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This page gives you some examples of the telescope
formulas. The first table shows how aperture relates to
light gathering and magnification. The RedNeck scope will
make an excellent finder, as soon as I can rig up some rings for
it. Even the TT76 would make a good compact finder with
the ability to zoom in with different eyepieces. Again,
there's the problem of getting a ring mount for it.
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|
Scope |
Eye |
RedNeck |
FS80 |
TT76 |
XT8 |
| Aperture (mm) |
7 |
63.5 |
80 |
76 |
200 |
| Focal Length (mm) |
17 |
216 |
400 |
600 |
1200 |
| Light Gathering |
1.0 |
82.3 |
130.6 |
117.9 |
816.3 |
| Focal Ratio |
2.4 |
3.4 |
5.0 |
7.9 |
6.0 |
| Min.
Magnification |
1.0 |
9.1 |
11.4 |
10.9 |
28.6 |
| Max.
Magnification |
1.0 |
100.0 |
127.0 |
120.6 |
317.5 |
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The second table shows effect of focal length on exit pupil,
magnification, and field of view when combined with various
focal length scopes. The TableTop 76 uses puny .96 inch
eyepieces, so it is not listed in these examples.
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It is interesting to compare the scopes with the 1
degree FOV:
|
| Scope |
Focal Length |
Focal Ratio |
Eye- piece |
EP AFOV |
Exit Pupil |
Mag. |
FOV |
| Redneck |
216 |
F/3.4 |
4.5 |
50 |
0.8 |
48.0 |
1.0 |
| FirstScope 80 |
400 |
F/5 |
9 |
50 |
1.8 |
44.4 |
1.1 |
| SkyQuest XT8 |
1200 |
F/6 |
25 |
50 |
4.2 |
48.0 |
1.0 |
Notes:
1) Notice that the magnification is
similar, despite using different eyepieces. This is an
artifact of using eyepieces with the same AFOV.
2) In general, magnification and FOV are linked - A
narrower FOV equals higher magnification. The equation is
varied by the AFOV of the eyepieces in question.
3) The exit pupil is larger in a longer
focal ratio scope. This is why long focal length scopes
are used for planetary viewing - The images appear larger at the
same magnification. |
| |
| Aqua = narrowest
field of view & highest usable
magnifications |
| Lime = most useful range for
the scope |
| Red = FOV of 1 degree - |
| Telescope |
Eyepiece
Foc. Lgth. |
Eyepiece
AFOV |
Exit
Pupil |
Mag |
FOV |
|
30mm_Orion_Ultrascopic
|
|
|
| Redneck Scope |
30 |
52 |
5.0 |
7.2 |
7.2 |
| FirstScope 80wa |
30 |
52 |
6.0 |
13.3 |
3.9 |
| SkyQuest XT8 |
30 |
52 |
5.0 |
40.0 |
1.3 |
|
25mm_Orion_Plossl |
|
|
| Redneck Scope |
25 |
50 |
4.2 |
8.6 |
5.8 |
| FirstScope 80wa |
25 |
50 |
5.0 |
16.0 |
3.1 |
| SkyQuest XT8 |
25 |
50 |
4.2 |
48.0 |
1.0 |
|
20mm_Orion_Ultrascopic |
|
|
| Redneck Scope |
20 |
52 |
3.3 |
10.8 |
4.8 |
| FirstScope 80wa |
20 |
52 |
4.0 |
20.0 |
2.6 |
| SkyQuest XT8 |
20 |
52 |
3.3 |
60.0 |
0.9 |
|
15mm_Orion_Ultrascopic |
|
|
| Redneck Scope |
15 |
50 |
2.5 |
14.4 |
3.5 |
| FirstScope 80wa |
15 |
50 |
3.0 |
26.7 |
1.9 |
| SkyQuest XT8 |
15 |
50 |
2.5 |
80.0 |
0.6 |
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9mm_Orion_Plossl |
|
|
| Redneck Scope |
9 |
50 |
1.5 |
24.0 |
2.1 |
| FirstScope 80wa |
9 |
50 |
1.8 |
44.4 |
1.1 |
| SkyQuest XT8 |
9 |
50 |
1.5 |
133.3 |
0.4 |
|
4mm_GTO_Plossl |
|
|
| Redneck Scope |
4.5 |
50 |
0.8 |
48.0 |
1.0 |
| FirstScope 80wa |
4.5 |
50 |
0.9 |
88.9 |
0.6 |
| SkyQuest XT8 |
4.5 |
50 |
0.8 |
266.7 |
0.2 |
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