Why Do Objects Look Brighter in Your Telescope Then with Your Naked Eye?
This was a question asked at a recent astronomy club meeting which I attended. An interesting question and one I haven’t thought about for a long time. Our eyes can see vast distances unless, of course, we suffer from one of the many eye conditions that inhibit our eyes ability to work correctly. Then we have to wear corrective lens, like I do to see well but, we still can see vast distances. Take for instance M31, the Andromeda galaxy, some 2.5 million light years away. Under a dark sky away from light pollution, the Andromeda galaxy appears as a smudge of light. But, as massive as the Andromeda galaxy is, it still appears as a small point of light to our eyes.
So, it makes sense that an object further away is harder to see than one that is closer to us. The further away an object is, less of its light reaches our eyes. It simply takes up less space on our eyes retina. Your retina is the light sensitive tissue at the back of our eyes. This makes the object smaller and the details harder to see. Figure 1 below, shows how light is transferred through the human eye to the retina.
Figure 1. How light is transferred through the human eye.
If we want to make a distant object appear brighter, we need a bigger eye to collect more light. More light, of course, makes the object appear brighter and we can magnify that light so that it takes up more space on our retina. So, with a telescope, the objective lens (the big lens) collects more light from a distant object than our eyes can and focuses that light to a point inside the telescope called the focal point. The eyepiece (smaller lens), magnifies the light from the focal point so that more light hits your retina.
The amount of light a telescope collects is dependent on the size of the objective lens which gathers and focuses light from a very narrow section of the sky. The eyepiece then magnifies that light like a magnifying glass magnifies the words on a page, making the object appear brighter.