WFPDB DEVELOPMENT: RESTORING CHARACTERISTIC CURVE FROM DIGITIZED

IMAGES OF SCANNED PHOTOGRAPHIC PLATES

VII-th Bulgarian-Serbian Astronomical Conference

1 - 4 June, 2010, Chepelare, Bulgaria

H. Markov, M. Tsvetkov, A. Borisava, N. Petrov

Institut of Astronomy and National Astronomical Observatory - BAS

saving astronomical photographs

Today astronomical photography is rarely applied, but

for the sake of this a huge amount of photographic

plates and films with valuable and still only partially

utilized information for the astronomical objects and

phenomena have been accumulated in the archives of

the astronomical observatories. The preservation and

utilization of this treasure is of great importance for

contemporary astronomy. To retrieve the information

stamped there we need to refine not only the digitisation

process but also some approaches wich ensure us the

posiible highest photometric quality.

digitization of the photographic material gives the opportunity

these astronomical observations to be processed and analyzed in

the manner of the CCD observation

one serious disadvantage of the photographic emulsion is its

nonlinearity. In order to overcome this problem one have to

build-up the so called characteristic curve – the relation between

photographic density and the relative intensity.

one particular task

The profesional astronomy practice requires on every

astronomical plate a photometric wedge to be

stamped additionally. Most oftenly when the object

of scientific interest is a globular cluster such wedge

is missing. In these cases the recovering of the

emulsion characteristic curve is tricky in some sense.

the wedge used in the NAO

the approach

our main goal was to develope a method for building-up the

emulsion characteristic curve only by means of apropriate tasks

available in IRAF packages. Here we demonstrate the principles of

the method we developed and would like to emphasize that no

additional software or routines were used.

our presentation is not a review of the available methods to recover

characteristic curves but will demonstrate one of them – a method

offered by de Vacouleuers in 1982

The principle of bulding characteristic curve using radial profiles of

two stars with known magnitudes. The step between every two points

is Δm – the difference in the magnitudes of the couple.

To apply the method of de Vaucouleurs one needs to identify in the field of the

investigated plate some stars with known magnitudes. Here we demonstrate a

scaned plate with a wedge. The stars identified by means af GSC are marked

Radial stellar profile – the base for recovering the characteristic curve.

The requirement for two stellar profiles to be used in a couple is to merge in

the same background. This could be influenced not only from the variable

sky illumination but also from the non-equal chemical fog caused by un-

proper treatment of the plate during the development process.

Concerning here demonstrated method every couples of stellar

profiles contains information for the characteristic curve of the

emulsion. Following the method the more close in transparency

profiles will produce more points on the curve.

On the right, the char. curve derived by means of stellar profiles is marked by point. The continuous curve shows the char. curve derived by the wedge stamped on the plate. A good coincidence between two relations is achieved if the magnitude difference of the stars in a couple is accepted to be 0.21 mag instead 0.29 mag adopted from GSC.

stellar profiles of the coupleGSC Δm = 0.29 mag

These couple demonstrates the ability of the method to reveal inconsistency of stellar magnitudes. In our opinion the magnitude difference of 1.67 mag is not realistic

stellar profiles of the coupleGSC Δm = 1.67 mag (???)Δm = 0.5 mag is more realistic

One particular case of the outlined method to recover the characteristic curve of the emulsion uses rich sample of stars with known magnitudes. This approach relies on the fact that the top pixels of the stellar image(or the max values in the profile) are well proportional to the brightness of the star. So if we have rich sample of stars with known magnitudes which cover the whole dynamic range in transparency(density) we can construct the char. curve. The most appropriate fields filling such requirements are the regions of globular clusters. where we can find many stars with well established brightness(Stetson 2001, Catalog of CCD standards).

another approach

Part of the field near the center of the globular cluster M3 where about

400 CCD standard stars from the Stetson catalog were identified and

used to buld-up the characteristic curve for this plate

characteristic curve derived by means of the CCD standard of

Stetson in the field of globular cluster М3. The points' scatter in

the middle is caused by stars on the horisontal branch of the

cluster which are more blue than others cluster members.

some basic IRAF packages and routins

DAOPHOT(DAOFIND, PHOT)

APPHOT(RADPROF)

MATH(CURFIT)

IMAGES(DISPLAY, IMEXAM)

TABLES

So feel free to use the photographic archive of the National Astronomical Observatory. We can help you to retrieve the maximum

quality information from the astronomical photographic plates

THANK YOU FOR YOUR ATTENTION

Това изследване е извършено с помощта на договори с ФНИ ДО 02-273, ДО 02-85, НИК-05