Eyepiece Selection (Focal Length Selection) - Application

Applied to My Telescopes | My Eyepieces | Resume | References

On this and further pages, I will deal with the selection of suitable eyepiece focal lengths for your own (or planned) telescopes as well as with the assessment of whether existing (or planned) eyepieces are suitable for these telescopes and fit together. Starting point are recommendations on the basis of the size of the exit pupil, which I found in the literature or on the Internet and which I have "consolidated" in my own recommendations.

Overview

  • On the preceding page, I present a more or less "theoretical" introduction to the topic, three recommendations for selecting eyepiece focal lengths that are based on the size of the exit pupil, as well as an own "consolidated" one, which I derived from it. If you are not interested in the background of my recommendations, feel free to skip this page and proceed to the next one.
  • On this page, I apply my "consolidated" recommendations to my own telescopes as an example. Readers should have no trouble applying the recommendations to their own (or prospective) telescopes. All you need to know is the focal ratio (for calculating the focal lengths of the eyepieces) and the aperture (for calculating the magnifications) of your telescopes!
    I also take my existing eyepieces as an example and ask how they fit those recommendations. Readers should also be able to follow this step with their own (or prospective) eyepieces and telescopes.
  • The last page on this topic (in preparation) deals with the question of how prospective eyepieces fit to the existing ones. In addition I choose an interactive approach, which was pointed out to me by a starfriend.

Once again, I would like to emphasize that I am still an astronomy beginner who has found these criteria in the literature or on the Internet, and now tries to apply them to his own equipment, hoping that this information is useful for other beginners in astronomy, too. I am far from giving recommendations for specific eyepieces, because I lack the respective experience. But be assured that the Internet is full of such recommendations ...

Note: For definitions in a small glossary, see page Quick & Dirty Astronomy Glossary.

 

Applied to My Telescopes

Rehearsal...

Here are, as a reminder, my recommendations for the selection of eyepiece focal lengths derived and "consolidated" from several sources:

Category Deep Sky Application Area
Exit Pupil (mm)
Minimum Magnification / Maximum/Large Field of View Search
7...10
Overview, large-area nebulae
4.5...5...6 (7)
Normal Magnification Optimal for large-area, faint nebulae; nebulae, star clusters
3.5...4
Perceptibility optimal for many objects, e.g. for most galaxies, globular star clusters, and mid-size deep sky objects
(1.5) 2...3
Maximum Magnification / Maximum Resolution Actually, the "normal" upper magnification limit...
1
With perfect seeing, achieves maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies; maximum magnification for planets that makes sense
0.6...0.7...0.8
Separation of narrow double stars, small planetary nebulae; at the extreme limit of the telescope, to perceive the faintest details
0.4...0.5

Now all I have to do is multiply the exit pupil values with the focal ratios of my telescopes to arrive at the focal lengths of the eyepieces that suit my telescopes. I will demonstrate this step exemplarily with my own telescopesin the following!

Application

In the following, I apply my selection criteria to my current as well as three of my former telescopes.

For a better overview, I compile the results in tabular format. The following table is based on an Excel spreadsheet, in which I calculate the eyepiece focal lengths (exit pupil * focal ratio) and telescope magnifications (aperture / exit pupil) for my telescopes.

Here are the values transferred from the Excel spreadsheet (the focal lengths of the telescopes are just given for information purposes!):

Criteria Exit
Pupil
Focal Length of Eyepiece
  Exit
Pupil
Magnification
GSD 680
150PDS
P130
100P
PS72
102
127
 
GSD 680
150PDS
P130
100P
PS72
102
127
Category Application Area
f >
6
5
5
4
6
12.75
11.81
 
