INTERNATIONAL PLANETARIUM SOCIETY
ALL-SKY STANDARDS COMMITTEE
PROPOSED SPECIFICATIONS OF ALL-SKY PROJECTION EQUIPMENT
First Draft: September 18, 1990
PURPOSE
The purpose is to establish a standard set of guidelines for All-Sky Projection systems. This standard will allow users to exchange All-Sky slides with a minimum of modification.
PROJECTOR ALIGNMENT
Many of the sixty planetaria which use All-Sky Systems will not be able to modify their projectors to any great degree. However, some of the basic factors, such as projector height and alignment, can indeed be standardized. New facilities can follow these guidelines to ensure compatibility:
The six projectors should project on-axis and be equally spaced sixty degrees apart at the base of the dome, with an accuracy of 0.5 degrees.
All-Sky projectors should be numbered progressively clockwise.
The "number one" projector should project to exactly due south.
When projected, the center of a standard ASC (All-Sky Standards Committee) alignment grid slide should correspond to a forty-five degree altitude on the surface of the dome.
The vertical center line of the projected All-Sky grid should be parallel with the azimuth lines.
The front of the projection lens should be even with the surface of the dome or as close as possible.
The projection lens should be as close to the spring line as possible.
The standard slide mount will be a Wess #2.
The IPS Standard Alignment Grid will be produced by the President's All-Sky Standards Committee. Instructions for sizing the grid will be provided to users who use a non-standard projection lens size.
The projection system should use a matched set of 35mm lenses.
PROJECTOR NAMING
A set of guidelines for the naming of projectors will help in identifying the position of the projectors:
The first or primary projection bank (A) should project directly across the theater diameter.
The second projection bank (B) should project from the immediate right of the primary projection bank (A).
The third projection bank (C) should project from the immediate left of the primary projection bank (A).
If the projectors are to be stacked vertically, the upper projector will be considered the primary bank (A).
In essence, this standard allows for one projection bank to be in the best position as a standard and allows for growth in the number of projection banks.
The alignment grids projected by the second and third banks will align directly to the primary grid.
The format of the All-Sky grid will be in the shape of the pie wedge.
The size and location of the grid within the slide mount will be described in the following ways:
The correct three corner points of the All-Sky grid will be provided on a pin-registered Oxberry standard grid.
The correct three corner points of the All-Sky grid will also be provided as locations within the Wess #2 glass slide mount.
PHOTOGRAPHY
Different photographers have had success using different techniques. These guidelines represent a loose consensus of what works in most planetarium domes:
The camera tilt should be 45 degrees from the rotational plane.
Some people have had remarkable success pointing the camera much lower than forty-five degrees.
The lens should be the equivalent of a Nikon 16mm full frame fisheye or a Nikon 15mm flat field lens.
SOFTWARE
Now that All-Skies can be produced and processed using computers, All-Sky images can be exchanged using tape backups or floppy disks. Although there are dozens of graphic file formats currently in use, the Standards Committee has decided to adopt a few of the most popular (and readable) formats.
TARGA FILE FORMAT (TGA):
The images should be uncompressed in either 16 bit, 24 bit or 32 bit. Resolutions can range from 512 x 400 pixels to 4k x 4k pixels. Thirty-two bit images should only be used when an alpha channel is necessary.
TAGGED IMAGE FILE FORMAT (TIFF):
The images should be uncompressed in either 16 bit, 24 bit or 32 bit. Resolutions can range from 512 x 400 pixels to 4k x 4k pixels.
ADOBE PHOTOSHOP FILE FORMAT:
If a Macintosh is used for image processing, the Adobe Photoshop format should be used. As with the TGA and TIFF file formats, the resolutions can range from screen width to 4K x 4K pixels, and the color depth can range from 16 bits to 32 bits.
COMMITTEE MEMBERS
Several individuals contributed to the first draft of specifications. Requests for information can be sent to the following individuals.
DENNIS MILBRANDT
Calgary Centennial Planetarium
P.O. Box 2100, Stn. M., Location 73
Calgary, Alberta T2P 2M5
CANADA
TOM WUJEC
McLaughlin Planetarium
100 Queen's Park
Toronto, Ontario M5S 2C6
CANADA
Sky-Skan, Inc.
51 Lake Street
Nashua, NH 03060
USA