How to compute dark master with ISIS

 

 

The dark (thermal signal) is an unwanted signal induced by the production of thermal charges in the detector. We observed this signal in an image taken in a long exposure time in the dark. The dark signal is present in all acquired images. It must be subtracted in the process of extract the signal characteristic of the observed star.

For a pure image signal of darkness, remove the offset of the image obtained long exposures with the camera shutter closed (or block to ligth the telescope aperture). This is the reason for which you must first calculate the offset image,

Right, the acquisition of a sequence 11 dark images with the software Audela. The exposure time is of 300 seconds. These data will be appropriate to process spectra acquired with a 300 seconds exposure. We will see later how fit the dark map has different exposure times.

Of course, the detector temperature is the same as in the acquisition of the spectra of objects.



 
From the "Master images" tab, the calculation of a high quality dark image from individual images n300-1, n300-2, ..., n300-11. The
dark300.fit result writed in in the working directory.


Tip: make a habit of adopting the same name for your calibration images. A generic name such as "No-xxx"is easy to type
during acquisition. Do not complicate your life. A name like
"dark300.fit" is so explicit. It means that the dark master
corresponds to an exposure time of 300 seconds.



 
How to adapt the master dark map to a specific exposure time?


You have the masters dark300.fit. You must process spectral sequence of images taken with an integration time of  180 seconds. I do not have a map of the dark current corresponding to the same exposure time.


The ratio of exposure time is 180/300 = 0.6. To obtain the marter dark image associated with an exposure time 180 seconds, just multiply the dark for 300 seconds by 0.6.

Whether to produce a dark deal to put pictures only 1.5 seconds, the coefficient here is 1.5 / 300 = 0.005. In this case you call the image produced dark1_5.dark.

This can be done from the tab "Tools #1". Note we produced the image file name "dark180.fit", which is logical. Always remember to name your images consistently, and stick to the protocol.

An alternative is to calculate the dark180  images directly from the "Image masters" tab, from individual 300 seconds exposure. It is shown on the right how to proceed.



 

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