QColor On-Line
User's Manual
Preliminary Release for QColor Version 20d
QColor employs
a sophisticated four-image method of making color pictures that helps you
beat the exposure time and noise limitations of color imaging. From one
no-filter image and three color-filter integrations, QColor produces a
24-bit TIFF or 24-bit BMP color image file. With a color gamut of 16.7
million colors, output from QColor is of publication quality. QColor can
also make exceptional images from standard tri-color image sets. And QColor
accomplishes all this using a standard PC with a standard VGA card.
What
QColor Does
QColor combines one image taken with no filter (the No-Filter, or "NOF"
image) with a set of three images taken through standard red, green, and
blue tri-color filters. The NOF image gives you the opportunity to create
a color image that combines the outstanding signal-to-noise ratio of the
no-filter image with the vivid, realistic color characteristic of tri-color
imaging. If you have standard red-green-blue image sets without a no-filter
image, QColor allows you to create a synthetic no-filter image, so that
you can enjoy the benefits of quad-color imaging.
QColor supplies flexible tools that allow you to resize, equalize, and
precisely co-register you image set in microscopically fine steps, and
color tools so that you can fine-tune the color balance of the celestial
object you have imaged for a precise, true-color output. Because QColor
uses numeric inputs, you can duplicate trusted color synthesis "receipes"
precisely, for reliable and repeatable imaging success.
QColor does not change the source files that it works from. This means
that you can experiment freely without fear of corrupting your original
image data. When you load an RGBN image set, QColor automatically creates
three "generic" color files with the names RED.FTS, GRN.FTS,
BLU.FTS, and NOF.FTS and only these are altered during the creation of
a color image.
Sample Images
Here are two sample images made with QColor:
Quad-Color Sagittarius made at the 1995 Oregon
Star Party by Richard Berry, using an 18mm wide angle lens on a Coobook
245. Each image set was four images of 60 seconds each, and the filters
were standard Wratten filters #47, $58, and #25.
NGC 157 in Quad-Color by Al Kelly, using
a Cookbook 245 on his 32-inch f/4 Newtonian reflector. Al stacked 5 red
images, 6 green images, 8 blue images, and 8 no-filter images of 60 seconds
integration each.
QColor
Basics in a Jiffy
Follow these steps to create a quad-color image:
- Load your RGBN images. Use the Load RGBN (LR) command to specify
the red (R), green (G), blue (B), and no-filter (N) source images. For
optimum results, you should prepare these images for compositing using
CB245 or Multi245 software. (For tri-color imaging, specify the red, green,
and blue images, and load the best and sharpest of these as the no-filter
image.)
- Resample the RGBN images. Images should be resampled to "square-pixel"
dimensions. Use the Resample RGBN (RS) command to resize the images. Images
taken in the Cookbook 245's 252-wide mode should be resampled to 320x240,
and images in 378-wide mode should be resampled to 378x284 pixels. Focus-mode
images taken with the camera in 378-wide mode should be resampled to 215x240
pixels for maximum resolution.
- Equalize the RGBN Images. The source images may have different
exposures, but the Equalize Color (EC) command adjusts them to the exact
same specifications. For deep-sky images, a low point of .01 and a high
point of .9999 usually work well. If the gamma value is greater than 1,
the images will be brighter; below 1 the images become darker. For the
most accurate color, set the gamma value to 1.0. Note that equalizing automatically
creates good color balance for most images.
- Blink-Register the RGBN images. Bring the three color images
into rough alignment by blinking. Blink Registration (BR) blinks the red,
green, and blue images against the no-filter image; you simply press the
arrow keys until there is no motion left. If the images need to be rotated,
press the [home] key and enter the desired rotation in degrees; positive
angles are clockwise and negative angles are counterclockwise. When your
have the images reasonably well registered, press the [end] key to go on
to the next color. When you have blinked the three colors against the no-filter
image, you will see a table that you can edit, if necessary. QColor then
registers the R,G, and B images to the N image. (If you are making a tri-color
image, you'll be registering the other two images to the one that you designated
as the no-filter image.)
