Scope DOCTOR Steve Richards
Our resident equipment specialist cures your optical ailments and technical maladies CARL CLARKE
Are there any sodium light pollution filters I can use with my Canon EOS 700D ‘virtual telescope’, made up of camera lenses?
RICHARD A MAJOR
Sodium streetlights are the plague of many an astronomer, whatever apparatus they use. Luckily, they only emit light across a very narrow wavelength band, and so eliminating the glow can be easily done with a filter without sacrificing your light gathering power. As you have discovered, conventional screw-in 2- and 1.25-inch light pollution filters can’t be fitted to your camera but there are some solutions.
Assuming that you use an unmodified DSLR camera, the choices are a little limited. However, Hutech produces an excellent light pollution filter called an IDAS LPS-D1 light pollution clip filter, which is compatible with your Canon DSLR camera. This filter controls sodium light pollution very convincingly without contributing much in the way of a colour cast to your images. The filter is housed in a special holder that installs inside the camera between the camera’s lens bayonet and the flip mirror. This filter is also very suitable for a fullspectrum modified DSLR.
Some people modify their cameras to remove the standard infrared filter as it cuts out a lot of red light, including the all important hydrogenalpha wavelength. If you have modified your camera, then consider the Astronomik CLS clip filter as it gives a very good boost to contrast to alleviate light pollution. As an alternative, you could use Astronomik hydrogen-alpha and oxygen III narrowband clip filters to capture mono or false colour images. Observing through a telescope can certainly make you place your body in a whole range of contorted positions, guaranteed to give you back and neck pains! Observing from a seated position will help enormously but the type of telescope can dictate the seating arrangements.
Short refractors, SchmidtCassegrains and MaksutovCassegrains are easier to accommodate than Newtonian reflectors as the range of movement of the eyepiece with regard to the elevation of the object is less. A popular solution is to use a drummer’s stool, as this is comfortably padded but has a reasonable range of height adjustment. You should position and adjust the stool so that your back is straight and you are not leaning forward to look into the eyepiece.
For a Newtonian reflector with its wide range of eyepiece positioning, you should adjust the focuser so that you look down into it at an angle and use an astronomer’s observing chair like the Berlebach Charon, TS Astro Chair or Sky-Watcher Anti-Tip Observing Chair.