Observation Astrophotography

Astrophography is the technique of photographying celestial objects. It is with no doubt a fine and rewarding amateur astronomer activity. It combines astronomy and photography. Since the beginning of that new century, astrophotography have seen the irruption of new, digital technologies. Astrophotography until then was using camera and roll films as it has almost completely passed to digital cameras and processing techniques. The question has to be asked whether, generally, the digital era features the same rigorous simplicity than the classical astrophotography techniques. Digital imagery had been born in the 1960's based upon electronics and space exploration. Digital techniques generally are now allowing to capture pictures which needed until then 1 to 5-meter professional telescopes. In terms of astrophotography, the F/D number is what the diaphram of a objective lens is to photography. The less the number, the more luminous the picture. Thence a low F/D number is better in terms of stellar astrophotography

Digital Imaging Instruments

Digital era astrophotography is needed digital-type cameras and a PC, or a printer. Digital photography came together with the personal computers era. In a digital camera, film is replaced by electronic devices which convert light photons into electric signals. A chip of silicon captures photons and translates them into electric signals. These signals are stored into a memory chip as that stored content is then transmitted to a computer where pictures may be edited and computer-processed. They are viewed like such or printed (or a digital camera may be directly plugged into a printer to get a printed picture). 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. 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 capture boxes based upon a sensor or 'chip,' the size of it tiny compared to a 35 mm film. CCDs need to be cooled
• dedicated astrophotography cameras are expensive however. That's why the amateur community first 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 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
• a simplest tool still is a electronic ocular which provides on a laptop a simple image similar to the one seen in a ocular with the eye. The image quality however may be cheaper than with the other systems

What is Possible With Digital Imaging?

As far as what is possible with digital imagery is concerned, it depends upon the device used

• 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
• tweaked webcams or manufacturers' astrowebcams are widely used for planetary, lunar, and solar 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 allows to select the best images, hence balancing the poor native quality of each image or poor seing conditions, for exemple, and providing strikingly fine pictures. Webcams however do not provide for long exposure times and mostly cannot be used about deep sky objects
• 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. As far as star-trails imagery is concerned, DSLRs are needing multi-exposures and editing software processes
• 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

More general statements may be stated, like: albeit often scorned in tutorial simple digital cameras are useful enough however like basic tools and allow to planetary closenesses, general views of the Moon and the Sun as handheld behind a ocular (wich is useful for eclipses). Digital DSLRs are mostly tools for tripod or parallel -the DSLR is piggy-backed to the telescope- astrophotography. They give access to constellations and to deep sky too. Astro videocams (they are needing a computer and a dedicated software) are providing for remarkable results in planetary astrophotography. CCD cameras, at last (idem), are tool dedicated to deep sky astrophotography. All digital imagery techniques are necessitating specific processes in terms of image capture or image processing. Most advanced digital tools like the CCDs and the DSLRs -like the simple or advanced easy-to-use CCD imagers at Celestron or Meade- 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). A 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 those techniques, generally, are needing that images obtained be processed by dedicated softwares like 'stacking' several images together or further improving the rendition of the picture, etc.

Traditional Methods in Non-Digital Astrophotography

Methods in non-digital astrophotography traditionally range from tripod to eyepiece projection. The practice of it now tends to decrease because imaging devices and films are less and less available. Most used pre-digital era cameras in astrophotography were cameras (most often of the 35-mm format, and SLR ones) and lenses. The SLR system allowed a view of the object to image through the camera's visor

• 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. The '500 rule' allows how to avoid apparent star movement for pictures taken of the sky without a telescope. Take 500 and divide it by the focal length in millimeters of your lens. The resulting number is the length of time in seconds that you can keep your shutter open before seeing star trails. For example, if you’re using a 20 mm lens, 25 seconds (500 divided by 20) is the longest you can set your exposure time
• 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 used the ancient, 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. Slides films often are favoured by amateur astronomers because they yield a fine rendering of colors