- Centroid Register the RGBN images. For precise registration,
type (CR). You will see the NOF image. Use the arrow keys to move the cursor
to a nice round isolated star, and then press the S key to select the star.
QColor will center on the star and then display each of the color-filtered
images in turn.Again, use the arrow keys to move the cursor to the selected
star and then press the S key to select the star. Each color image is precisely
aligned to subpixel accuracy to the no-filter image.
- (Tri-Color Only: Create a Synthetic No-Filter Image. Use the
Synthesize No-Filter (SN) function to combine the red, green, and blue
images into a no-filter image. You can select the your own color weightings,
or use the theoretical default values. Use the save function (SA) and save
this image to disk as NOF.FTS.)
- Enhance the No-Filter Image. At this point, you can use the
Load function (LO) to load and the no-filter image. You can apply linear,
gamma, gammalog, log, or histogram shaping filters, simple stretch scaling,
as well as Gaussian Binomial, or Power-Law unsharp masks to make the no-filter
image sharper and crisper. Save the no-filter image and you're ready to
make color!
- Adjust the Color. The Adjust Color (AC) function produces a
long and delicate scale of colors well suited for creating a precisely
balanced color image. Using this function, you can tell how your color
image looks, and perform the fine-tuning corrections to make the color
or brightness of the image just what you want. You can adjust the brightness
and contrast of each color image individually, apply powerful techniques
that enhance image color, and specify the effect that the no-filter image
will have on the final color image. Here's how:
- For each color, you can set the add parameter. Adding
a positive number (such as 20) increase the strength of that color in the
image, while adding a negative number (such as -10) decreases the strength
of that color. For example, if the image looks too green, add a negative
number and view the image again -- it should appear less green. The add
parameter is usually a value between 0 (no effect) and several hundred.
- For each color, you can set the con parameter to change
the color contrast. A contrast value of 1.0 has no effect, while numbers
greater than 1 increase the contrast of a color and numbers smaller than
1 decrease the color contrast. Con is a very powerful parameter that seldom
takes values below 0.7 or over 2.0. It is normally best to make the contrast
value of all three color the same, and for good results most of the time,
leave all the contrast values set to 1.
- For the overall color strength, you can set the saturation parameter.
For true, natural color, the saturation should be 1. Values greater than
1 make all colors stronger; values less than 1 made all colors weaker.
The image is rendered in shades of gray when saturation is set to zero.
For nebulae which tend to be colorful, is is best to set the saturation
at 1, but for galaxies which tend to be weak in color, you may wish to
set the saturation to 1.3 or 1.4. Higher saturation values tend to make
your images look fake.
- For overall color brightness, you can set the gamma parameter.
A value of 1 leaves the brightness untouched; values greater than 1 make
the image brighter, and values less than 1 make it darker. Gamma should
normally be set to 1 because the best way to control the overall image
brightness is to adjust the add and con values
for the NOF image. If you have done a good job processing the NOF image,
the color image will be the right brightness.
- Finally, you can set the NOF parameter. This determines
how strongly the no-filter image will effect the color image. If NOF is
0, then the nof image has no effect whatever; if NOF is 1, then the NOF
image has total control of the color image brightness. For a 50/50 blend,
you can set the NOF parameter to 0.5. This is useful because you can make
an over-enhanced NOF image and then blend it with the more subtle image.
NOF has one other function: if you set NOF to -1, then the image is rendered
in pure colors with no brightness information at all. This is useful for
assessing the overall color balance of the image, since the sky looks best
when it's a neutral to bluish gray.
- After the color image forms on the screen, you have three options:
Accept It, Try Again, and Escape. If you press the
"y" key, the add and con parameters you have specified will be
applied to the images and you will be returned to the main menu. If you
press [enter], you can adjust the add, con, saturation, gamma, and NOF
parameters and see what changes occur. Finally, if you press the [esc]
key, you exit to the main menu without making any changes. If the image
is much too dark or light, for example, you should exit and change the
NOF image directly.
