Collimation and Star Testing An RVAS Meeting Program

Collimation and Star Testing An RVAS Meeting Program By Frank Baratta March 20, 2017 Horsehead Nebula: Public domain image at Pixa. Bay. com © F. Baratta, 2017; Graphics and images © F. Baratta, 2017, except as noted

. . . bad collimation is the number one killer of telescopes, world wide. . . Walter Scott Houston September 8, 1987 At a minimum, the star test provides a "tool" for checking "collimation. “ Star Testing a Telescope RVAS Monthly Meeting March 20, 1995

Program Caveat There is a huge number of footnotes, sidebars and nuances that can be added on the topics of collimation and star testing. This program reaches just the basics. My experience is primarily with “obstructed” telescopes (such as reflectors and Schmidt-Cassegrains), and so the content will skew this direction. Here and there I’ll make references to “unobstructed” telescopes (such as refractors). But first, some preliminaries. . .

Recurring Terms Reflector Telescope Components • Objective (or “Primary”) Lens or Mirror Refractor • Diagonal (or “Secondary”) Mirror – Reflectors only • Focuser (or “Eyepiece Holder”) • Eyepiece (or “Ocular” [outdated]) Graphics credit: Starizona. com, with permission

Recurring Terms Collimation*: the aligning of a telescope’s lenses and/or mirrors along its light path (optical axis) in order to minimize image distortions. * from a misreading of the Latin “collineare”: to direct in a straight line “Inside” and “Outside” of Focus: If the center silhouette of a “focuser, ” or “eyepiece holder” (shown without an eyepiece inserted) is its “at” focus position, then at left is “inside” focus and at right is “outside” focus. “racked in” at focus “racked out”

We all love pictures of the night sky where the stars appear sharp as a tack. But what does a star look like in a telescope? It’s not really a point of light! Diffraction: The bending of light passing through an opening (aperture) or when encountering an obstacle. Diffraction Pattern: The spreading (as a result of diffraction) of light from a point source into a bright central area surrounded by a series of decreasingly bright concentric rings. The “Airy Disk”: The bright central area of the diffraction pattern. Decreases as aperture increases. Theoretical FOCUSED images of a star, highly magnified, with a perfect telescope that has no central obstruction (top) and a perfect telescope with a central obstruction (bottom). Note that, due to the additional diffraction effects of its obstruction, the obstructed telescope transfers light into the first ring. Impact? Star Field Image: George Hodan, Public. Domain. Pictures. net Unobstructed Telescope Pattern: uncopyrighted image posted on Cloudy Nights Forum Obstructed Telescope Pattern: Gold Run Partners, inc. and Gold. Astro. com; with permission

What’s our destination? Illustration of the view in a well-collimated reflecting telescope looking down the empty eyepiece holder at the secondary mirror: a series of concentric circles. Illustrations of DEFOCUSED stars seen at high magnification in a perfect, wellcollimated telescope with a 30%* obstruction (left) and well-collimated unobstructed telescope (right); both show a series of concentric rings. * Diameter of diagonal compared to diameter of primary

Collimation

Peering Down the Empty Focuser Interpreting what you see. . . Multiple misalignments – to distinguish features in view. Our task is to align all the elements form a set of Note: You’reto seeing both the actual Diagonal diagonal and holder as well as holder concentric circles. reflections of these and other components off the primary back to the diagonal. Spider vanes Graphic Credit: New Perspectives on Collimation, Menard and D’Auria; with permission.

Collimation Tools Sight Tube Top Views Collimation Cap Bottom Views Cheshire Pen Collimation Cap Image: Agena. Astro. com; with permission Combo Cheshire/Sight Tube Tool: Starizona. com, with permission

Step 1: Diagonal Centered Under Focuser? Checking with the Sight Tube Crosshairs Bottom Edge of Sight Tube Diagonal Mirror Graphic Credit: New Perspectives on Collimation, Menard and D’Auria; with permission.

