Taking Bias (Zero) Frames

1) be sure all light sources are OFF in the dome, including the eyepiece reticle lights. The dome slit should be closed, and the dome as dark as possible. The covers should be on the telescope.

2) on the CCD PC, take an exosure (flash camera icon). In the dialog box,

• set exposure time to 0.01 sec,
• select Exposure Mode = Grab,
• select Frame Mode = Dark Frame Only,
• select Resolution = 1024x1024, and
• hit OK

3) once the image is read out and has appeared on the screen, save it to disk (floppy disk icon). For the first image of the night, you will have to change directories to the one you cleared/created earlier. Call the image "biasNN.fit", where NN is the running number 01, 02, 03, ... . If you take another set of biases later in the night, be sure not to repeat numbers -- you may want to call them 11, 12, 13 , ... .

4) repeat Step2 seven times, giving 8 bias frames in the sequence.

NOTE: you should check the number of counts in your first bias (slide the pointer around the image and note the (X,Y,Counts) readout in the lower left, or do a histogram on a bias frame), you should see a peak around 974 counts. This is the usual bias level for our chip. If it is different by more than 10 or 20 counts, please report it to Dr. Layden or Dr. Laird.

DEFINITION: A "bias" or "zero" frame measures the amount of signal, or charge, on the frame due simply to the read-out. It has zero exposure time, and recieved no illumination. In addition to determining and correcting for the mean level of this signal, the bias can also be used to correct the object images for any pattern in this signal which is constant in time (often referred to as "fixed-pattern noise").

WHY A BIAS? The bias raises the background level of counts on the chip. Imagine if there was no bias applied. If you took a frame with a very short exposure time or in very dark conditions, the counts due to the sky background would be about 0. However, there is noise associated with reading out the chip, so some pixels would want to have negative counts. Since the chip only records integer counts between 0 and 16384 (2^14 -- we have a 14-bit chip), these pixels would get the value 0, and the statistics of the sky noise would be corrupted. Putting a bias on the chip ensures that all pixels will have a positive value, and the statistics will be computed correctly.

Updated 1999 June 03