Specifications

Section 3 - At the Telescope with a CCD Camera
Page 38
Because the Research Series cameras have regulated temperature control, you may
prefer to take and save separate dark images, building up a library at different temperatures
and exposure times, and reusing them on successive nights. At the start it's probably easiest to
just take the dark frames when you are taking the image. Later, as you get a feel for the types
of exposures and setpoint temperatures you use, you may wish to build this library of dark
frames.
3.7. Displaying the Image
The image can be displayed on the computer screen using the graphics capability of your PC.
Auto contrast can be selected and the software will pick background and range values which
are usually good for a broad range of images or the background and range values can be
optimized manually to bring out the features of interest.
The image can also be displayed as a negative image, or can be displayed with
smoothing to reduce the graininess. Once displayed, the image can be analyzed using
crosshairs, or can be cropped or zoomed to suit your tastes.
3.8. Processing the Image
If not done already, images can be improved by subtracting off a dark frame of equal exposure.
You will typically do this as part of the Grab command although it can also be done manually
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 mode 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.