most SCTs. Make the star just slightly defocused inside focus . You will start to see the diffraction pattern (Fig. 12). Now turn the focuser to get it slightly outside focus (Fig. 13). Don’t be surprised if the images look lousy and off-center, as your scope is not yet fully collimated.
Out-of-focus fine collimation will center the star inside these out of-focus patterns. As with coarse collimation, adjust the screws, this time making tiny turns, perhaps cumulatively 1/8 turn. Center the brightest point inside the pattern. Again, the whole image will shift, perhaps out of the FOV. Be sure to recenter it as needed. Now the inside and outside focus images should look much better (Figs. 14 and 15). Notice that the bright spot (Airy disk) is centered within an elliptical overall diffraction pattern. The ellipse is caused by the off-axis placement of the artificial star light source relative to the convex mirror reducer and the OTA. It’s an artifact of configurations A and B that does not interfere with collimation.
4) In-focus final collimation
Final collimation will center the Airy disk inside the in-focus star’s diffraction pattern. You'll want to switch to a higher magnification. The idea is to be able to clearly see the diffraction pattern. I used a 2.5mm eyepiece giving 500x for this close-up examination of the diffraction rings. Again, your adjustment procedure will use just tiny tweaks to the collimation screws to center the in-focus Airy disk inside the diffraction ellipse. Your end result will look like Fig. 16.
You can directly examine the pattern without the mirror as a check (Fig. 17). Because removing the convex mirror reducer enlarges the apparent size of the star, it may now be larger than the scope’s diffraction limit and it won’t truly be an artificial star. (NOTE: the collimation images were taken with a DSLR and a 2X Barlow attached to the C5+. They are substantial enlargements of the original image size to show details emulating the visual appearance of the process through the eyepiece.)
|Fig. 12: Defocused image inside focus before fine collimation.||Fig. 13: Defocused image outside focus before fine collimation.||Fig. 14: Defocused image inside focus after fine collimation.|
|Fig. 15: Defocused image outside focus after fine collimation.||Fig. 16: In-focus view of the centered diffraction disk and pattern after final collimation.||Fig. 17: Direct inside focus post-collimation view of the Hubble 50-micron pinhole from 25 feet with the C5+.|
Observations with scopes collimated this way have been very pleasing. With my C8 at 400x, Mars 2010 opposition revealed details like the dark thread just above the south polar cap. The diffraction disks of the stars in the Double Double look circular and clean in my C5+. During the 2012 Transit of Venus, I observed the planet's faint back-lighted atmospheric halo with the C5+. The image quality of both scopes has generated fine compliments at star parties.
Variations on the theme
Even though my example is for a SCT, indoor artificial star collimation can be used with other telescope designs. Newtonian reflectors would use exactly the same procedure, adjusting the collimation screws of the primary instead of the secondary. Refractors have different diffraction patterns and lack any central obstruction from a secondary. A scope with a large near focus will be a challenge to find an indoor space with a long enough sight line for the technique to work with a single (reducing) mirror. If you don't have a SCT secondary mirror, you can use a good convex rear-view mirror from an auto supply store or a Christmas tree ornament instead, as long as they have the right radius of curvature to properly reduce the apparent size of the artificial star. Ball bearings aren't recommended as they often have numerous small scratches that can interfere with reflection.
Artificial Stars and Spherical Mirror Reduction
1) Artificial star definition
What is an artificial star? It's a man-made light source that looks like a real star through a telescope. Since a real star looks like a point of light through a telescope, an artificial star must appear as a point source. Whether or not a light is a point source can be determined by comparing the apparent size of the artificial star at a given distance from the telescope – the pinholes in the Hubble flashlight for my setup - versus the diffraction limit or resolution of the scope. The apparent angular diameter d (in arc-seconds) of a pinhole used as an artificial star is (Eq. 1):
where P is the pinhole size and D is the distance from the scope to the pinhole in the same units.
