Polaroid ST-8E, ST-9E, ST-7E manual Guiding

Section 2 - Introduction to CCD Cameras

(High = 9µ2 pixels, Medium = 18µ2 pixels, Low = 27µ2 pixels). When binning is selected the electronic charge from groups of 2 by 2 or 3x3 pixels is electronically summed in the CCD before readout.. This process adds no noise. Binning should be used if you find that your stellar images have a halfwidth of more than 3 pixels. If you do not bin, you are wasting sensitivity without benefit.

The halfwidth of a stellar image can be determined using the crosshairs mode. Find the peak value of a relatively bright star image and then find the pixels on either side of the peak where the value drops to 50% of the peak value (taking the background into account, if the star is not too bright). The difference between these pixel values gives the stellar halfwidth. Sometimes you need to interpolate if the halfwidth is not a discrete number of pixels.

Another important consideration is the field of view of the camera. For comparison, the diagonal measurement of a frame of 35mm film is approximately 43mm, whereas the diagonal dimension of the ST-7E chip is approximately 8 mm. The relative CCD sizes for all of the SBIG cameras and their corresponding field of view in an 8" f/10 telescope are given below:

Camera	Array Dimensions	Diagonal	Field of View at 8" f/10
Tracking CCD	2.64 x 2.64 mm	3.73 mm	4.5 x 4.5 arcminutes
ST-5C	3.20 x 2.40 mm	4.00 mm	5.6 x 4.2 arcminutes
ST-237	4.74 x 3.55 mm	5.92 mm	8.2 x 6.1 arcminutes
STV	4.74 x 2.96 mm	5.58 mm	8.2 x 5.1 arcminutes
ST-6	8.63 x 6.53 mm	10.8 mm	14.6 x 11 arcminutes
ST-7E	6.89 x 4.59 mm	8.28 mm	11.9 x 7.9 arcminutes
ST-8E	13.8 x 9.18 mm	16.6 mm	23.8 x 15.8 arcminutes
ST-9E	10.2 x 10.2 mm	14.4 mm	17.6 x 17.6 arcminutes
35mm	36 x 24 mm	43 mm	62 x 42 arcminutes
	Table 2.2 - CCD Array Dimensions

2.4.6. Guiding

Any time you are taking exposures longer than several seconds, whether you are using a film camera or a CCD camera, the telescope needs to be guided to prevent streaking. While modern telescope drives are excellent with PEC or PPEC, they will not produce streak-free images without adjustment every 30 to 60 seconds. The ST-7E, ST-8E and ST-9E allow simultaneous guiding and imaging, called self-guiding (US Patent 5,525,793). This is possible because of the unique design employing 2 CCDs. One CCD guides the telescope while the other takes the image. This resolves the conflicting requirements of short exposures for guiding accuracy and long exposures for dim objects to be met, something that is impossible with single CCD cameras. Up to now the user either had to set up a separate guider or use Track and Accumulate to co-add several shorter images. The dual CCD design allows the guiding CCD access to the large aperture of the main telescope without the inconvenience of off-axis radial guiders. Not only are guide stars easily found, but the problems of differential deflection between guide scope and main scope eliminated.

Track and Accumulate is another SBIG patented process (US #5,365,269) whereby short exposures are taken and added together with appropriate image shifts to align the images. It is supported by the ST-7E, ST-8E and ST-9E camera software, but will generally not produce as good as results as self guiding, where the corrections are more frequent and the accumulated

Page 11