Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://vega.inp.nsk.su/~inest/OCAAS/ocaas11-16 Äàòà èçìåíåíèÿ: Fri Mar 16 13:11:54 2001 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 19:54:55 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: occultation

Introduction

1 Observatory Control and Astronomical Analysis System.

1.1 Introduction.

OCAAS is a complete UNIX software package for the local and remote operation of an
astronomical observatory. It supports both interactive real­time operation and unattended
batch­scheduled operation. It controls all aspects of the telescope mount, CCD camera, filter
wheel, focuser, weather instrumentation, power supply, GPS receiver, internet or phone line
communications, dome and shutter hardware. It automatically performs image corrections, WCS
coordinate calibration, compression and data transmission. OCAAS includes programs for off­line
photometric and astrometric image analysis of static and time­varying phenomena.

1.2 Main Control Features.

1.2.1 Supports equatorial and altitude­azimuth mounts.

OCAAS allows the telescope axes to be oriented in any orientation. Thus, equatorial,
altitude­azimuth or any mount situation is acceptable. This flexibility accommodates portable
operation particularly well. German equatorial mounts are supported. A fast 3­star alignment
procedure computes the orientation to good precision, and can be refined to higher accuracy by
automatically taking, analyzing and applying a sky mesh. OCAAS accommodates field rotation,
which occurs for any orientation other than equatorial, by generating commands to operate a field
rotation motor.

1.2.2 Tracks planets, comets, asteroids, and Earth satellites.

Ephemerides are included for all planets. Orbital elements for current comets and all numbered
asteroids are included and may be easily extended and updated. Earth satellites are fully
supported and may be tracked if very current NASA Two­Line­Element sets have been retrieved
and the mount is capable of tracking rates on the order of degrees per second.
1.2.3 Unattended scheduled observing
A simple language is used for specifying details of an observation request, such as filter, duration,
source name or RA/Dec, offsets, special calibrations, repeat count and delays. Files of requests
may be created using any text editor, with a Web page form, or with a GUI application, mksch.
Collections of individual observing requests are then combined using a scheduling tool, telsched,
into an efficient interleaved command sequence which will be executed unattended throughout a
night by the telrun system. Images will automatically be corrected and calibrated in parallel with
acquisition. All details of each observation are logged individually, in addition to a continuous log
of all engineering data. This automated sequencing may also be interrupted at any time for an
unrelated set of requests, then resumed as necessary.

1.2.4 Remote Internet or Phone operation.

All control software is fully network­aware using the X Windows protocol. Thus, real­time remote
operation and monitoring can be performed using any viable network connection just as easily as
operating locally. Remote scheduled operation is a matter of sending a command sequence file
before the night begins, then retrieving the images and logs the next morning.
Ethernet connections are preferable, but ISDN and PPP connections to an ISP via modem are
also supported. Just be aware of large data sets which can be generated.

1.2.5 Automated powerup and axes calibration sequencing.

When power is first applied to the OCAAS computer, it can automatically initiates a basic self­test
sequence. Then each axis is sent to find its home position to calibrate the motor step and
encoder positions. When these procedures complete, typically in less than two minutes, the
system is fully operational and, if batch observations are pending, observing commences (or
resumes) automatically.

1.2.6 Simple Basic­Alignment procedure.

When a telescope running OCAAS is first installed at a new site, the telescope mount orientation,
basic flexure and non­perpendicularity of the principle axes must be determined as a set of
descriptor coefficients. This is all accomplished using a simple procedure which requires only that
three known stars be located in sequence in an eyepiece or the CCD camera. The operator is
prompted during each step of this procedure which takes about 15 minutes to perform. This
procedure need only be repeated if the telescope mount is disturbed or modified.

