Nearly all the information we obtain from the cosmos is from telescopes. The first telescope invented for scientific purposes was invented by Galileo almost 400 years ago, a refractor telescope consisting of two lenses. Through this device Galileo observed the moons of Jupiter and the surface features of Earth’s moon. This began a remarkable journey in astronomy toward understanding the universe.
Reflecting telescopes were soon to follow. Improvements in manufacturing and design has led to a wide array of amazingly powerful telescopes we have today, used for studying objects in non-visible parts of the spectrum (x-rays, infrared, etc.) in addition to visible light. CCD cameras and spectrographs allow us to study planet formation, discover exoplanets, the surface of the Sun, activity near black holes, and countless other phenomena.
Most professional telescopes today are reflection telescopes, more specifically of the Cassegrain-Schmidt variety because of the ability to mount additional research equipment at the end of the light path. In fact, many new telescopes built these days are not even designed to have an eyepiece!
Activity
We will be using the “oculars” plug-in in Stellarium to take a tour of the night sky using combinations of three different telescopes and three different lenses. Stellarium has a rather large database of images corresponding to known celestial objects, which allows us to simulate the magnification and field of view for various combinations.
One may assume that more magnification makes for better viewing, but that is only partially true. In many cases, the field of view from the primary mirror/lens in combination with the eyepiece might not capture the entire celestial object. Additionally, greater magnification can sometimes produce blurry images. For an amateur astronomer, or an astrophotographer, the more options for telescopes and eyepieces, the more likely they are to get an image that is just right. The magnification can be calculated using the following equation:
, (1)
where fo is the focal length of the objective lens/mirror and fe is the focal length of the eyepiece. Note that an objective lens/mirror with a larger focal length will result in a greater magnification, and an eyepiece with a smaller focal length will result in a greater magnification. Magnification is a unitless number and is often written with a times symbol. For example, a magnification that doubles the size of an object (M = 2) will be written “x 2” or “2 x” to indicate the object looks twice as big.
In this lab, we are the amateur astronomer with our backyard telescopes, but we will happen to have access to the Cassegrain 24.5” telescope housed at Golendale Observatory, located in Goldendale, Washington.
Procedure
Follow the steps outlined below. Note: if you are using a school computer, another student may have already set this up, so you’ll be in luck. However, make sure you check the information they entered because people make mistakes.
Step 1 – Open Stellarium. On the left menu there is a an icon that looks like a wrench, click this to open the configuration window. Click on the “Plugins” tab. There are all sorts of cool plugins you can may want to explore on your own time for fun. Find the “Oculars” plugin from the list. Check the box that says “Load at Startup,” then close Stellarium.
Step 2 – Open Stellarium again. Now you will see a new menu in the upper right portion of the screen. This is the oculars menu. Also, a new button that looks like a square with a circle cut out of it, this will allow you to switch back and forth with the telescope view from now on (called “ocular view”).
Step 3 – On the upper right menu, there will be a a button with a wrench in the middle of the circle. Click it to open the Oculars menu. We will be using the “Eyepieces” and “Telescopes” tabs. This is where we will will enter our own information for telescopes and eyepieces. Enter in the following information after you click “Add” (all units are in mm).
Telescopes:

Name: Nexstar 102 SLT
Focal length: 660.0
Diameter: 102.0
Check-box: Vertical flip
Name: Celestron 11”
Focal length: 2800
Diameter: 279.4
Check-box: Horizontal flip
Name: Goldendale Observatory 24.5”
Focal length: 9023.0
Diameter: 622.3
Check-box: Horizontal flip
Eyepieces:

Name: 11 mm aFOV: 82.0 Focal Length: 11.0
Field Stop: 17.0
Check-box: none
Name: 24 mm aFOV: 82.0 Focal Length: 24.0
Field Stop: 37.0
Check-box: none
Name: 40 mm aFOV: 82.0 Focal Length: 40.0
Field Stop: 47.0
Check-box: none
Step 4 – It is convenient to delete all the built in oculars and telescopes. Keep the options you just entered and delete the rest. You’ll find as we go through this lab that all those extras become distracting and not helpful if you don’t delete them.
We are now ready to explore the night sky. The information I listed above is pulled directly off manufacturer’s websites (except for Goldendale, I actually emailed them to obtain the focal length because it was custom built).
I chose these telescopes because the Nexstar was the refraction telescope I showed in the lecture, the Celestron 11” were the student telescopes we had at Northern Arizona University, and Goldendale is the largest public viewing telescope in Washington. If you like this plug-in, own a telescope, or are in the market for a telescope, you can always find the specifications on the manufacturers website and get a sense of what you can see by using this Stellarium oculars plugin.
Stellarium Tips:
• Searching an object and hitting enter will always center the object.
• You can manually adjust the position of the object by using the arrow keys (useful for planning astrophotography).
• Occasionally, the center object button is helpful for getting the object back in the middle of the field of view.
Exploring the Solar System
Set the date to 10/29/2020 at 11:30:00 and stop the clock. Turn the ground and atmosphere off so we don’t have to worry about the time of day throughout the rest of this lab (unless otherwise specified). Also, make sure ”scale moon” is off under ”sky and viewing options.” Type “moon” into the search. This should put the moon in the center of the screen with crosshairs. Click the “ocular view” button, either on the upper right portion of the screen or on the horizontal menu (square with a circle cut out). You are now simulating the view through the telescopes. The circle indicates the field of view for the ocular/telescope combination. You would only see what is inside the circle through an actual telescope.
