Starry Sky Austin

Light Gathering Power of Telescopes

October 10, 2017 Uncategorized 0

This past June, I was accepted to the Astronomy in Chile Educator Ambassador Program and had the amazing opportunity to travel to Chile to learn about the astronomical research being done there.

ACEAP 2017

Our group of 9 consisted of astronomy writers, amateur astronomers,  astrophotographers, science teachers, astronomy educators and planetarium professionals from across the United States and from Chile. Together, we make up the 2017 ACEAP team. Each of us brought a love of astronomy and an affinity for communicating that love. The trip also served as an opportunity for us to connect with one another and learn from one another; a way to combine our efforts to better spread knowledge of astronomy to the public.  No doubt, we were all looking forward to seeing the southern hemisphere night sky,  learning about the intriguing astronomical research being done in Chile, and most of all, sharing our experiences with our communities when we return.

Something that I experienced that I want to share with everyone is the immense size of the telescopes! The light gathering power they possess is monumental compared to what our eyes can see. It is for this reason that research telescopes keep getting bigger and bigger. The more light they gather, the deeper we can see into our universe’s past.

The most important property is a telescope’s light gathering power. Today’s research telescopes maximize this important property. The larger the aperture (the opening at the top of the telescope tube), the more light the telescope will gather. These large ‘light buckets’ are collecting photons of light. The more photons of light they can gather the better, and the bigger their aperture, the finer detail they can resolve in very distant objects.

To get a feel for what light gathering power means, let’s start with our eye. What is the light gathering power of your pupil? Let’s figure it out!

The light gathering power is proportional to the area of the main mirror of the telescope.  To compare the difference in the light gathering power of our eye to different sizes of telescopes, you calculate the ratio of the areas of their main mirrors (objective lenses).

The mathematical equation reads like so: 





Our human pupil has a maximum diameter of about 8 millimeters  in dim light.

Let’s compare the light gathering power of our human eye to the size of the telescope mirrors we use in my classes.


The Funscope mirrors have a diameter of 76 millimeters.

Reimers 15″

The largest telescope mirror at Reimers Observatory, our 25″,

has a diameter of 635 millimeters.

The Gemini telescope I visited in Chile has a mirror with a diameter of 8000 millimeters.


Plug those numbers into the equation for light gathering power and compare them to the light gathering power of our human eye and this is what you get:

The Funscopes have about  90   times the light gathering power that the human eye.

The largest telescope at Reimers Observatory has  6,300  times the light gathering power than the human eye.

The Gemini telescope has 1,000,000 times more light gathering power than the human eye!

I wanted to introduce the idea of light gathering power to my students and also wanted them to experience what it was like to be in the presence of such large mirrors that can gather that much light, so I made a model of the Gemini telescope mirror, located on Cerro Pachón adjacent to the Cerro Tololo Inter-American Observatory in Chile, to use in my classes:

They really enjoyed it! They were blown away by the size of the mirror. Lots of them asked if we could use this exact mylar emergency blanket version like a telescope. That was a perfect opportunity to explain why we couldn’t and why telescope mirrors need to be precise, smooth  and also not easily moved by the wind, like the mylar version you see here.

I can’t wait to bring this to more programs so people can appreciate the work that goes into attempting to peer into the deepest reaches of our incredible universe!

To learn more about the ACEAP program visit: ACEAP and the Gemini telescope, visit:

#ACEAP #NSFfunded #GeminiObs  #cerrotololo #ALMAObs_esp