The ultimate astronomical photography has
to be through-scope imaging, but this is undoubtedly the hardest to get right and indeed
some amateur astronomers have spent thousands of pounds trying to get good quality images.
Once you can image through the scope, it is possible to take impressive images of deep sky
objects as faint as those visible by eye through the 200" Palomar telescope and
probably as good as the pictures taken with film through that ‘scope in the 1960s !!
Faint objects, invisible by eye through the ‘scope, jump to life with impressive
detail in spiral arms, dark rifts etc. Higher magnification can also permit the imaging of
the moon and planets showing surface detail only grasped in those very clear moments by
eye.
Indeed the moon and the planets are easier to image that the deep sky
objects, at least as far as the equipment requirement is concerned. The reasonably
accurate RA drive described above will do the job as exposures will be short meaning that
tracking inaccuracies will not be noticeable. Using film based cameras, the moon can be
imaged in a second or so and the major planets in a few seconds. Results can be
disappointing, as turbulence in the air during exposures of that length will blur detail.
CCD cameras, however, require much shorter exposures; I find that the moon can be imaged
in less than 0.1 second and Jupiter and Saturn around half a second or less. In these
shorter times, fairly sharp images can be obtained, but it is trial and error. I take lots
of images, viewing each after downloading and make a judgement on whether it looks good
and is a keeper or should be erased. This way it is easy to accumulate dozens of images in
one evening for subsequent processing, which will probably whittle down the perhaps two or
three really good ones.
The CCD camera is usually used in place of the eyepiece, although it is
possible to image through the eyepiece to achieve high magnifications. Adaptors to hold
the camera will then be required. Prime focus imaging without eyepieces is more normal,
but to control magnification requires the use of focal reducers (to decrease magnification
and increase field of view) and Barlow lenses (to increase magnification and reduce field
of view). For the planets I use a Barlow lens that gives me the equivalent of f24 which I
find provides an acceptably sized image. For imaging deep sky objects I use a focal
reducer to give me f6.3 and a greater field of view than the standard f10.
Imaging deep sky objects through the telescope requires longer
exposures, for CCD cameras typically a few minutes and ten times this for film based
cameras. This necessitates higher quality equipment, otherwise tracking inaccuracies
quickly become apparent as star trails on the images. However, when using CCD cameras,
exposures of a minute or so can yield reasonable images. There are essentially two
approaches to longer exposure imaging through the telescope. The first involves the use of
a secondary telescope or guidescope to guide the main scope and keep it fixed on the filed
of view and to correct for any tracking inaccuracies in the RA drive system. Alternatively
an off-axis guider can be used. This is just a beam splitter that lets some of the light
through to the camera and some to the guider. Guiding can be done manually where the
observer adjusts the position of the scope to keep a star in the middle of the field of
view (using cross-hairs) with the drive correction system of the scope. An alternative is
to use a CCD camera to do this, when it is usually referred to as autoguiding. As it
sounds, this is all becoming complicated and EXPENSIVE !!
My solution to imaging through the ‘scope is firstly to use a
telescope that has an accurate drive system and to take short exposures, before star
trailing and tracking inaccuracies become noticeable. But how short do the exposures need
to be ?? This will depend on the telescope, but do bear in mind that none are perfect.
Even the Meade LX200 with its periodic error correction cannot track perfectly. I find
that my LX200 will permit exposures of 3 minutes without star trailing. My previous
‘scope, an Orion Optics GX250, allowed about one minute, which is still very good. If
you can get an image, without trails, with an exposure of a minute or more then you are in
business !! Using the summation method described previously, it is easy to take several
short exposures then sum them electronically to get the equivalent of 10 or 20 minute
exposures. This is sufficient to image almost any deep sky object, certainly all the
Messier objects and most of the NGC objects, a hobby for a lifetime !!
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