Črni Vrh Observatory

The ACIT Imaging System

The Automatic Comet Imaging Telescope (ACIT) is an imaging device designed for Sky Survey Program and is now jointly run by University of Ljubljana and Crni Vrh Observatory. It utilizes a 20-cm, f/2 automatic Baker-Schmidt telescope and HiSys-44 CCD imaging detector. The goal of the project is to scan larger areas of sky for several times on each clear night and search them for the presence of any transient objects. The whole system is being operated in Linux and may be run either in-site or remotely over the Internet link. An observing schedule, containing field coordinates, exposure time and number of scans is prepared for each night in advance and sent to the main computer. The image acquisition, processing and searching for new objects are fully automated. At the end of observing session, the suspect objects are checked either locally or remotely over the data link. On an average 10- hour night, our system is able to scan twice an area of 20^o x 20^o.

The system consists of a fork-type friction driven mounting, the 20-cm, a f/2 Baker-Schmidt telescope and a custom designed stepping motor controller.

The telescope was designed by H. Mikuz, D. Zgavec, B. Dintinjana and J. Skvarc and built in a local workshop.

CCD camera performance

CCD Sensor	KAF-1600 1536 x 1024 pixels, 9 um x 9 um pixel size
Cooling	liquid, two stage Peltier at -40^oC below ambient
Dark current	1e-/sec
ADU Conversion	3.0e-/ADU
Field of view	1.8^ox 1.2^o (2.2 square degrees)
Data format	FITS (16-bit)
Limiting magnitude	about 15.6^m in one minute unfiltered exposure

Software

The TSERVER program takes care for the communication between hardware controllers for both telescope and CCD camera and application programs. It controls the telescope positioning and image acquisition, while the image processing (Dark, Bias, Flat) is done by XVista immediately after image aquisition. Each image header contains various data such as date-time of acquisition, duration of exposure, WCS field coordinates and other information about the observation. The images are analyzed by the Fitsblink software. The program is able to do accurate astrometry of any moving object automatically by comparing the field stars with GSC or USNO-SA 1.0 catalogues. A MPC computer format report can be generated and sent to the Minor Planet Center.

TSERVER program was written by Bojan Dintinjana. Fitsblink is a freeware program, written by Jure Skvarc.

Search strategy

Each region of sky is searched at least twice on each night. As soon as the second scan is over, the software compares the lists of objects found by Fitsblink and checks for the presence of any moving objects. Finally, a list of image pairs, containing candidate objects, is generated. The candidate objects are then visually checked.

Telescope

Mechanics

The polar and equatorial disks are driven by 1:15 and 1:20 rollers, respectively, through 1:180 worm gears and stepper motors. Both motors are operating in micro stepping mode yielding microstep resolution of 0.11 arcseconds. The telescope slews across the sky at 0.9o/sec and has a pointing accuracy of 1-2 arc min over the entire sky.

Optics

Baker-Schmidt, 20-cm, f/2 (400mm f.l.) with fully corrected flat field. The camera has a special bar cage structure, made of low-expansion Invar alloy that holds the mirrors at a permanent distance. This prevents shifting of the focal plane due to ambient temperature changes and enables to begin observations without losing time for focusing.

Electronics

Stepping motor electronics is built around a 80535 microcontroller which provides signals for two pairs of LMD 18245 stepping motor drivers and inputs for stop switches and hand controller. It communicates with the central computer through a RS-232 port. During the operation it maintains its position for both axis and takes care for all the low-level signals.