Image Link Overview
What is Image Link?
Image link
is a set of tools which make it easy to identify database objects in your
CCD images or scanned photographs. Image link places a digital image on
the "Virtual Sky," then
provides a set of tools to quickly orient the data from the guide star catalog’s
database so that objects from the image directly overlay the graphically plotted data.
Once this alignment has been achieved, your images become interactive star charts rather
than static pixel data. In your image you can label objects, add grid lines, identify
objects (such as galaxies, asteroids and nebulas), determine the coordinates of any
object, and much more.
There are two basic methods for establishing a link between a digital image and the TheSky Display. The Link
Wizard uses stellar pattern recognition to automatically align the image with
cataloged celestial data. "Object Dragging" provides a tools for manually adjusting
the Sky Display to match the image.
The following steps are common to both automatic and manual
alignment:
-
Current Sky Display (zoom level and
center of field). This is especially important when using the
automatic alignment function since the pattern recognition uses the objects present on the
display when searching for a match.
-
Copy the image to the clipboard using CCDSoft image processing
software. Image Link uses CCDSoft
image processing software to copy the image to the Windows clipboard.
Paste the image onto the Sky Display
using the Paste Image command in the Edit menu of TheSky. This image is copied as an 8-bit BMP.
TheSky contains a very sophisticated pattern recognition function for aiding the
process of aligning pasted images (Edit | Paste using Window's Clipboard) with the underlying stellar data from the
UCAC2. USNO-B1, Hipparcos/Tycho and Guide Star
Catalog data.
The image alignment function has proved successful on hundreds of different
images taken with various CCD sensors with fields of view from 8 arc minutes to over 30
arcminutes. Once a match is found, it computes the necessary rotation, translation and
scaling so that the sky data and coordinates match that of the image.
Although the Alignment Wizard will work on a wide variety of images
with varying image scales and star densities, there are certain requirements which must be
met if it is to be successful.
One-to-One Aspect Ratio
CCD cameras which do not have a 1:1 aspect ratio must be scaled so that they do. For
example, the ST-6 camera has pixels which are 23x27 microns so they are about 15% longer
in the Y direction. To compensate, you need to scale the image from 375 pixels in the X
direction to 320 pixels. Choose the Re- sample command from the Modify menu in CCDSoft to re-scale your image.
Sufficient Stars
There must be at least six or more well defined stars in the image which are also part of
the GSC database. Since most imagery covers fields of 10 or more arc-minutes, and
the GSC goes to nearly 15th magnitude in sparse areas of the sky, this requirement is
usually easy to meet. If you are working with an extremely long focal length
instrument and your fields of view are a couple of arc minutes or less, then you will
probably have to manually align the image. The latest version of TheSky can also use the stars
from the USNO
version 1.0 or 2.0. For details on accessing the USNO version 1.0 or 2.0 from
within TheSky please see
the following page TheSky and
the USNO data.
White Stars on a Black Background
The image must not be an inverse or negative image which shows stars as small pixels
values and the background as high pixel values.
Image Size
The image used for the link should be at least 100 pixels on a side, preferably larger.
Smaller images make it difficult for the pattern recognition functions to differentiate
patterns.
Using the Link Wizard
Once the image is displayed on the screen as described above, choose Tools Image Link to
display the image link dialog. The various controls are explained below.
The Image Link Dialog
To display this dialog, choose Tools Image Link. This dialog contains commands for
changing the display status of the digital image, performing small (single pixel)
rotations and executing the Link Wizard.
The following image status buttons control how the image is
shown on the Sky Display.
Image Link Setup
Image Status
Enter your image scale if you know it. If
unsure of your optical system's image scale, you can leave it blank and let Image Link attempt to find it for you.
Or, better yet
let TheSky6 compute your image size and scale
for you.
Not Displayed
Choose this button to erase the image from the Sky Display. It is important to note that
the image will still be present in memory belonging to TheSky (i.e. not just in the
clipboard) so that it may be turned on later even if the clipboard contents are changed.
Choose Transparent to display the image on the Sky Display while allowing the data
(stellar and non-stellar) to show through the image. This is very useful when manually
aligning an image and trying to see the relationship between sky data and the actual
image. You can toggle between Not Displayed and Transparent quickly without redrawing the
Sky Display.
This image of the Trifid Nebula is displayed using the Transparent display mode. Note the
black star positions from the Guide Star Catalog, Hipparcos/Tycho and USNO (if present)
showing through the imaged stars indicating a good alignment between the two. Also note
the extended labels are difficult to read since the Transparent mode “meshes”
the screen and image together.