Focal Length >
1200
750
650
400
432
1300
1500
Maximum FOV Search from
10
60
50
50
40
60
127.5
118.1
  from
10
20.0
15.0
13.0
10.0
7.2
10.2
12.7
to
7
42
35
35
28
42
89.3
82.7
  to
7
28.6
21.4
18.6
14.3
10.3
14.6
18.1
Minimum Magnification / Large FOV Overview, large-area nebulae from
6.5
39
32.5
32.5
26
39
82.9
76.8
  from
6.5
30.8
23.1
20.0
15.4
11.1
15.7
19.5
...
6
36
30
30
24
36
76.5
70.9
  ...
6
33.3
25.0
21.7
16.7
12.0
17.0
21.2
...
5
30
25
25
20
30
63.8
59.1
  ...
5
40.0
30.0
26.0
20.0
14.4
20.4
25.4
to
4.5
27
22.5
22.5
18
27
57.4
53.1
  to
4.5
44.4
33.3
28.9
22.2
16.0
22.7
28.2
Normal Magnification Large-area, faint nebulae; nebulae, star clusters from
4
24
20
20
16
24
51
47.2
  from
4
50.0
37.5
32.5
25.0
18.0
25.5
31.8
to
3.5
21
17.5
17.5
14
21
44.6
41.3
  to
3.5
57.1
42.9
37.1
28.6
20.6
29.1
36.3
Best for many objects, e.g. for most galaxies, globular star clusters, and mid-size deep sky objects from
3
18
15
15
12
18
38.3
35.4
  from
3
66.7
50.0
43.3
33.3
24.0
34.0
42.3
...
2
12
10
10
8
12
25.5
23.6
  ...
2
100.0
75.0
65.0
50.0
36.0
51.0
63.5
to
1.5
9
7.5
7.5
6
9
19.1
12.7
  to
1.5
133.3
75.0
86.7
66.7
48.0
68.0
84.7
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit  
1
6
5
5
4
6
12.8
11.8
   
1
200.0
150.0
130.0
100.0
72.0
102.0
127.0

Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets

from
0.8
4.8
4.0
4.0
3.2
4.8
10.2
9.4
  from
0.8
250.0
187.5
162.5
125.0
90.0
127.5
158.8
...
0.7
4.2
3.5
3.5
2.8
4.2
8.9
8.3
  ...
0.7
285.7
214.3
185.7
142.9
102.9
145.7
181.4
to
0.6
3.6
3.0
3.0
2.4
3.6
7.7
7.1
  to
0.6
333.3
250.0
216.7
166.7
120.0
169.9
211.7

Separation of narrow double stars, small planetary nebulae;
perception of faintest details

from
0.5
3.0
2.5
2.5
2.0
3.0
6.4
5.9
  from
0.5
400.0
300.0
260.0
200.0
144.0
204.0
254.0
to
0.4
2.4
2.0
2.0
1.6
2.4
5.1
4.7
  to
0.4
500.0
375.0
325.0
250.0
180.0
255.0
317.5
  Check  
F/EP
Focal Length of Eyepiece
   
EP/F
Magnification
  Minimum (Factor)
1.5
4.0
3.3
3.3
2.7
4.0
---
---
  Minimum
(EP)
6.5
30.8
23.1
20.0
15.4
11.08
15.7
19.5
2
3.0
2.5
2.5
2.0
3.0
6.4
5.9
 
7
28.6
21.4
18.6
14.3
10.29
14.6
18.1
  Maximum (EP)
6.5
39.0
32.5
32.5
26.0
39.0
82.9
76.8
  Maximum
(Factor)
1.5
300
225
195
150
108
---
---
7
42.0
35.0
35.0
28.0
42.0
89.3
82.7
 
2
400
300
260
200
144
204
254
                     
3*
600
450
390
300
216
306
381

*) According to Stoyan maximum for small extended objects (according to Stoyan, the maximum magnification is: Aperture * 2 / 0.7); italic: no longer in my possession

Simplification

At first glance, the above table does not look very helpful, but let not overwhelm you by the amount of data! First, limit yourself to one telescope at at moment. In the following, I will, however, not follow my advice, but just consider the telescopes that I actually own at the moment (as of March 2019).

Secondly, make sure to select only one eyepiece with a common focal length in the categories with a large field of view, maybe two in the category of "normal magnification," and maybe even three or more in the category of "maximum magnification," the latter in order to enable a better fit to the seeing conditions.

