This article is about exploring the world of light our eyes cannot see, using a modified digital SLR camera. I have a physics degree but I’m still finding my way with the practical applications of infrared and ultraviolet, so this article is mostly written from a theoretical point of view. Take everything I say with a pinch of salt! 😉
Our eyes can detect only a small portion of the electromagnetic radiation that’s floating around out there. It various from person to person, but the average human can see wavelengths from violet at 400 nanometres (nm) up to red at 700 nm. The human eye is most sensitive to green light, right in the middle of the visible part of the spectrum. However, the silicon sensors used in almost every single (digital) camera are natively sensitive to wavelengths from ultra-violet (UV) at 300 nm up to infra-red (IR) at 1200 nm. If these wavelengths were included in everyday photographs, they’d look extremely odd because that’s not what our eyes expect to see. To prevent this, camera manufacturers install a filter over the camera sensor that prevents UV and IR light from reaching the sensor. Now, the photos you take will look the same as you can see with your eyes. The red line on this graph shows what the built-in UV and IR reduction filter will allow to pass. It’s not very much, bearing in mind the camera is sensitive to almost the whole width of the graph. From an astronomy point of view, a lot of distant nebulae emit or reflect light, including UV and IR. These objects are quite faint so it is useful to collect as much light as possible when photographing them. You’re cheating the nebula if you allow the UV and IR light to travel across the universe for thousands of years, simply to discard it less than one millimetre before you could have captured it. A lot of astronomers modify their camera to remove the filter to allow all this extra UV and IR light to reach their sensor. They benefit by collecting more light, and by recording more information about a world the eye cannot see.
It’s usually not ideal to capture the whole range of wavelengths at the same time, otherwise soft images are likely to occur as different wavelengths are brought into sharp focus in different places. Most photographers overcome this by using additional filters over the lens of their camera which limit the wavelengths of light entering to only UV, only visible, only IR or possibly combinations of these, depending on the application.
For instance, the blue line on the graph above shows the transmission of a Baader ACF filter. You might wonder why someone would bother with a filter that doesn’t permit many more wavelengths to reach the camera, but the reason is because many nebulae emit strongly on the H-alpha emission line, which is at 656 nm and appears deep red to our eyes. As you can see, the built-in camera filter would block at least 75% of this valuable light. Removing the standard filter and replacing with the ACF filter suddenly means you can collect 4 or 5 times more of the H-alpha radiation. This is the real benefit and there are other filters available that permit only H-alpha and not the rest of the visible spectrum.
I decided to have my camera converted to “full spectrum” sensitivity, primarily for astrophotography but also for occasional use with infrared landscape photography. I normally use black & white infrared film but it is quite expensive and difficult to handle reliably. Using an IR-converted DSLR allows false colour infrared as well as black & white infrared, which is a bit different. I nosed around online for a while before deciding to employ the services of DSLR AstroMod and I am extremely pleased with the results. The chap behind DSLR AstroMod is an experienced astronomer and photographer himself, and was keen to share his first-hand experience, help me choose the right conversion package and give advice on how to get the best out of the camera – all for a reasonable price. If anyone else is considering a DSLR conversion to IR or full spectrum, I heartily recommend DSLR AstroMod.
At the moment it’s cloudy in this part of the UK so I haven’t actually been able to use the newly-converted camera for any astronomy – only a few test photos in the garden. I’ve got a lot of experimenting to do before I figure out what works best for astronomy, but this is exactly where I wanted to be. Before, my astronomy equipment was the limiting factor and a source of frustration. Now, with a good telescope, a good mount and a good camera, my equipment no longer limits me. I am the limiting factor, and this is my incentive to learn 🙂
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Silicon detectivity is from about 200 to 1100nm actually; the “usual” 300nm cutoff is due to glass absorption, while the long wavelength limit is set by the bandgap of Si, 1.1 eV, i.e. it’s pretty much transparent beyond 1100nm. Peak sensitivity for a Si photodiode is normally between about 800 and 900nm.