Nebulae red response and filter modification

11.3.1 DSLR spectral response

Figure 11.5 sums up a lot of information about the wavelengths of light to which cameras respond. Reading from the top, the first thing you'll notice is that color film has a gap in its response between green and blue, and DSLRs don't. This is important because the strong hydrogen-beta and oxygen-III lines from emission nebulae fall in that gap. That's one reason nebulae that look red on film may come out blue with a DSLR.

CAMERAS

FILTERS

CELESTIAL OBJECTS

LIGHT POLLUTION

DSLR (typical)

i Kodak Color

\ E200 film

Red and yellow filters

Dldymlum glass "(red) intensifier"

Broadband light pollution filter

Stars, galaxies, reflection nebulae

Emission nebulae

Sodium-vapor streetlights

Mercury-vapor streetlights

DSLR (typical)

i Kodak Color

\ E200 film

Red and yellow filters

Dldymlum glass "(red) intensifier"

Broadband light pollution filter

Stars, galaxies, reflection nebulae

Emission nebulae

Sodium-vapor streetlights

Mercury-vapor streetlights

350 nm

400 nm

450 nm

500 nm

550 nm

600 nm

650 nm

700 wavelength nm

VIOLET

BLUE

GREEN

YELLOW

Color

Figure 11.5. How the visible spectrum is emitted by streetlights and celestial objects, transmitted or blocked by filters, and detected by film or DSLR sensors. Data are approximate; consult actual specification sheets when possible.

DSLR makers don't publish spectral response curves, so the curves at the top of Figure 11.5 are estimated from Sony CCD data sheets plus a number of published tests. What is most uncertain is the effect of the infrared-blocking filter in front of the sensor. We know that with this filter removed, the sensor has strong response up to 700 nm or further.

11.3.2 Filter modification

There are two reasons astrophotographers often have their DSLRs modified for extended red response. One is the strong hydrogen-alpha (Ha) emission from nebulae at 656.3 nm. The other is that working in deep red light, with filters blocking out the rest of the spectrum, is a good way to overcome skyglow in cities.

Canon's EOS 20Da (now discontinued) and some Fuji DSLRs have been manufactured with extended red response. Usually, though, astrophotographers rely on third parties to modify their cameras. Reputable purveyors of this service include Hutech (www.hutech.com), Hap Griffin (www.hapg.org), and LifePixel (www.lifepixel.com). For those who have confidence in their ability to work on delicate precision instruments, there are also modification instructions published on the Internet.

The simplest modification is to simply remove the filter. This has three drawbacks. One is that the red response increases so much that the camera no longer produces realistic color images; red always predominates. The second is that the camera no longer focuses in the same plane; neither the autofocus mechanism nor the SLR viewfinder gives a correct indication of focus. The camera can only be focused electronically. The third is that, with such a camera, camera lenses may no longer reach infinity focus at all.

It's better to replace the filter with a piece of glass matched in optical thickness (not physical thickness) to the filter that was taken out. Then the focusing process works the same as before. Better yet, replace the filter with a different filter. Firms such as Hutech offer a selection of filters with different cutoff wavelengths, as well as options for inserting additional filters for daytime color photography.

How much more Ha response do you get? An increase of x 2.5 to x 5, depending on how much Ha light was blocked by the original filter and whether the new filter is fully transparent at the wavelength. The original IR-blocking filter always has nearly full transmission at 600 nm and nearly zero transmission at 700 nm.

11.3.3 Is filter modification necessary?

Figure 11.6 shows what filter modification accomplishes. In this case, the modified camera is a Canon EOS 20Da, whose sensitivity at Ha is 2.5 times that of the unmodified version. The 20Da was designed to work well for daytime color photography, so it doesn't have as much of an Ha boost as it could have had.

Figure 11.6. Effect of filter modification. Top: unmodified Canon XTi (400D). Bottom: Canon 20Da with extended hydrogen-alpha sensitivity. Each is a single 3-minute exposure of the Orion Nebula (M42) with 420-mm f /5.6 lens, processed with Photoshop to give similar color balance and overall contrast. See the back cover of this book for the same pictures in color.

Other modified cameras show somewhat more of an increase. In the picture, the right-hand edge of the nebula is representative of thinner, redder hydrogen nebulae elsewhere.

What is striking is how well the unmodified camera works. Yes, the modification helps - but it's not indispensable. The unmodified camera shows the nebula reasonably well.

Table 11.1 Intensities of main spectral lines from emission nebulae (as percentage of Hp from the same nebula).

M42

M16

NGC 6995

Theoretical

Wavelength

(Orion

(Eagle

(Veil

thin H II region

Line

(nm)

Nebula)

Nebula)

Nebula)

(2500 K)

O II

372.6 + 372.8

119

157

1488

Ne III

386.9

13

2

118

Hy

434.0

41

36

44

44

O III

436.3

47

486.1

100

100

100

100

O III

495.9

102

29

258

O III

500.7

310

88

831

N II

654.8

26

104

124

Ha

656.3

363

615

385

342

N II

658.3

77

327

381

S II

671.7 + 673.1

9

116

68

O II, OIII, etc., denote ionization states of elements, each of which emits more than one wavelength. The Ha, H^, and Hy lines of hydrogen are all from H II.

M42 is a dense, strongly photoionized H II region; measurements were taken just north of the Trapezium, very close to the illuminating stars. Data from D. E. Osterbrock, H. D. Tran, and S. Veilleux (1992), Astrophysical Journal 389: 305-324.

M16 is a photoionized HII region less dense than M42 and considerably reddened by interstellar dust. Data from J. García-Rojas, C. Esteban, M. Peimbert, M. T. Costado, M. Rodriguez, A. Peimbert, and M. T. Ruiz (2006), Monthly Notices of the Royal Astronomical Society 368: 253-279.

NGC 6995 is part of the Veil Nebula or Cygnus Loop, a supernova remnant, and is ionized by the expanding shock wave from the supernova, rather than by light from nearby stars. Each value shown in the table is the mean of the authors' reported fluxes from five positions in the brightest part of the nebula. Data from J. C. Raymond, J. J. Hester, D. Cox, W. P. Blair, R. A. Fesen, and T. R. Gull (1988), Astrophysical Journal 324: 869-892.

Theoretical values for a thin, weakly excited region of pure hydrogen are from D. E. Osterbrock and G. J. Ferland, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (Sausalito, Calif.: University Science Books, 2006), p. 72.

To understand why this is so, remember that Ha is not the only wavelength at which nebulae emit light. There are also strong hydrogen-beta (H^) and oxygen-III (O III) emissions which color film does not pick up. Depending on the type of nebula, there may be more than that. As Table 11.1 shows, the Veil Nebula is actually brightest in blue and near-ultraviolet wavelengths, not hydrogen-alpha.

For that matter, a x 2.5 or even x 5 difference in Ha sensitivity is not gigantic. In photographic terms, it is a difference of 1.3 to 2.6 stops, no more than a third of the usable dynamic range of the image.

And, most importantly, deep red sensitivity matters only when you are photographing emission nebulae or using a deep red filter to overcome light pollution. Even then, excellent work can be done with an unmodified DSLR (Figure 11.8 p. 140). The rest of the time, when photographing galaxies, star clusters, or reflection nebulae, modified and unmodified DSLRs will certainly give the same results.

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