Once you have achieved a “link” between the image and the Sky Display, you may
want to see the image on the screen undistorted by the data base stars showing through.
Choose Opaque to display the image “on top” of catalog data. All labels and grid
lines (if present) will be drawn on the image but the corresponding stars and non-stellar
object data will be hidden. This allows you to have your image on screen uncluttered by
the celestial data, yet still be able to identify, add labels and lines!
This image of the Trifid Nebula is displayed using the
Opaque display mode. Note the only stellar objects showing are those from the image since
the image “hides” all of the database objects in this mode. Also note that the
extended object labels are now plotted on top of the image rather than merged with it.
Getting the Scale from your Image
After Image Link has found a match you can
get detailed information about the image. This includes the exact coordinates of the
image, the scale of the image, and the rotation of the image. Very useful
information. In fact, CCDSoft will use this data along with the catalogs present to
perform accurate automatic plate constant astrometry.
Using TheSky6 you can also
compute your image size and scale provided they are already on the
list supplied OR provided you have the necessary information, i.e. focal
length, apparent field of view, f-number, CCD detector size, etc.
CW and CCW Rotation Buttons
These buttons are used to rotate the Sky Display in very small increments, approximately
one pixel of motion near the edge of the image. See Fine Tuning the Sky Display below for
more information on adjusting the Sky Display to match exactly the displayed image.
Whenever the Image Link Dialog is displayed, all of the motion buttons and zoom buttons
cause the sky display to move in single pixel movements rather than 10% of the screen size
as it normally does. Additionally, the rotate buttons on this dialog box permit single
pixel rotation. This permits fine tuning the position of the background Sky Display to
achieve a very accurate match between image and data.
Link Wizard
Choosing Link Wizard from the Image Link dialog initiates the pattern recognition
algorithm which searches for relationships between the star patterns on the image and the
star patterns on the screen. The search algorithm has a number of phases which attempt to
find a match.
If all of the requirements of the image and the Sky
Display position have been met as described above, Link Wizard should find a match in the
first phase of looking. If the area of the sky is not known very well and the Sky Display
shows considerably more of the sky than the image, it may require all phases to establish
a link.
The single most important factor in helping the Alignment
Wizard determine the orientation and scale of your image is setting the screen size so
that it closely matches that of the image. Once you have determined this, you can set an
eyepiece indicator so that it is not necessary to reset each time you use alignment
feature. “Closely matches” means it should be within 50% of the size of the
image. The closer the match, the quicker Link Wizard will find the match.
As the Alignment Wizard looks for patterns, a progress
bar indicates that it is looking for matches. If it is not found on phase one, the chances
are reduced that it will find a match. The subsequent phases partition the screen and look
in smaller “chunks” so they are more likely to find matches if the Sky Display
field size is significantly larger than that of the image.
When a match is found, the Sky Display is adjusted to
match the image. The image display mode is set to Transparent so that it is easy to see
the relationship between the Sky Display and the image. This is important for a couple of
reasons. First, in some cases (although rare) Link Wizard will think it has found a match
when it really did not. Transparent mode makes it easy to see if it found an
incorrect match. Second, you may
want to “fine-tune” the position of the Sky Display for very accurate matches.
Here is a working example image taken by me
using the Paramount in Chile. The object is the Trifid Nebula NGC6514. The CCD
camera is the Apogee AP-7 512 by 512 array. The scale is 1.9 arc
seconds/pixel and the field of view is 16.2 arc minutes square. You
can also have TheSky add a field of view indicator
based on the above size.
Example Image: Trifid Nebula
Right Mouse Click on the above image and Save
it to the hard disk and load it OR better yet just Right Mouse Click and
choose Copy it to the Windows Clipboard (Copy) then Edit | Paste
it into TheSky and attempt an Image Link.
The size of the image is approximately 16
arcminutes square. Zoom to a 20 arcminutes field in TheSky first centered on
the Trifid nebula NGC6514 first then attempt the link.
Here is the proper scale and position angle.
Best to try a known working image first before trying your own.
Using the above example image
Find and center on the Trifid nebula in
TheSky. Edit | Find Trifid and center the field. Zoom to 20 arc
minutes using Edit | "Find Z20m". This means Zoom absolute to 20
arcminutes.
Next Edit | Paste the above image taken from
the Clipboard (Right Mouse Click image and Copy) into TheSky.
Note: Rotation of TheSky display
does not matter! Only the center of the field and zoom level are
important! Remember you must be able to see the entire field first!
If you are zoomed in too far Image Link will fail!
Center on the Trifid Nebula first
(Edit | Find Trifid)
Zoom to approximate field of view
(16-20 arcminutes)
Once you have centered TheSky Display on the
object and set the field of view accordingly you can start the Image Link. Tools | Image Link | Link Wizard. Or use the Icon on the Image Link
Tool Bar.
