Amateur astronomy is fueled by a constant wonder about the skies above and an expanse of space outside of the Earth. For ages people appreciated celestial phenomena with just unaided eyes and also today a view of a star studded dark sky never fails to amaze even a casual spectator. With the technology affecting and expanding every aspect of human activity, amateur astronomers were also embracing new tools enabling them to pursue their hobby at new levels.
The first major development was the invention of the telescope by Galileo in 1610. However, it took a few centuries for that instrument to spread beyond the sphere of wealth and influence. It was not until after the First World War that amateur telescope making gained momentum and made optical instruments available to almost anybody willing to spent some modest resources plus quite a bit of time and sweat. While typical commercial telescopes were for decades restricted to small aperture refractors, amateur telescope makers built large aperture reflectors and innovative design instruments already in the twenties. Check out "Amateur Telescope Making" books edited by Albert E. Ingalls ...
Large instruments recquired massive mounting, however, and it was not until John Dobson introduced his briliantly simple alt-azimuth design in the eighties, that large Newtonian reflectors gained popularity. The side effect of that development was an "epidemic" of aperture fever. So called Big Dobs (with apertures of 30 and more inches) still draw huge crowds at star parties and rank on the top of desired "toys" of many.
The availability of telescopes tremendously expanded the universe accessible to an amateur astronomer. Of course a naked eye still reveals thousands of stars in our own Milky Way (typically down to 6th magnitude, though 7.5 is not unheard of at prime dark sky spots like Texas Star Party) as well as planets and an occasional bright comet. There are just three outer galaxies visible without optical aid: Magellanic Clouds (two close satellites of Milky Way) and over 2,000,000 light years distant Andromeda Galaxy (known as M31).
Telescope lets one see fainter objects by collecting more light. While the iris of the eye can in the dark open to up to 7 mm aperture, a 70 mm telescope will collect 100x more light. That will let you see (at high power) stars almost as faint as magnitude 13. Typical 8 inch reflector will reveal 15.2 magnitude stars and a 36 inch monster will reach down to magnitude 18.5. In general, 2.5-fold increase in aperture lowers the limiting magnitude by 2. So, for visual use, an instrument with 80-inch aperture would be recquired to see magnitude 20.5 star!
Of course it is not just the nearby stars one could look at with a telescope. In fact, the main drive behind aperture fever is a desire to see fainter and fainter galaxies and other extragalactic objects. Again, a typical 8 inch telescope will rich in such case over 1,000 times farther than M31 revealing, for example, a quasi-stellar object 3C273 in Virgo.
There are also some other practical limitations on amateur telesopes. The majority of galaxies hardly ever present anything beyond a vague hints of structure even in large aperture telescopes . Only a lot closer nebulae (residing inside Milky Way) will display intricate details - however, just a few brightest examples will show any color. The main reasons are in the physiology of human vision: our eyes have significantly lower resolution of faint objects (which we see mainly with a so-called periferal vision) and perception of color does not function at low light levels.
Obviously, replacing an eye with a different detector solves the above problems. Photographic film can be placed at the focal plane of the telescope to record the image. The smallest details visible in faint galaxy will now only depend on the "grain" of the film. Color film can be used to record hues of the deep sky objects or separate black-and-white images (taken through color filters) can be combined into a color photograph in a darkroom. As a bonus, the limiting magnitude in astrophotography (once optimal lenght of exposure is achieved) depends only on focal lenght. For example a photograph taken with 1,000mm focal length telescope will reveal 17th magnitude stars - and the limit will be exactly the same for a 4 inch (f10) or an 8 inch (f5) instrument.
Of course, there are few problems which prevent spread of astrophtography as an alternative to aperture fever. First of all, the telescope needs to be on an equatorial mount. That means building a permament observatory or spending a larger part of observing session just setting you scope up and polar aligning it acurately. Worst than that, the recquired exposure times are in the range of hours - at which point not only the observer's ability to guide but even the film's ability to record light breaks down.
Against all those obstacles, there is a number of accomplished amateur astrophotographers and many of their achievenments are truly amazing. No longer one has to try to describe in words or render in pencil a wispy beauty of a nebula or a glittering splendor of a star cluster.
However, while the use of film got adapted by quite a number of amateurs in the seventies and eighties, professional observatories (which used film to record astronomical objects since the end of the nineteenth century) at the same time started to abandon photographic media in favour of charge-coupled devices (CCD). Those silicon-based detectors have following advantages as a light-recording detector:
|no loss of sensitivity to light during exposure||reciprocity failure beyond a few second exposure|
|no minimal light intensity needed to detect target||minimal light intensity recquired to detect target at all|
|high efficincy of light detection (up to 90%, though device and wavelength-dependent)||low quantum efficincy (max. 4% at optimal wavelengths)|
|signal is proportional to light intensity||response to light is non-linear|
|large dynamic range (typically 16-bit)||low dynamic range (6-bit)|
|picture elements (pixels) are regularly spaced||picture elements (grain) are randomly distributed|
|ready for digital processing||needs to be processed in a chemical darkroom|
What all that means for an amateur astronomer (professional astronomers obviously made their minds already)?
Most important for amateur, by replacing an eyepiece with a CCD camera one can instantly cure aperture fever by virtually increasing that current scope's aperture almost 10-fold. Even with the relatively short exposures, 8 inch telescope can reach 20th magnitude - the visual limit of the telescope with an almost 80-inch mirror! In that respect CCD camera is probably the most economical way of expanding your hobby - after all 80-inchers are not cheap these days, not to mention a mount and an observatory to go with it (or a tractor trailer to get it to the nearest star party).
Calibration and digital processing are sometimes viewed with suspicion even though there is no such thing as a "true image".
So, if after all we usually do believe what we see with our eyes or what is shown on a photograph, then there is no extra reasons to distrust a CCD image!
I have observed skies visually for years but it was adventure with CCD camera that transformed my hobby. While CCD imaging seems highly technical and complicated, it is important to remember that even the longest journey starts with that first step. CCD imaging can be enjoyed at different levels and lead to different experiences. Why I do provide examples of encountered problems and solutions, there are multiple ways of addressing them. After all no two people look at any star in exactly same way! If you venture into CCD imaging do not be afraid to ask questions and question the answers! And most of all, have fun!
This webpage contains (by definition incomplete) guide to deep sky objects available to an observer using a CCD camera. As there is in fact a limitless choice of imaging tagets, I have concentrated my efforts on providing examples of images of the best known objects like those featured in Messier Catalogue. Also objects listed in two additional observing lists (Finest NGC Objects and Deep Sky Challange Objects) compiled by Royal Astronomical Society of Canada are represented, as well as more obscure targets demonstrating that it is worth to look beyond familiar and popular to discover unexpected.
Clear skies to you all,
This work is dedicated to my loving and patient wife, Malgosia.
My thanks go to Dave McCarter, Eric Schandall, Richard Berry and many friends from London, Victoria and Kingston centers of Royal Astronomical Society of Canada and a wider amateur astronomy community. I would have never embarked on this journey without their inspiration ...
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© Jan Wisniewski