AST195 PROJECT

SPRING STARS SEEN THROUGH A TELESCOPE

Stars are so far away that no telescope has sufficient power to show their actual real bodies.

What you see when you look at a star with a telescope is the light from the star, formed into a false shape that is caused partly by the telescope, and partly by the star. You do not see a true image of the star. The details involve the physics of light and are beyond this class, but the following two general statements are true:

1.   the telescope itself determines the shape (round, as the telescope has a round lens or mirror) and general size of the image

2.   the star's brightness determines the exact size of the image (brighter stars look larger than fainter ones).

The theory of this was worked out by the British astronomer George Biddell Airy in the 1830's so his named is attached to this phenomenon.

A telescope forms a round false (or spurious) image of a star, called a “spurious disk” or an “Airy disk”. In the case of a bright star the Airy disk can be surrounded by concentric rings or halos of light. The basic rule is that, the larger the aperture (that is, the main lens or mirror) of the telescope is, the smaller the Airy disk. The Airy disk is sort of like the “pixel size” of the telescope. So telescopes with larger apertures have smaller Airy disks, or smaller pixels, and therefore form sharper, higher-resolution images. On top of this is that the Airy disks of fainter stars appear smaller than those of brighter stars. In modern telescopes the Airy disk is usually so small that you can't see it, so the star just looks like a point of light, like a sparkly diamond.

We will try all this out with our telescopes.

For this project you will need your large and small aperture masks that you made in a previous project. You will need your red light and your clipboard. You will need a sharp pencil and eraser. You will also need to make sure that your finder is in perfect alignment for this. Thus you must complete the “Telescope Purchasing”, “Basics A”, “Basics B”, “Basic Observing Equipment”, and “Aperture Masks” projects prior to attempting this project.

You are to look at three stars:

· Two bright stars that differ in colors as seen with the naked eye -- Sirius (the brightest star in the night sky) and Betelgeuse (in the constellation Orion). Click for an identification chart -- here for colors on black, here for black on white.

· Mizar, the middle star in the handle of the Big Dipper. Mizar is a double star -- a star that looks like it is just one star as seen by the naked eye, but turns out to be two when viewed with the telescope. Note that there is a star close to Mizar, called Alcor, that can be seen with the naked eye. This is not the “double”. Mizar itself is a double. Click for an identification chart -- here for colors on black, here for black on white.

You are also to observe a star cluster or Nebula. Choose either the Pleiades star cluster or the Orion Nebula.  Click here for an identification chart.

Note, if you don’t like these charts, you may also find internet aids or phone apps that will help you find these stars.

Observe Sirius with your telescope at high power. Adjust your focus until the star looks as small and as sharp as you can get it (out of focus stars tend to look like large circles). Draw what you see. Use a colored pencil or note the color on your drawing. Take a photo.

Then observe Sirius with your telescope at high power, with the large aperture mask, and draw what you see (before you take a photo).  Use a colored pencil or note the color on your drawing.  Take a photo.  Do not expect much from the photo – just do the best you can.

Then observe Sirius with your telescope at high power, with the small aperture mask, and draw what you see (before you take a photo).  Use a colored pencil or note the color on your drawing.  Take a photo.  Do not expect much from the photo – just do the best you can.

Do the same for Betelgeuse and Mizar, except taking photos is optional for these (probably only Sirius will be bright enough for your camera to record). You will have nine drawings in all. In some cases, all you may see is a point of light and your drawing will just be a firm pencil dot, or multiple dots if there are multiple stars in your field of view.

Galileo himself observed Mizar with one of his better telescopes, so when you observe Mizar with the large aperture mask on your telescope you will be seeing something very much like what Galileo saw.

Lastly, observe the Pleiades or the Orion Nebula with your telescope at low power, with no aperture mask. Draw what you see.  It is unlikely that you can get a photo of either of these.

Use a drawing sheet for your drawings.

In two typed paragraphs, discuss what you saw. What do your drawings show?

Turn in

· Your ten drawings.

· Your three Sirius photos.

· Your typed paragraphs.

Make sure your name is on everything.