Specifications

Section 2 - Introduction to CCD Cameras
Page 31
Using an STL-11000M or STL-6303E camera with their 9 micron pixels, a telescope of
~75 inches focal length will produce a single pixel angular subtense of 1 arcsecond. A 0.5X
focal reducer would shorten the effective focal length to 36 inches and produce images of 2
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 system with 36 inches focal length
would have a larger field of view and more faint detail due to the faster optic. The STL-1001E,
with its 24 micron pixels would have the same relationship at roughly 195 inches focal length.
See table 2.2 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. For example, the STL-1001E with its large 24 x 24 micron
pixels captures seven times as much light as the STL-6303E and STL-11000M's 9 micron square
pixels. For this reason we provide 2x2 or 3x3 binning of pixels on most SBIG cameras. With the
STL-11000M and STL-6303E, for instance, the cameras may be configured for 18 or 27-micron
square pixels. Binning is selected using the Camera Setup Command. It is referred to as
resolution (High = 9µ
2
pixels, Medium = 18µ
2
pixels, Low = 27µ
2
pixels). When binning is
selected the electronic charge from groups of 2x2 or 3x3 pixels is electronically summed in the
CCD before readout. This process adds no noise and may be particularly useful on the STL-
4020M with its very small 7.4 micron pixels. Binning should be used if you find that your
stellar images have a halfwidth of more than several (3 – 4) pixels. If you do not bin, you are
wasting sensitivity without benefit. Binning also shortens the download time.
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. The relative CCD
sizes for all of the Research Series cameras and their corresponding fields of view through a
telescope with a focal length of 100 inches are given below. The field of view is inversely
proportional to focal length. So, for example, cutting the focal length to 50 inches will result in
a field of view that is twice the value shown below:
Camera Array Dimensions Diagonal Field of View at 100” FL
TC-237 Tracking CCD 4.93 x 3.71 mm 6.17 mm 8.2 x 6.2 arcminutes
STL-4020M 15.2 x 15.2 mm 21.4 mm 29 arcminutes
STL-1302E 20.5 x 16.4 mm 26.2 mm 35.5 arcminutes
STL-1001E 24.6 x 24.6 mm 34.8 mm 47.1 arcminutes
STL-6303E 27.6 x 18.4 mm 33.2 mm 44.9 arcminutes
STL-11000M 36.1 x 24.7mm 43.7 mm 59.1 arcminutes
35mm Film 36 x 24 mm 43 mm 59 arcminutes
Table 2.2 - CCD Array Dimensions