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Eyepiece Calculator
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Initial Conditions
| Focal Length: | 800.0mm |
| Aperture: | 200.0mm |
| Focal Ratio: | 4.0 |
| Include 2-inch data: | True |
| Table corrected for Paracorr: | False |
| List only DIOPTRX eyepieces: | False |
| Max recommended mag. for aperture: | 350x |
GSO 200/800mm
| Design | Focal Length (mm) | Apparent Field (°) | Eye Rel. (mm) | Barrel Size | Mag. (x) | Exit Pupil (mm) | True Field (°) | Star Drift Time (sec) | True Field @ 1000 yards (feet) |
Mag. w/2x Barlow (2x) | Mag. w/2x Big Barlow or Power- mate (2x) | Mag. w/2.5x Power- mate (2.5x) | Mag. w/3x Barlow (3x) | Mag. w/4x Power- mate (4x) | Mag. w/5x Power- mate (5x) |
| Plössl | 55 | 50 | 38 | 2" | 14.5 | 13.8* | 3.3 | 789 | 172.5 | -- | 29.1 | -- | -- | 58.2 | -- |
| Panoptic | 41 | 68 | 27 | 2" | 19.5 | 10.3* | 3.3 | 789 | 172.5 | -- | 39.0 | -- | -- | 78.0 | -- |
| Wide Field (disc) | 40 | 65 | 24 | 2" | 20.0 | 10.0* | 3.0 | 706 | 154.5 | -- | 40.0 | -- | -- | 80.0 | -- |
| Plössl | 40 | 43 | 28 | 1¼" | 20.0 | 10.0* | 1.9 | 463 | 101.3 | 40.0 | 40.0 | 50.0 | 60.0 | 80.0 | 100.0 |
| Panoptic | 35 | 68 | 24 | 2" | 22.9 | 8.8* | 2.8 | 663 | 145.1 | -- | 45.7 | -- | -- | 91.4 | -- |
| Wide Field (disc) | 32 | 65 | 19 | 2" | 25.0 | 8.0* | 2.4 | 566 | 123.8 | -- | 50.0 | -- | -- | 100.0 | -- |
| Plössl | 32 | 50 | 22 | 1¼" | 25.0 | 8.0* | 1.9 | 463 | 101.3 | 50.0 | 50.0 | 62.5 | 75.0 | 100.0 | 125.0 |
| Nagler 5 | 31 | 82 | 19 | 2" | 25.8 | 7.8* | 3.0 | 720 | 157.5 | -- | 51.6 | -- | -- | 103.2 | -- |
| Panoptic | 27 | 68 | 19 | 2" | 29.6 | 6.8 | 2.2 | 523 | 114.4 | -- | 59.3 | -- | -- | 118.5 | -- |
| Nagler 5 (disc) | 26 | 82 | 16 | 2" | 30.8 | 6.5 | 2.5 | 600 | 131.3 | -- | 61.5 | -- | -- | 123.1 | -- |
| Plössl (disc) | 26 | 50 | 18 | 1¼" | 30.8 | 6.5 | 1.6 | 379 | 82.9 | 61.5 | 61.5 | 76.9 | 92.3 | 123.1 | 153.8 |
| Plössl | 25 | 50 | 17 | 1¼" | 32.0 | 6.3 | 1.5 | 363 | 79.5 | 64.0 | 64.0 | 80.0 | 96.0 | 128.0 | 160.0 |
| Panoptic | 24 | 68 | 15 | 1¼" | 33.3 | 6.0 | 1.9 | 463 | 101.3 | 66.7 | 66.7 | 83.3 | 100.0 | 133.3 | 166.7 |
| Wide Field (disc) | 24 | 65 | 14 | 1¼" | 33.3 | 6.0 | 1.8 | 423 | 92.6 | 66.7 | 66.7 | 83.3 | 100.0 | 133.3 | 166.7 |
