The Arrival of Affordable Digital SLRs

In June 2002 Canon released its D60 D-SLR model, one of the first 6.3-megapixel cameras. Initial tests published for this camera showed much lower noise in long exposures than all previous models. I acquired one of the first available models.

Figure 5.1. Johannes Schedler in his observatory with his 4-inch TMB "Apo" and Celestron C11 Schmidt-Cassegrain.

Early trials soon showed a dramatic improvement in reducing noise for long exposures. For the first time, exposures of more than 60 seconds could be used and even at summer temperatures. What could be achieved with these new D-SLR cameras at that time was reviewed in the Sky & Telescope October 2002 and June 2004 issues, with the D-SLR results featured in the "Gallery" section. For the following year, I used the D60 extensively to image many of the brighter deep-sky objects, most of them at prime focus of my 4-inch TMB refractor and Celestron C11 (at f/6) telescopes (see Figure 5.1).

In June 2003 the Canon D60 was replaced with the new model 10D (see Figure 5.2). Although basically the same chip design, there were several key improvements introduced with it, namely:

• significantly better noise performance for long exposures for all ISO ratings;

• nearly complete elimination of the red amplifier glow on the right side of the image;

• higher ISO settings up to 1600 (3200 not really usable);

• higher review magnification of up to 10x for analyzing images via the integral display;

• improved remote capture software allowing multiple interval exposures up to a maximum of 30 sec each (but not in bulb mode!).

See Figure 5.3 for the noise comparison of the 10D compared to the D60 showing a 300-sec ISO800 dark at 22°C, cropped from the center to the right corner.

Figure 5.2. Camera Canon D60 and Canon 10D.

The new cheaper Canon Rebel (300D in Europe), built with a plastic casing, shows similar results for deep sky applications but two major drawbacks have to be noted:

1. No mirror lockup is possible, but a firmware update, available from the Canon homepage, enables a mirror lockup.

2. No direct connection to remote interval timer is possible - the plug for the timer TC80N3 must be exchanged to enable its use.

The SLR-type camera body provides an easy coupling to any telescope at prime focus. This connection is usually by means of a T-ring that attaches to typical telescope accessories. The large chips of the new D-SLRs offer better quality and wider coverage of the telescopic field than is possible with the smaller chips of most astronomical cameras. Field flatteners/reducers are very helpful in achieving the desired image quality into the frame corners.

What are the biggest advantages of such cameras compared to film SLRs? A clear advantage, especially for photographers with little experience, is the

Canon 10D

Canon D60

Figure 5.3. Comparison image of the noise performance - Canon 10D vs. D60.

immediate review of the image by getting the result quickly after the exposure. This allows problems related to focus, composition and sky background level to be resolved during the imaging session itself. A second advantage is the relatively low noise and high resolution compared to a typical slide or negative color film. New cameras like the 10D not only save the images in jpg mode but also in raw mode that utilizes the internal 12-bit-per-channel format. This can further be converted to 16-bit-per-channel tiff images. This is a big advantage for the wide contrast

Table 5.1. Overview on

current D-SLR

camera models (compared in early 2CCS for DSO

capability)

Model

Effective

Deep-Sky

Street Price

Comment

Pixel

Capability

Approx. in $ (€)

Canon IDS

ll MP

+++

7CCC

Top model, 24 x 36mm chip. Replaced with MKII model with 16.7 MP

Canon 10D

ó.a MP

+++

lSCC

Best all-round performance. Replaced by 20D model with 8.2 MP

Canon Rebel

ó.a MP

+++

lCCC

Best price/performance

(300D)

ratio, limited functions

Fuji S2 Pro

ó.a MP

++

l800

Good all-round performance. Replaced by S3 model with 6 MP (x2)

Kodak DSC

la MP

+

óCCC

Bad noise behavior at

Pro14

higher ISO

Nikon D100

ó.a MP

++

lSCC

Good all-round performance

Nikon D2H

4.l MP

++

aCCC

Good all-round performance

Nikon D70

ó.a MP

lCCC

Low noise, but amplifier glow, best alternative to Canon

Olympus E1

S MP

++

22CC

Good all-round performance, some noise

Pentax *ist D

6.l MP

++

lasC

Good all-round performance, some noise. DS model similar at lower cost

per color

llCC

Foveon X3 sensor, weak low-light performance, no bulb mode

per color

+

lóCC

Foveon X3 sensor, improved performance at ISO400

Note: The noise behavior at long exposures is not comparable to daytime behavior. All the

listed cameras

are excellent for use during daytime.

range that has to be accommodated with many deep-sky images, e.g., faint nebula structures embedded in bright foreground stars. Compared to 35mm film, when using lenses, the focal length has to be multiplied by a factor of 1.6 to get the equivalent field coverage. See Table 5.1 for an overview of current DSLR cameras.

As expected, the noise behavior very much depends on the ambient temperature. As is the case with cooled CCDs, the sensors of digicams are subject to dark current, which doubles for every 6°C increase in temperature. However, some of the dark noise is compensated for by the internal calculations of the CMOS chip. During the warm season a well-matched dark frame subtraction is essential to get the best results out of the raw images. During wintertime, even at high ISO ratings, the dark current and noise are reduced to a minimum so that a dark frame subtraction is not as essential, since the residual noise is basically of a random structure.