Launch the Wizard!
Link Wizard Tool Bar
After a few seconds you should see the following.
Image Link Match!
Note Scale and Position Angle!
NOTE: Always make sure the scale and
north angle make sense.
Image is now linked in TheSky!
The color Malin AAO Photograph has been disabled
for clarity!
NOTE: The stars from TheSky6 are showing through the CCD image above
causing the "holes" in the stars.
Image Link Information
Click anywhere within the
Image Linked image to display the Object Information
dialog.
Image Link Information
Mouse Click Anywhere!
NOTE! The
Image Link Information is not the
first entry in the Object Information dialog. Drop down the combo box and
click on the entry titled Image Link Information.
Object Information
Drop down list to
Image Link Information
In the Object information dialog note the
name is Image Link Information.
Object name: Image Link
Information
Equatorial: RA: 18h 02m 43s Dec: -23°03'00"(current)
Equatorial 2000: RA: 18h 02m 23s Dec: -23°02'52"
Horizon: Azim: 25°57'31" Alt: -71°47'48"
Visibility: Rise 21:16, Set 06:39
Transit time: 01:56
Position angle: 01°07'
Hour angle: -11h 25m 28s
Screen x,y: 615.00, 439.00
Sidereal time: 06:37
Click distance: 500.0000
Scale: 1.9046 <-- Image Scale in arcseconds per
pixel
Height: 16.2051
Width: 16.2580
If you are unsure of your image scale use the
Unknown scale option. That is better then entering the WRONG scale
which makes things worse garbage in garbage out! In this above example you can use the known scale of
1.9 arcseconds/pixel but you don't have to, Image Lin can find it. Either way.
Best to compute your scale using TheSky's Field of
View Indicators.
Tools | Image Link |
Setup
Choose Unknown scale if not sure
If the Link Wizard cannot find a match when you think that it should, check the following:
-
Make sure that ST-6 images are
re-sampled down by 15% in the X direction i.e. 319x242 instead of
375x242 (square). BETTER yet.
Get the EVENT to resample ST-6 Images automatically!
-
Make sure you know your field of view
and the screen size is set properly. It is better to make the
Sky Display field slightly larger than the image field size.
-
Make sure the CCD area you have imaged is contained on the Virtual Sky.
-
Try the
example image provided above that will just work
Techniques for Manual Alignment
While the image is displayed on
TheSky Display you can often see the correspondence
between data base objects and those in the image. It can be difficult to orient the Sky
Display to match the positions of the corresponding objects in the image using the
standard move, rotate and zoom functions. The Object Drag functions make this type of
screen adjustments easier.
We have encountered a number of images which were taken
at a very small scale (i.e. the field width is a few arc-minutes across) which had too few
corresponding stars in the GSC. As long as there is an easily identifiable object such as
a galaxy or relatively bright star, manual alignment becomes easy using the Drag functions
and a bit of practice.
Orthographic Projection
The first step in performing a manual alignment is to choose the Orthographic option under
the Options menu. When the Orthographic option is checked, an orthographic projection is
used to plot all of the celestial objects. This projection proves to be the best
projection for matching the actual geometry of CCD images. If you attempt a manual
alignment without this option set, you will be able to get stars to match in some parts of
the image and not others. This option is automatically set when Link Wizard is used.
The orthographic projection is slower than the default
projection used by TheSky. Therefore it is usually preferable to turn this option off
(i.e. unchecked).
Dragging a Single Object
To drag an object (such as a star) to a corresponding position on the displayed image,
press and hold the shift key, then using the mouse, drag from the object to its new
desired position. You will see a green “destination arrow” drawn on the screen
to the new position. To accept this “drag”, press the left mouse button. To
reject it, press the right mouse button.
Dragging Two Objects
If you can visually recognize where two objects should be moved so that they are in line
with image pasted on the Sky Display, then you can add two “destination arrows”.
Using two arrows is completely different from using a single arrow. A single arrow is used
for translation purposes only (i.e. moving the entire screen a specified amount). In
addition to translation, two arrows also performs scaling (or zooming) and rotation.
For those bold enough. Here is a
sample VB script that will compute the scale in arcseconds/pixel for you. You need only know the size of the pixels found in the camera and your
telescope information. Save the following text with the extension VBS
then run the script by double clicking on it.
NOTE: Feel free to use the
script anyway you like or even make changes. The script is provided as
is with no warranty or guarantees. If you wish to report problems have
concerns or questions about it email
me. There are other similar programs available for this
on the web.
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