- You may have to go "around the loop" dozens of times until
you get the hang of color balancing. In general, it is best to start with
all add at 0, all con at 1, saturation
at 1, gamma at 1, and NOF at 0 and
see what you have. Next, increase the saturation to 1.5 and
adjust the red, green, and blue add parameters until the
color looks about right, but much too strong, then set saturation
to a value between 1.01 and 1.3 depending on how strong you want
the color. Next, set NOF to -1 and check that the sky is
gray or blue-gray, and if it is not, adjust the color add parameters
until it is reasonable. Next, set NOF to 1 and see how the
quad-color image looks. You can fine-tune the image brightness by adjusting
the no-filter add and con parameters, but keep
the changes small. When you have everything right, press the Y key to accpt
it and exit.
- Check your color output. The ViewColor (VC) function is your
second "window" on your color image. It shows a different "take"
on how the image will look by creating a palette of 216 colors that is
bright and contrasty on the screen.
- Save the Color Image. Using the Save Color (SC) command, enter
the file name for the output image and approve the saturation and gamma
correction parameters. QColor creates your choice of a 24-bit color TIFF
or a 24-bit Windows Bitmap (BMP) file. If you type the filename with the
.TIF extension, it will be saved as a TIFF file; if you type the .BMP extension,
it will be saved as a Windows bitmap (BMP) file. Both formats are widely
used and can be loaded into other graphics programs.
Color
Balance Your Monitor
Test the color balance of your monitor by giving the name of one image
for all three colors. The screen display should appear in shades of neutral
gray. If the image has a green, blue, magenta, or other color cast, adjust
the color balance of the monitor so that you see neutral gray image. If
these controls are internal, have a technician adjust the monitor for you.
Most monitors distort the brightness that they display, with the middle
values appearing too dark. Using the Set Display (SD) function, you can
correct this distortion. For most monitors, a monitor gamma value of 1.9
does an excellent job.
How
to Make Precise Quad-Color Images for QColor
Although color imaging is simple in theory, it is difficult to put into
practice. For realistic results, you must use a filter set that closely
approximates the spectral response of the display phosphors on your computer
monitor and the tricolor sensors in the human eye. Wratten filters #25
red, #58 green, and #47 blue have long been considered the standard filter
set for photographic tricolor work.
- Quad-color imaging is just like tri-color imaging except that you make
one image with no filter. Because more light reaches the CCD, the resulting
no-filter (NOF) image has a much higher signal-to-noise ratio than the
three images made through color filters. The quad-color technique allows
you to use the luminosity from the high-quality no-filter image with color
derived from the noisier, less sharp color-filtered images.
- Recently Edmund Scientific has introduced red, green, and blue dichroic
color separation filters with ~90% transmission that do an excellent
job at a very reasonable cost ($60). Because the Edmund filters transmit
more light than the Wratten filters, exposure times are considerably shorter
with Edmund dichroics than with Wratten filters.
- However, the high infrared sensitivity of the TC245 CCD chip means
that neither Edmund nor Wratten filters cannot be used by themselves
make accurate color images. This is because standard blue, green, and
red filters transmit a great deal of near infrared light. CCD tricolor
images shot with standard filters come out with a strong purple cast.
- To block the infrared, you must add an extra
filter. The Corion NR-400 infrared rejection filter does an
excellent job. Murnaghan also offers an IR exclusion filter. Both are expensive,
but because their transmission is 90% or more across the visible spectrum,
these filters cause little loss of light. Used with an infrared rejection
filter, both the Wratten or Edmund filters give accurate CCD color rendition
of daytime scenes, planetary images, and stellar images.
- With the Cookbook CCD camera, you can shoot one or more images of the
object through each color filter combination, that is, through the red
plus IR rejection, the green plus IR rejection, and the blue plus IR rejection.
For true color, an IR rejection filter must always be used. You will soon
discover that the TC245 has low sensitivity to blue light. To make a blue
image with the same pixel values, you need to integrate at least twice
as long in blue as in red. For this reason, multiple imaging stack-and-track
software like Multi245 is recommended. Blue integrations up to one hour
are quite feasible.
