Image Link fly out menu (Tools menu)

TheSky6's Image Link uses pattern recognition to precisely align a CCD image, photograph, or Digitized Sky Survey image with the simulated sky. The Image Link fly out menu contains tools that make orienting your CCD images or scanned photographs with the simulated stars in the Virtual Sky simple.

Image Link places a digital image directly over the Virtual Sky, then provides a set of tools to orient the Virtual Sky (by matching star patterns in the Hipparcos/Tycho, Guide Star Catalog, USNO, UCAC or other stellar catalogs) so that objects from the image directly overlay the graphically plotted data.

Image Link even works on images that are mirrored!

The Link Wizard works best on images taken of smaller fields of view (60 arcminutes or smaller).
The image must have at least six well-defined stars to perform an Image Link.
The image should have sufficient signal so that stars are clearly identifiable.

See Image Requirements below for details.

Image Link of the Horsehead Nebula.

Once this alignment is 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, minor planets and nebulae), determine the coordinates of any object, and much more.

Image Requirements

Although the Link Wizard works on a wide variety of images with varying image scales and star densities, the following requirements must be met so that the Link Wizard can successfully find a match.

Image Requirement

Description

8-bit (256 color) Grayscale Image

TheSky6 Edit | Paste command accepts images from the Windows Clipboard that have 8-bit color depth with 256 shades of gray (grayscale) only.

Images that do not use this format must be converted to the correct format using CCDSoft or your favorite image processing software.

One-to-One Aspect Ratio

Images taken with cameras that do not have a 1:1 aspect ratio must be scaled. For example, the Santa Barbara Instrument Group (SBIG) ST-6 camera has 23x27 micron pixels, meaning the pixels are about 15 percent longer in the Y direction.

To compensate for this rectangular pixel, you need to scale the image from 375 pixels in the X direction to 319 pixels. To do this, use CCDSoft or any other image processing application than can re-scale or re-sample and enter a width of 319 pixels and 242 for the width. Copy this new image to the clipboard from the image processing application and paste it into TheSky.

Sufficient Stars

There must be at least six well-defined stars in the image and these stars must also be present in the Guide Star Catalog, UNSO or UCAC databases.

The image may have to be aligned manually if you are working with an extremely long focal length instrument and your fields of view are less than five arcminutes.

White Stars on a Black Background

The image must not be an inverse or negative; that is, one that shows stars are shown black on a white background. If your image does show black stars on a white background, use the Negative command from CCDSoft to reverse the colors.

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 algorithms to differentiate star patterns.

Also, Link Wizard is best suited for images taken of smaller fields of view (less than 60 arcminutes across).

Performing an Image Link

There are three methods to link a digital image to the Virtual Sky.

The Create Digitized Sky Survey command overlays images from the Digitized Sky Survey on the Virtual Sky.
The Link Wizard command uses pattern recognition to automatically align stars in the image with cataloged celestial data.
You can also use the manual alignment tools to manually adjust the orientation of the Virtual Sky.

The following steps are common to both automatic and manual alignment.

Move to the approximate area of the sky where the image was taken. Using the Find or Move To command, adjust the Virtual Sky so that the area of the sky contained in the celestial image is also on the Virtual Sky. This is especially important when using the automatic alignment, since the pattern recognition uses only the objects currently on-screen when searching for a match.
Copy the image to the clipboard using CCDSoft CCD Astronomy Software, or any other application that can paste device-independent bitmaps (DIBs) to the clipboard.
Paste the image onto the Virtual Sky using the Paste command from the Edit menu of TheSky.

Link Wizard: Automatic Image Alignment

TheSky contains a very sophisticated pattern recognition function for aiding the process of aligning pasted images with the underlying stellar data from the Guide Star Catalog. Once a match is found, it computes the necessary rotation, translation and scaling so that the orientation of the Virtual Sky very closely matches that of the image.

