Technical specifications of the APO-modified C102-HD refractor of Celestron.

I. General/Original.

The original outfit of the C102-HD achromate with equatorial mount (CG-4) consists of two eyepieces, a 7.6 mm Plössl, series 3000 of Meade and a 20 mm of Celestron, and a Celestron polar axis finder scope with cross wire illumination. The visual finder telescope is a 6x30 Celestron with a cross wire.

II. Astrophotography.

Images of sky-objects are photographed with a PC-controlled, 1-stage cooled(-30°C) CCD-astrocamera, the 12 bits Starlight-Xpress MX5-C, which makes it possible to record repetitively single-shot multicolour pictures. Monitoring and control is performed with the program STAR2000/MX5-C or ASTROART2.0 with the MX5-C plug-in. Raw images of exposures are colour-converted to LRGB images and processed with ASTROART, too.

III. Optical aids and Sky-data.

Optical options have been extended by replacing the 6x30 finder scope with a 9x50 one and fitting a 0.1 lux CCD-videocamera at prime focus to it. This greatly enhances finding and focusing objects up to Magnitude 7. The 0.33 inch B/W sensor of the 12V camera has 725(H)x528(V) pixels, gives 380 TV lines in horizontal and 450 lines in vertical direction, has an automatic iris and shutter speed (1/50 to 1/32000), has a 2:1 interlace scanning system and a S/N ratio better than 50dB.

The real-time image of this combination is depicted on a TV-monitor on which screen a centering cross has been drawn. The circular FOV of this combination is 2.4° and is, together with the square FOV of the single-shot CCD-astrocamera projected at the Earth Centered Universe(ECU)-screen, showing the relevant part of the sky. The centers of both FOV's are made to coincide at the center cross of the TV monitor and the center of the CCD-chip of the astrocamera. This simplifies finding and centering sky-objects to a great extent. General information on sky-objects is retrieved from the well-documented REDSHIFT-3 code.

Further on a telecompressor lens (Meade 0.63x), two apochromatic Barlow's (Vixen Deluxe, 2x) and a 12 mm Kellner eyepiece with illuminated double crosswire, mounted in a Flip Mirror Finder (True Technologies) equiped with an IR-filter, have been added. For testing optimized visual focusing, a dummy eyepiece with a 7 lines per mm Ronchi grating is available.

IV. Modifications.

A. Electromechanical.

The CG-4 mount, of which the bearings and drives have been re-adjusted to satisfaction, has been upgraded with:

1. an autonomous controlled stepper motor for the right ascension axis (RA), max. 8x sidereal speed,

2. a variable speed DC declination motor (JIM's Mobile) to set/control the declination and

3. a variable speed DC focusing motor (JIM's Mobile) to focus the image plane.

The backlash in the two drives of the mount for RA- and DEC rotation at reversal is 6.6s and 51"(arc), respectively. Hysteresis is well within these values.

The focuser has beeen modified, too. The upper silicone strip has been reinforced with a metal backing, while longer set-screws have been inserted in the focuser body.

All accessories to be used at the focuser draw-tube are fitted with three setting screws to facilitate alignment of the optical train, for which a Cheshire eyepiece and a TL-lightsource are used.

B. Telescope control.

1. RA- and DEC-motion.

Actuation of both the DC DEC-motor and RA-stepper motor is being executed via the STAR2000 interface module in combination with the accompanying Relay Box. The speed of the DEC-motor can be continuously controlled manually, while their are next to sidereal rate, two(2) speed options for the RA-motor; 2x and 8x sidereal speed in both East- and West-direction. The lower speeds are used for self-guiding and tracking of a chosen celestial object in the FOV of the telescope, while the higher speeds are used for slewing purposes.

The STAR2000 interface module from Starlight-Xpress is operated by either the STAR2000 software (telescope control panel) of the MX5-C astrocamera or by the telescope control window (move panel) of ASTROART2.0.

Finding an object, slewing to an object and centering it is performed by combining the telescope control panel window with the real-time local sky-chart of ECU. Chosing an object in ECU gives its co-ordinates and deviation of the present position the telescope is pointing at. The latter information is coming from the DSC’s on the equatorial mount and their "Astro-mouse" interface with ECU. By activating the relevant buttons in the control/move panel, the deviations are zeroed and the chosen object arrives near or at the optical axis of the telescope as observed in the continuously updated image field of the astrocamera MX5-C. For the latter situation , the camera operates in the focus-mode under ASTROART2.0. At that moment the position of the optical axis of the telescope and the position of the chosen object are synchronized and centered in ECU.

2. Focusing.

Focusing is performed with the motorized rack-and-pinion focuser and the DFRO. The focus-motor is actuated via the North- and South-buttons of the move-panel of the telescope control window in STAR2000 and the result is real-time observed in the regularly updated image field of the "focus"-window in ASTROART2.0.

C. Tracking and Guiding.

During tracking or self-guiding of an object, a real time ECU-window shows the possible deviation in RA- and DEC-values of the actual telescope position (ie. pointing position of it's optical axis) from the coordinates of the chosen object. Corrections can be performed both manually by actuating the hand controllers of the RA- and/or DEC motors and automatically by using the "guide"-option of ASTROART. In that case an image window shows the initial and actual position of the guide object and the STAR2000-protocol derives correction commands from the deviations of the actual position and actuates the motors to correct the position. The guiding information is updated every 0.2 seconds or at longer intervals. Control is within ±1 pixel (about 0.6"at F/40) of the CCD-chip of the astrocamera for an indefinitely period of time.

For long exposures, accurate control of the position of the object to be studied on the CCD-chip of the camera is also done with the automatic self-guide option in ASTROART2.0. Control can be obtained to the 1 pixel level for an adequately picked guide-star , which suppresses trailing on the image, completely.

Raw MX5-C images at F/40 of Upsilon-29. Left: 15 minutes exposure without self-guide. Right: 15 minutes exposure with ASTROART2.0 automatic self-guide.

VI. Numerical values of the set-up.