- On the same night you shoot your images, obtain a set of dark frames,
and if possible, make flat field frames also. In an ideal world, you
would shoot separate flat fields for each of the colors because the color
sensitivity of the chip may vary over its surface. If because of time constraints
you can shoot only one flat field, shoot it through the green filter.
- Calibrate each of the images. If you are using CB245 or Multi245,
be sure to use Dark Matching to obtain the lowest possible noise level.
Good calibration is important to avoid sky backgrounds that change in color.
- If you have shot multiple frames in each color, use Multi245 to
register all of the images to a common star, then average the images
taken in each color. This will give you red, green, and blue tricolor frames
with very high signal to noise ratios and already in perfect register.
- For the best color, avoid the non-linear scaling laws. Use stretch
or linear scaling even though this usually entails some loss of highlight
detail. Non-linear scaling laws have high gain at low PVs and low gain
at high PVs, so you get intensely colored noise in the sky background and
weak or washed-out color in the celestial object, which is the opposite
of what you want.
- The color-related functions ask for a saturation parameter. It is tempting
to set this to some value other than 1 (normal color), but resist if you
can. Strong saturation usually looks fake. If you must enhance the
saturation, stick to values under ~1.4.
Hint, Tips,
and Things to Try
QColor is designed to make accurate, honest, astronomical color CCD
images quick and easy to produce. The images resulting from QColor are
rich and full. By departing from the default color, saturation, and gamma
settings, you can create exaggerated and false color images, but doing
so is not the primary purpose or intent of QColor. If you stick with the
default settings and are surprised by the softness and delicacy of color
output from QColor, consider the possibility that you are seeing what the
object really looks like.
The images that QColor produces are 24-bit color TIFF and BMP images
that are 100% compatible with all major graphics, word-processing, and
desktop publishing programs. Use any graphics program to convert them to
GIF, JPEG, TGA, or other format of your choice. LViewPro is recommended
as an outstanding shareware program for making file format conversions.
Quad-Color
Imaging Tutorial
On the distribution diskette you will find sample images of Jupiter,
Saturn, Mars, and M51, plus the three-image set used in this tutorial.
The files are called N2146RED.FTS, N2146GRN.FTS, N2146BLU.FTS, and N2146NOF.FTS.
Al Kelly took these images with his Cookbook 245 camera on his 32-inch
f/4 telescope in Texas. These were taken in 252x242 mode, and they have
already been stack-and-tracked and calibrated.
- Quad-Color Step 1: Load the RGBN set using the LR command. Enter
the path name of the directory where the files are located and the file
names N2146RED.FTS, N2146GRN.FTS, N2146BLU.FTS, and N2146NOF.FTS. The Each
of the files will be loaded and immediately cached as RED.FTS, GRN.FTS,
BLU.FTS, and NOF.FTS. QColor does not access the original files again,
so the original images are never altered.
- Quad-Color Step 2: To correct the aspect ratio, resample the images
to 320 x 240. You can enter any size you wish, but to display correctly,
the sample and line pixel counts must be in a 4:3 ratio. 320x240 is the
default for images taken in 252-wide (.PA/B) mode, and 378x284 is the default
for images taken in 378-wide (.P1/2/3) mode, however, you may find that
378x284 requires too much memory to run. If that is the case, resample
to a smaller size such as 320x240. Each of the images will be resampled
to the new size. Resample Cookbook 211 (.PIX) images to 256 x 256, and
378-wide-mode focus images (.PC) to 215x240.
- Quad-Color Step 3: Equalize the images. This simply means that
you apply the same scaling law to all three images. The default values
are .01, .9999, and 1. It is best to keep the gamma value at 1 for maximum
color fidelity. After equalization, the four images are roughly the same
brightness, and may appear rather dark. Don't worry: this is normal.
