Polaroid ST-7E, ST-8E, ST-9E manual Flat Field Images, Pixels vs. Film Grains

Section 2 - Introduction to CCD Cameras

minute, eliminating the many "hot" pixels one often sees across the image, which are simply pixels with higher dark current than average.

2.4.4. Flat Field Images

Another way to compensate for certain unwanted optical effects is to take a "flat field image" and use it to correct for variations in pixel response uniformity across the area of your dark- subtracted image. You take a flat field image of a spatially uniform source and use the measured variations in the flat field image to correct for the same unwanted variations in your images. The Flat Field command allows you to correct for the effects of vignetting and nonuniform pixel responsivity across the CCD array.

The Flat Field command is very useful for removing the effects of vignetting that may occur when using a field compression lens and the fixed pattern responsivity variations present in all CCDs. It is often difficult to visually tell the difference between a corrected and uncorrected image if there is little vignetting, so you must decide whether to take the time to correct any or all of your dark-subtracted images. It is always recommended for images that are intended for accurate photometric measurements.

Appendix D describes how to take a good flat field. It's not that easy, but we have found a technique that works well for us.

2.4.5. Pixels vs. Film Grains

Resolution of detail is determined, to a certain degree, by the size of the pixel in the detector used to gather the image, much like the grain size in film. The pixel size of the detector in the ST-7E and ST-8E is 9 x 9 microns (1 micron = 0.001mm, 0.04 thousandths of an inch) and in the ST-9E it's 20 x 20 microns. However, the effects of seeing are usually the limiting factor in any good photograph or electronic image. On a perfect night with excellent optics an observer might hope to achieve sub-arcsecond seeing in short exposures, where wind vibration and tracking error are minimal. With the average night sky and good optics, you will be doing well to achieve stellar images in a long exposure of 3 to 6 arcseconds halfwidth. This will still result in an attractive image, though.

Using an ST-7E or ST-8E camera with their 9 micron pixels, an 8" f/10 telescope will produce a single pixel angular subtense of 0.9 arcsecond. A 8" f/4 telescope will produce images of 2.5 arcseconds per pixel. If seeing affects the image by limiting resolution to 6 arcseconds, you would be hard pressed to see any resolution difference between the two focal lengths as you are mostly limited by the sky conditions. However, the f/4 image would have a larger field of view and more faint detail due to the faster optic. The ST-9, with its 20 micron pixels would have the same relationship at roughly twice the focal length or a 16 inch f/10 telescope. See table 4.4 for further information.

A related effect is that, at the same focal length, larger pixels collect more light from nebular regions than small ones, reducing the noise at the expense of resolution. While many people think that smaller pixels are a plus, you pay the price in sensitivity due to the fact that smaller pixels capture less light. The ST-9E with its large 20 x 20 micron pixels captures five times as much light as the ST-7E and ST-8E's 9 micron square pixels. For this reason we provide binning on the ST-7E and ST-8E that configure the camera for 18 or 27-micron square pixels. Binning is selected using the Camera Setup Command. It is referred to as resolution

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