Use Case:  Adjust Coude Focus

Description:  The actual focus position is measured on the stellar image using the FAS or the FSS. The M2 position is compared with the reference position stored in the database and any deviation is corrected with the M2 focusing mechanism.

Role(s)/Actor(s):
Primary: User(Operator, Maintenance)
Secondary: M2, FAS, FSS, Nasmyth Wheel

Priority: Critical. (alternate= desirable)

Performance: A few minutes. Time to chop between two Half-mask position and to compute error vector <=2 sec.

Frequency: Whenever necessary.

[Design Description Entry]

Package Assignment:  Optical Adjustments.

Preconditions:

Basic Course: Nasmyth Free Hole Alternate Course: Nasmyth Half-Mask chopping

Exception Course: Command failed

Postconditions:

Issues to be Determined or Resolved:
FAS & FSS Strap setups must be defined
Minimum (optimum) number of data for confident focusing
Will STRAP provide astigmatic terms? (astigmatic optics)

Notes:
As part of the AT Optical Alignment Parameters,
add the focus positions Coude-Focus-FAS, Coude-Focus-FSS and Coude-Focus-VLTI.

How the focus is handled remains to be detailed. The basic approach (priority=Major) is that the above values are stored in the database as such. Since the focus will need to be re-done when altitude is changed (more than 2 deg) or temperature changes (more than 0.2°C), the next approach (priority=Major -Warning: priority was set to Desirable during FDR discussion, but it is indeed very important to avoid loosing observation time-> it is changed here to Major!-) is to have a look-up table which compensate for altitude and temperature changes (using ONECAL corrections). The output of the Focus Adjustment Use Cases shall then be used to define a correction offset to be added to the look-up table (the look-up table could be updated off-line "by hand" but without dedicated AT SW tool). The ultimate approach (priority=Desirable) is to have the same set of tools as used for "pointing Model" but applicable to "Focus Modeling". In that case, the above focus parameters are used, together with altitude and temperature values, to update the "Focus Model" when desired.
 
 

Last modified: Thu Jun 28 13:39:23 UTC 2001