| Nagler 4 | 22 | 82 | 19 | 2" | 36.4 | 5.5 | 2.2 | 533 | 116.6 | -- | 72.7 | -- | -- | 145.5 | -- |
| Panoptic (disc) | 22 | 68 | 15 | 2" & 1¼" | 36.4 | 5.5 | 1.8 | 429 | 93.8 | 72.7 | 72.7 | 90.9 | 109.1 | 145.5 | 181.8 |
| Ethos | 21 | 100 | 15 | 2" | 38.1 | 5.3 | 2.6 | 621 | 135.8 | -- | 76.2 | -- | -- | 152.4 | -- |
| Plössl (disc) | 21 | 50 | 14 | 1¼" | 38.1 | 5.3 | 1.3 | 310 | 67.9 | 76.2 | 76.2 | 95.2 | 114.3 | 152.4 | 190.5 |
| Nagler 2 (disc) | 20 | 82 | 12 | 2" | 40.0 | 5.0 | 2.0 | 470 | 102.8 | -- | 80.0 | -- | -- | 160.0 | -- |
| Nagler 5 (disc) | 20 | 82 | 12 | 2" | 40.0 | 5.0 | 2.0 | 470 | 102.8 | -- | 80.0 | -- | -- | 160.0 | -- |
| Plössl | 20 | 50 | 14 | 1¼" | 40.0 | 5.0 | 1.2 | 293 | 64.1 | 80.0 | 80.0 | 100.0 | 120.0 | 160.0 | 200.0 |
| Panoptic | 19 | 68 | 13 | 1¼" | 42.1 | 4.8 | 1.5 | 365 | 79.9 | 84.2 | 84.2 | 105.3 | 126.3 | 168.4 | 210.5 |
| Wide Field (disc) | 19 | 65 | 11 | 1¼" | 42.1 | 4.8 | 1.4 | 336 | 73.5 | 84.2 | 84.2 | 105.3 | 126.3 | 168.4 | 210.5 |
| DeLite | 18.2 | 62 | 20 | 1¼" | 44.0 | 4.6 | 1.4 | 327 | 71.6 | 87.9 | 87.9 | 109.9 | 131.9 | 175.8 | 219.8 |
| Radian (disc) | 18 | 60 | 20 | 1¼" | 44.4 | 4.5 | 1.3 | 314 | 68.6 | 88.9 | 88.9 | 111.1 | 133.3 | 177.8 | 222.2 |
| Delos | 17.3 | 72 | 20 | 1¼" | 46.2 | 4.3 | 1.5 | 363 | 79.5 | 92.5 | 92.5 | 115.6 | 138.7 | 185.0 | 231.2 |
| Ethos | 17 | 100 | 15 | 2" | 47.1 | 4.3 | 2.1 | 507 | 111.0 | -- | 94.1 | -- | -- | 188.2 | -- |
| Nagler 4 | 17 | 82 | 17 | 2" | 47.1 | 4.3 | 1.7 | 417 | 91.1 | -- | 94.1 | -- | -- | 188.2 | -- |
| Plössl (disc) | 17 | 50 | 12 | 1¼" | 47.1 | 4.3 | 1.0 | 245 | 53.6 | 94.1 | 94.1 | 117.6 | 141.2 | 188.2 | 235.3 |
| Nagler 5 | 16 | 82 | 10 | 1¼" | 50.0 | 4.0 | 1.6 | 379 | 82.9 | 100.0 | 100.0 | 125.0 | 150.0 | 200.0 | 250.0 |
| Nagler 2 (disc) | 16 | 82 | 10 | 2" & 1¼" | 50.0 | 4.0 | 1.6 | 375 | 82.1 | 100.0 | 100.0 | 125.0 | 150.0 | 200.0 | 250.0 |
| Panoptic (disc) | 15 | 68 | 10 | 1¼" | 53.3 | 3.8 | 1.2 | 293 | 64.1 | 106.7 | 106.7 | 133.3 | 160.0 | 213.3 | 266.7 |
| DeLite | 15 | 62 | 20 | 1¼" | 53.3 | 3.8 | 1.1 | 274 | 60.0 | 106.7 | 106.7 | 133.3 | 160.0 | 213.3 | 266.7 |
| Wide Field (disc) | 15 | 65 | 9 | 1¼" | 53.3 | 3.8 | 1.1 | 264 | 57.8 | 106.7 | 106.7 | 133.3 | 160.0 | 213.3 | 266.7 |
| Plössl | 15 | 50 | 10 | 1¼" | 53.3 | 3.8 | 0.90 | 216 | 47.3 | 106.7 | 106.7 | 133.3 | 160.0 | 213.3 | 266.7 |
