ASTROPHOTOGRAPHY: DO PHOTOGRAPHIC PLATES STILL HAVE A PLACE IN PROFESSIONAL

ASTRONOMY? ROBERTO BARTALI

ABSTRACT Before 1727, when the first attempt to record pictures was made, astronomers only can do visual observation and drawn their impression of the object, about after a century, the application of the photographic plates to astronomy made a big difference; thousand of new stars and very fine details on nebulae allows many new discoveries and made possible direct measurements without subjective errors. In the beginning of the XX century, a new improvement, color photography, show a new face of the universe. Until the third quarter of the XX century, many film manufacturers made special films for scientific and astronomical purposes increasing the spectral sensitivity on both sides of the visible, in the infrared (IR) and ultraviolet (UV).

A new revolution take place in astronomical imaging with the introduction of the Charge Coupled Device (CCD). In the last 30 years, electronic technology grows in giant steps and the development of more sensitive CCD, with an enormous number of pictures elements (pixels), together with powerful computers and software algorithms for image processing, seems to let photographic films as an obsolete technique. The aim of this essay is to show the advantage or disadvantage of the CCD over photographic films and try to see if the electronic imaging can be the best choice for astronomers. PHOTOGRAPHIC FILMS

Figure 1 Spectral sensitivity of different Kodak films High speed infrared Technical Pan 2415 Spectroscopic 103aO Spectroscopic 103aF

Photographic films came in a variety of formats, from large size plates to rolls and they are divided in two type: color and black and white. For professional astronomical purposes, color film is not as interesting as for amateurs. The reason is the spectral sensitivity of each different film. If we take a picture with the same telescope, same

exposure time and on the same night, we get very different result, so there is no reason for using, scientifically speaking, color films. Instead of that we can

use several techniques to get the best from black and white pictures, as false color

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processing, contrast enhancement or contour processing (2)(3)(4)(5)(6)(7). Another way to analyze the picture is to scanning it and then process it electronically using some computer software like Adobe Photoshop (23) (for general purpose) or other specifically developed for astronomy like Iris (21), DS9 (19), Cadet (20), Astra (22), etc. Between 1940 and 1980 many film manufactures developed special films for scientific imaging, Kodak, Ilford and Agfa are most important. Their workhorse products, series “103a” spectroscopic films from Kodak (8) and “HP5+” from Ilford was the most used by professional astronomers. These films was replaced by Kodak Technical Pan 2415 (8) with better spectral response (2)(8). In figure 2 we can see the different sensitivity of some films from Agfa (10), Konica (12), Ilford (9) and Fuji (11), all are professional type and they have, in theory, similar characteristics.

Figure 2 Spectral response of the following films Agfa APX400 Ilford Delta Professional Ilford HP5+ Fuji Neopan Konica INF750

Due to the chemical and physical reactions that took place in the film when it is exposed to light, the sensitivity is best on the blue and UV side of the visible spectrum

(Figure 1 and 2). This is a problem because Hydrogen, the most common element in the universe, emit at wavelengths in the red side of the spectrum. The sensitivity of the film depends on the size of the silver

bromide crystals, there is a direct proportional relationship between size and the capability to catch photons. Most sensitive films have large crystals (2), so the picture loose resolution and when enlarged present a grainy effect. Many films present another drawback, the reciprocity effect: that is the loss of sensitivity as the

exposure increase (2)(3)(5). Due to the very low light received from celestial object, the exposure time can be several hours for reaching the limiting magnitude, it is absolutely necessary the use of a free reciprocity effect film for do that. Many techniques are used in order to increase the sensitivity and the performance before and after the exposition (1)(2)(3). The most used is the hyper sensitization before exposing the film, several other are used in the developing stage, such as pushing and contrast enhancement. All are chemical reactions and needs a special environment, security rules and procedures to be observed strictly. Even for the best film, the quantum efficiency (the relation between input light and the number of crystals in the emulsion sensitized, in case of film) is very low, about 3% and can be increased to 10 % with hyper sensitization. Resolution for a medium speed astronomical film (400 ASA) is about 20 micron, to achieve more resolution we have to reduce sensitivity (2)(8)(10), but the exposure time must increase, the reciprocity effect also increase with time, so the end result is a poor picture.

