There are many factors that can effect the quality of a telescope session: clouds, turbulence, haze, light pollution, moonlight, and so on. Most of these the astronomer cannot do anything to change, but one of them – night adapted vision, you do at least have some control over. You cannot control everything your eyes do, but you can help them a lot.
Many beginning astronomers often make a common mistake of going from a brightly lit house with their telescope out into the darks skies and start viewing through their telescope immediately. This can be a bit unsatisfying if you are looking at dim deep space objects. The will be extremely faint simply because your eyes are not adapted to the darkness. This isn’t just about the pupils of your eyes dialating, either.
Harken back to your high school biology class. You may recall that the human eye has two light-sensing parts (known as photoreceptors): The rods and the cones. What they may not have mentioned is that most of the time, in modern civilization at least, the cones do most of the work. When you look around or at this screen you see a wide range of colors – the entire visible spectrum is seen via the cones (barring genetic issues like color-blindness, etc). During the daylight hours, or around any light at all the cones are picking up the light and sending the information to your brain.
By comparison the rods seem a bit lazy. But in fact they are quite necessary for getting around at nighttime. Rods are very sensitive to low light levels and don’t work when there is a lot of light around. They also do not sense color. This is why when you walk around at night you might notice things seem a lot less colorful and more in shades of gray. The maxim of ‘at night, all cats are gray’ is a demonstration of rods doing their job. Trouble is, it takes some time for your eyes to adapt to the darkness. In some cases it may take as much as 30 minutes to get them up an running and getting any bright light during that process will put them down again and you will have to start over.
This does cause some issues as you might expect as while you could possibly set up your telescope with night adapted vision you really cannot read a star chart or planisphere with them, nor check on dials on an equatorial mount. Even reading the focal lengths on eyepieces can be troublesome. Turning on an ordinary flashlight will ruin all that time it took to wake up the rods in your eyes. So you can see why astronomers get annoyed with people who shine flashlights at them or drive cars with active headlights onto where they are viewing.
There is a way around this: The Rods have a blind spot in the red portion of the visible spectrum. Somewhere around 610-620nm the rods ‘cut out’ and light in that part of the spectrum is unseen by them. Hence a light that shines on the far red end of the visible spectrum will not agitate the rods and you can keep your night vision.
This is where red flashlights come in. In the past astronomers had to take some fairly drastic methods to get red light. Tricks like painting the lens of a flashlight with red
fingernail polish or using a pieces red cellophane taped over the end were common tricks. But they did have limitations that made them less than ideal. Nail polish could ruin a flashlight, cellophane probably did not block all the light it should and all versions of this were clunky.
Telescope manufacturers started producing red flashlights specifically designed for the job. The early models were usually just lower-powered incandescent bulbs with red filters. Often these were not ideal solutions but anything more effective would be very costly.
LED flashlights helped the field a lot. Now low-powered LEDs could be used with light in the proper part of the spectrum.
But that brings the question: Do I need a red flashlight?
The answer as is often the case is: it depends. Ask yourself the following questions:
1) Are their any other direct light sources at your viewing sight that you cannot avoid?
2) Do you have light pollution even if there are not in direct view of any other lights?
3) Are you viewing bright objects such as the Moon, Jupiter, Saturn, Venus or Mars?
If the answer to any of these questions is ‘yes’ then you probably do not need a red flashlight.
That being said, you should still probably use one unless you have an unavoidable beam of light pointing at your viewing area (Hey, it happens – sometimes you just have to make do). The red flashlight is dim enough that it will not make your pupils constrict, which while not entirely related to your Rod-based night adaptation is still important. Plus it is simply a good habit to get into.
So, you figure, I should have a red flashlight? You might have to resist the urge to go online and buy the first shopping result that come sup when you enter “RED LED FLASHLIGHT” and instead purchase a model made by an astronomy oriented company. Why? Here are the reasons:
1) Colored flashlight makers use LEDs that are too powerful – You will often see this in the specifications of the flashlight (if it has any). Things like ‘Superbright LED) and other terms mean that the manufacturer chose the most powerful LED they could get for the price point. They aren’t concerned with low-power because the people buying these are probably not using them for astronomy, but other lighting needs.
2) They probably have the wrong wavelengths – A cheap LED flashlight probably uses a bulb that does not filter out light on the other parts of the spectrum. It will still look red, because most it is still mostly red light, but there can be a lot of light in other part of the spectrum coming out that can put your rods to sleep.
3) No adjustability: Sometimes you really need to turn your red flashlight down low, and other times you need to crank it up a bit. Many flashlight made by astronomy companies have the ability to adjust the brightness level. Having this flexibility can help for there is a definite difference between the light you need for setting up a telescope and just reading a star chart.
So stick with the tools meant for the job. Get a red astronomy flashlight made by an astronomy company.
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