top of the lawn: Coulter CT-100

Session name: 20071012.2000
Weather: Cool temps at 46°F/7°C with gentle [3] to moderate breeze.

Telescope: Coulter CT-100, 4.25" f/4 (FL=432mm)
Mount & Tripod: Manfrotto/Bogen 410 on Manfrotto/Bogen 3221s
Eyepiece (magnification / fov): Kellner ~20mm (21.6x / 2°)

The Coulter CT-100 reflector is one of the scopes I occasionally use to observe. Charlie has the same scope and has dubbed his S.A.R. Mine is aptly called at CiTy-100 befitting an urban observer. It is a small richest field scope, packs handy in its own case, sets up in seconds. This scope was mentioned in Gary Seronik's Telescope Making column in Sky & Telescope, Dec 2003, page 126, including a copy of an old advertisement. I like this scope for its portability and ease of use.

This session's skies were one of the best recently with clear, dry skies. NELM was about magnitude 5 in zenith, counting 6 stars of Ursa Minor with Pherkad Minor making 7. eta UMi was not seen. Transparency was good across most of the sky, even in our gegenschein to the south. In the eyepiece of CT-100, the Milky Way glowed as the background sky beneath the brighter field stars while panning through Cygnus all the way down toward Albireo.

The CT-100 consists only of essential elements required for a telescope. The observer is provided an optical train whereby two assemblies mount on a single strut with beveled edges. The strut serves as a rail where the two assemblies, outfitted on the underside with teflon strips, slide to rough focus the telescope. The CT-100 front assembly houses the spider, secondary mirror and eyepiece chimney. The front assembly also has a surface-mounted slot to hold a finder scope, although I don't have one. The CT-100 rear assembly holds the mirror and its cell.

The advertisement copy shown in the S&T issue states that scope comes with a "27mm helical focusing eyepiece with 1 1/4" barrel provides 14x". The one I purchased came with a helical focusing eyepiece but I estimate it to be a Kellner 22mm with 40° field of view. This yields a magnification of 19.6x and a 2.04° true field of view. This agrees with a field measurement taken last night by placing stars of the Alpha Persei Association at the edge of the field stop. The magnification and field of view is similar to that of the Takahashi 22x60 binoculars. Telescope limiting magnitude was 9.06 using stars within the area of HIP 16118, -16210, and -16244 (34 Per).

The front assembly provides no relief extra tubing beyond the spider attachment points. It suffers from extraneous light intruding directly into the eyepiece but this could be corrected with a fold of paper. This scope was purchased without a shroud so I use black flocked paper and rubber bands. The same paper and bands have held up over three years, even on some dew-filled evenings. I store the flocked paper in an empty paper towel tube. Sometimes I'll attach this paper towel tube to use as a simple finder scope, otherwise I sight over the top of the tube.

Following are some daytime shots out the window. With camera zoomed out fully, one can see vignetting and the shadow of the secondary. Following that are shots zoomed in. Off axis is not so bad but in the field with stars coma is noticeable. The scope could have used tweaking to the focus to sharpen up the scene. Also the eyepiece adapter sat a little off center where I had to tug on the camera to center the shadow in the low power shot. In the higher power shots I let the camera rest naturally.

Zoomed out showing shadow of central obstruction. When observing low power on large, bright objects, like the Moon, one can see the shadow float in the middle of the image.

Same shot as above but zoomed in. And as in the field when looking at star fields and clusters the obstruction is not noticed. In practice, I use only two size of eyepieces: a Tele Vue Plossl 32mm or a Plossl 20mm/Kellner 22mm.