The graph (Figure 5.5) illustrates the standard deviation for 300-sec darks at two different temperatures (22°C vs. -4°C). These darks have been taken in raw mode, converted to 16-bit-per-channel images by ImagesPlus and examined in Astroart.

The values in Figure 5.5 demonstrate that there is little advantage in using higher ISO ratings for achieving maximum signal to noise (S/N). For my typical semi-rural sky conditions, I use ISO 200-800 for unfiltered images and ISO 400-1600 for narrowband filtered images. For long exposures, I use 5 minute

Temperature influence on Dark Noise for the Canon 10D

(300 sec exposure]

Temperature influence on Dark Noise for the Canon 10D

(300 sec exposure]

100 200 400 800 1600

ISO Rating

100 200 400 800 1600

ISO Rating

Figure 5.5. Standard deviation of darks at 22°C vs. -4°C.

CANON EOS 1GD - SPECTRAL RESPONSE

"D

e malri G.4

or N

4GGG 45GG 5GGG 55GG óGGG Ó5GG 7GGG

Wavelength (A)

Figure 5.6. Relative spectral response of the Canon 10D. Note the low response at the important 6563A H-alpha emission line.

exposure for raw frames with the white balance set to "sunny." For shorter exposures (less than 2 minutes), the lower ISO settings prove the best.

There is one major disadvantage for the D-SLRs compared to film imaging: the limited sensitivity in the far red where the H-alpha emission line at 6563 A reveals most detailed structures in the majority of emission nebulae (see Figure 5.6). The spectral sensitivity investigations from Christian Buil on the 10D are showing that the normalized sensitivity falls off from 100% in the green to approximately 6% at 6563 A. Considering a 40% average absolute quantum efficiency for ABG chips, the absolute quantum efficiency at 6563A will be near 2%.

The integral IR blocking filter of the 10D reduces the far red end of the visible spectra and therefore is the main reason for the poor red response. Preliminary tests on D-SLR cameras with the IR filter removed showed a significant increase of the red response. However, carrying out this modification cannot be recommended because the filter is an important part in the optical path. Removal will make the auto-focus useless and the color balance will be severely disturbed for daylight imaging. Hutech is now marketing a modified Canon Rebel camera, with an exchanged IR-cut filter optimized for dedicated astro-imaging purposes, for approximately $1500. Because the Bayer color filter pattern used on the chip contains 50% green pixels and 25% for both blue and red pixels, the expected absolute sensitivity without the IR-cut filter will not exceed 10% at 6563 A over the total chip area, even for the modified cameras.

CANON EOS 1GD - SPECTRAL RESPONSE

4GGG 45GG 5GGG 55GG óGGG Ó5GG 7GGG

Wavelength (A)

The biggest disadvantage of CCDs compared to film, until now, has been the much smaller chip size compared to the film format (typical 36 x 24mm). This has changed, however, in the past two years. Like the previous D60, the Canon 10D uses the same 22.7 x 15.1mm 6.3 megapixel CMOS sensor (3072 x 2048 final image size). This corresponds to the size of APS film format. The big chip size compensates for the reduced red sensitivity and the noise at high ambient temperatures.

The already mentioned limitations of the D-SLRs are valid again when comparing them to dedicated astronomical CCD cameras. These typically monochrome cameras are cooled to a constant low temperature of approximately 30°C below ambient. They are optimized for quantum efficiency and they can reach between 50% and 80% over the visible wavelengths. In a monochrome CCD camera every imaging pixel contributes for maximum resolution as no interpolation between the different colors filters has to be performed. CCD astro cameras show real 16-bit-per-pixel resolution and are close to ideal, even under heavily light-polluted skies. My opinion is that for the foreseeable future, dedicated astronomical cameras will have significant advantages for narrowband imaging and for reaching the faintest structures. Additional features that D-SLRs cannot provide are binning, subframe readout, focus support routines, integrated guiding and automated image acquisition, as well as links to other astronomical programs and hardware.

What features should one look for in a digital SLR for astrophotography? The camera must have a bulb mode and connectivity to a remote interval timer, either hardware like the TC80N3 for the Canon D-SLRs or software based via the PC connection for long controlled exposures. One should select a model with low noise design, as shown in Figure 5.3. I would recommend the current standard line of D-SLRs; the top-end models will not repay the high investment, as technical progress will push forward the features like chip size, transfer speed and sensitivity in high speed.

The model should be able to use existing high-quality lenses. Focal lengths between 20 and 300 mm are able to frame most of the possible deep-sky targets. Figure 5.8 shows my typical setup for deep-sky imaging using the C11 with a reducer for f/6, mirror lock and helical fine focuser for the D-SLR. A 4-inch TMB refractor at f/12.5 is used as a guide scope with the MX7C autoguiding. The setup is also used with the telescope roles reversed. The D-SLR is permanently connected to the PC to control focusing, while the timer TC80N3 automatically takes the exposure sequence.

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Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

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