Step 1 (cont’d): Align Diagonal & Focuser Issue: The diagonal is not concentric with bottom edge of sight tube • Definitely off center • Diagonal shape is not circular– could be due to several possible orientation problems And, hopefully, relocating the spider or focuser won’t be needed!

Step 2: Objective and Diagonal Aligned? Checking with the Cheshire What you might see. . . Unlike with a separate Sight Tube, the Cheshire can be used at night by shining a red light into its opening.

Enlarged View Through Cheshire Bottom Edge of Cheshire Reflection of Edge of Diagonal Actual Edge of Diagonal (concentric with bottom of Cheshire) Reflection of Mirrored Surface of Cheshire Reflection of Primary (not concentric with diagonal) Reflection of Cheshire Peep Hole Opening Reflection of Edge of Diagonal Holder Reflection of Primary Mirror Center Spot (“donut”)

Step 2 (cont’d): Aligning the Primary Turn the primary mirror tilt adjustment knobs or screws in or out to center the primary mirror spot on the Cheshire peep hole opening. You can break a sweat trying to align everything with this set-up! Mirror Adjustment Images: (top) New Moon Telescopes; (bottom) Schlatter. org; both with permission

Sounds like a lot of work! Are there other methods that can simplify matters?

Step 2 (cont’d): Aligning the Primary Using Laser Collimators What’s a laser collimator? A laser device emitting a red dot or other shape beam used to align the secondary mirror, with a rear-facing bull's-eye target cutout on which the position of the beam’s reflection is adjusted in order to align the primary mirror. Considerations: • Requires donut center spot on the primary to reflect beam • Newer versions allow for adjusting brightness of the laser beam • Recommend at least rough visual alignment before use of laser (caution needed if reflection of beam misses secondary) • Looseness of collimator in focuser can introduce misalignment • Cost varies widely (SVBONY, $29. 95 [Amazon]; Hotech Crosshair Self-Centering, $114. 95 [Starizona])

Star Testing

What is a star test? • Examining a defocused star’s appearance in order to assess the alignment and quality of a telescope’s optical components. • Considerations: • Should be done on nights of good “seeing” and after the telescope has adjusted to the ambient temperature. (So, manually collimating the telescope—as we just discussed—generally precedes star testing. ) • Can be done under the stars or using an artificial star. • Should be done at the highest magnification practical for your telescope. Some sources recommend about 25 x per inch of telescope aperture. But even at this moderate magnification, using relatively unmoving Polaris for the test gets interesting if your telescope doesn’t track!

Collimating by the Star Test For Misaligned Reflectors: As for collimating with Cheshire tool, use primary At Left: Well collimated, perfect telescope with adjustment knobs or screws to reorient mirror tilt 30% obstruction. until ring pattern is completely concentric. At Misaligned Right: Well Schmidt-Cassegrains: collimated, perfect unobstructed For telescope. is generally limited to reorienting the Collimation secondary’s tilt using the three adjustment screws on exterior of the corrector plate. Disassembly and accessing the primary mirror is generally considered a factory repair. At Left: Perfect telescope with 30% obstruction For Misaligned Refractors: showing a misaligned primary. If there adjustment screws on the lens cell, At Right: Perfectholder, unobstructed telescopein the remove eyepiece insert eyepiece showing visual back, misaligned defocus a objective. star and turn screws until rings are completely concentric. If adjustment screws are lacking, it’s generally considered a factory job. May also involve focuser adjustment. Refractor Image: Guido Santacana, 2017; uncopyrighted

Collimating by the Star Test An Alternative Approach: Defocused star’s diffraction pattern shows the telescope needs • Move the defocused star around the edge of the field to the to be collimated. spot where the rings are (most) concentric. • Using the adjustment screws, move the image to the center of the field of view. • Shrink the defocused star image (which ups collimation sensitivity) and repeat the procedure, and then repeat again. • Then refocus slowly; if collimated, the rings will collapse concentrically around black spot. Graphics: Sky & Telescope Magazine, with permission

