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Creating a basic set of eyepieces for your new telescope

by Graeme1858

By: Lady Fraktor

One of the most asked questions by beginners, What eyepieces should I get?

There is always a lot of discussion from people on what eyepieces to get with their new telescopes.

Most people seem to want to get the most magnification possible right away, but unfortunately, reality in the form of current atmospheric conditions has a lot to say on that subject.

All telescopes, no matter the type, have an “optimal” range where they perform the best. As you gain experience, you will understand when and how to properly go outside those limits.

This discussion will show you how to generate a good optimum set to start out with and get the most from your scope without the pitfalls of buying something that will not be of use to you.

There are 3 formulae that you will need to know to work this all out. For the beginner, it will show the relationship of telescope focal length, eyepiece focal length, exit pupil, and magnification.

AFOV (Apparent Field Of View) is not taken into account as it is more subjective to the target and style of telescope.

The formulae you need to know to do this are:

1) Focal Length of Eyepiece ÷ Focal Ratio of Telescope = Exit Pupil
2) Focal Length of Telescope ÷ Eyepiece Magnification = Eyepiece Focal Length
3) Focal Length of Telescope ÷ Focal Length of Eyepiece = Eyepiece Magnification

The first thing you need to do is find your starting exit pupil. I prefer to start at 0.5mm, which is usually around ½ the Focal Ratio of the telescope.
The scope we are going to use in the example is my SV 80mm f/6.9 (552mm), so we will start with an eyepiece focal length of 3.5mm

HINT: An eyepiece focal length equal to the Focal Ratio of your scope is normally a 1mm Exit Pupil.

1) 3.5mm ÷ 6.9 = 0.507 or 0.51mm
2) 552mm ÷ 3.5mm = 157.7 or 158x magnification

For spacing the eyepieces, we are going to use a magnification decrease of 1.5x so,

158x ÷ 1.5 = 105x magnification

The next step is to use formula 3.

3) 552mm ÷ 105x mag = 5.2mm (eyepiece focal length)

Use the above formula to work out your exit pupil for this eyepiece and then carry on with the next step.

105x ÷ 1.5 = 70x magnification.

As you work through you will have a list that looks like this.

Set 1 (1.5):
3.5mm -- 0.51 ep -- 158x
5.2mm -- 0.75 ep -- 105x
8mm -- 1.16 ep -- 70x
12mm -- 1.74 ep -- 47x
18mm -- 2.58 ep -- 31x
26mm -- 3.81 ep -- 21x
39mm -- 5.71 ep -- 14x
55mm -- 8 ep -- 9x (unusable)**

Work down till you get to just over a 7mm exit pupil as most people will never be able to use anything larger than 7mm.

It can be used in a refractor as it will not affect the view, in a reflector the shadow of the secondary mirror will start to show as a black spot in the view if you exceed the size of your dark adapted pupil.

Notice also in Set 1 that there is no real need for a barlow lens as dividing any of the focal lengths by 2 will put the new focal length within 1-2mm of the next eyepiece.
For a starter set the range of 8mm - 39mm (substitute a 40mm in place of the 39mm) will work very well.

Afterwards if desired you could add one in the 5.5 or 6mm range and a 32mm on the low range.

Now to work out a second set but this time using 1.6 instead.

Using the example scope and formulas/steps above your chart will look like this:

Set 2 (1.6):
3.5mm -- 0.51 ep -- 158x
5.5mm -- 0.80 ep -- 99x
9mm -- 1.31 ep -- 62x
14mm -- 2.05 ep -- 39x
23mm -- 3.33 ep -- 24x
36mm -- 5.33 ep -- 15x

In Set 2, there is potential for using a 2x barlow as the focal lengths will be in between all the calculated focal lengths.

Most eyepiece manufacturers space their focal lengths at 1.4, 1.5 or 1.6x

Congratulations, you have just generated a useful range of eyepieces for your telescope!

The only thing you need to decide on now is what AFOV you wish to use and do you want a barlow or not.

I have found over time that I prefer to view using fixed focal length eyepieces over using a barlow.

I still have a couple and occasionally will use them, normally in a “grab and go” situation of a barlow and 2-3 eyepieces.

In conclusion:

Most new telescopes come with a 9mm and 25mm eyepieces which unfortunately are usually not of the best quality. Most of these are poorly made Ramsden, Huygenian, Kellner or Plossl which is a shame because when made properly they are actually fine eyepiece designs in the right telescope.

The 9mm and 25mm eyepieces can be considered “work horse” focal lengths. They will always work in any style of scope. When considering buying better quality eyepieces right away, my recommendation is to change these for better quality first.
They are eyepieces that will be in your kit for a long time. They do not have to be the same type as supplied with the telescope. This is where you can play with the AFOV, depending on what you normally want to view.

Using your telescope at the lowest possible magnification has a lower limit for every scope which is normally dictated by the size of your dark adapted eye pupil.
This is considered around 7mm which can and will decrease as you get older.
You can exceed it with a refractor as it will not affect the view, though really would just be wasting money as it just becomes “wasted” light.
In a reflector, the shadow of the secondary mirror will start to show as a black spot in the view if you exceed the size of your dark adapted pupil size.

Highest magnification possible will be dictated by the prevailing current atmosphere and style of scope.
Physics says this is what your scope can theoretically do while the atmosphere says this is what it will be allowed to do. Occasionally, when the two are feeling civil towards each other, you will be able to exceed those limits, but it is not an occasion that happens very often, unfortunately.

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