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CCD IMAGING

The first attempt to use electronic imaging was

made in 1935 when Lallemand took planetary pictures with an electronic camera (1), but the first CCD used for astronomical purpose was in 1975 (6). CCD is a collection of photosensitive semiconductor elements organized in rows and columns forming a matrix (Figure 3). Each photo element (pixel) is capable to free electrons when incoming photon hit the semiconductor element. Electrons are then stored

into another part of the semiconductor until the exposure time ends. Electrons (charges) are then shifted, applying a voltage, from one site to the next until all of them reach the output amplifier (4)(6)(7)(8).

Figure 3 2048x2048 pixel Kodak KAF4202 CCD Courtesy of Kodak

Quantum efficiency of CCD (the of incoming photons to the output

ey are very

to ghigh

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There are many different CCD, we can divide them into 2 types: linear and matrix array. Linear arrays have not application in astronomy (only the camera on first vehicle landed on Mars have one) and each type may be for color or monochromatic picture capture. Color CCD are effectively a set of three identical monochrome arrays each one covered by a tiny color filter (red, blue and green) this way the image is formed by the quantity of each color is present in the incoming light. As stated in the Photographic Film Section, is better to use monochrome sensor instead of color, because image processing can do a best job revealing many more details.

ue to the semiconductor ntrinsic characteristics, CCD are much ore sensitive to the red and IR part of

he spectrum (Figure 4, 5, 6), only ecently manufacturers improve the blue ensitivity, in most cases covering the ensitive part with some fluorescent aterial making some kind of avelength conversion; (5)(6)(8) when V and blue wavelength photons hit that aterial, it emits photons at longer avelength where the semiconductor is ore sensitive.

ratio electrons) is very high, so th

FiSITe S100A CCD spectral response From SITe web

gure 4

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(each photon is capable enerate a free electron). Dynamic range, or the number of different gray levels is very too, reaching 16 bits (65536 different levels).

itive, manufacturer are working now on sensors with 100% QE

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Most CCD have an integrated output amplifier and some have all the electronic for analog to digital conversion. Full frame CCD are best suited for astronomy and many of them are divided into 2 or 4 section for increasing the readout speed. As the number of pixels in the

array increase, the time needed for reading all the sensor increase, even at high speed (10 Mhz or more) for reading a 8K x 8K pixel CCD we need more than 6 seconds.

Figure 5Kodak KAF4202 CCD spectral response From Kodak web page

The resolution of CCD can be very high, some manufacturers make sensors with square pixel of 6 micron, the average pixel size is 20 micron. The

sensitivity of the CCD permit very short exposure time and can show stars with much faint magnitude than photographic films can do.

Figure 6 Thomson TH7888A CCD spectral response

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drawbacks. As all electronic components,

CONCLUSION how a comparison of the characteristic and performance of CCD and

ng is impera

With all of this good characteristic, CCD seems to be a marvel, but they have their own they suffer from heat. For exposure times longer than a few seconds they must be cooled, normally at -20 or less degree Celsius. Another problem is the blooming, when the full capacity of the pixel is exceeded, due to long exposure or when light from bright object strike the sensor, electrons jump to neighborough pixels. Some CCD manufacturers place a special cell to collect the excess of electrons (antiblooming).

From Thomson web

Table 1 sphotographic film. There are many advantages using CCD for image detection against films; sensitivity, QE, resolution, red spectral response, capability of see the final image almost instantaneously and the possibility of further processing, are most important.

