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This entry was posted on 31 August 2016 by Filippo Bradaschia.
When we use a camera to take a normal
picture, we can select the magnification which best suits the object of
our image, switching from wide-angle shots (for example 20mm focal
length) to higher magnification shots (for example 200mm focal
length). This freedom of choice is not possible in astrophotography as,
due to telescopes special optical design and the specific
characteristics of the objects they are used for, they need a fixed
focal length that can be varied with the use of additional accessories
but only to some extent.
For example, apart from expensive
solutions mostly for professional use, a short focal length telescope,
which may be used to photograph deep-sky objects, can hardly be used to
capture images of the planets or the moon. Because of this, when we
have to select the best telescope for our needs we have to choose
between a specific-purpose telescope (do we need to photograph the
planets of nebulae?) or do we want to have a more general one.
It's possible to use a standard
DSLR camera (properly coupled to a telescope) to capture the first
images of any celestial object, however to obtain high-quality images
different acquisition techinques are used and, therefore,telescopes
with different characteristics are suggested. Astrophotography is in in
fact divided into 2 main classes:
Let's see now how to capture different objects and which telescopes are the most suitable for each situation.
These objects are very bright and, in
order to obtain excellent images, it is necessary to enlarge the image
to capture all the smallest details (for example Jupiter's clouds, Mars
polar cap or a thin Moon rima). In this instance, the technique will be
that of using the so-called “planetary cameras” with the telescope to
record a video on the computer: the video will be composed by different
frames and processed by softwares specifically designed for
astrophotography (such as Registax) in order to select the best images
(namely the images less affected by atmospheric turbulence). The
pictures will be averaged to reduce the noise, allowing us to apply
special filters to improve the contrast level.
A large-diameter telescope is the most
suitable to capture images of the planets, the Moon and the Sun, because
it allows high-magnification and, therefore, it captures smaller
details. Furthermore, longer-focal length instruments are preferable,
because, by adding a planetary camera, it makes easier to obtain high
magnifications ( a specific Barlow lens is usually applied in the
telescope focuser, in front of the camera to increase the focal length,
above 2000mm). Among the variety of telescope models, the Makutsov-Cassegrain telescopes (such as SkyWatcher MAK150PRO, SkyWatcher MAK180PRO or OrionOpticsUK OMC140 and OrionOpticsUK OMC200)
are the best. They offer not a wide flat field but a high contrast in
its center (which is where we most need it as a planet will always
be small against the extent of the telescope framed field). Medium-long
focal length Newton telescopes, such as OrionOpticsUK VX6L,
OrionOpticsUK VX8L, OrionOpticsUK VX10L and OrionOpticsUK VX12L (focal
ratio higher than f/5), are also suitable for this kind of objects, as
the small size of the secondary mirror limits the obstruction, and,
therefore, improves the image contrast.
This object have a very low brightness
and their apparent dimension in the sky may also be quite big.
Therefore, they usually don't need high magnification (many objects are
framed with telescopes with focal length between 500mm and 1000mm) but
it is important to use a low focal ratio (not higher than f/8). With
a telescope with high focal ratio is, in fact, very difficult to capture
the less bright details of the objects, even after a long exposure.
Moreover, it is important to consider the flat field extension since the
big size of the object captured requires big-sized sensors.
In order to capture great pictures of
these objects, the used technique is to record one or several
long-exposure photographs (generally, a long exposure allows to obtain a
more detailed picture). The photograph should be carefully tracked
during the exposure (that is, the mount must not have tracking errors
since they produce blurred images) and therefore, especially for new
users, it is important to use telescopes with not too high focal length
to possibly reduce tracking errors. For this reason, the apochromatic
refractors (such as PrimaLuceLab AIRY telescopes)
are the most suitable and they are often used, since they offer high
optical quality (because of the apochromatic lenses), a proper focal
ratio (generally between f/6 and f/7) and a wide flattened-field because
of dedicated field flatterers. Short focal length Newton telescopes
(focal length around f/4) are as well suitable solutions, especially
when equipped with a good coma corrector. SkyWatcher Newton Wide Photo
200/800 f4 and SkyWatcher Newton Wide Photo 250/1000 f4 are an excellent
solution because they offer good features at a moderate price;
PrimaLuceLab NEWTON 200CF f4.5, NEWTON 250CF f4.8 or NEWTON 300CF
also be considered for this kind of images: they offer, compared to
SkyWatcher ones, better-corrected optics, carbon fiber tubes and
high-quality mechanics (but with a higher price).
