By Al Fressola
Our guest speaker at the September general meeting was Shannon Calvert, a professional photographer and graphic designer, having a business known as Hire Imagination, a full service marketing and design agency.
Shannon gave the audience a fascinating look into the world of astrophotography. His talk was directed to techniques for photographing astronomical objects, in particular, deep sky objects, such as nebulae, galaxies, and star clusters, as well as planetary and lunar photography.
Shannon gave a recounting of his own introduction to astrophotography, including his earliest days when he tried to photograph astronomical subjects with limited success. Shannon discussed how he gradually became more proficient, learning the techniques needed to generate astonishingly beautiful images of astronomical objects. He showed the audience an amazing assortment of subjects he has imaged, including the Ring Nebula, Trifid Nebula, the Triangulum Galaxy, the Dumbbell Nebula, the Owl Nebula and many other deep sky objects in the Messier catalog of objects (originally discovered by French astronomer Charles Messier).
In his talk, Shannon went over in detail the techniques for taking multiple exposures of an object of interest, such as between 30 and more than 200 exposures, each typically with a relatively short time exposure, such as 20-30 seconds. The images can be taken through a telescope such as an apochromatic refractor, on a tracking equatorial mount, (including the one he uses, which he brought to the talk) or through a telephoto lens. Shannon described how he uses a digital camera, such as the Canon 7D to record the images. He stated that to ease the taking of such images, use is made of what is known as an intervalometer, some of which are built into modern digital cameras. An intervalometer allows the user to, in essence, program the camera to take multiple images each of a certain time duration. Shannon indicated that this greatly relieves the need for being at the telescope while these multiple images are made, other than to insure that the telescope is accurately tracking the deep sky object being photographed.
Similarly, with images taken directly through a telephoto lens, the intervalometer is used where the camera itself is attached to a tracking device, such as the Star Adventurer tracking device, which can be mounted to a tripod. The device allows the camera to accurately track an astronomical object over a period of time, thus compensating for the rotation of the earth while the multiple images are being taken.
Shannon mentioned that such images are normally recorded in the raw data format so as to obtain the maximum amount of information for each image recorded.
He also discussed the techniques for what is known as “stacking” the multiple images, so as to form a single image of the deep sky object being photographed. He demonstrated how a single image which may barely show the object of interest in the photograph is transformed into a stunning final image after the multiple images are stacked into one. Shannon explained that stacking is more than aligning (via software) the multiple images together to form a composite image of the desired object, but also the various software techniques used to reduce the various types of noise associated with each image, including dark noise associated with the image sensor (sometimes known as dark field subtraction), as well as light pollution. He discussed various techniques for color balance, reduction of luminance noise, chrominance noise, stretching of the histogram and other processing steps which are used to enhance the resulting stacked image.
Shannon discussed the use of a Bahtinov mask, which is a device used to help focus a telescope or telephoto lens accurately. He also talked about modifying the image sensor on the camera to remove a built-in filter used on digital cameras, which blocks some of the light spectrum particularly associated with some deep sky objects and, in particular, light of wavelengths associated with hydrogen-alpha light.
Shannon indicated that although he has done some of his photography in areas with very little light pollution, many of his images have been taken either at the Westport Astronomical Society Observatory in Westport, Connecticut or in Easton, Connecticut. Thus, the image processing can clearly reduce a lot of the light pollution to acceptable levels resulting in astrophotographs of stunning detail.
Shannon stated further information about the Westport Astronomy Society can be found at https://www.was-ct.org and that they have public viewing on Wednesday nights, weather permitting.
In short, Shannon’s talk was fascinating and informative and clearly showed his outstanding ability in creating beautiful images of the celestial objects.