Ritchey-Chrétien (RC) Telescope collimation

• Optically align 2 hyperbolic mirrors and a focuser
• Set correct mirror distance

1. Adjusting one of the mirrors or the focuser affects the alignment of the others
2. Mechanical axis is rarely also the optical axis
3. Most tools wrongly assume that the optical axis is the same as the mechanical axis
4. Mirror distance is critically important
5. Some models have the focuser attached to the primary mirror assembly, some even without collimation capabilities.
6. Central rings in the mirrors are not always at the optical center
7. Most models have no adjustment options to center the mirrors

Collimation using stars at night

• Deepsky Instruments method http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf
• Mirror spacing by measuring focal length via platesolving.
• SkyWave-Collimator, AI based wavefront sensing https://www.innovationsforesight.com/
  • https://innovationsforesight.com/PDF/InnovationsForeight_RC_Collimation_NEAIC_2024.pdf
  • https://www.innovationsforesight.com/Wavefront/SKW_RCT_Alignment_011522.pdf from 2021
• ASTAP image inspection for the primary (after having aligned the focuser and the secondary with a laser during daylight).
• Tri-Bahtinov-Mask. This should be usable for the primary (after having aligned the focuser and the secondary with a laser during daylight)
• Mirror spacing with a star
• scrutinize a star at high mag and adjust the PM/SM despace until the spherical aberration goes away by Tom Dey
• Put the sensor at the specified backfocus and then use mirror spacing to come to focus.

Collimation during daylight

• Hall of Mirrors method, symmetrical bisected crescent moons https://www.cloudynights.com/topic/782014-rc-collimation-with-no-laser-or-stars/page-6#entry11276346
  • Not really usable with a 30cm model, the secondary size leaves only a few reflections to work with.
• Kitchen table collimation of a Cassegrain-type reflector by Russ Slater https://britastro.org/2018/kitchen-table-collimation-of-a-cassegrain-type-reflector
• RC collimation with no laser or stars https://www.cloudynights.com/topic/782014-rc-collimation-with-no-laser-or-stars/
• Paper or plastic disk with a central hole for the primary
• https://www.cloudynights.com/topic/782014-rc-collimation-with-no-laser-or-stars/?p=11260952
• Collimating_GSO_Ritchey_Chretien_with_a_plastic_disc_V2-for-old-RC6-RC8-designs.pdf by David Davies
• Cheshire eyepiece for the secondary
• Takahashi TKA00443 collimator
• Takahashi TCEE0100 collimator
• http://www.alluna-optics.com/Alluna-Blog/113-collimation-RC-telescope.html
• HOTECH Advanced CT Laser Collimator http://www.hotechusa.com/category-s/23.htm
(aka Hotech ACT)
• Manual V8, 7 pages https://www.hotechusa.com/v/vspfiles/assets/images/ACT%20Laser%20Collimator%20Manual%20V8.pdf
• Collimation video https://www.youtube.com/watch?v=wgDBktsX3fI&list=PL80AADE80AB76336D
• https://www.cloudynights.com/topic/509460-daytime-collimation-of-a-cassegrain-scoperc/
• https://www.cloudynights.com/topic/714340-collimation-experience-with-hotech-act-and-ritchey-chr%C3%A9tien/#entry10288647 use the ACT on an RC
• http://jropics.com/collimation.html use the ACT (and SCA and HG) on an RC
• ../hotech-act/ my own page on how to collimate the Hotech ACT itself.
• Howie Glatter with the Concentric Circle attachment
• https://stargazerslounge.com/topic/349932-help-please-collimating-an-rc-with-a-howie-glatter/
• http://stevesastro.blogspot.com/2013/05/collimating-my-gso-rc8-telescope.html
• Mirror spacing with a Ronchi
• Ronchi-testing-the-focal-length.pdf by David Davies
• TSRCKOLLI, https://www.teleskop-express.de/shop/product_info.php/language/en/info/p11188_TS-Optics-2--LED-Collimator-for-RC-Telescopes-and-all-other-Types-of-Telescopes.html
• Bench Collimating a GSO RC8 https://www.ancientphotonsastro.com/post/bench-collimating-a-gso-rc8
• OCAL web cam with software that can draw lines and circles http://www.ocalworld.com/en/product-tutorial.html RC Collimation Tutorial

