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.

The best telescope may not be the most powerful, but one you will use the most: find out what is the telescope for astrophotography suitable for you.

Telescope for astrophotography: left, a planetary image (Jupiter planet), right a deep-sky photography (M33 galaxy). Both recorded by the author.

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:

1. Planetary photography (planets, Moon, Sun): objects such as planet disc or a detail of the Moon surface have a small apparent dimension and, therefore, images have to be recorded with an high magnification;
2. Deep-sky objects photography (galaxies, nebulae, star clusters): since these objects have low brightness it's important to capture the higher amount possible of light and therefore images are captured with long time exposure (several minutes or hours).

Let's see now how to capture different objects and which telescopes are the most suitable for each situation.

Telescopes for planets, Moon and Sun at high magnifications

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.

Telescope for lunar and planetary astrophotography: left SkyWatcher MAK150PRO telescope, right OrionOpticsUK VX10L telescope.

Telescopes for deep-sky objects (galaxies, nebulae or star clusters)

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 f4 should 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).

Telescope for deep-sky astrophotography: left PrimaLuceLab AIRY APO80 telescope, right SkyWatcher Newton Wide Photo 200/800 f4 telescope.

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.

Telescope for deep-sky astrophotography: OrionOpticsUK ODK10 telescope.

Telescopes for (almost) all uses

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- Cassegrain telescopes.

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.

Telescope for (almost) all applications: left Celestron EdgeHD 800 telescope, right SkyWatcher Newton 250/1250 f5 telescope.

Dimension is not everything: portability

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.

Telescope for astrophotography: dimensions comparison of a PrimaLuceLab AIRY ED90 refractor (left), Celestron EdgeHD 800 (center) and SkyWatcher Newton 200/1000 f5 (right).