Roper Photometric manual Intermittent Exposure, Good Exposure Time

Intermittent Exposure

This script takes a stellar image of a faint galaxy. The observatory is located under the flight path of a local airport and incoming planes occasionally pass through the field of view. The observatory has at least 30 seconds warning before a plane passes over, so the camera needs a signal to close the shutter, stop exposing (temporarily), then begin exposing again. The camera supports MPP mode, so the dark current can be drastically reduced during the several-hour exposure.

A switch, connected to the camera's trigger input pin, controls when the shutter opens and closes. (The switch is simply an open/close switch inserted between the ground and the trigger pin on the camera's trigger port.) When the switch is turned on, the camera opens the shutter and exposes. When the switch is turned off, the shutter closes and the exposure waits. (The camera is still exposing, but it has a closed shutter and minimal dark current.)

script_begin();
shutter_close();	/* good precaution—who knows what state it's in	*/
clear_parallel(5);	/* clear out any residual charge	*/
clear_serial(2);	/* might as well clear this out too	*/
shift_mode_ism();	/* force the CCD to shift and EXPOSE in MPP mode	*/
clear_until_trig();	/* wait for the trigger to be turned on...	*/
loop_begin(100);	/* this allows exactly 99 interruptions	*/
expose_until_trig();	/* shutter is closed, waiting for a trigger	*/
shutter_open();	/* open the shutter and begin exposing	*/
expose_while_trig(0);	/* GOOD EXPOSURE TIME	*/
shutter_close();	/* interruption...	*/
loop_end();
clear_serial(100);	/* an evening’s worth of charge has accumulated	*/
pixel_readout(0,1024,1,1024,1);	/* readout the entire CCD (1024x1024)	*/
pixel_display(1024,1024);	/* display the entire CCD	*/
script_end(0);	/* DO NOT CLEAR the CCD	*/

There is one problem. If there have only been a half-dozen interruptions, how do you finish the readout? The best way is to turn the switch off (which closes the shutter), put on a lens cap, and toggle the switch about a hundred more times. A somewhat riskier method would be to:

•Turn the switch off, thus closing the shutter

•Use the application to ABORT this script (which shouldn't erase the image)

•Run the next script...

script_begin(); shutter_close(); clear_serial(100); shift_mode_ism(); pixel_readout(0,1024,1,1024,1); pixel_display(1024,1024); script_end(0);

/* FINISH READOUT OF A 1024x1024 CCD	*/
/* just to make sure	*/
/* DO NOT clear the parallel register	*/
/* do all shifting in MPP mode	*/
/* readout the entire CCD (1024x1024)	*/
/* display the entire CCD	*/

This last script needs to be typed into its own file, since everything after the script_end instruction (including that second script) is treated as a comment.

18Advanced Camera Operation Manual