Disclaimer: The Iowa Robotic Telescope title was transferred in September 2003. It is now operated by Winer observatory. This page is here for reference only.

 

Optics and Optical Tube Assembly 

The telescope optics consist of a 0.5m diameter Ritchey-Chretien primary, a remotely controlled focussing secondary and a 45 degree flat tertiary (Nasmyth desgin). The nominal focal ratio is f/8. The measured focal length is 4.06m, resulting in a plate scale of 19.6 microns per arcsec. The optical surfaces were manufactured by Torus Technologies of Iowa City (now OMI). The optical tube assembly, shown at left, was designed and built at the University of Iowa by Robert Winsor. It uses low thermal expansion carbon fiber struts and aluminum mirror cells. The thermal design predicts less than 1 micron change per degree centigrade of the primary-secondary separation. The primary mirror support is a 36 point floating system (click for a number of CAD renderings of the OTA). The focus motor assembly is mounted behind the secondary mirror. We have chosen a Nasmyth design for the optical path for ease of mounting CCD cameras and filter wheel. A set of detailed photographs illustrate the design.

Mount

The mount is a compact azimuth-elevation design with friction wheel final drives. Both axes are driven with NEMA size 34 stepper motors which drive a 60:1 harmonic gear reducer, a 1:1 zero backlash belt, followed by a 12:1 friction drive. The motors are controlled by an Oregon Microsystems PC-39 intelligent controller. Each axis is equipped with incremental optical shaft encoders (Gurley). We use a very accurate magnetic home switch and count motor microsteps (1 microstep is ~0.4 arcsec) to determine position. The design specifications call for blind pointing accuracy of less than 1 arcmin RMS and a tracking accuracy of less than 0.005 arcsec per second. Click for a sketch of the mount and OTA.

CCD Camera

The CCD Camera is an Apogee Instruments AP-8 with a SITe Si-503a thinned, visible AR coated CCD sensor. This chip has 1024x1024 format with 24 micron pixels and peak quantum efficiency of ~90% and excellent blue sensitivity. The read noise is about 15 electrons, while the dark current is less than one electron per pixel per second. The field of view is 21x21 arcminutes with a pixel size of 1.235 arcsec. This is less than ideal for photometry under good seeing conditions, but represents a compromise between field of view and pixel resolution

Filter Wheel

The filter wheel is a ten position stepper motor controlled wheel with 50 mm diameter filters. There is a set of Johnson standard BVRI filters, a clear filter, and several narrow band (1nm) filters, including H alpha, H beta, OIII, and an 'offband' reference filter.

Telescope Enclosure

The Observatory enclosure is a roll-off roof design. This is ideal for seeing (minimal air convection), is mechanically much simpler (no tracking required), but can be susceptible to wind loading on the telescope.

Site Description

The IRO is located at the Winer Observatory, a privately owned facility about 3 miles SSE of Sonoita, Arizona. The Observatory is in the high Sonoran desert about 15 miles E of Mt. Hopkins (see map). The MMT can be seen on the mountain ridgeline west of the site. The site is owned and operated by Mark Trueblood, an engineer at NOAO's Gemini project. The building consists of a 20x40 foot service area and a 20x40 foot telescope siting area with roll-off roof. The control computers are located in a warm room (appx. 20x10 foot) within the service area.