In the upper right portion of the screen you’ll now see a list with ocular (eyepieces), telescope, and lens. This will allow us to flip through by using the right and left double arrows. For this lab, KEEP “LENS: NONE,” we will only be using the oculars and the telescopes. Make sure the telescope icon on the bottom is lit, this will keep the telescope tracking the object so it does not drift in the field of view.
1) Which eyepiece and telescope combination creates the most “zoomed in” view of the moon (smallest field of view)?
2) Calculate the magnification using Eq.(1) with your result from 1.
3) Which eyepiece and telescope combination(s) allows you to see the entire moon and
Mars in the same field of view (both within the circle)? Tip: use the arrow keys on your keyboard)
4) Calculate the magnification with your result from 3.
5) Which eyepiece and telescope combination gives the most magnified image of the moon while the entire moon remains inside of the field of view (inside the circle)?
Now let’s explore some other objects in the solar system. Type “Jupiter” into the search. This should center you on Jupiter.
6) Which eyepiece and telescope combination produces the largest (and most detailed) image of Jupiter?
7) Which eyepiece and telescope combination is the minimum magnification I would need to see Jupiter’s bands (not just a bright object)? (Note: You don’t have to calculate the magnification, just list the eyepiece and the telescope).
Let’s take a look at Saturn. Type “Saturn” into the search, this should put Saturn in the center of the field of view.
8) Which eyepiece and telescope combination is the minimum magnification I would need to see Saturn’s rings?
(Now for fun, take a look at Saturn using the Goldendale telescope with the 11 mm eyepiece).
Let’s take a look at Neptune, the furthest away of all the large planets. Type in “Neptune” into the search. Neptune looks like a blue disc in even the most powerful of ground based telescopes. On this date, we are fortunate enough to see Neptune’s largest moon Triton. If you look through the Goldendale telescope with the 11 mm lens, you get a nice image of Neptune and Triton. Triton is the little dot next to Neptune (click on it to confirm).
9) Which eyepiece and telescope combination is the minimum magnification I would need to glimpse Triton (it will look like a point of light)?
Exploring Deep Sky Objects
During the 18th century, astronomer Charles Messier identified and catalogued mysterious “fuzzy” objects that he observed in the night sky. Although not known at the time, these objects consisted of galaxies, nebula (star forming clouds), and planetary nebula (rings formed by dying stars ejecting material into space). These objects have been given an “M” (for Messier) number in the Messier catalogue. Thousands more have been found and are now classified in the NGC catalogue, which encompasses the Messier catalogue. Modern telescopes will have most of the catalogues stored in their database. We will explore some of these objects, as well as open clusters, globular clusters, and binary stars.
Type “M81” into the search. This should center M81 in the field of view of your telescope.
10) What is this object (binary star, nebula, planetary nebula, galaxy, etc.)?
11) M81 actually has a friend in the night sky, M82 (both are located in the big dipper). What is this object? (Note: use the arrow keys on your keyboard to adjust the object’s positions in your field of view)
12) Which eyepiece and telescope combination will give me the smallest field of view with both M81 and M82 still in the field of view?
13) Type M45 into the search, what is this object?
14) What is the eyepiece and telescope combination that will give me the smallest field of view while still being able to see all of M45 in the field of view
15) Type M57 into the search. What is this object?
16) Which eyepiece and telescope combination that with give me the smallest field of view while still being able to see all of M57 in the field of view?
17) Albireo is a favorite among astronomers because it is a binary star with two stars of vastly different temperatures. Type Albireo into the search. Which star has a higher temperature (recall blue stars have higher temperatures than red stars or yellow stars), β1 or β2?
18) Click the ocular view off for a moment, can you tell that Albireo is a double star without using a telescope?
19) (Click the ocular view back on) Type M42 into the search. What is this object?
20) What is the eyepiece and telescope combination that will give me the smallest field of view while still seeing all of M42 in the field of view?
This is just a small sample of the objects we could look at in the night sky. I’ll let you explore these on your own. For now, lets summarize our findings with some conceptual questions.
21) (Multiple choice) Which eyepiece and telescope combination gives me the largest field of view?
A. 24 mm eyepiece with the Goldendale 24.5” (fo = 9023mm)
B. 11 mm eyepiece with the Nexstar 102 SLC (fo = 660mm)
22) (Multiple choice) Which eyepiece and telescope combination gives me the most magnification? (Note: you could calculate this or use Stellarium, but you are encouraged to answer by just using reasoning)
A. 11 mm eyepiece with the Celestron 11” (fo = 2800mm) B. 40 mm eyepiece with the Nexstar 102 SLC (fo = 660mm)
Share your favorite image
This is an extra activity in which you can earn all your participation points for the week in one post. Find an object not used in this lab that you like. Find an eyepiece and telescope combination that gives you your favorite field of view of this object.
You will receive a 10/10 for this week’s participation if you post all of the following on the Lab discussion board:
1) The name of the object
2) The eyepiece and telescope combination you used. 3) A screen shot of your field of view.
Tip: Turn the oculars view off. Open the “Sky and Viewing” options from the left menu. Click the checkbox in the sky tab that says “Nebulas.” This will label many deep sky objects in the sky. To view the object through the telescopes, click on an object, then click the oculars view back on.