Nexstar 11 GPS Astronomy

Eyepiece Information

There are a number of different eyepieces available for the telescope. The first thing you will want to do is to define the full range of magnifications available to you. To do this visit the Calculations page by clicking this link: Calculations page. You will want to have ready the diameter of your objective mirror or lens, and the focal length of the instrument you intend to use, the focal length of the eyepieces you want to get information for, and the Apparent Field of View. If you have those items you can learn a lot about your telescope. Here are the items you can learn about your eyepiece/telescope combination:

1. System Magnification: What the total magnification is with your telescope and the eyepiece you selected
2. Exit Pupil: How large is the cone of light leaving the eyepiece
3. Eyepiece FOV: Just how much of the sky can you really see
4. Lowest Magnification: What is the lowest power you can use with your telescope
5. Highest Magnification: What is the highest power you can use with your telescope
6. F Ratio: What fratio is your system operating at at prime focus
7. Diffraction Disk Size: Size of the diffraction disk created by the telescope and eyepiece
8. Smallest Lunar feature visible: What is the smallest thing you can see on the Moon

You will need to know what some of these terms mean before you start... here is a brief description of the important terms to this discussion:

Lets look at each one of these above and find out what they really are... "System Magnification", is exactly what it says, the current number of times your telescope will enlarge something as seen by your eye.

Exit Pupil is the size of the cone of light leaving the Eyepiece, EP is important because if it is larger than your pupil of your eye, (7 mm for a kid in full dark, 5 for us older folks), you are wasting light. You NEVER want to waste light.

Eyepiece FOV is the size of the circle you view in the sky using the selected eyepiece and telescope, in degrees. The Moon is around one half degree, so if your eyepiece telescope combination come out to .5 degree, you will know that you can see anything the size of the moon in your eyepiece.

Lowest Magnification is the lowest magnification that your Nexstar11GPS will allow. The center obstruction of the Nexstar11 will hide some part of things, and as you reduce the power, the central obstruction will become more and more apparent. At some point the amount of area the central obstruction covers will become objectionable.

Highest Magnification is the maximum power your telescope can perform at on a perfect night, with great seeing, and some luck. It works out to around 50 power per inch of aperture.

F Ratio is the ratio of the diameter of the primary mirror to the focal length of the primary mirror. In the Nexstar11GPS case, the mirror is 280 mm in diameter, and it focuses the light to a point 2800 mm from the glass of the mirror. That works out to f10. 280/2800=10

The first thing you may want to do is to define the full range of magnifications available to you. Subtract the Lowest power your Nexstar will work at from the highest power your Nexstar will work at. In my case I have a Nexstar11GPS. The highest power the Nexstar11GPS will work at is 551, the lowest is 35. That means I will need a set of eyepieces that cover a range of 516 power, or 551-35=516.

I have decided that I would like to cover this range in seven steps. However I don't want to buy seven eyepieces, at a hundred plus bucks per eyepiece. So I will use a Barlow lens. A Barlow lens will make whatever eyepiece you are using act like it has a shorter focal length. OK time for a bit of math.

Magnification is the focal length of the primary mirror divided by the focal length of the eyepiece you are using. Lets do a quick problem to make sure we understand this...

If you consider your Nexstar11GPS you will find that the focal length of the primary is 2800 mm. If you dig around in the Instruction manual you will also find that it is operating at an f ratio of 10:1. That means that the focal length is ten times the diameter of the primary mirror, or f10! Now that you know how that works, lets work some numbers...

Your low power eyepiece should be 35 power. If you take the focal length of the telescope and divide the required power into that, you get the eyepiece focal length. So your first calculation should look like this: 35/2800, or 80. That means you want an eyepiece of 80 mm focal length to obtain 35 power. See table 1 below for the full range:

Table 1

Eyepiece focal length vs power for a Nexstar11GPS

Note1: The 40 mm eyepiece comes with the Nexstar11GPS.

