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 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, open star clusters
3.5...4
Perceptibility optimal for many objects, e.g. for most galaxies, and mid-size deep sky objects
2...3
Maximum Magnification / Maximum Resolution Actually, the "normal" upper magnification limit; globular star clusters
1...1.5
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!):

Part 1: Focal Length of Eyepiece

Criteria Exit
Pupil
Focal Length of Eyepiece
C8
C5
C8R
C5R
GSD 680
150
PDS
P130
100P
PS72
TLAPO
1027
102
127
Category Application Area
f >
10
6.3
6
5
5
4
6
7
12.75
11.81
Maximum FOV Search from
10
100
63
60
50
50
40
60
70
127.5
118.1
to
7
70
44
42
35
35
28
42
49
89.3
82.7
Minimum Magnification / Large FOV Overview, large-area nebulae from
6.5
65
41
39
32.5
32.5
26
39

45.5
82.9
76.8
...
6
60
38
36
30
30
24
36
42
76.5
70.9
...
5
50
31.5
30
25
25
20
30
35
63.8
59.1
to
4.5
45
28
27
22.5
22.5
18
27
31.5
57.4
53.1
Normal Magnification Large-area, faint nebulae; nebulae, open star clusters from
4
40
25
24
20
20
16
24
28
51
47.2
to
3.5
35
22
21
17.5
17.5
14
21
24.5
44.6
41.3
Best for many objects, e.g. for most galaxies, and mid-size deep sky objects from
3
30
19
18
15
15
12
18
21
38.3
35.4
to
2
20
13
12
10
10
8
12
14
25.5
23.6
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit; globular star clusters from
1.5
15
9.5
9
7.5
7.5
6
9
10.5
19.1
17.7
to
1
10
6.5
6
5
5
4
6
7
12.8
11.8
Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets		 from
0.8
8.0
5
4.8
4.0
4.0
3.2
4.8
5.6
10.2
9.4
...
0.7
7.0
4.4
4.2
3.5
3.5
2.8
4.2
4.9
8.9
8.3
to
0.6
6.0
3.8
3.6
3.0
3.0
2.4
3.6
4.2
7.7
7.1
Separation of narrow double stars, small planetary nebulae;
perception of faintest details		 from
0.5
5.0
3.2
3.0
2.5
2.5
2.0
3.0
3.5
6.4
5.9
to
0.4
4.0
2.5
2.4
2.0
2.0
1.6
2.4
2.8
5.1
4.7
  Check  
F/EP
Focal Length of Eyepiece
  Minimum (Factor)
1.5
6.7
4.2
4.0
3.3
3.3
2.7
4.0
4.7
---
---
2
5.0
3.2
3.0
2.5
2.5
2.0
3.0
3.5
6.4
5.9
  Maximum (EP)
6.5
65.0
41.0
39.0
32.5
32.5
26.0
39.0
45.5
82.9
76.8
7
70.0
44.1
42.0
35.0
35.0
28.0
42.0
49.0
89.3
82.7

Part 2: Magnification

Criteria Exit
Pupil
Magnification
C8
C8R
GSD 680
150
PDS
P130
100P
PS72
TLAPO
1027
102
127
Category Application Area
Focal Length >
2032
1280
1200
750
650
400
432
714
1300
1500
Maximum FOV Search from
10
20.3
20.3
20.0
15.0
13.0
10.0
7.2
10.2
10.2
12.7
to
7
29.0
29.0
28.6
21.4
18.6
14.3
10.3
14.6
14.6
18.1
Minimum Magnification / Large FOV Overview, large-area nebulae from
6.5
31.3
31.3
30.8
23.1
20.0
15.4
11.1
15.7
15.7
19.5
...
6
33.9
33.9
33.3
25.0
21.7
16.7
12.0
17.0
17.0
21.2
...
5
40.6
40.6
40.0
30.0
26.0
20.0
14.4
20.4
20.4
25.4
to
4.5
45.2
45.2
44.4
33.3
28.9
22.2
16.0
22.7
22.7
28.2
Normal Magnification Large-area, faint nebulae; nebulae, open star clusters from
4
50.8
50.8
50.0
37.5
32.5
25.0
18.0
25.5
25.5
31.8
to
3.5
58.1
58.1
57.1
42.9
37.1
28.6
20.6
29.1
29.1
36.3
Best for many objects, e.g. for most galaxies, and mid-size deep sky objects from
3
67.7
67.7
66.7
50.0
43.3
33.3
24.0
34.0
34.0
42.3
to
2
101.6
101.6
100.0
75.0
65.0
50.0
36.0
51.0
51.0
63.5
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit; globular star clusters from
1.5
135.5
135.5
133.3
75.0
86.7
66.7
48.0
68.0
68.0
84.7
to
1
203.2
203.2
200.0
150.0
130.0
100.0
72.0
102.0
102.0
127.0
Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets		 from
0.8
254.0
254.0
250.0
187.5
162.5
125.0
90.0
127.5
127.5
158.8
...
0.7
290.3
290.3
285.7
214.3
185.7
142.9
102.9
145.7
145.7
181.4
to
0.6
338.7
338.7
333.3
250.0
216.7
166.7
120.0
169.9
169.9
211.7
Separation of narrow double stars, small planetary nebulae;
perception of faintest details		 from
0.5
406.4
406.4
400.0
300.0
260.0
200.0
144.0
204.0
204.0
254.0
to
0.4
508.0
508.0
500.0
375.0
325.0
250.0
180.0
255.0
255.0
317.5
  Check  
EP/F
Magnification
  Minimum (Factor) Minimum
(EP)
6.5
31.3
31.3
30.8
23.1
20.0
15.4
11.08
15.7
15.7
19.5
7
29.0
29.0
28.6
21.4
18.6
14.3
10.29
14.6
14.6
18.1
  Maximum (EP) Maximum
(Factor)
1.5
---
---
300
225
195
150
108
153
---
---
2
406.4
406.4
400
300
260
200
144
204
204
254
   
