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D. Guide Bias Properties


If one needs to use the guide CCD images, one should correct them for bias just as for the main CCD. The guide CCD will have the same sort of bias-temperature relation as the main CCD, so one must obtain guider bias frames at the same detector temperature as the guider images. The guide CCD is small, so it takes very little time to obtain a set of 5 to 9 guide bias frames. One then median-combines these images just as discussed for the main CCD in Section 2.3. Figure 6 gives an example of a single guide bias frame and the median of 16 guide biases. The two images are shown at the same contrast.

  • Figure 6: Left: A single guide bias frame. Right: A median combination of 16 guide bias frames. Both images are shown at the same contrast. They were obtained on 20 July 2007 as part of the spatial scale tests discussed in Section 6.

    Inspection of Fig. 6 shows that median-combining the bias frames produces a less noisy master bias. There are many hot pixels and warm columns on the guide CCD, as well as the hot upper-right corner. Figure 7 shows a pair of guide images before and after correction by the median-combined bias shown on the right of Fig. 6. The result of bias-correction is to remove some of the warm columns and hot pixels from the image. The hot corner persists after bias-correction. Also, note the large number of ghost images on the frame. There are only two stars in the field. The rest of the ``sources'' are image artifacts.

  • Figure 7: Left: A raw guide CCD image of a double star. Right: A bias-corrected version of the same image. Both images are shown at the same contrast. They were obtained on 20 July 2007 as part of the spatial scale tests discussed in Section 6.


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    Updated: 2009 August 20 [pbe]