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  The detector parameter file detpars allows the astronomer to specify how the chip is to be formatted, how much preflash is to be used, a number specifying the gain, and the chip identification so that the correct microcode is loaded. An example is shown in Figure 2.

Many of the parameters that the user is likely to change are displayed on the ``status" line that appears at the beginning of each integration: the chip name, the format and binning factors, and the gain.

Digression: How To Choose A Gain

  ``How do I choose a gain setting?" is one of the most asked questions. All of our chips (other than TI5) have gains which are adjustable by the software. Why would you want to adjust the gain? The Tektronix chips all have a full-well capacity of >100,000 , and some as high as 250,000 , before there is any detectable deviation from linearity. However, the A/D converters are limited to 16-bits, and cannot output data that is greater than 65,535. Thus to make full use of the dynamic range you would like the gain factor to be about 4. Why not simply set it to 4? The reason is that most of our chips also have very low read-noise, and thus if the gain is greater than the read-noise, you will be undersampling the read-noise---in effect, increasing the read-noise to the level of the gain simply because of digitization noise. (You can't very well recover a read-noise of 3.5 if each data unit is equivalent to 4.) So like most things in life, there are trade-offs. Up-to-date values of the read-noise and linearity limits for all of our chips can be found in the CCD Characteristics Manual in each dome.

This was more of a problem before the new 16-bit A/D converters were implemented; now, most users will be happy by just accepting the default gain settings (see Sec. Q).

If you are attempting to do 1% stellar photometry of stars in a cluster, you are probably interested in covering as large a magnitude range as possible, and furthermore, your noise is going to be primarily photon-noise, not read-noise. Go for the largest value of per ADU as you can without exceeding the linearity of the particular chip. Generally, this will be the default gain, which is also the gain that will give you the least amount of horizontal bleeding from very saturated stars.

If you are doing surface brightness studies of objects through narrow-band filters, and the read-noise is significant but the dynamic range of your objects is limited, you may wish to stay with the largest gain number (smallest number of per ADU). Similarly in some very low-signal spectroscopic applications you are limited by the read-noise.

Note: Some of our CCDs show some very low-level ``streaking" to the right of the most heavily exposed stars. This problem is significant with S2KA and T2KB. The electronics has been adjusted to minimize this problem for the default gain setting. If you are concerned about the effects of very saturated stars on your direct imaging data, you would do well to stay with the default gain settings.

Digression: The Power of ccdinfo

  Once your detpars is setup, you can verify all of these important parameters by simply typing the command


You will get a response that resembles that of Figure 3.

If you ever wish to get information about all the (major) chips available, one can do a dcapinfo show=list. Information about a particular chip can be found by typing dcapinfo t1ka, for example.

next previous contents
Next: instrpars Previous: The Parameters Files
Updated: 26Aug1996