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© 2019 Door Peninsula Astronomical Society

16 Inch Ritchey–Chrétien

The new telescope is a variant of the Cassegrain
design, a reflector with a large primary mirror and a
centered, smaller secondary mirror which reflects the
magnified image through a hole in the middle of the
primary mirror to an eyepiece behind. Some reflector and
refracting telescope designs produce comas, or distortions
of objects appearing near the edge of the field of view.
These aberrations appear in a symmetrical way, but they
are distortions nonetheless. The Ritchey–Chrétien (RC)
design we have at the observatory has a primary concave mirror and secondary convex mirror both of which are
hyperbolic, a particular shape designed to eliminate off-
axis optical errors. [Large telescopes like the Keck in
Hawaii and the Very Large Telescope (VLT) in Chile use
this design.] With a diameter of 16 inches, the primary
mirror has, subtracting the area of the 4-inch center hole,
~ 188 square inches of light-collecting surface. Under
excellent seeing conditions, this telescope can pull in faint
stars to a magnitude of 14 when viewing through a glass
eyepiece with a widish angel of view. If a high-quality
digital camera is installed in stead of an eyepiece,
gathering the output from the twice-reflected image, the
light can be summed or integrated over time to gather
even more light from the target. Objects with a magnitude
as large as 18(!) can be imaged with a such a large
primary mirror and a quality digital camera.
Our 16” Ritchey–Chrétien model - sold by High
Point Scientific - has a carbon fiber truss tube design to
reduce its weight to 84 lbs (see photo). The smallish f/8
focal ratio gives a wide field-of-view but also decreases
the depth-of-field, not usually a concern in astronomy
because celestial targets are all at the same focus -
infinity. Both mirrors have a highly reflective dielectric
coating so they reflect 99% of the incident light. From
highpointscientific.com: “This design provides superb
image correction across the entire field as well as very
sharp images on-axis.” Our new RC should provided
outstanding images for years to come.

180mm Maksutov Cassegrain

Another telescope rides piggyback on the Ritchey–
Chrétien: a 180 mm (7.08 Inch) aperture Maksutov-
Cassegrain optical tube by Sky-Watcher. The tube is only
572 mm in length but the telescope has a focal length a
much longer 2700 mm, the total distance the twice-
reflected light travels inside the telescope to the point
where eyepieces and cameras go. Focusing is done by a
knob which moves the primary mirror slightly - the
eyepiece (and a star diagonal, if you use one) never
move. The 7’ aperture allows for “bright images of
celestial objects” and the “long 2700 mm focal length will
provide refractor-like contrast.” [optcorp.com]. The Sky-
Watcher will be best used for imaging the moon and
planets; the 16” Ritchey–Chrétien best used to image
dimmer targets such as galaxies. The two telescopes and
accompanying gear are mounting to and controlled by a
muscular Bisque Optics mount, which is bolted to a level
concrete pillar. The program that controls where the
telescopes are aimed is The SkyX Professional & Serious
Astronomer Edition, which contains extensive maps of the
heavenly targets.
Both telescopes can utilize eyepieces for optical
viewing, but looking through the bottom of a large
instrument situated in a small round room and angled any
which way is awkward at best and frequently a strain on
one’s neck and back. This is why our telescopes will be
used primarily with an astronomy-quality HD digital/video
camera, much more convenient and functional. We use a MallinCam SkyRaider DS2.3PLUS color video/imager/
autoguider, containing a Sony EXmor CMOS sensor
measuring 13.4 mm diagonally with 2.38 effective
megapixels. The SkyRaider provides near-instantaneous
downloads to the computer, making it easy to displaying
live images while simultaneously capturing video or long-
exposure digital images. Astronomy-quality digital
cameras sum the light coming in to a specific location
(typical pixel size: 6 square μm) over time. Like a
photographic plate exposed to an image for a duration, the
feeble light from objects too faint to see with just the eye
are “etched” onto the pixel field in the camera. Thus
images of much fainter celestial targets, such as far-away
galaxies and dim nebula, can be composeda/c and saved
and displayed. The astro-images can be viewed on any
suitable computer or video display, such as our new 80-
inch Sony HD big screen TV in the Astronomy Center via
fiber optic cable. (Did I mention the 80-inch big screen HD
TV before?). When the telescope is operating, we
frequently upload our telescope video to the Night Skies
Network, the original worldwide astronomy broadcast site.

-- Written by Tom Minahan