Note -- you have reached the original astrophotographs.com website. Thanks very much for visiting. Logo contact information has been updated (i.e. we moved to Taos, NM) as below:
12 Rabbit Valley Road / P.O. Box 1515
El Prado, NM 87529
303-903-8996 or 575-758-3670
You may also want to visit Willis' new astronomy site at: taosastronomer.com/
to CCD Imaging"
photographic film is placed at the focal plane ("focus") of the
telescope's optical system. With CCD imaging, an electronic CCD
(charge coupled device) chip consisting of many pixels (or photosites)
is placed at the focal plane. Parameters with film include, most
importantly, speed (sensitivity to light) and attributes of color
and grain. Parameters of CCD chips include the abnormalities of
the individual photosites (similar to film grain) and the inherent
existence of electronic noise. The differences noted above are due
to a few facts:
most astro-imagers enjoy CCD imaging because they can record
dim galaxies and nebulae in just a few minutes (as opposed to
hour-long exposures with traditional film) and by using various
image-calibration routines can produce fabulous astro-images
of objects once thought to be the domain of only large observatory
1 -- Film is
essentially "slow;" that is, it takes a relatively long time for
deep-sky images to be recorded. This phenomena is further complicated
by what is called "reciprocity failure," or the film's non-ability
to record the dim image in a linear way over time. In other words,
the film records less information in the second minute of an exposure
than it does in the first minute. Because of this, astrophotographers
have been forced to limit their time exposures to compensate. Further,
the "sky background" (light pollution and just the general brightness
of the sky) increases over the time of the exposure, so exposures
must be kept relatively short so as to not have excessively bright
2 -- Film's
grain still increases with speed, although the technologies of film
grain have become very advanced. Nevertheless, to use a fast film
is to use a grainy film.
objects (like nebulae and galaxies) with film, then, introduces
some compromise. Enter the field of electronic CCD imaging. Actually,
this sort of imaging has been around a long time, as we have been
receiving and processing electronic images from spacecraft for decades.
Also, common home video cameras work on somewhat the same principle.
The limiting parameters for CCD astronomy applications include:
1 -- Although
the CCD chip is very much more sensitive to light, and it records
the light in a linear fashion (no reciprocity failure here) other
compromises exist. First of all the chip needs to be super-cooled
to possess such sensitivity. Further the sensitivity of individual
photo sites (think "grain") is variant.
2 -- The photosites,
even under the super-cooled conditions described above, aren't perfect.
They vary in attributes from site to site. Because of this, electronic
calibration must be performed to overcome these flaws.
3 -- The thermal
signal (or "dark current") of the CCD chip increases over time.
If the chip is cooled, this will compensate to some degree; however,
final compensation can only be accomplished by additional electronic
4 -- Because
your image is previewed, focused and saved on a "local" laptop computer,
CCD imaging requires a lot more expensive equipment. Also, even
after the above-mentioned electronic calibrations (accomplished
by "shooting" additional frames as described below in the typical
CCD observing session checklist) are done, what you are left with
is a digital file. Are these "real" photographs? Probably yes, but
when you additionally "modify" them by using ultra-complex image-processing
routines, are you then "creating" additional data, or just "adjusting"
A typical CCD
Imaging checklist follows:
Set up telescope
Create new image directory for observing session
Let camera cool and initialize
Check and align all finder scopes
Locate object in main scope
Focus using one of a variety of methods
Shoot image set for each object
Shoot additional calibration frames to correct for electronic noise,
all raw data on hard drive and back up data
Shut down camera
Shut down computer
Combine calibration frames with image set data
Manipulate photographs with digital image-processing tools if desired
(checklist courtesy Richard Berry)
Probably most astro-imagers enjoy CCD imaging because they can record
dim galaxies and nebulae in just a few minutes (as opposed to hour-long
exposures with traditional film) and by using various image-calibration
routines can produce fabulous astro-images of objects once thought
to be the domain of only large observatory instruments. In planetary
imaging, CCD cameras really excel because they record the moments
of perfect seeing that are generally wasted with even a modest real-film
exposure of just a few seconds. Also, with the advent of vast affordable
computing power in the hands of the ordinary amateur astronomer,
the complex image-processing algorithms required to post-process
the raw CCD data are now available to all.
If you're still
interested, explore the links included within this web site, or
feel free to e-mail me with questions.
I'm only a novice on this subject, but perhaps I could point you
in the right direction.
doesn't really scratch the surface of this very complex subject.
As noted, please refer to the links provided here and in the "Cool
Links" section for many more details. Also consider trying CCD
imaging for yourself.
Contact us at 303.903.9886
All images and
narratives copyright Willis Greiner, all rights reserved.
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