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ALFOSC shutter

Analysis based on CCD8 data, but timings still valid for new CCDs

ALFOSC shutter is "butterfly"-type and is located between the filter and grism wheels.

The minimum time to open and close the shutter is about 0.3-0.4 seconds (Figure 1). So, one ***can _not_ take*** an exposure shorter than ~0.4 sec.

In addition, the actual shutter opening time is about 0.1 seconds longer than the requested time.

Timing accuracy measurements from IAWN campaigns show that the time given in the ALFOSC headers have a delay of 0.4 seconds.

The shutter pattern is virtually non existent. Several 0.2 and 5 seconds overscan subtracted images were coadded and normalised to one second. The ratio of the long and short images indicates that there is a slight gradient from bottom left to top right hand corner. However, the difference between the corners is less than 1 part in 1000 for a 0.2 sec exposure.

Click here to see the image only.
Top panel, integration time versus measured count level (using Beta light) from overscan subtracted image.

Below, linearity residual versus integration time using the requested integration time (blue) and with 86 msec correction (green).
The dotted and dashed lines show 0.5% and 1% lines, respectively.

Back to top Last modified: April 19 2023