3*
609.6
609.6
600
450
390
300
216
306
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 November 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 (in blue):

Criteria Exit
Pupil
Focal Length of Eyepiece
C8
C8R
150PDS
    PS72
TLAPO
1027
        127
Category Application Area
f >
from...to
10
6.3
5
6
7
11.81
Maximum FOV Search 7
10
70-100
---
44-63**
40
35-50
35-56*
42-60
40-56*
49-70
40-56*
83-118
---**
Minimum Magnification / Large FOV Overview, large-area nebulae 4.5
6.5
45-65
40-56
28-41
25-40
22-33
20-32*
27-39
28-38*
31-36
32-35*
53-77
---**
Normal Magnification Large-area, faint nebulae; nebulae, open star clusters 3.5
4
35-40
35-40*
22-25
20-25
17-20
16-20*
21-24
20-25*
24.5-28
24-28*
41-47
 40**
Best for many objects, e.g. for most galaxies, and mid-size DSO 2
3
20-30
20-32*
12.5-19
12.5-18
10-15
10-15
12-18
12-18*
14-21
15-20*
24-36
24-32
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit; globular star clusters 1
1.5
10-15
10
6.3-9.5
6-9.5
5-7.5
5-7.5
6-9
6-9
7-10.5
7-10,5
12-18
12-18
Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets		 0.6
0.8
6-8
6-8
3.5-5
3.5-5
3.0-4.0
3-4
3.5-5
3.5-5.0
4.2-5.6
4-6
7-9.5
7-10
Separation of narrow double stars, small planetary nebulae;
perception of faintest details		 0.4
0.5
4-5
4-5
2.5-3.2
2.5-3.5
2.0-2.5
2-2.5
2.4-3.0
2.5-3.0
2.8-3.5
2.5-3.5
4.7-6
4.5-6

*) Partly available as 2" eyepiece; **) typically no suitable 1,25" eyepieces available; problems with viewing at 40 mm; italic: not possible; red: magnification too high; blue: commercial focal lengths

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

In the following, I will not only consider just those telescopes that I currently own (January 2020; plus my previous Explorer 150PDS), but also only my current and some borrowed eyepieces. As of January 2020, I own the following eyepieces:

    
     
   

Photos: Top row (from left to right): 40 mm Lacerta ED eyepiece, 35 mm TS UFL ED eyepiece, 32 mm DigiScope Plössl eyepiece, and 24 mm TeleVue wide angle eyepiece; bottom row (from left to right): 16 mm, 7 mm, and 4 mm TS UWA eyepieces, and 10 mm TeleVue Delos eyepiece.

My 12.5 mm Plössl and 23 mm crosshair eyepieces are not shown and will not be considered.

Application of the Recommendations

Criteria Exit
Pupil
Focal Length of Eyepiece
C8
 150PDS
PS72
TLAPO1027
127
Category Application Area
f >
10
6.3
 5
6
7
11.81
from to Rec Is Rec Is   Rec Is Rec Is Rec Is Rec Is
Maximum FOV Search 7
10
70-100
 ---
44-63
 ---
35-50
35, 38, 40
42-60
35, 38, 40, 56
49-70
40
83-118
32
Minimum Magnification / Large FOV Overview, large-area nebulae 4.5
6.5
45-65
 40, 56
28-41
32
22-33
24, 26, 28, 32, 32
27-39
26, 28, 32, 32, 35, 38, 40
31-36
32, 35
53-77
32
Normal Magnification Large-area, faint nebulae; nebulae, open star clusters 3.5
4
35-40
 35, 38, 40
22-25
24
17-20
16
21-24
24
24.5-28
24, 26, 28
41-47
32
Best for many objects, e.g. for most galaxies, and mid-size DSO 2
3
20-30
 18, 24, 26, 28, 32, 32
12.5-19
16
10-15
10, 16
12-18
10, 16, 18
14-21
16
24-36
24, 32
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit; globular star clusters 1
1.5
10-15
 10-16
6.3-9.5
 7, 10
5-7.5
7, *
6-9
7
7-10.5
7, 10
12-18
10, 16
Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets		 0.6
0.8
6-8
 7
3.5-5
4
3.0-4.0
4
3.5-5
4
4.2-5.6
4
7-9.5
7, 10
Separation of narrow double stars, small planetary nebulae;
perception of faintest details		 0.4
0.5
4-5
 4
2.5-3.2
 *
2.0-2.5
*
2.4-3.0
*
2.8-3.5
*, 4
4.7-6
4, 7

*) Possible with Explore Scientific focal extenders (2 x, 3 x, 5 x); bold: 2" eyepiece; italic: not possible; magenta: borrowed for evaluation (2"); gray: no longer in my possession

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. For the C8, I found a 2" eyepiece around 25 mm quite a useful addition, and therefore acquired one. For the C8 and also for the PS 72/432, a 56 mm eyepiece (2") would fit. Thus, for the search function, a 2" eyepiece with a focal length of 56 mm would be a useful addition. However, I was not satisfied with the 56 mm Plössl eyepiece that I had borrowed, and returned it finally to its original owner. For the TLAPO1027, a 20 mm eyepiece might to be considered.

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.

Here, I do not consider the fact that my eyepieces have different fields of view so that there can be "overlaps", that is, redundancies, regarding 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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07.10.2024