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

2. . . . 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

3. 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 . . .

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Eyepiece (or “Ocular” [outdated])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

5. 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”

6. 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, PublicDomainPictures.net
Unobstructed Telescope Pattern: uncopyrighted image posted on Cloudy Nights Forum
Obstructed Telescope Pattern: Gold Run Partners, inc. and GoldAstro.com; with permission

7. 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, well-collimated 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

8. Collimation

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

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

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

12. 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!

13. 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.

14. 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 Primary Mirror Center Spot
(“donut”)
Reflection of Edge of Diagonal Holder

15. 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 every-thing with this set-up!
Mirror Adjustment Images: (top) New Moon Telescopes; (bottom) Schlatter.org; both with permission

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

17. 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, $ [Starizona])

18. Star Testing

19. 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 25x 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!

20. Collimating by the Star Test
For Misaligned Reflectors:
As for collimating with Cheshire tool, use primary adjustment knobs or screws to reorient mirror tilt until ring pattern is completely concentric.
For Misaligned Schmidt-Cassegrains:
Collimation is generally limited to reorienting the 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.
For Misaligned Refractors:
If there are adjustment screws on the lens cell, remove eyepiece holder, insert eyepiece in the visual back, defocus a 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.
At Left: Well collimated, perfect telescope with 30% obstruction.
At Right: Well collimated, perfect unobstructed telescope.
At Left: Perfect telescope with 30% obstruction showing a misaligned primary.
At Right: Perfect unobstructed telescope showing misaligned objective.
Refractor Image: Guido Santacana, 2017; uncopyrighted

21. Collimating by the Star Test An Alternative Approach
Move the defocused star around the edge of the field to the 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.
Defocused star’s diffraction pattern shows the telescope needs to be collimated.
Graphics: Sky & Telescope Magazine, with permission

22. 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

23. 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 25x 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.

24. 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!

25. 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

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

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

28. 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.
CloudyNights.com Forum
Posted by Randy
12/8/2007

29. Some Concluding Comments
Explore Your Telescope’s Directions and “Edges”
While looking through the empty focuser:
Hold your hand over the front of the ‘scope and point at the secondary.
Reach into the front of the ‘scope and (carefully) run your finger along the edge of the secondary.
While 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).
The graphics of star tests included in this talk are computer generated simulations offered to give a sense of the conditions described. At the eyepiece, star test views are much more subtle and soft, as in this actual refractor star test image.
Refractor Star Test Image: Cor Berrevoets, aberrator.astronomy.net, with permission
Graphic credit: Starizona.com, with permission

30. A Few Resources Books and Articles: Websites:
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:
– Free star test simulator and more.
– Extensive range of topics.
– Help for beginners and useful refreshers.
– General and technical topics.

31. Questions ???
Comments ???
Thank You !!!