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Do You Need a Modified Camera For Astrophotography?

by Roger N. Clark

Some astrophotographers say a stock digital camera is a poor choice for astrophotography because of reduced sensitivity to red hydrogen-alpha emission. They say you need a special modification or you can't detect the many emission nebulae in the night sky. Is this true? NO!


The Night Photography Series:


Contents

Introduction
Emission Nebulae
Filter Transmission and Hydrogen-Alpha Response Conclusions
References and Further Reading


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Introduction

Most unmodified (stock) digital cameras have plenty of H-alpha response. The problem is that the amateur astronomical community seems to consistently use post processing methods that suppress H-alpha. Specifically, people seem to be doing a histogram equalization (auto white balance). Try the same processing work flow on a red sunset image and it will come out awful.

The stars in the Milky way galaxy are predominantly yellow and red. You wouldn't know that from all the amateur astro photos out there, as the histogram equalization turns yellow stars blue. It is because the average astro scene is predominantly yellow/orange/red that the histogram equalization steps are suppressing H-alpha response.

See my processing challenge in part 3c above) and try your methods. If you can't get at least as much H-alpha as I show on that page and in Figure 2 below, your methodology is the cause, not that unmodified cameras lack H-alpha response.

Emission Nebulae

Because the H-alpha line is very strong, and in a modified camera H-alpha line is so intense, the histogram equalization is just bringing the intensity of H-alpha back to a reasonable level.

Hydrogen emission nebulae have both H-alpha and H-beta (blue), and often some oxygen and emission lines from other elements. The true color of hydrogen emission nebulae is more like pink to magenta, depending on the abundance and reddening by dust, not the blood red we see from H-alpha modified cameras. An unmodified camera shows these colors and their varying intensities better so you can discern chemical processes. Tables 1 and 2 show some of the colors in the deep sky.

Table 1

Table 2. Emission Lines in Nebulae in the Visible Spectral Range

The relative abundances of gases and their temperatures determine the observed strengths of the emission lines. Dust preferentially absorbs blue, so the dust abundance creates reddening so the abundance of dust further changes the relative intensities, and therefore the colors recorded by a camera.

With a modified camera, the H-alpha line usually dominates in hydrogen-containing nebulae so that the relative abundances of other gasses and effects of dust are swamped by the strong red H-alpha emission, making many/most hydrogen containing nebula mostly just red. Then one can't see the multitude of colors and thus there is loss of compositional information beyond the existence of hydrogen in the nebula.

The following figures (1-8) illustrate the diversity in color and composition that can be recorded by a stock DSLR. The examples are all made histogram equalization to squash the reds. The key to bringing out the varying colors, including red H-alpha, is post processing. The dominant reason astrophotographers can't bring out the H-alpha is due to post processing methods that suppress reds. See parts 2 and 3 of the series for more details on processing.


Figure 1. The Horsehead and Great Nebulae (M42) in Orion. Compare the colors here with those described in Table 1. The region shows pervasive dust (orange), hydrogen emission nebulae (reds to pinks and magenta), emission by oxygen (green/teal in the core of the Great Nebula), and Rayleigh scattering by fine dust (sky blue). Figure 8 shows a closer view of M42. The is a 2-frame mosaic made with a stock Canon 7D Mark II 20-megapixel digital camera and 300 mm f/2.8 L IS II lens. Total exposure times were 70 minutes for the left half, and 27.5 minutes for the right half. See the Gallery Page for more info and a larger image.


Figure 2. The very faint heart nebula with only 18 minutes of exposure time with an unmodified (stock) Canon 7D Mark II 20-megapixel digital camera and 300 mm f/2.8 L IS II lens. A spiral meteor speared the heart. This image was an NASA Astronomy Picture of the Day (September 15, 2016). See the Heart Nebula Gallery Page for more info and a larger image.


Figure 3. M8, the Lagoon Nebula and M20, the Trifid Nebula. Very faint hydrogen alpha nebulosity was recorded with only 19 minutes total exposure time with an unmodified Canon 7D Mark II 20-megapixel digital camera and 300 mm f/2.8 L IS II lens. See the Gallery Page for more info and a larger image.