| Delos | 14 | 72 | 20 | 1¼" | 57.1 | 3.5 | 1.2 | 297 | 64.9 | 114.3 | 114.3 | 142.9 | 171.4 | 228.6 | 285.7 |
| Radian (disc) | 14 | 60 | 20 | 1¼" | 57.1 | 3.5 | 1.0 | 247 | 54.0 | 114.3 | 114.3 | 142.9 | 171.4 | 228.6 | 285.7 |
| Ethos | 13 | 100 | 15 | 2" & 1¼" | 61.5 | 3.3 | 1.6 | 382 | 83.6 | 123.1 | 123.1 | 153.8 | 184.6 | 246.2 | 307.7 |
| Nagler Original (disc) | 13 | 82 | 18 | 2" & 1¼" | 61.5 | 3.3 | 1.3 | 305 | 66.8 | 123.1 | 123.1 | 153.8 | 184.6 | 246.2 | 307.7 |
| Nagler 6 | 13 | 82 | 12 | 1¼" | 61.5 | 3.3 | 1.3 | 302 | 66.0 | 123.1 | 123.1 | 153.8 | 184.6 | 246.2 | 307.7 |
| DeLite | 13 | 62 | 20 | 1¼" | 61.5 | 3.3 | 0.99 | 237 | 51.8 | 123.1 | 123.1 | 153.8 | 184.6 | 246.2 | 307.7 |
| Plössl (disc) | 13 | 50 | 9 | 1¼" | 61.5 | 3.3 | 0.78 | 187 | 40.9 | 123.1 | 123.1 | 153.8 | 184.6 | 246.2 | 307.7 |
| Nagler 4 (disc) | 12 | 82 | 17 | 2" & 1¼" | 66.7 | 3.0 | 1.2 | 293 | 64.1 | 133.3 | 133.3 | 166.7 | 200.0 | 266.7 | 333.3 |
| Nagler 2 (disc) | 12 | 82 | 11 | 2" & 1¼" | 66.7 | 3.0 | 1.2 | 281 | 61.5 | 133.3 | 133.3 | 166.7 | 200.0 | 266.7 | 333.3 |
| Delos | 12 | 72 | 20 | 1¼" | 66.7 | 3.0 | 1.1 | 257 | 56.3 | 133.3 | 133.3 | 166.7 | 200.0 | 266.7 | 333.3 |
| Radian (disc) | 12 | 60 | 20 | 1¼" | 66.7 | 3.0 | 0.90 | 216 | 47.3 | 133.3 | 133.3 | 166.7 | 200.0 | 266.7 | 333.3 |
| Nagler 6 (disc) | 11 | 82 | 12 | 1¼" | 72.7 | 2.8 | 1.1 | 255 | 55.9 | 145.5 | 145.5 | 181.8 | 218.2 | 290.9 | 363.6† |
| Nagler Original (disc) | 11 | 82 | 15 | 2" & 1¼" | 72.7 | 2.8 | 1.1 | 255 | 55.9 | 145.5 | 145.5 | 181.8 | 218.2 | 290.9 | 363.6† |
| DeLite | 11 | 62 | 20 | 1¼" | 72.7 | 2.8 | 0.84 | 201 | 43.9 | 145.5 | 145.5 | 181.8 | 218.2 | 290.9 | 363.6† |
| Plössl | 11 | 50 | 8 | 1¼" | 72.7 | 2.8 | 0.65 | 156 | 34.1 | 145.5 | 145.5 | 181.8 | 218.2 | 290.9 | 363.6† |
| Plössl (disc) | 10.5 | 50 | 7 | 1¼" | 76.2 | 2.6 | 0.62 | 149 | 32.6 | 152.4 | 152.4 | 190.5 | 228.6 | 304.8 | 381.0† |
| Ethos | 10 | 100 | 15 | 2" & 1¼" | 80.0 | 2.5 | 1.3 | 303 | 66.4 | 160.0 | 160.0 | 200.0 | 240.0 | 320.0 | 400.0† |
| Delos | 10 | 72 | 20 | 1¼" | 80.0 | 2.5 | 0.91 | 218 | 47.6 | 160.0 | 160.0 | 200.0 | 240.0 | 320.0 | 400.0† |
| Radian (disc) | 10 | 60 | 20 | 1¼" | 80.0 | 2.5 | 0.75 | 180 | 39.4 | 160.0 | 160.0 | 200.0 | 240.0 | 320.0 | 400.0† |
