mountain profile Institute for Astronomy University of Hawaii

Day - Night Seeing Monitor [DNSM] Telescope System

THEORY OF OPERATION

The image quality at a telescope focal plane is directly related to the statistical perturbations of the wavefront, from the source, entering the telescope pupil.  By using various wavefront sensing methods, it is possible to examine and measure these incoming wavefront fluctuations, and thereby measure the net seeing conditions, independently of the telescope in use.

The method employed consists of measuring the stellar wavefront slope angle differences over two or more small pupils, separated by some distance.  This method, being a differential one, is inherently insensitive to telescope tracking errors.  This feature is extremely important in the case where relatively small telescopes are used in exposed outdoor conditions, thus subject to wind buffeting.

The primary telescope optics consist of an all- spherical- surface design, modified- Maksutov system, having a primary mirror, secondary mirror, and a thick meniscus corrector lens assembly, mounted close to the secondary.  These optics form a conventional Cassegian focal plane image of the star through the central perforation in the primary mirror.  The NSM Hartman screen attached at the front of the telescope tube, contains four (4) sub apertures, in an equally spaced array.  Each aperture is 10 cm in diameter.  The screen can easily be removed from the front of the telescope tube if necessary, for example, to replace it with one having different- sized apertures.  The ancillary optics located in the backplane area form a relayed image of the pupil and hence the sub apertures at a convenient location on the backplane optical bench at this reimaged pupil plane.   A precision adjustable optical wedge assembly is located at the pupil image plane.  The wedges diverge the respective pupil beams to form four separate images at the camera.

A seeing measurement is computed from a set of 100 images taken at approximately three frames per second (presently being upgraded to ~200 frames per second).  The positions of the four star images are measured in each frame of the set and the differential motion between frames computed.  The atmospheric condition required to produce this level of movement is interpolated from a pre- computed simulation of the system and the atmosphere.  The result is the Freid parameter, Ro, for the sample period.

TELESCOPE AND ANCILLARY EQUIPMENT

The DNSM telescope is a 1500lb assembly mounted on a hydraulic elevator within the top of a 35 foot steel tower attached to the Zodiacal Light Observatory building at tehe Haleakala observatory site.
Additional equipment incorporated in the system:
J-box:  A small electronic cable junction which is located within six feet of the telescope assembly.  The size of the J-box is 16Hx14Wx6.25D.
Electronic Housing:  A 1/2 rack- sized electronics housing with front and rear covers.  This housing contains computer and electronics equipment, located no more than 30 feet from the J-box (due to cable length restrictions).  The size of the electronic housing is 36Hx21Wx27D.
Nitrogen Gas:  A standard metal cylinder of compressed nitrogen gas in order to operate a mechanical brake on the telescope assembly.
Computer System:  A PC- computer system with graphical display CRT, several television monitors.. t