Saturday, December 3, 2011

Cardboard Telescope (Kepler)

• 3'' dia. x 8'' cardboard tube
• 1.5'' dia. carbboard tube, one 8'' length (the eyepiece tube) and one 1.25'' length (the eye piece cuff)
• 2.9'' plastic meniscus objective lens (we get ours from, see CAT# lns-16)
• 19.5mm DCX eyepiece lens (we get ours from, see item # 68095 )
• 3-6 foamcore spacers 3/4''x3/4'' (we used the scraps from the tube spacer fins)
• 2''x 2'' mat board or foamcore eye piece holder (black works the best as it doesn't reflect light)
• 3 foamcore spacer fins, two 3/4'' x 6'' and one 3/4'' by 8.5'' (making the fins a bit larger than 3/4'' wide is a good idea as you can always make them smaller and a tight fit is desirable)

For prep:
• Utility knife
• Straight edge
• Optional: a band saw, makes quick work of cutting the tubes to size, a 3/4 leather punch can be used to put the hole in the eyepiece lens holder quickly

For assembly:
• Scissors
• Hot glue gun

The first step is to place and secure the objective lens in its tube.  In order to do this you need to make a shelf for the lens to sit on about an inch from the top of the tube.  In the photo above we glued 2 short length of 3'' tube with slices cut out of them on top of each other to form a shelf, but gluing 3 or 4 3/4''x3/4'' pieces of foamcore at the same level works just as well.

To secure the lens hot glue at least 3 more small foamcore pieces above the lens to prevent it from falling out.

Next hot glue the eyepiece lens on to its holder being careful not to get any glue on the center of the lens.

Add some hot glue to the bottom of the eyepiece tube and glue it onto the eyepiece lens holder with the lens centered in the tube. It does not optically matter if you put the lens inside or outside of the tube, but putting the lens on the inside allows you to place the eyepiece holder flat on a table and then look down the eyepiece tube to center the lens. Once the lens is firmly in place cut off the extra mat board material the sticks out around the tube.

To finish the eyepiece tube glue the eyepiece cuff over the eyepiece holder.  The eyepiece cuff functions  to position your eye the correct distance from the lens in order to see through the telescope easily. The length of the eyepiece cuff is specific to the lenses we used and may need to be adjusted to accommodate
lenses with different focal lengths if the same lenses are not available.

Glue on the spacer fins distributed equally around the tube like so:

Test fit the eyepiece tube in the objective tube either add tape to make them larger or rub the fins on the edge of a table to shorten them until they fit snugly. You want the tubes to be able to slide in and out with with some resistance, but do not lock up.

When you're happy with the fit use you scissors to punch a hole in the back of the objective tube and the end of the long spacer fin and tie them together with yarn to prevent them from being separated.

Things to do and notice:
The two most obvious things you'll notice is that everything is bigger, and it's all upside down and backward. Kepler's refracting telescope works by collecting a lot of light with the objective lens focusing it to create an image at the back of the telescope tube and then focusing the eyepiece on that image to magnify the the middle of it. The degree of magnification depends on the ratio of the objective lens' focal length to the eyepiece lens' focal length. In the case of this telescope the objective has a focal length of about 290mm and the eyepiece has a focal length of 66mm giving a magnification of about 4.4X. Note that for the image to come into focus the objective lens must be about 356mm (14'') from the eyepiece lens, so the telescope will not appear to work at all when the eyepiece tube is fully retracted into the objective tube. Because you want to keep the sliding action pretty stiff you may want to mark the approximate focal length on the long fin when you find it for ease of use in the future.
In general the light path of a Kepler style telescope will look like this:

        One of the consequences of this design is that the image is inverted as the light paths cross at the image plane. While this is not particularly desirable for looking at terrestrial objects, it was a great improvement over the previous Galilean Telescope. Galileo's  telescope could only achieve small magnification values without significant distortion, but did not invert the image as it used a positive and a negative lens.  The clear but inverted image of Kepler's telescope (with two positive lenses) is good for looking at things like the moon, however it can be a challenge to look at things on the ground. Panning right looks like panning left, similar to working on something while looking through a mirror.

Another interesting effect you can sometimes notice when looking at brightly lit objects is chromatic aberration: a thin line of rainbow will appear around the object. This occurs because lenses produces images by bending light to a point of convergence, however not all colors bend at the same rate so a rainbow will appear on the edges of the images where the colors do not overlap.      

Safety Note:      
A telescope is a device for collecting and concentrating light, looking at the sun or any other extremely bright light source may potentially cause serious damage to your vision, as it will focus the light much like a magnifying glass on your retina. So have fun but be careful.