European Southern Observatory New Technology Telescope

General :
Observations :

MOS Operations

3 MOS SW maintenance

   3.1 MOS Offset calculation
   3.2 Reset of punching counter

3.1 MOS Offset calculation

The requirement is that the slit punched should be aligned with the columns of the detector once the MOS plate is in the observing position.

To ensure this the mosOffset should be adjusted. This operation is only required when either the CCD or the starplate are dismounted.

Here follows a procedure to measure and calculate this offset:

• Punch a plate loading a file that will contain coordinates which will vary just in y, while x is maintained fixed.
• The easiest to perform this step is to define using emos MIDAS context a mask containing a single long slit from top to bottom of the punchable area.
• Take an exposure with the telescope room lights on, using this plate. (The reference light can also be used).
• Call Midas with "inmidas" and give the following commands:

 
• intape/fits 1 mos xxxxx (where xxxxx is the name of the file created by emmi)
• load mos000x (where x is the name of the file created by previous command)
• center/gauss
  
  select with the mouse a point in the upper part of the slit image
  select another point in the lower part of the slit image
  click right mouse button to get out of center/gauss command
  
  
• Now we can calculate the difference in position between upper and lower part
•  take the difference between the two center values in pixels like
 
diff = (Xlow - Xup)/(Yup - Ylow)
• this difference gives the offset using the following formula:

 
tan (angle) = diff/1000
angle = arctan(diff/1000) deg.
Since 1 deg = 1000enc units then
 
offset = angle*1000 enc
(the offset is positive if the slit image is inclined as 11:25)
 
• new_mosOffset = old_mosOffset - offset

where old_mosOffset can be read from the database issuing the command:
getval stap,mosOffset
 
• Load new mosOffset value on MOS with:

 
• setval stap,mosOffset,xxxx (where xxxxx is the calculated offset -> new_mosOffset)
• check that new value was correctly loaded with

getval stap.mosOffset or with getconf stap,true

Check that the offset was correctly calculated repeating the procedure with a slit which is on the  same plate but shifted to one side:

•   punch a new slit on the same plate using the "over-punch" option.
•   take a new exposure with this new slit check the alignment again
•   if OK. Save the configuration with "emiBackup"

A reference value has been measured on July 01st, 1997 (mosOffset = 215374) corresponding to an alignment of the slit of +0.4 pix/1000 pix clearly within specifications.

3.2 Reset of punching counter

It might happen that a punch head breaks during operation. As there is an internal counter that keeps track of the cumulative number of punches performed by each head, this counter should be reset to 0 for the given head (let's name p the id of this head). Here is the command that allows you to read first the value and then reset it:

dbRead "@lemmi:emidevServer:motors:strz.values(p+1,4)"
dbWrite "@lemmi:emidevServer:motors:strz.values(p+1,4)" 0

The counters of all the punching heads available can be also checked using the ccsei message window on wemmi by selecting lemmi environment and emilcuServer process. Then issue the following command to get the data you are interested in:

getconf strz,true

The line count correspond to the number of punches performed by a given punch head. Be careful
as the first number corresponds to punch id 0 which does not exist, and then 1, 2 ,3 .....
 

Send comments to: Gabriel Martin <gmartin@eso.org> Last Update: Feb. 06, 2000 (GM/EC)