…continuedOf Pixel Size and Focal Reducers
The previous discussion only highlights ways to maximize the field of view for deep-sky imaging with todays popular CCDs. There are many other considerations when it comes to matching telescopes and detectors. First, nowhere is it chiseled into stone that you must have an image scale of 2" per pixel. Anyone doing lunar and planetary imaging will get superior results with scales of 1/2" or less per pixel. Even for deep-sky imaging, any site with good seeing will benefit from scales of less than 2". Some image-processing techniques, especially those involving resolution-enhancing algorithms like maximum-entropy deconvolution, work better with images that have large image scales (so-called oversampled images).
Conversely, excellent deep-sky imaging has also been obtained with pixel scales of 4" or more, especially in the case of large, bright objects. Indeed, many stunning images are produced with conventional camera lenses attached to CCDs. The resulting image scales (tens or even hundreds of arcseconds per pixel) may not yield the best-looking stars, but they can render remarkable views of huge nebulae.
Another consideration is that some desirable features are found only on large-pixel chips. Take, for example, the back-illuminated SITe CCDs that are currently available in cameras manufactured by companies such as Apogee Instruments. Having 24-micron-square pixels in arrays with 512 and 1,024 pixels on a side, these chips have exceptional sensitivity, especially to blue light, compared to their front-illuminated cousins. The blue sensitivity alone makes these detectors very attractive to people who are interested in photometry and tricolor imaging.
The number and size of pixels in a detector are only two considerations when you are planning the purchase of a CCD camera. In the coming months well look at other important issues involved with getting the best performance from todays state-of-the art digital-imaging equipment.
|Specifications for Popular CCDs|
|Kodak||KAF-0400||6.9 x 4.6||768 x 512||9 x 9||390,000|
|Kodak||KAF-1000||24.6 x 24.6||1,024 x 1,024||24 x 24||1,000,000|
|Kodak||KAF-1300||20.5 x 16.4||1,280 x 1,024||16 x 16||1,310,000|
|Kodak||KAF-1600||14.0 x 9.3||1,552 x 1,032||9 x 9||1,600,000|
|Phillips||FT-12||7.7 x 7.7||512 x 512||15 x 15||260,000|
|SITe||SI502A||12.3 x 12.3||512 x 512||24 x 24||260,000|
|SITe||SI003A||24.6 x 24.6||1,024 x 1,024||24 x 24||1,000,000|
|Sony||ICX027BLA*||6.4 x 4.3||500 x 256||12.7 x 16.6||13,000|
|Sony||ICX055AL*||4.9 x 3.6||500 x 256||9.8 x 12.6||145,000|
|Texas Instruments||TC-211||2.5 x 2.5||192 x 165||13.75 x 16||32,000|
|Texas Instruments||TC-215||12.3 x 12.3||1,024 x 1,024||12x12||1,000,000|
|Texas Instruments||TC-241*||8.6 x 6.5||375 x 242||23x27||91,000|
|Texas Instruments||TC-245*||6.4 x 4.8||378 x 242||17x19.75||91,000|
|Texas Instruments||TC-255||3.2 x 2.4||320 x 240||10x10||77,000|