The best long focal length telescopes to
capture deep-sky objects (to capture even the smallest details of small
objects like planetary nebulae and galaxies) are Ritchey-Chretien or
Dall-Kirkham (such as OrionOpticsUK ODK).
They offer at the same time medium-long focal length (to obtain a
high-magnification), a not too high focal ratio (to maintain a high
brightness result in the image) and a wide flat field by using adequate
field-flatterers (to obtain sharp point-like stars, even while using
big-sized sensors). These type of instruments are quite expensive and
require high quality tracking mounts: as will be described in the next
article, a longer focal length generate a higher magnification which
requires a more accurate tracking.
Then do we have to choose a suitable
telescope for only one specific kind of imaging, when we are looking for
an appropriate telescope for astrophotography? No, we don't need to.
First, please note that if you are considering to buy an apochromatic
telescope to record long-exposure pictures, you can use it for planetary
and lunar imaging either, even though it will offer lower performances
compared to a telescope which suits best for that role (such as a
larger diameter Makutsov Cassegrain telescope).
If you are looking for a good-quality telescope for almost all uses, models such as Celestron Schmidt-Cassegrain or Celestron EdgeHD aplanatic Schmidt-Cassegrain might be the most appropriate solutions, since they are used on both visual and photographic applications.
These telescopes though, have a high
focal length and therefore they could not represent the best solution
when used for long-time exposure photographs of deep-sky objects, even
by applying a focal-reducer. In fact, models such as standard
Schmidt-Cassegrain telescopes, don't offer a wide-flattened field when
are used with a reducer (which allows to pass from f/10 to
f/6.3). Models such as aplanatic Schmidt-Cassegrain telescopes offer as
an optional dedicated focal reducer that reduces the focal ratio from
f/10 to f/7, while maintaining a wide flat field. This solution is the
most expensive though and, since the telescope still has high focal
length, it requires to use the telescope (also for 8 inches diameter
models) with high-quality tracking mounts (which are the most expensive
usually). It is important also to consider that Schmidt-Cassegrain or
aplanatic Schmidt-Cassegrain telescopes use an internal mechanism (which
moves back and forth the primary mirror) for focusing. As a result, in
case of a long time exposures, the primary mirror could slightly tilt (mirror flop),
causing a slight shift of the framed field. This is not a big issue
until a guide telescope is installed in parallel to the SC for the
autoguide. In fact, due to the “mirror flop”, the field captured by SC
telescope will slightly move, while the tracking camera will keep
pointing in the direction of the guide star: as a result, the stars will
appear slightly blurred. For this reason, the autoguide system should
be better performed using an off axis guider: this allows to guide using
the main telescope and it also balance the possible small movements of
the primary mirror caused by the mirror flop. In one of the next
articles will be discuss how to properly set this system with Schmidt-
An alternative that you can consider
when you look for an "universal" telescope but also with a low cost is
the one of medium focal ratio Newton telescopes like the SkyWatcher 150/750 f5 Newton, SkyWatcher Newton 200/1000 f5 and SkyWatcher Newton 250/1250 f5.
These telescopes have less precision optics and mechanics than a
Schmidt-Cassegrain or aplanatic Schmidt-Cassegrain telescope, and
they're also larger and less portable. But having a very low price, they
could be the ideal choice to start, even for visual use.
One more parameter to consider when
choosing a telescope is its portability. In fact the best telescope for
you could be the one you can easily carry around and use rather than the
most powerful one. Because of this, it is important to consider what
you are going to use your telescope for and what it will be necessary to
do to use it. For example, if you live in the centre of town and you
have to travel searching for low light polluter areas, the best
telescope could be an apochromatic one to image deep-sky objects
with long exposure time, or a Schmidt-Cassegrain in case of the Moon or
planets imaging with high magnifications.
If portability is not an issue (for example, if you live in a house with
garden, away from too many lights), you might consider larger diameter
telescopes like the Newtons which, if provided with advanced technical
features (like the PrimaLuceLab NEWTON) can be used with maximum
enjoyment in any kind of astrophotography or for observation.