A telescope with an aperture A (in inches or centimeters) has a resolution r in arc-seconds for a given wavelength of light λ (in or cm) of (Eq.2):
Set the apparent diameter of the pinhole equal to the telescope resolution or Eq. (1) = Eq. (2) and then find the distance D. The result is (Eq. 3):
This is the minimum pinhole distance for diffraction-limited collimation. Tables 1 and 2 give the distances in more convenient feet and meters for popular apertures in inches and centimeters and pinhole sizes in microns.
Inside I use the 50-micron pinhole. As a conservative, metric rule of thumb for this pinhole size, the minimum distance in meters is roughly the same as the scope aperture in centimeters (a 5cm (50mm) refractor should be at least 5 meters distance, a 20cm (200mm) SCT should be at least 20 meters distance, etc.). If a pinhole is too close to the telescope, it can be too big (as inferred above) and it also can have light with a significant amount of divergence. This will introduce spherical aberration into the image, even as far away as 20 focal lengths. This is really important when testing the quality of the optics or making them but much less important here. For collimation purposes, it can be much closer than that without causing any problems. We know (or assume) the optics are good. We just want the ability to accurately align them using a diffraction pattern in or out of focus. Most of the time, introduced aberrations that don't grossly distort the image won’t matter.
2) Spherical mirrors and reduction
Tables 1 and 2 show that larger scopes need larger distances for diffraction-limited collimation. A 12-inch reflector needs at least 86 feet, an indoor space that is hard to come by. Or perhaps you want to use a larger size pinhole than 50 microns. What then? You can use spherical mirrors to shrink the size of the star at shorter distances. A convex spherical mirror acts on light from a distant object and creates a smaller, upright virtual image that looks like it comes from behind the mirror. How much is the virtual image reduced in size? It depends on f, the focal length of the mirror and how far the object is from the mirror. From the optics equation (Eq. 4), just as for a lens, the mirror's reduction is the image distance p compared to the object distance o (Eq. 5, p and f are negative numbers):
The radius of curvature (ROC, twice the focal length f of a convex mirror) for many commercial SCT secondaries is approximately equal to the scope aperture in inches. The 11” SCT secondary I often use has about a 13” ROC, hence a 6.5” focal length. Used inside the original scope, it had a reduction factor of 0.2 as it diverged the primary's light for a longer focal length. (The inverse number, 5, is often called the magnification factor of the scope needed to achieve f/10 with the common f/2 SCT primary.) With configuration A at 18” from the Hubble's 50-micron pinhole, the pinhole is reduced by the factor 0.27. Closer mirror-pinhole separations will give larger factors, while greater distances will give smaller factors.
Divide the values in the table by the reduction factor (Eq. 5) for your mirror and setup to get the effective distance with the spherical mirror reducer. With this technique, you can easily align a 14” SCT inside a typical home. If the distance between the mirror and pinhole is large compared to the mirror's focal length, the the virtual image is formed at the reducing mirror's focal point and the reduction is just the reducer's focal length divided by the mirror-pinhole separation (Eq. 6). This is the case when configuration B is used with a modest focal length mirror like the 11” secondary.
For a given mirror-pinhole separation, a smaller focal length reducer will give a greater reduction factor as well as a fainter star. This applies if you are using a Christmas tree ornament, since the focal length will be ¼ the diameter. If the focal length of your convex mirror is unknown, you can use templates created with a compass and compare curvatures to help determine the approximate ROC and the focal length. Configurations A and B that I have used are easy to setup with simple supports or none at all for the light and mirror. Because they are off-axis, they do produce elliptical diffraction patterns (most noticeable with configuration A, minimal with configuration . This is usually not important; however, if desired (for example, to more easily see the astigmatism of an out-of-collimation refractor), the shape can be remedied with more intricate stands placing the telescope, light and pinhole all in a line along the optical axis of the scope (configuration C, Fig .18). For some scopes and positioning, the flashlight will obstruct part of the visual field.
|Fig. 18: Configuration C, used for circular diffraction patterns. (This example is for illustrative purposes only to show general placement. In a real setup the components would be farther apart and on stands assuring positive coaxial alignment and clear visibility of the pinhole.)|
Copyright © 2013 Derek Wallentinsen
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