Human CCD
Topic/Concept: Astronomical imaging, CCDs (basic functions and imperfectios)
Type of Activity: large group, get up out of your chairs!
Prerequisite knowledge required: light is a particle (photon)
Resources required: LOTS of confetti (at least a garbage bag full). A bucket for each student (a large yogurt container would work). A tarp. A large sheet of paper that represents the CCD image. Some extra people to help, that can be ‘planted’ in the CCD as faulty pixels and who can help read out the CCD.
Learning Objectives: Basic understanding of how a CCD works (grid of light buckets that read out to the computer). Introduction to a point-spread function. Awareness of imperfections in data that can affect astronomers.
Common misconceptions:I wonder if they understand that a CCD is a lot like film, just electronic and better? It would probably be good to compare imaging progress in astronomy to that of their personal cameras—we started off using photographic plates, but CCDs are better for a score of reasons, so now we use them. Just to give it a bit more context.
Also, a lot of people think that the secondary mirror will create a dark spot in the center of the image. This activity does not deal with this misconception directly, but maybe it should?
Detailed description of activity: Put the tarp down, and have all the students stand on it in even, equally-sized rows. You want them to look like a grid!! We had 15 kids + 1 teacher as a ‘planted’ pixel, so we had 4 rows of 4. Give each student a bucket, and have them hold it above their heads. One teacher now stands on a desk, above the students heads, and throws confetti down at them into their buckets. This is messy, but that’s ok! The first time we had a simple image, just a star. The confetti teacher aimed most of the confetti at one student, but some confetti purposely got into adjacent buckets to represent a point-spread function.
When the teacher is done raining down the confetti, say something like “Telescope shutter closing”. Now it is time to read out. The teacher in charge of reading out should have prepared the large sheet of paper to mimic the setup of the student pixels—so on our sheet, we had four rows of four squares. The read-out teacher should have a bucket. Run to the nearest student and tell them to pour their confetti into your bucket. Go dump this confetti onto the square representing this student’s pixel. Return to the nearest student, and have all the students in that student’s row pass their confetti down, so that now, the nearest student’s bucket is again full. Go empty the bucket into the squre that corresponds to the origin of this confetti. Continue this ‘bucket brigade’. When all the confetti has been emptied from this row, have all the rows pass their confetti down to their adjacent row, so that now the emptied row is again replenished, and you can repeat the process.
When all the students’ buckets have been emptied, talk about the image, and have them guess what the confetti teacher was trying to make. Don’t forget to draw their attention to the confetti that was lost during all those bucket transfers! This is “charge transfer efficiency”.
Repeat this process several times, each time focusing on some new elements:
-bias (a teacher runs around and puts some confetti in everyone’s basket before the confetti teacher starts throwing it)
-sky (the confetti teacher makes sure everyone gets some confetti, and after you read out, you can try to subtract this)
-bad pixel (the planted-pixel teacher has a hole in the bottom of their bucket, so that no confetti is retained)
-hot pixel (the planted-pixel teacher has a bag of confetti in her pocket, and she fills up her bucket ‘illegally’)
-saturated star (the confetti teacher rains so much confetti down on the planted-pixel teacher that her bucket starts to overflow, and the planted-pixel teacher throws the overflow into adjacent buckets, while drawing attention to this by saying things like “Oh no! I have too much light! You have to take some!!!”)
-cosmic ray (a teacher with a bag of confetti attacks an innocent student, filling up their bucket entirely)
-galaxy (the confetti teacher can try her very hardest to make complex shapes. We tried to make a little spiral, we failed, but the kids were very understanding)
After the first couple times, we no longer did the whole reading-out process, because it was pretty time consuming. We just had all the students come and dump out their own confetti onto the appropriate square.
After an image is all constructed, be sure to discuss with the students about what happened!
Assessment: None, really. If you have to actually use CCDs with your students in the future, you’ll probably be able to tell if they have a basic understanding of the images, or not.
Associated files and images:
Comments: This is an activity created by the COSMOS staff at University of California—Santa Cruz/Center for Adaptive Optics. We taught it to 15 high school kids, in about an hour.
The students LOVED this activity. It was an excellent change from lecture, and actually a really awesome community builder. I’m not really sure how deep of an understanding it gave them of astronomical images—I think it was a great overview, but when we actually sat down and started talking about bias frames, flat fields, etc, they definitely had a really hard time still. However, I think they had the basics DOWN—that CCDs are light buckets, etc.
I would definitely recommend this activity for any age group! It was great fun! And the huge amount of confetti isn’t actually that much of a problem—you should be able to collect the vast majority of it at the end, and reuse it (use a tarp, really! this makes clean up so much easier). I think the biggest problem is, this activity really does require three teachers.