- Quad-Color Step 4: Register the images by blinking. Use the
cursor arrow keys to move the small image until the features match. With
these images, the color images are rotated with respect to the no-filter
image. Press the [home] key and enter a rotation value of -2 degrees. The
image will appear on the screen, then rotate, then begin blinking. Recenter
it, then press the [end] key. The red, green, and blue images are registered
to the no-filterimage. When you reach the checking screen, you should see
values for red of 3, 15, and -2; for green of 8, 18, and -2; and for blue,
2, 19, and -2. When you reach the bottom of this screen, the images will
be rotated and shifted.
- Quad-Color Step 5: Register the images by centroiding. Centroiding
is more accurate than blinking. You will see the no-filter image on the
screen. Center the cursor pixel on any star with a clean, round image and
press the [S] key. You will get a readout of the centroid coordinates and
be asked if you wish to continue. If you answer [Y], the red, green, and
blue images will appear on the screen in sequence. In each case, use the
arrow keys to move the cursor to the same star and then press the [S] key.
QColor shifts the red, green, and blue images into precise alignment with
the no-filter image. The shifts will be small: a fraction of a pixel.
- Quad-Color Step 6: Enhance the NOF image. This is where quad-color
really shines: you can enhance the image brightness scale without changing
the color balance. start by loading (LO) the NOF.FTS image. Brighten it
with a gammalog (GL) scaling of 0.01, 0.9999, 0.35, then use the Edit function
(ED) to check pixel values in the image. The sky background is around 270
and the nucleus of the galaxy is around 2500. Stretch scale (ST) the image
using 200 for the low point and 3200 for the high point. Next, use the
power-law unsharp mask (PU) with a radius of 2.5 and a contrast of 2. Finish
the job by saving (SA) the enhanced image as NOF.FTS.
- Quad-Color Step 7: Adjust the color by entering add and
con values for each of the four images, and iterating. This
set of images is close to the right color balance, so the red add
and con will probably be 0 and 1. To make the image
slightly less orange, for the green add and con,
try 10 and 1, and for the blue add and con,
try 20 and 1. The color is pleasingly saturated when the saturation
parameter is 1.3, gamma is 1, and NOF is
1.
- Quad-Color Step 8: View the color image using the VC command.
This produces a version of the image using intense and somewhat posterized
colors. Step across the room to see the effect. It is best to leave the
saturation at 1, and the dithering at a low value such as the default of
.1, but feel free to experiment with higher values to get grainy but more
blended color renditions.
- Quad-Color Step 9: Save the image in color using the SC command.
You have already determined the optimum parameters, so just press return
to accept the values. QColor can save the image as a TIFF file or as a
Bitmap file depending on the extension you give it, that is, whether you
type the name as N2146COL.TIF or N2146COL.BMP. Use whichever format is
more convenient. If your images consistently come out too dark, raise the
gamma value a few tenths and try again, and if they are always too light,
lower the gamma value a bit. Quad-Color Step 10: The finished image
is a 24-bit color image. Convert it to a GIF or JPEG image with a conversion
utility such as LViewPro, import it into word-processing or graphic-arts
software, or print it with one of the wonderful new low-cost ink-jet printers
now on the market.
- Quad-Color Result...and that is how the image N2146QUA.JPG
was made.
- Quad-Color Result...the NOF image used here received an additional
enhancement step: it was Gaussian Unsharp Masked (GU) using a radius of
24 and a contrast of 1.6. This step really pumped up the visibility of
the outer features. N2146ENH.JPG
Tri-Color
Imaging Tutorial
Even if you have a backlog of images shot using the tri-color method,
you can use QColor to improve the quality of the color images you can make
from them. The technique is very similar to that described for making quad-color
images above, except that you substitute the best one of your tri-color
images for the no-filter image, register the other images to it, and then,
after the color images have been palced in accurate alignment, combine
them to produce a synthetic no-filter image.
The best way to learn is to experiment. Since QColor does not alter
the original images, if you mess up something, you can always return to
the original images and try again. When it comes to judging color balance,
experience in the color darkroom is valuable, and you will appreciate the
computer's speed and precision compared to paper and chemistry.
Return to the begining of this document
