· Laptop Computer - Shielded: Naturally this will be a test of the laptop as I use it in the field. I have used a laptop for almost 10 years in the field, even for visual observing. The test will be done using the same laptop and a SkyTools 3 generated chart of M31 with the laptop screen at minimum brightness and covered with a sheet of fitted, dark red acrylic plastic. In addition, the "night vision" mode implemented by SkyTools 3 will be turned on.
· Regular Star Chart/Dim Red Light: A common red LED flashlight (Celestron Item #93588) with the dial set to "minimum" and a chart from Uranometria 2000.0. The light itself will be suspended over the chart about 8 inches (20cm) away from the paper to approximate a typical observer's use in the field.
· Regular Star Chart/Medium Red Light: The same LED flashlight with the dial set to its halfway point and a chart from Uranometria 2000.0. The purpose of the medium light test is to establish a result for a standard red light that is typical of an average observer's use.
· Regular Star Chart/Bright Red Light: The same red LED flashlight and Uranometria chart, this time with the brightness dial set to "maximum".
· The Eye: To provide an objective means of measuring the results I employed a Canon 60D DSLR camera operating at ISO 800 with an 18mm f/3.5 lens. This is in an attempt to approximate the optical specifications of the average human eye without a negative impact on the financial specifications of this particular observer's bank account.
· The Measurement: Each test subject will be photographed in a darkened room in aperture priority mode. By using aperture priority mode, the camera will adjust the length of the exposure based on the amount of light hitting the light meter (center weighted average metering). A shorter exposure indicates a more destructive light source. A longer exposure indicates a more night vision friendly light source.
· The Observing Conditions: Measurements will be taken from two distances. The first will be from 30 inches (76 cm) to approximate the effect of the light source on the observer that might be using it. The second will be from a distance of 10 feet (3 m) to approximate the effect of the light source on a nearby observer.
30 in. Exposure
10 ft Exposure
Dim Red Light
Medium Red Light
Bright Red Light
*FR Factor - Fried Retina Factor
My own subjective thoughts on the experiment:
Dim Red Light: The output created by the light source at this setting would have been unusable for me. Ten years ago I would have been able to use this light with a chart, but there's no way that I could do it now with my aging peepers. I believe that very few observers would be able to use the light at this dim setting.
Shielded Laptop: The laptop with all of the shielding methods in place is a about 1/2 f stop brighter than a dim red light. This is right in line with a typical observers red flashlight that I typically see on the observing field. I did find it interesting that at a distance of 10 feet the laptop appears to be marginally brighter than a red light at medium setting. This is almost certainly due to the fact that it's still a backlit light source.
Medium Red Light: This light setting is a touch brighter than what I see from most observers at a star party and would be too bright for me to use without damaging night vision unnecessarily. I actually added this test after completing the other four in an effort to provide a fairer comparison of real world conditions.
Bright Red Light: This light was really, really bright. I can say that I've never seen an observer use a red light to look at a star chart at this setting. I have used a light at this setting when crossing the observing field to keep an eye out for tripping hazards like dark colored dogs sleeping on the ground or random satellite dishes. Don't laugh. I've found myself face first in the ground because of both.
Figure 1: Comparison photo showing a 3.2 second exposure of a shielded laptop, and a medium brightness red LED flashlight.
In a case of astronomy imitating life, everything old eventually becomes new again. The laptop computer, go-to hand controller and smartphone are simply new expressions of an old issue: proper light control. A properly shielded laptop is no more damaging to night vision than a properly shielded flashlight. Even with the influx of technological advances to the observing field the age old task of maintaining proper dark adaptation is still the same, just in a different form. Without question, there are additional precautions necessary with many laptops. In my case I generally take four steps with my laptop on the observing field to properly shield it for myself and my fellow observers:
1. Red Fitted Acrylic Cover: I cover the laptop screen with a dark red fitted cover and tape it down to prevent any unfiltered light from leaking out of the screen. These can be custom ordered from http://www.tapplastics.com/shop/product.php?pid=519 . I also cover every indicator light on the laptop with electrical tape.
2. Red Light Mode: Most charting/observing programs have a night mode that will turn everything on the screen to a red light type of color scheme in an effort to help. I use SkyTools 3. The red light mode in Sky Tools 3 turns everything on the screen black and all of the relevant text to a deep red. In conjunction with a red plastic cover, the screen is very readable.
3. Adjust Screen Brightness: I adjust the screen brightness to be as dim as possible. These three steps were taken to create the results in these tests on a 16.4" laptop screen.
4. Point the screen to the North: I try to setup so that the laptop screen itself is always pointed to the north on the observing field. Few observers are ever trying to observe an object that is low in the north, so this setup puts my screen out of the line of fire of almost every observer on the field.
I don't expect that this will be the final word on the subject. Astronomers love a lively debate and this will continue to be debated long into the future. I simply hope that my admittedly unscientific experiment has added to the discussion in a positive manner.