The Barlow

You may have seen that the last three entries say XX with Barlow. What is a Barlow? It is a lens that extends the focal length of your Nexstar11GPS. It is placed between the eyepiece and the Nexstar11 telescope. So if you purchase a X 2 Barlow, your focal length becomes 5600 mm, or 2800 mm times 2. This is the same as halving the focal length of the eyepiece for calculation purposes.

You can purchase Barlows from 2 to 5 power, you can also purchase an adjustable Barlow. A Barlow lets you keep some of the good features of a low power eyepiece and use them at high power. As eyepieces get shorter in focal length the curves get steeper in the construction of the optics. This makes for smaller lenses. Using a Barlow lets you use a long focal length eyepiece, with a greater size of the eye lens, (the lens closest to your eye), better eye relief, etc., while reusing an eyepiece instead of purchasing another. Now that you have a rough idea what powers you want it's time to pick a few eyepieces based on some of your needs...

First lets look at the basic eyepieces out there...

Huygenian

The eyepiece design is free of chromatic aberrations, free of internal reflections, and has very short eye relief. Eye relief is how far your eye has to be from the eye lens. Further the field is highly curved, and this results in small fields of view. This eyepiece is normally the eyepiece of choice in a cheap telescope and should be avoided. This eyepiece is not a good choice for people that wear glasses.

Ramsden

A little less field curvature and more chromatic aberration, (color fringing), than the Huygenian, the same field of view is available, small... and short eye relief. Not a good eyepiece for eye glass wearers.

Kellner, Modified Achromat, or RKE

Chromatic aberrations are less than the Ramsden, but dirt tends to show more with this eyepiece. Still a rather low end eyepiece. You can get a field of view around 40 to 50 degrees with this design, and the eye relief is a bit better. Not a good eyepiece for eyeglass wearers.

Orthoscopic

This eyepiece is the minimum grade eyepiece I will use, (yes I know I am a eyepiece bigot), and I try and find others if possible. It is much better eyepiece than any of the above, and has wider apparent field of views. You will find that this eyepiece does have some internal reflections, but they are manageable. The eyepiece is fully corrected for distortions. Eye relief is better than all above as well. Use Orthoscopic eyepieces at high powers. Eyeglass wearers will have a better time of it with this eyepiece than any of the above.

Erfle

Erfle's are nice eyepieces for low power, wide AFOVs. More like Porthole in space! 60 to 70 degree field of view makes this eyepiece a low power choice. Also nice eye relief, good for those of you who wear glasses. Subject to internal reflections due to the increased number of surfaces. If you get one, make sure it has a good coatings group applied to it.

Plossl

Long eye relief and good design make this the workhorse of most people. That coupled with the fact that you can get a really nice eyepiece for around a hundred bucks makes this a pretty good choice at any power. They tend to be flat and distortion free and are good for wearers of eyeglasses.

Radian (Tele Vue)

The Radian is my favorite eyepiece for higher powers. I wear glasses and want long eyerelief, and large (60 degrees), flat fields, this eyepiece does both well. It is also parfocal which means that you don't have to refocus too far when changing eyepieces. Good clean design, but major bucks to purchase. I love this eyepiece!

Lets pick some eyepieces

Now that you know what eyepieces you need based on power, and how some of them act, it's time for you to make a few choices... Lets start at the high powers and work down. My choice for high power is a Tele Vue Radian, and a Celestron Barlow. The general consensus is that the Tele Vue Barlow and the Celestron Barlow are about the same, but the Celestron Barlow is way less money.

Low Power Eyepieces

I tend to stick to Plossl and Erfle designs. Both allow for nice eye relief, (remember I wear glasses), and both have nice AFOV's. So for my low power eyepieces I use Plossl, or Erfles.

High Power

Radians all around. I load them into a turret, and can switch between 4 of them without changing focus at high powers. Very nice look, flat and well corrected... Can't beat them so far!

If you are still interested in eyepiece construction a very good site covering all aspects of eyepiece design can be located by clicking here. After reading everything there, you will know more about eyepieces than you will ever need! Wonderful site!