Collimating by the Star Test Using an Artificial Star What’s an artificial star? It’s a battery-powered device which uses a bright, white light LED and a pin hole to project a point source mimicking a star. Considerations: • Indoor use is possible • Solves issue of star drift when collimating outdoors • Cost varies (Hubble unit, $29. 95; Astrozap (sold by Astronomics), $110. 00) • Need to separate unit and telescope sufficiently

Star Testing for Optical Issues The “Snap Test” A preliminary star test for roughly judging a telescope’s performance. Approach: • Use high magnification (no less than 25 x per inch of aperture) • Shift the focuser back and forth through best focus and watch how the image behaves. Good Result: The image appears to snap into best focus quickly. Issues Indicator: There’s no snap; rather, there’s a range of possible focus positions that leaves you uncertain which is best.

Star Testing for Optical Issues Basis: The diffraction pattern of a perfect, well-collimated telescope appears identical at the same distances inside and outside of focus. Inside Focus At Focus Outside Focus Theoretical appearance inside, at and outside focus at high magnification of perfect optics for a well-collimated telescope having a 30% obstruction (top row) and no obstruction (bottom row). As you might imagine, achieving absolutely identical inside and outside diffraction patterns is essentially impossible—and getting close is expensive!

Star Testing for Optical Issues (cont’d) Atmospheric Turbulence (may change over time) Tube Currents (subside as the scope acclimates to ambient temperature) Optical Surface Roughness (looks like Turbulence, but the fuzziness doesn’t improve) Surface Roughness Graphic: Astrosurf. com, with permission

Star Testing for Optical Issues (cont’d) Astigmatism (Is it the primary, the eyepiece or you? ) Spherical Aberration (undercorrected) Pinched Primary

Star Testing for Optical Issues (cont’d) Turned Down Edge Multiple aberrations combined (This is when grown men cry. )

Star Testing for Optical Issues (cont’d) What May Be the Best Advice. . . the best star test, IMO, is viewing Jupiter and Saturn. If you like what you see when the seeing is good and the scope is well cooled and collimated, then you have a winner! Everything else is academic. Cloudy. Nights. com Forum Posted by Randy 12/8/2007

Some Concluding Comments • Explore Your Telescope’s Directions and “Edges” While looking through the empty focuser: The graphics of star tests included in this talk • Holdare yourcomputer hand overgenerated the front ofsimulations the ‘scope and point at offered tothe secondary. give a sense of the conditions described. At • Reach into the frontstar of the ‘scope and (carefully) run your the eyepiece, test views are much more finger alongand the edge secondary. subtle soft, of asthe in this actual refractor star image. While test looking through a collimating tool: • After loosening the primary adjustment screws, sketch the direction tightening and loosening each screw moves the center spot in relation to the reflection of the tool’s reference mark (peep hole opening or crosshairs’ intersection). Refractor Star Test Image: Cor Berrevoets, Graphic aberrator. astronomy. net, credit: Starizona. com, withpermission

A Few Resources Books and Articles: Star Testing Astronomical Telescopes, Harold Richard Suiter, Willmann-Bell, Inc. , Richmond, Va, 2009 (second ed. ; first ed. , 1994) – The bible of star testing; overviews followed by in-depth treatments. No Tools Collimation, Gary Seronik, Sky & Telescope Magazine, October 2013 – Details of the method mentioned in this presentation. Websites: • http: //aberrator. astronomy. net/ – Free star test simulator and more. • http: //www. astronomyhints. com/collimation. html – Extensive range of topics. • https: //starizona. com/acb/basics/ – Help for beginners and useful refreshers. • http: //www. goldastro. com/goldfocus/technical. php – General and technical topics.

Questions ? ? ? Comments ? ? ? Thank You !!!