| Nagler 6 | 9 | 82 | 12 | 1¼" | 88.9 | 2.3 | 0.89 | 213 | 46.5 | 177.8 | 177.8 | 222.2 | 266.7 | 355.6† | 444.4† |
| Nagler Original (disc) | 9 | 82 | 12 | 2" & 1¼" | 88.9 | 2.3 | 0.88 | 211 | 46.1 | 177.8 | 177.8 | 222.2 | 266.7 | 355.6† | 444.4† |
| DeLite | 9 | 62 | 20 | 1¼" | 88.9 | 2.3 | 0.69 | 165 | 36.0 | 177.8 | 177.8 | 222.2 | 266.7 | 355.6† | 444.4† |
| Ethos | 8 | 100 | 15 | 2" & 1¼" | 100.0 | 2.0 | 1.00 | 238 | 52.1 | 200.0 | 200.0 | 250.0 | 300.0 | 400.0† | 500.0† |
| Delos | 8 | 72 | 20 | 1¼" | 100.0 | 2.0 | 0.71 | 170 | 37.1 | 200.0 | 200.0 | 250.0 | 300.0 | 400.0† | 500.0† |
| Radian (disc) | 8 | 60 | 20 | 1¼" | 100.0 | 2.0 | 0.59 | 142 | 31.1 | 200.0 | 200.0 | 250.0 | 300.0 | 400.0† | 500.0† |
| Plössl | 8 | 50 | 6 | 1¼" | 100.0 | 2.0 | 0.47 | 111 | 24.4 | 200.0 | 200.0 | 250.0 | 300.0 | 400.0† | 500.0† |
| Plössl (disc) | 7.4 | 50 | 5 | 1¼" | 108.1 | 1.9 | 0.44 | 106 | 23.3 | 216.2 | 216.2 | 270.3 | 324.3 | 432.4† | 540.5† |
| Nagler 6 | 7 | 82 | 12 | 1¼" | 114.3 | 1.8 | 0.69 | 166 | 36.4 | 228.6 | 228.6 | 285.7 | 342.9 | 457.1† | 571.4† |
| Nagler Original (disc) | 7 | 82 | 10 | 1¼" | 114.3 | 1.8 | 0.67 | 161 | 35.3 | 228.6 | 228.6 | 285.7 | 342.9 | 457.1† | 571.4† |
| DeLite | 7 | 62 | 20 | 1¼" | 114.3 | 1.8 | 0.54 | 129 | 28.1 | 228.6 | 228.6 | 285.7 | 342.9 | 457.1† | 571.4† |
| Ethos | 6 | 100 | 15 | 2" & 1¼" | 133.3 | 1.5 | 0.74 | 178 | 39.0 | 266.7 | 266.7 | 333.3 | 400.0† | 533.3† | 666.7† |
| Delos | 6 | 72 | 20 | 1¼" | 133.3 | 1.5 | 0.54 | 130 | 28.5 | 266.7 | 266.7 | 333.3 | 400.0† | 533.3† | 666.7† |
| Radian (disc) | 6 | 60 | 20 | 1¼" | 133.3 | 1.5 | 0.45 | 108 | 23.6 | 266.7 | 266.7 | 333.3 | 400.0† | 533.3† | 666.7† |
| Nagler Zoom | 6 - 3 | 50 | 10 | 1¼" | 133.3 - 266.7 | 1.5 - 0.75 | 0.37 - 0.19 | 87 - 45 | 19.1 - 9.8 | 266.7 - 533.3† | 266.7 - 533.3† | 333.3 - 666.7† | 400.0† - 800.0† | 533.3† - 1,066.7† | 666.7† - 1,333.3† |
| Nagler 6 | 5 | 82 | 12 | 1¼" | 160.0 | 1.3 | 0.50 | 120 | 26.3 | 320.0 | 320.0 | 400.0† | 480.0† | 640.0† | 800.0† |
| DeLite | 5 | 62 | 20 | 1¼" | 160.0 | 1.3 | 0.38 | 91 | 19.9 | 320.0 | 320.0 | 400.0† | 480.0† | 640.0† | 800.0† |
| Radian (disc) | 5 | 60 | 20 | 1¼" | 160.0 | 1.3 | 0.38 | 91 | 19.9 | 320.0 | 320.0 | 400.0† | 480.0† | 640.0† | 800.0† |
| Nagler Original (disc) | 4.8 | 82 | 7 | 1¼" | 166.7 | 1.2 | 0.47 | 113 | 24.8 | 333.3 | 333.3 | 416.7† | 500.0† | 666.7† | 833.3† |
