Technical Information on images
GALAXY
TYPING
The galaxy morphological types should all be consistent with the
"3rd Reference Catalog of Bright Galaxies"
(de Vaucouleurs+ 1991).
TELESCOPE
The pictures on this site were taken using the MDM Observatory
McGraw-Hill 1.3-m telescope located on Kitt Peak 50 miles west of
Tuscon, AZ.
The 1.3-m is a Cassegrain telescope and it was operated with its
secondary in f/7.5 mode.
A few images have been obtained from other sources on the Internet in
order to completely sample the Hubble Types. These will be
labeled as such.
CCD IMAGERS
Two CCD (charge-coupled device) chips were used at the focal plane of
the 1.3-m:
1) Echelle - manufacturer = SITE, type =
thinned,
backside ill.
format = 2048x2048, pixel
size = 24 micron
pixel scale = 0.50 "/pixel
field of view = 17.0'
2) Nellie - manufacturer = STIS, type = thick,
frontside
format = 2048x2048, pixel
size = 21 micron
pixel scale = 0.44 "/pixel
field of view = 14.9'
FILTERS
The Johnson B (blue) and R (red) filters were used for the majority of
galaxies. A few were measured in V(yellow) and I
(infrared).
IMAGE PROCESSING
The CCD data were processed in the standard manner: 1) the
bias was subtracted, 2) they were flat-fielded using a
combination of dome, sky and median flats, 3) multiple, dithered
exposures of the same galaxy were registered by shifting, and 4) images
were combined. We used the IRAF tasks CCDPROC, IMALIGN, and
IMCOMBINE for these procedures.
The images were displayed in SAOimage/DS9. We then use the GIMP
image editor to capture the DS9 window, crop it, and convert it into a
high-quality JPEG.
DISPLAY STRATEGY
The DS9 display can take on a vast range of appearances for a single
CCD frame. We generally used a logarithmic stretch rather than
linear, but not always. Our displaying strategy was different for
the B and R band frames. For the R-band, we sought to bring out
the faint outer parts of the galaxy and show surrounding satellite
galaxies. (R is better than B for this purpose because it gathers
more photons, and galaxies produce more red than blue photons.)
The B-band images were used to show detail in the central regions of
the galaxy. This was done by 1) zooming in on the central part of
the image, and 2) adjusting the display so that the maximum greylevel
(white) was greater than the peak count of the galaxy. Sometimes,
we reduced the number of greylevels allotted to the central count range
so that the isophotal contours would become visible (a "stepped"
appearance). This was purposefully done to show the presence of
dust, which shows up as a jag in an otherwise smooth elliptical contour.
In addition to the B and R images, we often present a third
image. Usually this is a ratio of the B and R image and in a few
cases, it is a continuum subtracted narrow band image taken in the
light of H-alpha emission (6563 Angstroms). The H-alpha
image reveals ionized gas clouds excited by hot stars and AGN (Active
Galactic Nuclei). The B/R ratio images indicate regions that are
relatively blue as bright white, and regions that are relatively red as
dark. In so doing, we reveal regions of recent star formation
where many blue stars remain, and the extent of the absorption by dust
(which makes the color redder). This is often more scientifically
useful than a "color" image that has been processed just to look
colorful.
FUTURE PLANS
We may add our HST WFPC2 images someday!
We will turn the tuning fork into an image map and do away with the
table! <-- DONE!!