azimuth enc1 failure during lightning


The encoder failure
Pointing errors while using azimuth2 encoder
Pointing check after everything restored

az1 encoder failed and the pointing errors from using az2 encoder (.ps) (.pdf)
pointing errors before/after the encoder board failure(.ps) (.pdf)

The Failure.  (top)

    On 14oct06 at 15:56:03 (ast) a lightning strike caused azimuth encoder 1 to fail. This is the azimuth encoder that is used to point the telescope. When the failure occurred, the software automatically switched to the az2 encoder (on the carriage house side). This caused a degradation in pointing (see point errors). We ran pointing with az2 encoder until 18oct06 at 16:00 AST.

    Debugging the azimuth 1 encoder failure:

    Since port one looks like it is getting zapped by lightning we should:

Pointing errors caused by running on az2 encoder:  (top)

    At 15:56:03 the az1 encoder failed and the  plc software switched to use the az2 encoder for pointing the azimuth. This encoder is on the carriage house side of the azimuth arm. This switch caused a pointing error: The plot shows  when the az1 encoder failed and the pointing errors from using az2 encoder (.ps) (.pdf):     From 14oct06 16:00(AST) thru 18oct06 16:00 AST we ran with the azimuth pointing error shown at the bottom of page two.
processing: x101/061018/

Pointing check after reinstalling encoder board.  (top)

    The encoder board failed on 14oct06. We ran using az2 encoder until 16:00 Ast 18oct06 when we plugged az1 encoder into encoder port 2 (that was being used for pointing). We reinstalled the repaired encoder board on 26oct06. The az1 encoder was replaced a few times during this period. When we replaced the az1 encoder,  we had to reset the az encoder pointing using the mechanical index. This has a pointer (mounted next to the encoder) that slides out and aligns with a mark at azimuth 270.
    The encoder is about 60 feet from the center bearing.  A 1 arcsecond error at 60 feet is about .003 inches along the arc. Including the sin(za) term, a 1 arcsecond error on the sky is about .01 inches at za=20 degrees and .02 inches at za=10deg. So the alignment of the pointer with the mark is non-trivial.

    To verify the alignment, data was taken on 27oct06. These were the same sources taken on 14oct06 prior to the failure. The plots show the pointing errors before/after the encoder board failure(.ps) (.pdf):

    The azimuth error before and after is about the same. The za error varies by about 10 arcseconds for the source B1040+123. This source transited around 9:40 AM AST. There may have been problems with the tiedowns going loose.  It looks like the azimuth position if fine.
processing: x101/061027/

<- page up