| Ethos SX | 4.7 | 110 | 15 | 2" & 1¼" | 170.2 | 1.2 | 0.64 | 153 | 33.5 | 340.4 | 340.4 | 425.5† | 510.6† | 680.9† | 851.1† |
| Delos | 4.5 | 72 | 20 | 1¼" | 177.8 | 1.1 | 0.40 | 96 | 21.0 | 355.6† | 355.6† | 444.4† | 533.3† | 711.1† | 888.9† |
| DeLite | 4 | 62 | 20 | 1¼" | 200.0 | 1.0 | 0.31 | 74 | 16.1 | 400.0† | 400.0† | 500.0† | 600.0† | 800.0† | 1,000.0† |
| Radian (disc) | 4 | 60 | 20 | 1¼" | 200.0 | 1.0 | 0.30 | 72 | 15.8 | 400.0† | 400.0† | 500.0† | 600.0† | 800.0† | 1,000.0† |
| Nagler Zoom (disc) | 4 - 2 | 50 | 10 | 1¼" | 200.0 - 400.0† | 1.0 - 0.50 | 0.24 - 0.12 | 57 - 29 | 12.4 - 6.4 | 400.0† - 800.0† | 400.0† - 800.0† | 500.0† - 1,000.0† | 600.0† - 1,200.0† | 800.0† - 1,600.0† | 1,000.0† - 2,000.0† |
| Ethos SX | 3.7 | 110 | 15 | 2" & 1¼" | 216.2 | 0.93 | 0.50 | 121 | 26.4 | 432.4† | 432.4† | 540.5† | 648.6† | 864.9† | 1,081.1† |
| Nagler 6 | 3.5 | 82 | 12 | 1¼" | 228.6 | 0.88 | 0.34 | 82 | 18.0 | 457.1† | 457.1† | 571.4† | 685.7† | 914.3† | 1,142.9† |
| Delos | 3.5 | 72 | 20 | 1¼" | 228.6 | 0.88 | 0.32 | 75 | 16.5 | 457.1† | 457.1† | 571.4† | 685.7† | 914.3† | 1,142.9† |
| Radian (disc) | 3 | 60 | 20 | 1¼" | 266.7 | 0.75 | 0.24 | 57 | 12.4 | 533.3† | 533.3† | 666.7† | 800.0† | 1,066.7† | 1,333.3† |
| DeLite | 3 | 62 | 20 | 1¼" | 266.7 | 0.75 | 0.23 | 55 | 12.0 | 533.3† | 533.3† | 666.7† | 800.0† | 1,066.7† | 1,333.3† |
| Nagler 6 (disc) | 2.5 | 82 | 12 | 1¼" | 320.0 | 0.63 | 0.24 | 58 | 12.8 | 640.0† | 640.0† | 800.0† | 960.0† | 1,280.0† | 1,600.0† |
| Design | Focal Length (mm) | Apparent Field (°) | Eye Rel. (mm) | Barrel Size | Mag. (x) | Exit Pupil (mm) | True Field (°) | Star Drift Time (sec) | True Field @ 1000 yards (feet) |
Mag. w/2x Barlow (2x) | Mag. w/2x Big Barlow or Power- mate (2x) | Mag. w/2.5x Power- mate (2.5x) | Mag. w/3x Barlow (3x) | Mag. w/4x Power- mate (4x) | Mag. w/5x Power- mate (5x) |
Table Notes
For further reading, see Eyepieces Advice.
†Tele Vue recommends maximum magnification of 60x / inch (2.5x / mm) of telescope aperture and 350x maximum regardless of aperture.
*An exit pupil greater than 7.5-mm may be too big for a telescope with central obstruction (most reflectors). Conversely, small exit pupils may reveal eyesight defects such as floaters. Consider binocular viewing to diminish the effect of an eye having floaters.