For orbiting telescopes, satellites and interplanetary ships, CCD imagitive. For terrestrial based telescopes the possibility to see the picture just a few

seconds after the exposure and transmitting it to a remote scientist is a big advantage, because the astronomer can take a quick decision, for example, take another picture, change the exposure time, change the field of view, without the need for wait to develop the film (operation that take a lot of time and must be made where the telescope is). This time delay can be the difference from discovery or not, for example, an object moving fast like an asteroid in close Earth orbit. A remotely controlled telescope is cheaper, because the

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astronomer don´t need to travel, be exposed to below zero temperature, etc. Other technological improvements, not possible with normal photographic films, are:

• the interferometric connection of various telescopes (like VLT) and summing the image take by each one into one picture.

• Active and adaptive optic systems do not work without the processing speed of a computer and the fast acquisition time of the images by a CCD

Storing electronic images do not need special environment, in a CD rom we can record hundred of pictures, saving space and money because photographic plates needs more space and a climatic controlled room for storing. Sending by mail a CD rom is as easy as send a letter, instead a photographic plate needs special package and care and it cost much more. Table 1 CCD and Film condensed comparison characteristics FILM CCD Type of reaction Chemical, physical Physical Quantum Efficiency <10% >80% Resolution 10 to 25 micron 6 to 24 micron Pixel matrix size 1200x1800 (24x36 format) 512x512 to 8192x8192 Spectral response 350 to 650 nm

can be extended from 250 to 950 nm

400 to 900 nm can be extended from 300 to 1100

nm Time from the end of the exposure to image

>30 minutes < 10 seconds (for larger format CCD)

Dynamic range <16 bits >=16 bits (65536 gray levels) Equipment cost Low High Auto guiding when exposing Not possible Yes Direct image processing Only after scanning the image Yes Remotely image acquisition Not possible Yes Automatic image capturing (no need for operator)

Not possible Yes

Need for cooling system No Yes Special chemical and physical processes and procedures for sensitivity increment

Yes No need

Special environment for developing

Yes No

Interferometric telescope connection capability

Not possible Yes

Automatic correction on adaptive and active optics capability

Not possible Yes

Automatic protection against saturation

Not possible Yes

Antiblooming capability Not possible Yes Reciprocity effect failure Yes No Loss of sensitivity if not cooled Yes but low Yes but high Binning capability Only after scanning the image Yes Resolution increasing Not possible Yes

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I really think, after comparing data, that CCD will make obsolete the photographic film in a very short time, not only for astronomy, but in all kind of imaging process, letting films only for artistic imaging purposes. As people use more CCD, their cost will be less and will be better. The day when manufacturers can do a really blue and UV sensitive CCD, there would be no reason to use films. REFERENCES 1) Muller P, DIZIONARIO DI ASTRONOMIA, S.E.I. 1972 2) Ferreri W, FOTOGRAFIA ASTRONOMICA, Il Castello, 1977 3) Covington M, ASTROPHOTOGRAPHY FOR THE AMATEUR, Cambridge, 1999 4) Buil C, CCD ASTRONOMY, Willmann Bell, 1991 5) Kitchin C, ASTROPHYSICAL TECHNIQUES, IOP, 1998 6) Howell S, HANDBOOK OF CCD ASTRONOMY, Cambridge, 2000 7) Berry R. et al, THE CCD CAMERA COOKBOOK, willmann Bell, 1994 Internet References 8) www.kodak.com Kodak technical literature (CCD, photographic films and filters) 9) www.ilford.com Ilford technical literature (film) 10) www.agfa.com Agfa technical literature (film) 11) www.fujifilm.com Fuji technical literature (film) 12) www.konica.com Konica technical literature (film) 13) www.ti.com Texas Instruments technical literature (CCD) 14) www.atmel.com Atmel technical literature (CCD) 15) www.st.com technical literature 16) www.sbig.com CCD camera 17) www.photonics.com CCD technology 18) www.site-inc.com CCD Image Processing Software References 19) DS9: http://hea-www.harvard.edu/RD/ds9/index.html 20) CADET: http://www.terra.es/personal2/oscarcj/introeng.htm 21) IRIS: http://www.astrosurf.com/buil/us/iris/iris.htm 22) ASTRA: http://www.phasespace.com.au 23) Photoshop: www.adobe.com