Most observational instruments for the Subaru telescope produce large and sometimes complicated data. For example, Suprime-Cam (Subaru Prime focus Camera) produces data with a size of 16MB10 for a single exposure and HDS (High Dispersion Spectrograph) produces images of complex echelle spectra whose locations are stored in an ASCII table extension (Baba et al. 2000). It is not easy to browse and check such large/complex data with existing browsers. We encounter this difficulty especially when we search for data from the archive. Preview images of smaller size are usually used to circumvent the difficulty. However, an image format based on simple compression as used in most data archival systems is not satisfactory to evaluate the quality of the original data.
To solve this problem, STARS (the Subaru Telescope data Archive System) (Takata et al. 2000) has implemented a new quick-look system. The system is comprised of two parts, ``Quick-look image Producer'' (QP) which generates the Quick-Look Images (QLIs) from the original images, and ``Quick-Look Image Server'' (QLIS) which processes the QLIs and displays them on web browsers. In this paper, we describe the outlines of them briefly.
The format of QLIs is defined within the FITS framework, which enables us to store several kinds of information derived from the original data, such as image statistics, typical profiles and spectra, into a single FITS-formatted file efficiently. Users can browse and examine QLIs using FITS browsers, if the browsers support all features of the FITS format.
To store the information of the original data into QLIs efficiently, we classified the original data into three categories, ``Calibration'' (C-mode), ``Image'' (I-mode) and ``Spectra'' (S-mode) and defined different QLI formats for each category. Because QLIs for I- and S-mode are needed to be visually inspected, a simple compressed image of the original data is stored as the primary array of the FITS formatted QLI. However, for C-mode, simple compressed images are not very useful. Therefore, we adopted a three-dimensional array of the statistics (mean, standard deviation, minimum and maximum calculated in every submatrix of pixels) in the original image for a primary image of the C-mode QLI. The first plane (mean) of the C-mode QLIs can be interpreted as a highly compressed image. We also added typical profiles from the original (C-mode) image into a binary table extension (BTE) to compensate for the low resolution of the statistics array. For the S-mode data, one-dimensional spectra are the most essential information. S-mode QLIs have spectra extracted from the original data, according to locations recorded in the ASCII table extension of the raw data, as an binary table extension.
The structure of the QLIs are summarized as below.
|I: Image||HDR||Compressed image(NAXIS=2)|
|S: Spectra||HDR||Compressed image(NAXIS=2)||HDR||spectra (BTE)|
|C: Calibration||HDR||Statistics matrix(NAXIS=3)||HDR||profiles(BTE)|
HDR: Header, BTE: Binary table extension
QP is written in Fortran90 using the CFITSIO (Pence 1999) subroutine library.
The flow of the program is as follows:
QP is triggered when the observation data are transferred from the telescope to the base facility at the Hilo campus by STARS. The return code is marked in the STARS database and is checked daily to see that the QLI is generated correctly.
QLIs produced by QP have a maximumsize of about 500 kB after gzip compression. While much smaller than the original data, it is still large for browsing via computer network. Moreover, standard web browsers cannot display images of FITS format. Quick Look Image Server (QLIS) transforms a FITS formatted QLI into a GIF imagea, or a smaller FITS image, according to the user's request, and displays it on the web browser.
Figure 1 shows the QLIS web page displaying the data of FOCAS (Faint Object Camera And Spectrograph) test data. When QLIS is first triggered by ``QLI'' button on the web page of STARS, QLIS creates a GIF image from the primary data of the QLI and displays it on the web browser. Then the user can choose a preferred image format (GIF/FITS), the image size, and spectrum IDs to be shown by HTML/FORM. If the FITS format is selected, a FITS browser using a Java Applet is evoked. in this case the user can manipulate the FITS image dynamically.
At present, QLIs in this format are available only for Suprime-Cam, HDS and FOCAS data but it will be extended progressively to other instruments. Some special treatments may be needed for the IR instruments.
Image manipulation functions of QLIS will be enhanced to allow users to magnify the image, extract an image section, etc.
We are also developing a simple standalone viewer to browse images and spectra/profiles of QLIs stored on users' local disks. Because existing FITS browsers such as FV and SAOimage cannot display the profiles/spectra in the binary table extension, this standalone browser may be useful in data reduction stage.
Baba, H., Yasuda, N., Ichikawa, S., Aoki, W., Taga, M., Horaguchi, T., & Hamabe, M. 2000, this volume, 486
Pence, W. 1999, in ASP Conf. Ser., Vol. 172, Astronomical Data Analysis Software and Systems VIII, ed. D. M. Mehringer, R. L. Plante, & D. A. Roberts (San Francisco: ASP), 487
Takata, T., Ogasawara, R., Kawarai, K., & Yamamoto, T. 2000, this volume, 157