How to focus

The focus should be checked at the start of the night and whenever there has been a significant change in temperature (> 0.5 °C).  A 7 - 9th magnitude star should be used for night observations and 4 - 6th for daytime observations.  Starcat can be used to find an appropriate star.  The T.O. usually sets the focus to an initial value of ~1.35.  A decrease in temperature will reduce the focus value.  The focused image is typically less than 1" (5 pixels) FWHM.  To focus:

1. Set devices to needed values.

A.  Lamps OFF and mirror OUT
B.  Filter NOT blanked
C.  Slit wheel OPEN
D.  Direct Imaging Mirror IN
E.  Shutter OPEN

2.  Take exposure

A.  Set Itime such that counts are in linear region of the chip
B.  Set Coadd to >1 to average out seeing
C.  Set Cycles to 1
D.  Turn Autosave OFF
E.  Set Obs Mode to Stare(A)
F.  Click GO

3. Determine FWHM

A.  Set a desired display to active (often 3)
B.  Draw line (shift middle-mouse button) across star image
C.  Click on Display Options tab
D.  Set Display Type to XLineCut
E.  Click Set Endpoint from Line
F.  Click Gaussian Fit

4.  Set Cycles to many (~100)

5.  Click GO

6.  Adjust focus until optimum FWHM is reached

A.  Click on Focus tab of T3Remote GUI
B.  Set focus increment value (usually 0.02-0.10) OR actual desired value
C.  Change focus with + or - button OR Set Focus button
D.  Redraw line across star as needed and redo step 3

t3 focus image

7. Click STOP in Cshell XUI when optimum FWHM reached

LAST UPDATE: February 1, 2010