Johns Hopkin~ University

Instrument Science Report CAL/FOS-081
March, 1992

1. Introduction

Onboard. (or Mode II) target acquisition refers to a set of methods
for acquiring and centering targets without real-tinie
intervention. The ultimate aim of each target acquisition method is to
accurately center the target in one of the small science
apertures. The tour onboard target acquisition strategies are:

1.	Binary seatch: a target is located in the 4.3 aperture by
  	stepping the diode array until the star is determined to be
  	located on tbe edge of the array.

2.	Peak-up: one or more raster scans are rnade of the target
  	region with a small aperture in place. The position where the
  	maximum count rate is observed. is determined to be the
  	position of the target.

3.	Peak-down: is the same as a Peak-up target acquisition except
  	that the target is determined to be the point where a minimum
  	count rate is observed.

4.	Firmware: the diode array scans the 4.3 aperture and the
  	internal microprocessor filters the image to determine the
  	position of targets within the. aperture.

Bach of these target acquisition strategies has their own features,
strengths and weaknesses. Detailed descript.ions and strategies can be
found. in the FOS target acquisition handbook. Here we report the
results of five target acquisition tests designed to verify the
operation and accuracy of the different target acquisition strategies.

Proposal 2195 (and its successor 3123), Combined Mode II Target
Acquisition verifies all tour target strategies on a single bright
star. Proposal 1314 (Faint Object Target Acquisition) verifies
Firmware on a single faint star and Binary search on a field of tour
faint quasar ilnages. Proposal 1428 (and its successor 3137) Blind
Offset Target Acquisition test the ability of the telescope to acquire
one target and perform a slew to another nearby object. Finally,
proposal 1427 WFC Assisted Target Acquisition tests & fifth target
acquisition strategy involving a real-tirne target acquisition with
the Wide Field Planetary Caniera (WFC) followed by a telescope slew to
place the star in the FOS aperture.

In the next section we discuss in detail the steps carried out in each
of these proposals. The following section presents the results of the
tests in tabular form along with an overall discussion and corninents
011 target acquisition strategies.