Fine-Tuning the Virtual Sky

Use the Rotate Clockwise and Rotate Counterclockwise commands on the Orientation menu to rotate the Virtual Sky in very small increments. Press and hold the Ctrl and Shift keys and click the orientation arrows for fine adjustments up, down, left and right. These options give you fine control for matching the image to the Virtual Sky.

Link Wizard

Once the image is pasted on the Virtual Sky, on the Tools menu, click Image Link, then click Link Wizard to start the pattern search. This involves searching for relationships between the star patterns on the image and the star patterns on the screen. The pattern recognition algorithm has a number of "phases" that attempt to find similar patterns between the stars in the image and the stars in TheSky's core stellar databases.

If all of the requirements of the image and the Virtual Sky position have been met as described above, Link Wizard will find a match in the Phase One. If the Virtual Sky's field width is much larger than the image’s, the search algorithm may require Phases One through Five to recognize the star patterns and establish an image link.

If no link is found, the message "Image Link not found" is displayed in the Status Bar. See the Troubleshooting section below for more information.

IMPORTANT NOTE

The single most important factor in helping the Link Wizard determine the orientation and scale of your image is to set Virtual Sky's field of view to closely match the image’s. Once you have determined your instrument's field of view, you can create and display a Field of View Indicator the represents the field width of your image.

A "close match" means the field width should be 100%-150% of the size of the image. The closer the match, the faster Link Wizard will find the match.

Transparent Image Link of NGC 891

This image of NGC 891 is displayed using the Transparent Image Display option. Note the black star positions from the GSC show "through" the image, indicating a good alignment. Also note the extended labels are difficult to read since the Transparent option "meshes" the screen and image together.

Opaque Image Link of NGC 891

This image of NGC 891 is displayed using the Opaque Image Display option. Note that only the image is displayed and all of the database objects are masked, or hidden by the image. Also note that the extended object labels are now plotted on top of the image rather than merged with the image.

As the Link Wizard looks for patterns, a progress bar indicates that it is looking for matches. If the field of view is close to that of the image, a match should be found in Phase One. Subsequent phases partition the screen and search smaller fields of view. A match is more likely to be found in later phases if the Virtual Sky field of view is significantly larger than that of the image.

When a match is found, the Virtual Sky 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 Virtual Sky and the image. This is important for several of reasons. First, in some cases (although rare) Link Wizard finds an incorrect match. The Transparent option makes this very apparent. Also, you may want to "fine-tune" the position of the Virtual Sky for very accurate matches.

Troubleshooting

If the Link Wizard cannot find a match, check the following:

Make sure you know the field of view of your image and set the Virtual Sky's field of view to slightly larger. Making the Virtual Sky field size larger than the image field size is best. Make sure the location of the image is matches the Virtual Sky.
ST-6 camera users: make sure ST-6 images are scaled down by 15% in the X direction, that is, 319x242 instead of 375x242.

Sometimes it is enjoyable, yet challenging to use the manual alignment on your images even if the Link Wizard is capable of finding the link.

Wide field ST-7 exposure of Cassiopeia and Perseus showing the Double Cluster, constellation lines and grid lines.

Manual Image Alignment

While the image is displayed on the Virtual Sky, you can often see the similarity between database objects and those in the image. Orienting the Virtual Sky to match the positions of the corresponding objects in the image using the standard move, rotate and zoom functions can be difficult. Object Drag feature makes this type of screen adjustments easier.

Object Drag

The position of the Virtual Sky can be adjusted by "dragging" an object from one part of the screen to another. Using this method with two stars, the field of view and orientation can be changed.

Dragging a Single Object

To drag an object on the Virtual Sky (such as a star) to its matching object on the image, press and hold the SHIFT key. Then, using the mouse, drag to its match on the image. You will see a green destination arrow drawn to the new position. To accept this drag, click the left mouse button. To reject it, click 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 an image pasted on the Virtual Sky, 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 (that is, moving the entire screen a specified amount). In addition to translation, two arrows also performs scaling (or zooming) and rotation.