DIGITAL SENSORS: THEIR SPECTRAL RESPONSE & LIGHT-POLLUTION (LPR) FILTERS
Today's digital camera offers astounding value with a broad range of shooting options, allowing a photographer to run wild with his or her imagination. This includes shooting of starry vistas and faint Milky-Way panoramas; a testament to the power and versatility of today's digital single-lens-reflex (DSLR) and mirrorless camera.
To understand how light-pollution (LPR) filters aid the night-sky photographer, we have to look at numerous aspects: How a digital sensor inside a camera senses light, how the sensor is configured to record colors, what colors celestial bodies in the night sky radiate their light in, what LPR filters transmit and reject, and finally, we need to be aware of the wavelengths of light where sky-glow occurs.
The Summary First:
Many emission nebulae have their light dominated by the deep red hydrogen-alpha wavelength. Examples are the Orion Nebula (M42), the Lagoon Nebula (M8), the Rosette Nebula in the constellation Monoceros, and the Heart-And-Soul nebula in the constellation Perseus.
Image sensors in digital cameras are overly sensitive to a large part of the spectrum which is why they have filters in front of them adjusting and curtailing their spectral response. A stock digital body will block most of the red light of the nebulae from being registered. A solution to this is to modify a CMOS camera by removing or replacing the stock filter, thus allowing the camera to register more red light.
LPR filters work well by blocking some sky-glow, however, they do NOT remove all the light-pollution from the image and they cannot make objects brighter.
Using a filter, you can take longer exposures of night-sky objects under various light-polluted skies improving the signal-to-noise ratio. When using an LPR filter with a stock camera, the color balance may be shifted to the blue-green. With an "astro-modified" camera this color shifting can be towards the red.
Broadband and CLS type filters are good overall light-pollution filters that block mostly the light from older light sources and natural airglow. At the same time, these filters allow the emissions of astronomical objects to pass through with a fairly high efficiency; typically with a 90% transmission. Currently, broadband and CLS type filters work better in the countryside, far from major urban areas. Filter makers have yet to produce good filters to adequately block LED lighting. Many filter types also come as filter-clips that insert into a camera body (behind the lens).
In order to determine the effectiveness of any filter under modern 21st century sky-glow conditions, it's essential to obtain an actual transmission curve for the filter, or in the least, a curve (or data) for its batch.
PART 1: Spectral Response of Sensors
Film Spectral Sensitivity
As the 'DSLR' acronym suggests, today's digital camera borrows a good deal from film attributes of the past. Color films used three distinct sensing layers to record blue, green and red light. A combination of the three colored layers yielded all other hues. The colored layers overlapped in their spectral sensitivity giving an overall uniform response with minor reductions in the blue-green and yellow-orange parts of the spectrum. The same scheme is maintained with color digital sensors, even though all silicon-based sensors cannot sense color.
Shown above are the spectral sensitivity curves for the layers in a good astro-friendly color film from the late 20th century. This particular film, Kodak Portra PRO 1000 (PMZ), was famed for its excellent red sensitivity. The most important nebula emissions are marked as vertical lines (visual & photographic).
CMOS or CCD Sensors
The two most common types of digital sensors are the Charged-Coupled-Device (CCD) and the Complementary-Metal–Oxide–Semiconductor (CMOS) sensor. The CCD is one of the oldest digital imaging technologies still offering advanced image quality with better dynamic range and excellent control of noise. The limited assembly and greater power consumption of CCD sensors have driven camera makers to replace them with much more versatile CMOS alternatives. With added built-in functionality, CMOS sensors work more efficiently than CCDs, requiring much less power with faster performance.
Modern "stock" DSLRs (on the left, with lens removed) contain a CMOS chip which is filtered TWICE! The blue appearance is that of the "hot-mirror" situated in front of the CMOS chip. CCD astro-cameras (on the right) may not have a filter at all, requiring the user to choose the filter to image in a desired spectral band.
