Heart of The Milky-Way

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DIGITAL SENSORS Part 1: Their Spectral Response

Today's digital camera offers astounding value, allowing a photographer to run wild with his or her imagination with a broad range of shooting options. This includes shooting starry vistas and faint Milky-Way panoramas; a testament to the power and versatility of today's digital single-lens-reflex (DSLR) or mirrorless camera.

To understand how light-pollution or LPR filters aid the night-sky shooter, 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 objects 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 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 hydrogen-alpha light of nebulae from being registered. A solution to this is to modify a CMOS body 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.

You can take longer exposures of night-sky objects under various light-polluted skies using a filter, 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 greater than 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.

Under modern 21st century sky-glow conditions, it's essential to obtain an actual transmission graph for the filter, or in the very least, data for the manufactured batch to determine the effectiveness of the filter.

The Spectral Response of Sensors

As the DSLR acronym suggests, today's digital camera borrows a good deal from film attributes of the past. Since digital sensors were made part-&-parcel of the first color one-shot cameras, we have to look as to how films of the past were made.

Film Spectral Sensitivity

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 was kept with color digital sensors, even though all silicon-based sensors CANNOT sense color.

Response of a good color film

Shown above are the spectral sensitivity curves for 3 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 amazing red sensitivity. The most important nebula emissions (visual & photographic) are marked as vertical lines.

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 cameras drove 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.

DSLR & CCD camera sensors

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.

Spectral response of CCD chips

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 large part of the spectrum, with sensitivity primarily in the green through the deep-red. 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.

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 for film, except for the red channel band which, unlike films of the past, 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 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. This is called a "hot-mirror".

Working spectral-response of Canon bodies.jpg

Above is the "working" (hot-mirrored plus Bayer-matrix) spectral range of the color channels for a few different CMOS Canon DSLRs (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.

The "Hot-Mirror"

Because of the extensive red and NIR responsiveness of digital sensors, all CMOS based cameras are equipped with a built-in dichroic filter 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 on the chip. CCD monochrome cameras do not have a hot-mirror, but color cameras do. A hot-mirror filter removed from an entry level DSLR is shown below...

Removed Hot-Mirror

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. 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 nebulae to be recorded.

Removing the hot-mirror in front of a Bayer matrix CMOS (plain facory) sensor, the channels sensitive in the visible range are also responsive in the IR. Therefore, many DSLR bodies can be used for Infra-Red photography provided that no filter or an IR-pass filter (to cut out visible wavelengths) is in front of the sensor.

Typical (naked) CMOS Spectral Response

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 camera 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 red 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 some 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.)

Cygnus-Cepheus nebulosity

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 (sometimes too much), simultaneously improving stellar images (by removing optical aberration bloating).

Baader's UV-IR astronomy filter

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 near 420 nm.

Part 2 of Digital Sensors & LPR filters.

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