November 12, 2004
The accompanying calculator gives an analytical approach to determining the minimum exposure time for image acquisition. This calculator is conservative in that it uses the Kodak-specified values for mean dark signal. That dark signal is skewed a bit due to the small population of hot pixels.
In order to effectively use the calculator, you will need some data.
I. Gain (g): For SBIG cameras, this is measured at the time of camera manufacture and entered in the cameraís firmware. It is reported in the FITS header as the value of the keyword EGAIN. Consult your camera control programís documentation for instructions on how to access the FITS header information. For other cameras or to verify the value, this value can be calculated by four frames. You will need to perform measurements in your camera control program to make this measurement. Here are the steps to measure the gain for your camera:
Note: Many programs do not handle the negative numbers that result from subtracting two similar images. In order to get the standard deviation of the difference accurately, proceed as follows:
a. Add 2000 counts to the first image
b. Add 1000 counts to the second image
c. Subtract the first image from the second image. The standard deviation ignores the added values and gives an accurate value for the standard deviation.
II. Readout Noise (Ron): You can either use the default value of the calculator or measure your own camera using the following steps:
III. Sky Flux (Esky): Take a 3-minute test exposure of a star poor area of the sky. Donít worry about guiding, star trailing, etc. It is best to take this image near the zenith to minimize the impact of any light pollution gradients. Of course, the moon should not be in the sky. Once the image is obtained, perform the following steps:
IV. Analysis: With the above data, proceed with the following steps:
Try different values of Allowable noise contribution from readout noise and dark current noise. Note how temperature tends to reach diminishing returns as your camera gets down to -20įC to -25įC. Note how the method of combining impacts the number of darks required. Min Max Clip is a combining method used in Mira. Similar results can be achieved by using Ray Gralakís Sigma program (www.gralak.com/Sigma)
You should repeat the calculation for each filter you use and each binning you use.
Note that narrow band imaging hardly ever gets to a sky noise limited condition without very long exposures. Here, 10-20 minute exposures at 1x1 binning are generally the norm. However, if you are interested in broad area narrow band information such as clouds of nebulosity, then 2x2 binning can help significantly in getting to a good SNR.
Lastly, dithering is an excellent tool to significantly improve your overall SNR.