Polaroid ST-8E, ST-9E, ST-7E Advanced Capabilities, Crosshairs Mode Photometry and Astrometry

Section 3 - At the Telescope with a CCD Camera

using the Dark Subtract command. By subtracting the dark frame, pixels which have higher dark current than the average, i.e., "hot" pixels, are greatly suppressed and the displayed image appears much smoother. Visibility of faint detail is greatly improved.

The CCDOPS program also supports the use of flat field frames to correct for vignetting and pixel to pixel variations, as well as a host of other image processing commands in the Utility menu. You can smooth or sharpen the image, flip it to match the orientation of published images for comparison, or remove hot or cold pixels.

3.9.Advanced Capabilities

The following sections describe some of the advanced features of SBIG cameras. While you may not use these features the first night, they are available and a brief description of them is in order for your future reference.

3.9.1. Crosshairs Mode (Photometry and Astrometry)

Using the crosshair mode2 enables examination of images on a pixel by pixel basis for such measurements as Stellar and Diffuse Magnitude, and measurement of stellar positions. The 16 bit accuracy of SBIG systems produces beautiful low-noise images and allows very accurate brightness measurements to be made. With appropriate filters stellar temperature can be measured.

In the crosshair mode, you move a small cross shaped crosshair around in the image using the keyboard or the mouse. As you position the crosshair, the software displays the pixel value beneath the crosshair and the X and Y coordinates of the crosshair. Also shown is the average pixel value for a box of pixels centered on the crosshair. You can change the size of the averaging box from 3x3 to 31x31 pixels to collect all the energy from a star.

3.9.2. Sub-Frame Readout in Focus

The Focus command offers several frame modes for flexibility and increased frame throughput. As previously discussed, the Full frame mode shows the entire field of view of the CCD with the highest resolution, digitizing and displaying all pixels.

The "Dim" mode offers the same field of view but offers higher frame rates by reducing the image's resolution prior to downloading. The resolution is reduced by combining a neighboring block of pixels into a "super pixel". This reduces the download and display times proportionately, as well as improving sensitivity. It is great for finding and centering objects.

The Planet mode is suggested if high spatial resolution is desired for small objects like planets. The Planet mode allows you to select a small sub-area of the entire CCD for image acquisition. The highest resolution is maintained but you don't have to waste time digitizing and processing pixels that you don't need. Again, the image throughput increase is proportional to the reduction in frame size. It can be entered from Auto mode.

2Under DOS the Crosshairs mode is accessed through the Display Image command in the Analysis mode.

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