1.2.7 Fine­alignment procedure.

Once Basic­Alignment has been completed, the telescope is typically capable of acquiring targets
within a few arc minutes of accuracy. If this is acceptable no further alignment is necessary.
However, if better acquisition accuracy is required then a fine­alignment procedure may be
performed. This procedure compensates for variety of systematic errors including unusual mount
flexure, incorrect location and drive train peculiarities. The first step is to schedule several
hundred images to be taken which cover the sky in a fine mesh. This is accomplished using a
single menu selection from the telsched batch preparation tool and starting the scheduled
acquisition system, telrun. As with all scheduled acquisitions, these images will be calibrated to
sub­arcsecond accuracy. When they are all finished, they are analyzed and combined into a map
of pointing errors using the tool pterrors. The resulting map is then installed and will be
automatically utilized for all subsequent pointing operations.
It usually requires four to six hours to acquire the fine­alignment mesh images, but it does not
require operator attention. The procedure need only be repeated if the telescope mount is known
to have been disturbed or modified in some way, or if recent images suggest pointing errors have
begun to reoccur. Note that uncertainties in atmospheric refraction will generally preclude very
high pointing accuracy when working near the horizon.

1.2.8 Automated temperature­compensated focusing.

OCAAS can perform an automatic focus procedure at any time under operator control. This
procedure acquires a short series of images at several focus positions and analyses each for
sharpness. These are then interpolated to compute an optimal focus position. The procedure
takes approximately five minutes and requires no operator assistance once it has been initiated
from the menu selection. The focus position thus found is logged along with the current ambient
air temperature. If this focus procedure is performed at least two different temperatures, OCAAS
control software will then use this log to automatically set the focus position based on the air
temperature before each image is acquired. The temperatures can also be detected directly on
the telescope using small sensors manufactured by Dallas Semiconductor.

1.2.9 All images automatically corrected and WCS calibrated.

As OCAAS acquires each raw image from CCD camera, they are automatically corrected with the
appropriate bias, thermal and flat frames. This can be true for both real­time operation and
scheduled operation. Cataloged reference frames may be used, or new bias and thermal frames
may be generated specifically just before each image. Dome flats may also be taken
automatically if desired. The details of this processing are always recorded in the FITS header.
After corrections have been applied, stars throughout the field are identified and pattern matched
to the Hubble Guide Star Catalog to compute a best RA, Dec and field rotation. This position
calibration is recorded in the FITS header using the standard World Coordinate System FITS
header keywords.

1.2.10 Lossless or Optimized image compression.

As images are acquired, they can be automatically compressed to reduce data storage
requirements. The compression can be lossless and will achieve approximately 3:1 savings in
space. Or a compression algorithm which is optimized for astronomical images can be specified
which can achieve file size reductions of 10:1 or more while preserving all quantitative
photometric and astrometric characteristics of the images. The algorithm used is known as H­
Compression and was built by the Space Telescope Science Institute for managing images from
the Hubble Space Telescope. The type and degree of compression can be specified separately
for each observation.

1.2.11 Field rotation control (required only for non­equatorial mounts).

Field rotation will occur during any extended exposure for a telescope mount whose polar axis is
not aligned very well with the celestial pole. Once at least the basic­alignment procedure has
been performed to determine the polar axis orientation, OCAAS control software can compute the
field rotation in real­time during each exposure. This can operate a stepper motor attached to the
camera to counter rotate and effectively remove the rotation effect from the exposure.
1.2.12 Dome, shutter and roof control
OCAAS can read an incremental encoder attached to a dome, and use the information to control
a bi­directional A/C motor to automatically maintain dome slit alignment with the current telescope
pointing position. OCAAS can also operate a motor to open and close a shutter curtain on the
dome, and pre­rotate the dome to a fixed position each time if necessary to align power take­off
wipers for the shutter motor power. Or, the shutter control can be used alone to activate a motor
for a roll­off roof.

1.2.13 Continuous Weather monitoring and logging.

OCAAS can monitor local meteorological data on a continuous basis and automatically terminate
further image acquisition and initiate shutter or roof closing immediately if preconfigured limits for
temperature, humidity or wind parameters are reached. When these conditions no longer exist for
a configurable period operation will automatically resume. As each image is acquired all weather
data are logged in the FITS header. The horizontal and vertical Full­Width­Half­Max statistics for
each image are computed and stored in the FITS header to facilitate quantitative investigations of
seeing. OCAAS stores all meteorological data to a log file when any parameter changes by a
configurable amount. These logs are compact and useful for studying the long­term weather
characteristics of a site.