Real Spectral Response of Digital Sensors
All silicon sensors are monochromatic detectors of light, meaning, no color information is registered. Each pixel on the sensor simply registers how many photons are striking it. Their somewhat bell-shaped spectral response results in sensing violet and deep blue photons near 430 nanometers (nm), with a considerably lower rate than in sensing deep red photons at 660 nm.
Shown above are the raw spectral responses for three different monochrome CCD sensors (blue, black or gray curves). The pixels respond to light from a broad segment of the spectrum with sensitivity primarily in the green through the deep-red parts. There's also significant sensitivity in the near infra-red (NIR). The most important nebula emissions are marked as colored vertical lines.
For digital sensors to detect and display discrete color, a few clever approaches are used. One method is to obtain three separate images for red, green and blue light using the same sensor. Placing a blue filter over the sensor, each pixel will obtain information as to how much blue light there is. Doing this again with a green and a red filter, each pixel knows how much green and red light there is. Combining and balancing the three data sets results in a color image. This process of creating pictures in color is called RGB imaging and is one of the best methods by which dedicated amateur astronomers create great color images of celestial objects.
CCD sensors can also be found in high-end digital video camcorders and some professional cameras. Incoming light is split into three beams via a high-quality & complex prism, while three different sensors, each one covered by a separate color filter, register the three different colors. CCD astro-cameras, however, are rarely filtered; the choice of filter is up to the user.
"One-Shot" Color Cameras
A much more common method for obtaining color images is to employ a Bayer-matrix mask directly on a CMOS sensor. This 1976 invention is named after Bryce E. Bayer of Eastman Kodak Company, where a single exposure generates a color image. The method involves a solitary pass of light onto one monochrome digital sensor coated with a Bayer-matrix mask (or a variant of this mask). Most color images created today are obtained using this simpler "one-shot" method using a CMOS chip.
Shown above is the difference between the configuration for the Bayer-matrix and for a Foveon X3 sensor used in some Sigma cameras. For Sigma cameras, the wafer consists of three vertically stacked photodiodes employing a vertical filtering pass for color sensing, similar to how films were exposed in the past. Every pixel site, therfore, senses all 3 colors. For cameras using a Bayer-matrix, every square of four pixels on a sensor collects information of only one color, yet each pixel is assigned red, green and blue values via an interpolation. This is called "debayering" or "demosaicing".
In modern digital cameras, which includes most smart-phones, I-phones, mirrorless and any single-shot cameras, the treatment for the CMOS sensor's spectral bands has been made to mimic the spectral response in films of the past. Again, three distinct sensing color channels (as opposed to layers in film) are employed to record blue, green and red light. The spectral range and response has been matched with that of film, more or less, except for the red channel band which is typically curtailed (reduced) in CMOS cameras. This is mainly due to the heightened sensitivity of any silicon-based sensor in the red part of the spectrum.
One problem with a CMOS sensor is the leaky nature of the Bayer-matrix which is factory layered directly on the sensor; the 3 color filters that make up the mask leak light in the near-InfraRed (NIR) part of the spectrum. Therefore, an additional filter is used which is called a "hot-mirror".
Above is the working (hot-mirrored plus Bayer-matrix) spectral range of the color channels for a few different CMOS Canon bodies (marked in the illustration). Virtually all of the spectral responses for these cameras have been constricted within the visible part of the spectrum. Notice the reduced red spectral channels which compensate for the heightened red sensitivity of CMOS sensors. The most important nebula emissions are marked in their colors as vertical lines.
Because of the extensive red and NIR responsiveness of digital sensors, all CMOS cameras are equipped with a built-in multi-layered dichroic filter just in front of the sensor. This is designed to block ultraviolet, some violet, NIR and some red light. This added hot-mirror is in addition to the Bayer mask. CCD monochrome cameras do not have a hot-mirror, but color cameras do. A removed hot-mirror filter from an entry level DSLR is shown below.
