Telescope for astrophotography, what to choose.
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 or deep-sky objects?) or 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.
It’s possible to use a standard DSLR or mirrorless camera (properly connected to a telescope) to capture the first images of any celestial object. However, in order to obtain high-quality images, different acquisition techniques are used and telescopes with different characteristics are suggested. Astrophotography is in in fact divided into 2 main classes:
- Planetary astrophotography (planets, Moon, Sun): objects such as planets or a detail of the Moon surface have a small apparent dimension and, therefore, images have to be recorded with high magnification;
- Deep-sky astrophotography (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 exposure times (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 increase magnification to capture all the smallest details (for example Jupiter’s clouds, Mars polar cap or a thin Moon rima). In this instance, the technique requires to use 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 or AutoStakkert!) in order to select the best images (namely the images less affected by atmospheric turbulence). The pictures will be averaged to reduce noise, allowing us to apply special filters and 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 Maksutov-Cassegrain telescopes are great. 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). Schmidt-Cassegrain telescopes are also great for this kind of objects since they provide a very good optical quality, easy to use (since they’re easy to collimate) at still an affordable price.
Telescopes for deep-sky objects (galaxies, nebulae or star clusters)
These objects have 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 fast focal ratio (not higher than f/8). By using a telescope with long focal ratio, it is in fact very difficult to capture the weakest details of the objects, even with a long exposure. Moreover, it is important to consider the flat field extension since the large size of the captured object requires large sensors.
In order to capture great pictures of these objects, the used technique is to record one or several long-exposure pictures (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 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/5 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 254/1000 f3.9 are an excellent solution because they offer good features at a moderate 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. They offer at the same time medium-long focal length (to obtain a high-magnification), a not too long 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 with large sensors). These instruments are quite expensive and require high quality tracking mounts: a longer focal length generate a higher magnification that requires a more accurate tracking.
Telescopes for (almost) all uses
Finally, do we have to choose a suitable telescope for only one specific type of astrophotography, when we are looking for an appropriate telescope? No, we don’t need to. First, please note that if you are considering to buy an apochromatic refractor telescope to record long-exposure pictures, you can use it even for planetary and lunar imaging, even if it will have lower performances compared to a telescope which suits best for that application (such as a larger diameter Maksutov-Cassegrain telescope). If you are looking for a good-quality telescope for almost all uses, Celestron EdgeHD aplanatic Schmidt-Cassegrain might be the most appropriate solutions, since they can be used for both visual and photographic applications.
These telescopes have a long focal length and therefore you have to add a specially designed focal reducer to reduce focal ratio from f/10 to f/7, while maintaining a wide flat field. It is important also to consider that Schmidt-Cassegrain or aplanatic Schmidt-Cassegrain telescopes use an internal focusing mechanism which moves back and forth the primary mirror. As a result, in case of long exposition times, the primary mirror could slightly tilt (mirror flop), causing a slight shift of the framed field. This is a big issue when a guide telescope is installed in parallel to the SC for 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 made by using an off axis guider: this allows to guide using the main telescope and it also balances the possible small movements of the primary mirror caused by the mirror flop.
An alternative and lower cost solution that you can consider searching for an “universal” telescope 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 lower quality optics and mechanics and they’re also larger and less portable. But, by having a very affordable price, they could be the ideal choice to start, even for visual use.
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 downtown and you have to move searching for low light polluted areas, the best telescope could be an apochromatic one to take pictures of deep-sky objects with long expositions, or a Schmidt-Cassegrain in case of planetary imaging at 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 Newtonians which can be used also for visual use.