Further reading

• http://interferometer-tests.blogspot.com/2013/06/2542000-10-ritchey-chretien-gso.html by Tommy Nawratil from www.teleskop-austria.at
• https://www.cloudynights.com/topic/703752-gso-16-inch-rc-collimate-with-an-autocollimator/?p=10961496 by Dr. Gaston Baudat from Innovations Foresight.
• laser collimator and test tilt by rotating
• laser collimator for the secondary mirror
• primary mirror tilt with a Takahashi collimation scope
• defocused star near zenith
• plate solving to measure mirror distance is a coarse predictor due to few percents of focal length differences between otherwise identical scopes
• offset of the secondary central obstruction with photo examples
• Shack-Hartmann (SH) wavefront analyzer
• intro to their ai based wave front sensing and collimation software
• https://www.cloudynights.com/topic/646374-not-another-gso-rc-collimation-thread/
• https://www.cloudynights.com/topic/514199-rcs-a-scope-for-masochists/
• http://www.hnsky.org/RC_collimation.htm
• https://www.cloudynights.com/topic/760224-rc-distortions/
• https://www.cloudynights.com/topic/657216-rc8-collimating-frustration/ by Robin Brown. Instruction set pdf: RC8-Collimating-Frustration-revision-1.4.pdf
• https://www.cloudynights.com/topic/693000-what-happened-to-my-collimation-on-a-12-inch-ritchey-chretien/
• https://www.cloudynights.com/topic/711668-adjusting-focal-length-of-16-inch-ritchey-chretien-scope/
• Cedric Raguenaud's Idiot-proof guide to Ritchey Chretien collimation RC-collimation-by-Cedric.pdf from https://raguenaud.space/2021/03/collimation-du-rc-partie-3/
• https://www.astroforum.nl/threads/collimeren-ritchey-chretien-artificial-star-of-collimator.1471289/
• https://deepspaceplace.com/gso8rccollimate.php
• https://www.dehilster.info/astronomy/collimating_a_ritchey_chretien.php by InFINNityDeck
• https://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/ by InFINNityDeck
• https://www.dehilster.info/astronomy/collimator.php by InFINNityDeck
• https://www.starry-night.nl/collimeren-met-een-collimator/ by InFINNityDeck
• https://stargazerslounge.com/topic/177312-altair-10-rc/
• Third-order astigmatism of a Ritchey-Chrétien telescope.https://spie.org/news/1763-when-excellent-on-axis-performance-is-not-good-enough?SSO=1
• SkyWave-Collimator by Dr. Gaston Baudat - Using AI for Telescope Collimation https://www.youtube.com/watch?v=u--0h5R7jBU
• A solution for focusers attached to the primary mirror assembly without collimation capabilities: https://www.cloudynights.com/topic/827768-finally-made-an-rc-collimation-adapter/
• https://www.cloudynights.com/topic/920444-how-to-determine-the-distance-between-mirrors-of-a-rc/
• chapter 5.2.1 Distance of Artificial Sources and Appendix F in "Star Testing Astronomical Telescopes." by Harold Suiter.
• pages 395 - 419 in the "The Astrophotography Manual, 2nd ed." by Chris Woodhouse.
  Topics discussed :
  • Cheshire
  • Takahashi collimating scope
  • Howie Glatter laser with concentric rings attachment
  • Hotech Advanced CT laser
  • Hall of mirrors
  • Theodolite
  • Artificial star
  • 9-ball tester
  • Classic star testing
  • GoldAstro collimation mask. (A tri-bahtinov mask.)
  • Mirror separation

Last page update : 2024-05-03T20:47Z -- Hans Lambermont