There are a number of reasons why camera makers place a hot-mirror in front of the sensor. For ordinary white-light photography, sensitivity in the any part of the UV or IR spectrum is undesired, otherwise funky colors and ghost images would result. Therefore, a UV-IR blocking filter must be used with any CMOS sensor.
Additionally, in order to register colors as humans perceive them, a bluish filter helps to balance out the high red sensitivity of CMOS chips. Consequently, CMOS sensitivity is kept fairly constant from the blue into the yellow, (from 430 to 570 nm), with a declining sensitivity into the red. Unfortunately for astro-imagers, since human vision is insensitive to Hydrogen-alpha (Hα) light at 656.3 nm, most of this wavelength is also filtered out before reaching the sensor. Nearly all modern cameras allow just about a quarter of the Hα wavelength of the nebulae to be recorded.
DSLR and mirrorless cameras rely on just three sensing channel bands for imaging in the visible part of the spectrum giving good color representation. The result is that the entire luminance information in a scene is collected in a wide spectral band comprised by the color over each pixel. For Bayer matrix CMOS sensors, the channels that are sensitive in the visible range are also responsive in the IR. Therefore, many DSLR bodies can easily be used for infra-red photography provided that no filter or an IR-pass filter is in front of the sensor.
Shown here is the spectral response for a naked CMOS DSLR, where the "hot-mirror" is removed. This particular low-cost and dated DSLR is the Canon Rebel XT body, but most other CMOS sensors have nearly a similar native response. Note the NIR response which is due to the leaky nature of the micro filters making up the Bayer-matrix. Once again, the most important nebula emissions are marked as vertical lines in their own color.
"Modding" a DSLR
Astro-imaging for red Hα light is still possible with any over-the-counter DSLR body containing stock filters, while other nebula wavelengths in the visible part of the spectrum are unaffected. Nonetheless, as soon as many sky-shooters discover the true red sensitivity of all CMOS sensors, they proceed to "mod" a DSLR body.
When economy minded intermediate and advanced astro-imagers want to obtain the best possible images for the red light of emission nebulae, they modify an inexpensive DSLR camera model or purchase a third-party vendor modified body. Astro-modifying a camera is by no means a simple task. It involves opening up a digital body full of minute components and circuit boards connected by delicate wires and removing the hot-mirror reducing Hα wavelengths.
When replacing the stock "hot-mirror" filter with an astronomical friendly filter, the sensitivity to red Hα light dramatically increases. A Canon 40D body modded with a Baader UV-IR cut filter is used here to capture a number of nebulas in the Milky-Way. (50mm lens, one frame, 300 sec, @ f2.8, 1600 ISO, on-camera noise reduction enabled, driven with an Ioptron tracker, tweaked with Photoshop.)
For a reasonable fee, several vendors can modify your shipped DSLR body. They can also supply a new or used one: www.spencerscamera.com/astro-conversions.cfm; dslrmodifications.com; imaginginfinity.com; and lifepixel.com. They can replace the hot-mirror with clear optical-grade glass, an astronomically friendly filter specifically manufactured to transmit the entire visible spectrum (about 420 nm to 690 nm), or no filter at all (a naked sensor). When applicable, they will also adjust for the focus by shimming the correct amount for any difference in a filter’s thickness and refractive index. Thereafter, the modified DSLR camera becomes faster overall and much better at recording the red light of emission nebulae, typically passing more than 90% of the Hα line. The shortest exposure times will be achieved with a filter-less (naked) sensor.
With a reputable astronomical filter replacing any stock filter, an "astro-modified" camera has the advantage of higher sensitivity for the red Hα wavelength, as well as for sulfur-II, at 672.4 nm. Additionally, the same filter is designed to cut out the UV and a significant amount of violet light, simultaneously improving stellar images (by removing optical aberration bloating).
Shown above are transmission curves for the stock filter of Nikon's D810 DSLR body (blue curve) and for Baader’s UV-IR Astronomy filter (red curve) used in modifying digital bodies. Various sizes are cut to accommodate the CMOS filter window of many different bodies. The cut-off in the violet is at about 420 nm.