1.2.14 GPS Location and time.

OCAAS can continuously monitor a GPS receiver to maintain the system time to much greater
than one second accuracy. It can also use the GPS receiver to initialize the geographic location
of the telescope. This is particularly handy for mobile applications.

1.3 System Requirements and Assumptions.

OCAAS is a software system only. It does not include any hardware, such as a computer, motors,
telescope, camera etc. The customer must choose and purchase these items separately. The
information regarding hardware in this section is intended primarily for planning purposes and for
initial engineering designs. Before purchasing decisions are finalized, it is recommended that
Clear Sky Institute be contacted to insure compatibility and clarify any issues.
Every effort has been made to allow OCAAS to function with many telescopes, domes and
cameras. However, it is clearly impossible to guarantee it will work in all cases. CSI works closely
with Torus Precision Optics, Inc., and Apogee Instruments, Inc., to insure OCAAS works with
their telescopes and CCD cameras, respectively. Choosing these suppliers for these components
will eliminate all risks with regards to compatibility with OCAAS software.

1.3.1 Pentium CPU.

OCAAS software drivers are currently written for controllers which function in the PC hardware
platform with at least two ISA slots. We can report excellent experience with the Dell XPS series
systems using a 166 or 200 MHz processor with 32MB of RAM. This system never shows CPU
utilization higher than 20% during all phases of image acquisition, correction, calibration and
compression.

1.3.2 Linux ELF 2.0.30 or newer

OCAAS software is written in ANSI C. GUI applications are written with the X Windows and Motif
API libraries. The software does not use any user process features specific to Linux and is known
to function on other UNIX systems, with the exception of the low­level device drivers which have
been optimized for Linux. We can report very reliable operation using the Linux 2.0.30 kernel and
the Slackware 3.3 distribution. We also know OCAAS works fine under Red Hat release 5.1,
which some might prefer for its graphical system administrative tools.

1.3.3 OMS PC­39­E6 Intelligent ISA Motion controller.

All motors and incremental encoders interface to the PC hardware using a PC39 intelligent
controller built by Oregon Micro Systems. This controller is a full­width card for the ISA bus
interface. It has high level commands for features such as cosine profile accelerations, home and
limit switch logic, motor pulse generation, encoder pulse quadrature detection and accumulators,
and multiple axis control. These facilities of this controller are important for off­loading the
time­critical pulse generation from the PC host. This particular model can operate six motors and
read two encoders. Other combination are available.
Other controllers, such as those from Galil or Parker, should also be suitable in principle although
Linux drivers for them are not yet available.

1.3.4 Stepper (or servo) motors on all axes.

OCAAS requires motors to control all axes, including HA (or Az), Dec (or Alt), focus, filter wheel,
and field rotator (if necessary). Through suitable amplifiers, the PC39 (mentioned above) can
drive either stepper or servo motors with no change in OCAAS software. OCAAS does not
perform periodic drive feedback, so friction drives for the principle axes are highly recommended.
We have had outstanding performance with servo motors and recommend these in general over
stepper motors for the main telescope axles.

1.3.5 Incremental encoders on mount axes.

OCAAS requires incremental shaft encoders on the HA (or Az) and Dec (or Alt) axes. This
assures accurate slewing acquisition should modest drive slippage occur. We strongly
recommend mounting the encoders directly coaxial with each shaft to eliminate the chance of any
slippage from intermediate coupling mechanisms. If the encoder resolution is sufficiently high
they can also be used as part of the feedback loop during tracking to refine the ideal velocity
commands issued to the motors.

