Moore Observatory's 24-inch Ritchie-Chretien Telescope

The 24-inch (0.6-meter) Ritchie-Chretien research telescope was manufactured by RC Optical Systems and installed in August 2006. It is currently in routine use for measurements of transiting exoplanets. The instrument features a very robust fork mounting with an absolute Renishaw precision encoder on the polar axis. It is capable of pointing with second-of-arc precision from a cold start under control of a version of XmTel including corrections for polar alignment.

The telescope's Zerodur optics were made by Paul Jones of Star Instruments. The f/8 system has a peak-to-valley wavefront variation of 0.242 wave, and 0.039 RMS, with a Strehl Ratio of 0.941. The mirrors have Spectrum Coatings enhanced aluminum coating with a reflectivity peaking at 96.9% at 528 nm, decreasing slightly to 95.7% at 450 and 650 nm. With efficient well-corrected optics useful from the atmospheric UV cutoff below 380 nm through the near infrared above 2 microns, the 0.18 arcsecond diffraction is achievable for visible light with very short "lucky" exposure times to minimize atmospheric turbulence.

The instrument is focused by a precision adjustment of the secondary mirror to bring the focal plane into coincidence with a fixed focal position 255 mm from the back of the primary mirror cell. Spacers are used to put the camera or other instruments at the proper position, and an encoded focus motor moves the focal plane 5 microns per encoder count. The image scale in the focal plane is 42.3 arcseconds/mm, so that 1 arcsecond corresponds to approximately 24 microns. At f/8, the telescope can focus precisely to within its diffraction limit since the least count on the focus encoder translates to a image size for a point source of 0.026 arcseconds. In long exposure imaging with averaging seeing of 1 arcsecond, focusing precision of 40 encoder counts is typical. A focal plane corrector and instrument rotator are available but are not used for photometry.

The telescope uses an Apogee U16M CCD camera with a 4096x4096 array of 9 micron pixels which oversample the seeing and permit very accurate photometry and imaging over a wide dynamic range. Interchangable filter wheels provide the Sloan filter set g',r',z', i' for photometry and color imaging. Narrowband interference filters including [SII], H-alpha, [O III], H-beta, and Na D are used for measurements of stellar activity and surface photometry of nebulae. Similar filter sets are also used at the CDK20 telescopes at Moore and Mt. Kent, and the CDK700 at Mt. Kent.

A sensitive Sensors Unlimited SU320KTX 320x240 InGaAs camera with 40 micron pixels is available for imaging in the J-band at 1.6 microns over a 9x7 arcminute field.

For high speed lucky imaging a 1024x1024 Photometrics Cascade II zero-noise camera captures photon noise limited images rapidly enough to freeze atmospheric turbulence.

The RC24 photo gallery has more views of the telescope and its instrumentation. Some taken with the telescope and others at Moore and Mt. Kent observatory are available in the Shared Skies Gallery.

Last update: May 8, 2010
kielkopf at louisville dot edu