QUESTION: 
What language of the sofwares that control the HST is written in?

ANSWER from Leslye Boyce on May 1, 1996:
There are three computers on Hubble right now and they are all 1970
technology;  the Hubble was built in the late 1970's and early 1980's.  
In the next servicing mission (2/97) we will be adding two new Science 
Instruments with powerful micropocessors.  Historically, we have used 
assembly language,  but now we are moving to C.  The issues we must 
consider are:
 
1) Memory limitations.  Our memory is typically plated wire and that 
technology use to be very expensive.  Assembly language applications allow 
us to utilize every memory location.  This also helps us make patches to the 
current programs. Prior to the First Servicing Mission (12/93) our main
pointing computer (DF-224) memory units were failing.  On this computer, we
had a total of 6 - 8K units.  We require 4 at any time.  We had lost 2 and
could not afford to lose any more.  A team of engineers (hardware & software)
designed and built a coprocessor which provided us with 64K x24 bits of 
additional RAM storage. That computer also contains an Intel 80386 
Processor.    
 
2) Performance.  In flight computers, we require the system to operate at real-
time.  This means that we want to read our sensors and send commands to our 
actuators immediately.  As a result, we often place heavy requirements on the 
CPU and that requires we track our the utilization very carefully.  In the 
older technology computers which did not run very fast, we typically go
above 90% CPU utilization.  That limits what we can do with the telescope.
We are in the process of rewriting some of Assembly code in C and migrating
those functions to our 386 processor.  It is a very exciting, yet busy time
for the flight software group. 
 
We are also going to replace the DF-224/Coprocessor with a new computer
in 1999.  It is a 80486 Intel Processor, with 33 MHz clocking.  The computer 
language of choice is C; we do not plan to use assembly.  We chose C because 
we believe, 1) it will be able to achieve our required performance with the 
increased CPU speed  and 2) it will help reduce our maintenance costs over 
the life of the mission (assy. programmers are sometimes hard to find).  
 
Hope this answers your questions and generates more!


ANOTHER ANSWER from Wayne Baggett May 16, 1996:
Since the are so many different parts to the Hubble Space Telescope, 
there are a large number of computer languages used in the programs.  
On the computers actually up in space, almost all of the code is written 
in assembly language, although the two new instruments being launched in 
1997 will use the C language for their software, with some assembler used 
when speed is of importance.  On the ground, the language used is simply 
the one which is best-siuted for the task: FORTRAN, LISP, IDL, C, 
assembler, and many others are used in various places.  It really pays to 
know as many programming languages as possible on this project.

Thanks for the questions.  I hope you have enjoyed the Live from HST program,
and have learned some interesting things about how these kinds of projects
really work.  Have a good summer!

Wayne Baggett
Computer Sciences Corporation
Space Telescope Science Institute