This would look as follows for my telescopes, given that I choose common focal lengths for commercial eyepieces:

Criteria Exit
Pupil
Focal Length of Eyepiece
  150PDS
    PS72
        127
Category Application Area
from...to / f >
5
6
11.81
Maximum FOV Search 7
10
35-56*
40-56*
32-40**
Minimum Magnification / Large FOV Overview, large-area nebulae 4.5
6.5
24-32*
28-38*
32-40**
Normal Magnification Large-area, faint nebulae; nebulae, star clusters 3.5
4
16-20
20-24
32-40**
Best for many objects, e.g. for most galaxies, globular star clusters, and mid-size DSO 2
3
10-15
12-18
24-32
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit  
1
5
6
12
Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets
0.6
0.8
3.0-4.0
3.5-5
7-10
Separation of narrow double stars, small planetary nebulae;
perception of faintest details
0.4
0.5
2.0-2.5
2.5-3.0
5-6

*) Available as 2" eyepiece; **) typically no suitable 1,25" eyepiexes available; problems with viewing at 40 mm

You should arrive at a similar table, however, with different numbers, for your own telescopes.

Discussion

The simplified table above provides me with a number of eyepiece focal lengths that I might purchase now for my individual telescopes. Fortunately, there are also several overlaps in the list, so that my telescopes could share certain eyepieces. And if I do not take it too seriously, then I might put together an eyepiece set that at least allows to use most eyepieces on all my telescopes...

In reality it was, of course, a completely different story! Together with some telescopes I acquired eyepieces, and also bought a Revelation eyepiece case with a larger number of Plössl eyepieces as a supplement. Since I did not like the Plössl eyepieces, I gave them away, except for the 32 mm eyepiece, and bought a set of three UWA eyepieces, which I later supplemented with two used UWA eyepieces. With the purchase of the Sky-Watcher 150PDS, I got a 2" eyepiece, which I later supplemented with one having an even longer focal length. So, in the course of time a more or less arbitrary set of eyepiece has accumulated, which I will present in the following and then check for "fit" and completeness on the basis of my recommendations "after the fact."

 

My Eypieces

I will not only consider just those telescopes that I currently own (March 2019), but also only my current eyepieces. Here is an overview of my eyepieces:

    
     
 

Photos: Top row (from left to right): 35 mm TS UFL eyepiece (ED), 32 mm DigiScope Plössl eyepiece, 28 mm Sky-Watcher Erfle eyepiece, and 10 mm TeleVue Delos eyepiece; bottom row (from left to right): 16 mm, 7 mm, and 4 mm TS UWA eyepiece, 24 mm TeleVue wide angle eyepiece

The 12.5 mm crosshair Plössl eyepiece is not shown and will not be considered.

Application

Criteria Exit
Pupil
Focal Length of Eyepiece
  150PDS
    PS72
        127
Category Application Area
f >
5
6
11.81
from to Rec Is Rec Is Rec Is
Maximum FOV Search 7
10
35-56
35
40-56
35
32-40**
32
Minimum Magnification / Large FOV Overview, large-area nebulae 4.5
6.5
24-32
24, 28, 32
28-38
28, 32, 35
32-40**
32
Normal Magnification Large-area, faint nebulae; nebulae, star clusters 3.5
4
16-20
16
20-24
24
32-40**
32
Best for many objects, e.g. for most galaxies, globular star clusters, and mid-size DSO 2
3
10-15
10, 12,5**, 16
12-18
12,5**, 16
24-32
24, 32
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit  
1
5
7, *
6
7
12
10, 12,5**
Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets
0.6
0.8
3.0-4.0
4
3.5-5
4
7-10
7, 10
Separation of narrow double stars, small planetary nebulae;
perception of faintest details
0.4
0.5
2.0-2.5
*
2.5-3.0
*
5-6
4, 7

*) Possible with Explore Scientific focal extenders (2 x, 3 x, 5 x); **) crosshair eyepiece; bold: 2" eyepiece

Discussion

If one looks at the blue numbers, one can see that, apart from very short focal length eyepieces, no eyepieces are missing in my equipment. Short focal lengths can be easily created by combining existing eyepieces with my Explore Scientific focal extenders (2 x, 3 x, 5 x). So there is actually no need for buying new eyepieces with a short focal length.

For the search function, however, a 2" eyepiece with a focal length of 56 mm might be a useful addition...

Here, I did not consider the fact that my eyepieces have different fields of view so that there can be "overlaps", that is, redundancies, as to the fields of view at similar focal lengths.

 

Resume

My "simplified recommendations" made it fairly easy for me to find suitable eyepiece focal lengths and to check my existing eyepieces against them. It was much more difficult for me, though, to work out these recommendations on the basis of existing recommendations...

 

References

 

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28.04.2019