1.3.6 Limit switches on principle axes.

For safety and peace of mind, limit switches are generally required at each extreme of travel on
each axis. The limit switches must operate independently on each axis. The switches should be
set to protect the mechanical and safety aspects of the equipment and installation. The limit
switches should not be used to restrain the observing circumstances for the site, such as
minimum altitude, as these are defined and enforced by the software system. When activated,
they immediately kill any pulses to the motor on their axis without requiring software cooperation.
The limit switches are also used during the initial power up sequence if the telescope was not
stowed before loss of power. Limits on the polar axis must be available for each direction of
travel. Clever mounting arrangements will be necessary to avoid a wedge­shaped dead zone.
Limit switches are required on the telescope main axes and are recommended on all other motion
controls. However, they can be eliminated if absolutely necessary for weight or size reasons if the
home switch for an axis is located at one extreme end of travel for that axis and the nominal
operating conditions for the axis is never near the other end of the travel. For example, the focus
mechanism is often a good candidate for choosing to not include limit switches.

1.3.7 Home switches on all axes.

Home switches are required for all axes. These are used to calibrate the stepper motor pulse
counts to known physical locations. The location of the home switches is arbitrary, although it can
reduce the time to perform the power up sequence if they are positioned to coincide with the
desired stow position. In special cases, they can also be located in such a way to eliminate the
need for limit switches on a motion control, as mentioned above.

1.3.8 2 GB disk space.

OCAAS system software and all databases require approximately 400 MB of disk space.
However, even one evening can accumulate on the order of 1 GB of data so a large amount of
disk space will be very handy. We have found 2 GB of space to be practical if the data can be
reliably off­loaded nightly.

1.3.9 SpectraSource HPC­1 or any Apogee CCD Camera.

The current release of OCAAS software includes drivers for the SpectraSource HPC­1 and all
CCD cameras produced by Apogee Instruments, Incorporated. These cameras interface to the
PC via their own custom ISA card controller cards.

1.3.10 Dome or Roll­Off roof.

OCAAS software is capable of controlling either a rotating dome with a motorized shutter, or a
roll­off roof observatory configuration. OCAAS software accommodates a rotating dome with a
bidirectional AC motor, an incremental azimuth position encoder, a home switch, and limit
switches. Since domes rotate rather slowly as a rule, however, it is usually worth the extra cost to
use an absolute encoder which eliminates the need to find the home switch on each power up.
OCAAS can accommodate either arrangement. The dome shutter requires a bidirectional AC
motor. OCAAS is capable of operating a dome shutter which is supplied power via a take­off
wiper that requires the dome to first be rotated to a particular azimuth.
A roll­off roof requires three output lines and three input lines. The output lines are asserted to
command Open, Close and Stop operations. The input lines listen for Opened, Closed and Error
feedback signals. OCAAS software can be configured to remove the Open and Close assertions
if the corresponding affirmation does not arrive within a configurable time period. The outputs are
always immediately deactivated if the Error input is asserted.
All current OCAAS dome software controls hardware capable of operating only at TTL levels; it is
up to the customer to use these to power their motors.
Since domes tend to vary significantly from one installation to another, the OCAAS design allows
for a custom dome ``plug­in'' to be installed on a case­by­case basis.

1.3.11 Peet Bros. Ultimeter 2000 weather station.

OCAAS software supports the Ultimeter 2000 weather station from Peet Bros. This system
provides data on wind speed and direction, temperature, humidity, air pressure and precipitation
via an RS­232 interface. A daemon process runs continuously to update OCAAS on current
conditions for its refraction model; saves all statistics to a file for logging purposes; and monitors
wind speed, temperature and humidity for alert conditions. If the latter occur, a Weather Alert is
issued and OCAAS stops observing and closes the dome (or roof). Each FITS image acquired
includes fields for all weather statistics.

1.3.12 GPS Receiver.

OCAAS can derive time and geographic location from any GPS receiver which can supply the
GPRMC NMEA data sentence to a serial port on a timely basis. To date, the Garmin models 45
and 36 have been tested and work well. Other qualifying brands are also very likely to work as
well.