Astrophotography

Astrophography, which is photographying celestial objects, is certainly a fine and rewarding amateur astronomer activity. It combines astronomy. And photography ;-) Debate today is about new digital imaging techniques (CCD, webcams, usual digital cameras) as opposed to film photography. This tutorial provide a first overview about today astrophotography

Traditional Methods in Astrophotography

Methods in astrophotography traditionally range from tripod to eyepiece projection

• tripod photography is just using a film camera with a lens and to shoot at sky. Such a method is mostly used to get what is called "stars trails", that is picture where the added light of stars during a long time exposure is forming curved light trails as the camera remains still and night sky revolves about Polaris. The longer the exposure, the longer the trails. Such a method is a good initiation to night sky. Using telephoto lenses is just theoreticallly possible as the size of planetary objects obtained this way e.g. is very small. Even with a 300 mm lens, Moon in 2.7 mm wide only on a 35 mm film
• piggyback astrophotography is the next step. The camera is attached to a telescope through an appropriate device. Various lenses (from wide-angle to telephoto) allowed by your camera brand may be used as features of the telescope and of the mount are available. Mostly, is an equatorial mount is available, this will allow long exposure photographs, removing the night sky motion effect, as the telescope may be used as a guider on another hand. Piggyback mounting allows very fine work about constellations, nebulae and Milky Way. Due to the small size of planetary object in 35 mm SLR such objects remain out of reach except in special circumstances like eclipses or transits e.g.
• prime focus photography is another major step. The telescope becomes the lens of the camera. Focal length of this lens is the telescope focal length! This is performed through a tube connecting the telescope and the camera. Such a tube is called a T-adapter. On one side the T-adapter attaches to the telescope -or fits into the eyepiece holder as on the other side is provides a photography industry standard 42 mm thread. Various T-rings allow the transition between the thread and the proprietary camera mounts. Prime focus astrophotography is mostly used for wide deep sky objects like nebulae as the image produced at the focus of any telescope is small. Jupiter e.g. will yield a 0.17 mm wide image only in a 4" f/10 telescope as the Moon will appear just 8.7 mm wide!
• eyepiece projection is optically linking the telescope and the camera too. But it is using an eyepiece. It is just linking the telescope bearing an eyepiece with the lens-devoided camera. Technically the eyepiece is said projecting the object image into the camera, unto the film plane. This is performed through a special T-adapter which is called a tele-extender. Like the previous adapter the tele-extender fist on one side to the telescope (attachment, eyepiece holder), on the other to the camera (42 mm thread, T-ring). The only differences are that an eyepiece may be inserted into the tube and that the tube's length may be made variable allowing various magnifications with a same eyepiece. The farthest the camera, the largest the magnification. This is the best method for planetary objects, for Moon, sunspots, and small deep-sky objects. Stars and deep sky object will require long exposure times hence the eyepiece projection method will require guiding. Most of the time a guiding will in turn require an equatorial mount. As seen above an equatorial mount is a mount where the rotational axis of the mount is pointing at the Polaris. The mount moving along that axis cancels the apparent sky motion. Planetary eyepiece projection astrophotography mostly does not require such long exposure times nor guiding

As far as films are concerned, traditional astrophotography use the ancient, usually used by anyone in the common life, chemicals-coated films. These films are reacting to light photons reaching film plane through the camera lens. A chemical process (called "development") reveals this reaction afterwards. Three categories of films may be used: black-and-white negative films, color negative films, or color slide films. First two requires a two-step process to get an image: the film is developped; a negative image appears on the original film support; this negative image is projected unto an appropriate photographic paper which is further chemically processed; a definitive readable image is thus obtained. Slide films are often privileged by amateur astronomers as they offer a finer rendering of colors. Slide films are processed once only: a positive color image appears directly on the processed original film support. Slides need an appropriate viewer (a dias projector e.g.). It's their primary use. Slides may be translated into a paper color image too

Digital Imaging Instruments

Digital instruments come mainly into three flavours: CCD, webcams, and usual and advanced digital cameras

• as far as astrophotography CCD cameras are concerned, they are electronic devices which convert light photons into electric signals. A chip of silicon (its size is tiny compared to a 35 mm film) captures photons and translates them into electric signals. These signals are stored into a memory chip. This stored content is then transmitted to a computer where pictures may be edited and computer-processed. A CCD sensitivity is measured in "pixels". Pixels are chips' light sensitive points. They are the equivalent of the "grains" of a chemical film. The more grains (or the finer the "grain"), the more pixels, the more accurate the picture
• dedicated astrophotography cameras are expensive however. That's why the amateur community quickly turned to do-it-yourself solutions. Through some modifications, webcams, as they feature a CCD chip too, were found useful for astrophotography imaging. Industry, now, is providing fine astrophotography dedicated webcams equipped with a 1.25" adapters, as the latest trend is the apparition of easy-to-use CCD imagers ranging from simple to advanced, with an affordable price, like the Celestron NexImage Solar System Imager or the Meade Deep Sky Imager PRO
• the industry is providing too digital cameras, ranging from mostly automatized small cameras to SLR-equivalent ones. Such cameras, even the low-end ones, may be used for astrophotography

What is Possible With Digital Imaging?

Whether or not the traditional astrophotography techniques, as described above, are useable with digital imagery depends on these new devices features

• usual digital cameras have mostly limited exposure time range and fixed objective lens. This forbids them long tripod exposures, piggyback, and prime focus and eyepiece projection methods. They may usefully used for telescope imaging of Moon and solar activity, or for "large scale" events like eclipses and transit (might they be seen through a telescope or directly at sky). These cameras provide fine general views too like auroras
• webcams are widely used for planetary imaging as they are imaging movie-camera fashion. They do not allow long-time exposures however. Webcams are working movie-camera fashion, that is they are taking a certain number of images per second. This feature, which is found too at manufacturers astrophotography dedicated webcams, allows to select the best images, hence providing strikingly fine planetary pictures
• SLR-equivalent digital cameras, that is digital SLR cameras (DSLRs) are allowing most of traditional astrophotography. The question resides mainly in their pricing. Another question is whether their sensitive chip match or not a surface equivalent to the one of a traditional 35-mm film
• dedicated astrophotography CCD cameras work attached to a telescope only and may work about any celestial object. They are expensive too. The new, easy-to-use, simple or advanced CCD imagers like the Celestron NexImage Solar System Imager or the Meade Deep Sky Imager PRO work attached to the instrument as, for some, dedicated softwares work with them

What is important is that the amateur astronomers community is making its choice. Recent trends are revealing that amateur astronomers mostly use webcams for planetary imaging, usual digital cameras for Sun, Moon, eclipses, transits, auroras, as dedicated astrophotography CCD cameras are used for deep-sky imaging. Digital SLRs are progressively emerging only, like the simple or advanced easy-to-use CCD imagers. An important by-effect of digital imagery is that it is still in the making about star-trails imagery. The latter need a multi-exposures and an editing software process. All these digital imagery techniques need additional digital image processes, like stacking several pictures and further improving the rendition of the picture. This is the job of dedicated softwares. On the other hand the most advanced digital tools like the CCDs and the DSLRs -like the simple or advanced easy-to-use CCD imagers- always need some tweaking like a dark frame (a picture is taken with the telescope covered) or a field flattening technique (a picture is taken of an uniformly illuminated scene). Such techniques are necessitated to eliminate the electronic noise (inherent unwanted illuminate pixels) or some vignetting (pixels at the edge of the picture are darker). CCDs need to be cooled