Figure 4. Colorful Nebulae in the Scorpio Region Rho Ophiuchus Region. Note the colors described in Table 1. Compare to Figure 1. The yellow means the dust grains are usually larger than when the dust is more orange/red. Notice the red nebulae (below the bright star Antares) compared to the pink in Figure 1. Here the dust in the region is absorbing blue, making the hydrogen emission redder than when H-beta is not absorbed. There is also less oxygen as no blue-green is contributing to the color. This image was made with only 9-minutes total exposure time with a Canon 6D 20-megapixel digital camera, with a Canon EF 100mm f/2 USM Lens. See the Gallery Page for more info and a larger image.


Figure 5. The North America and Pelican Nebulae in Cygnus show red, pink and magenta colors from hydrogen emission nebulae and surrounded by orange dust. Made with a Canon 7D Mark II 20-megapixel digital camera and a 200 mm f/2.8 lens with 52 minutes total exposure. The image was made in strong moonlight from a first quarter Moon. See the Gallery Page for more info and a larger image.


Figure 6. The Pleiades star cluster is surrounded by fine dust, creating Rayleigh scattering and the blue colors. Twenty six total exposure time with an unmodified Canon 7D Mark II 20-megapixel digital camera and 300 mm f/2.8 L IS II lens. See the Gallery Page for more info and a larger image.


Figure 7. The Great Andromeda Galaxy, M31 shows Population I stars as slightly bluish in the outer spiral arms, and Population II, yellower stars around the galactic core. The same color distribution occurs in our Milky Way galaxy. Made with a Canon 7D Mark II 20-megapixel digital camera and 300 mm f/2.8 L IS II lens. with 36 minutes total exposure. See the Gallery Page for more info and a larger image.


Figure 8. The Great Orion Nebula (M42). Compare the colors here with those described in Table 1. The region shows pervasive dust (orange), hydrogen emission nebulae (reds to pinks and magenta), emission by oxygen (green/teal in the core of M42), and Rayleigh scattering by fine dust (sky blue). The image was made with a stock Canon 7D Mark II 20-megapixel digital camera and 300 mm f/2.8 L IS II lens and 27.5 minutes total exposure time. See the Gallery page with more details.

Filter Transmission and Hydrogen-Alpha Response

Not all stock cameras have good hydrogen-alpha response. Sometimes the IR blocking filter cuts off at too short a wavelength, blocking hydrogen-alpha (656 nm) light. So before choosing a stock camera, see if you can find online the spectral response, especially of the IR filter. Here is one web site that shows the IR filter transmission of many digital cameras: Kolari Vision Internal Cut Filter Transmission . For example, look at the Nikon D90, which has just a few percent transmission at 656 nm, versus the D80 with over 20% transmission. Or compare the Canon 20D (over 30% transmission at 656 nm) versus a Canon T2i (only a couple of percent transmission at 656 nm). For astrophotography, transmission at the hydrogen-alpha line should be above about 20%.

Conclusions

Modifying digital cameras is not necessary to obtain great astrophotos. Many stock cameras have good hydrogen-alpha response, e.g. recent Canon DSLRs. More important than a modified cameras is proper post processing methods that will bring out and not suppress hydrogen alpha emission, and cameras with good sensitivity and very low thermal dark current. The best digital camera for deep sky astrophotography that I have evaluated has good sensitivity, including hydrogen-alpha, and amazingly low dark current is the Canon 7D Mark II 20-megapixel digital camera. A great full frame digital camera for astrophotography is the Canon 6D 20-megapixel digital camera.

The advantage of a stock digital camera in astrophotography is that the color balance is close to that of the human eye, and shows compositional differences better. Modified digital cameras are too sensitive to hydrogen alpha emission, making scenes containing hydrogen too red, swamping colors from other compositions. Often this shows in amateur astrophotos as dominantly red. The choice of course is personal. I prefer images with more colors to show more processes and chemistry. I believe such images are more interesting, so I only use stack digitalcameras for my astrophotography.


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References and Further Reading

Clarkvision.com Astrophoto Gallery.

Clarkvision.com Nightscapes Gallery.

The Night Photography Series:


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http://www.clarkvision.com/articles/astrophotography.image.processing

First Published July 30, 2015
Last updated February 22, 2017