A star hop moves from a star you know through patterns you can match on a chart
- Pick a starting star bright enough to see without optics, near the target.
- Plan the route on a chart: a line of stars, a triangle or a distinctive pair that leads toward the target, in steps no larger than your finder's field.
- Center the starting star in the finder and match the stars you see to the chart.
- Move one field at a time, matching patterns at each step, until the target's position is centered.
- Look in the telescope with your lowest-power eyepiece. A faint target is usually seen first with averted vision: look slightly to one side of it.
Charts help most when they match what you see. A constellation chart gets you to the starting star; a finder chart of stars to magnitude 8 or 9 covers the hop.
Know the size of your finder and eyepiece fields before you start
Every hop is counted in fields, so measure yours once. The true field is the patch of sky an instrument shows, in degrees.
| Finder or eyepiece | Field of view | How it shows the sky |
|---|---|---|
| Red-dot finder | no magnification | A dot projected on the sky; aim at naked-eye stars |
| Telrad and similar reflex finders | rings of 0.5°, 2° and 4° | Rings on the sky; lets you measure offsets from naked-eye stars |
| 6x30 straight-through finder | about 7° | Upside down; awkward near the zenith |
| 8x50 right-angle correct-image finder | about 6° | Matches the chart; comfortable to look through |
| 32 mm eyepiece in an 8-inch f/6 Dob (38x) | about 1.3° | Mirror-reversed or upside down, depending on the telescope |
To measure a field, center a star near the celestial equator, turn off the drive, and time how long the star takes to cross the field. Seconds divided by 240 gives degrees.
Your hand at arm's length measures degrees on the sky
- Little finger: about 1°, twice the width of the full Moon
- Three middle fingers together: about 5°
- Closed fist: about 10°
- Spread hand, thumb tip to little finger tip: about 20° to 25°
Hands and arms scale together, so these work for most adults. The pointer stars of the Big Dipper, 5.4° apart, are a good check.
Five worked hops to showpiece Messier objects
Separations are computed from the positions in this site's bright star and Messier catalogs. All five suit any telescope with an 8x50 finder.
| Target | Constellation | Start | The hop | Start to target (°) |
|---|---|---|---|---|
| M31, Andromeda Galaxy | Andromeda | Mirach (beta And), magnitude 2.1 | From Mirach go northwest 3.9° to mu And, then on 2.9° to fainter nu And. M31 is 1.3° beyond nu, in the same finder field. | 7.7 |
| M13, Hercules Cluster | Hercules | Eta Herculis and Tianji (zeta Her), the west side of the Keystone | The two stars are 7.3° apart. M13 sits a third of the way from eta to Tianji (2.5° from eta), just off the line. | 2.5 |
| M57, Ring Nebula | Lyra | Sheliak (beta Lyr) and Sulafat (gamma Lyr) | The two stars are 2.0° apart, both in one finder field. M57 lies on the line between them, 0.8° from Sheliak: look for a tiny gray smoke ring at 100x or more. | 0.8 |
| M81 and M82 | Ursa Major | Phecda (gamma UMa) and Dubhe (alpha UMa) | Draw a line across the bowl of the Big Dipper from Phecda to Dubhe (10.4°) and extend it the same distance again. M81 is 10.1° from Dubhe, slightly off the line; M82 is 37′ north of it in the same eyepiece field. | 10.1 |
| M27, Dumbbell Nebula | Vulpecula | Telum (gamma Sge), the tip of the arrow | Center Telum in the finder and move 3.2° due north. M27 is in the same finder field, a soft glow about 8′ across, a quarter the width of the full Moon. | 3.2 |
When a hop fails, the usual causes are a reversed view or too faint a target
- Reversed view: Newtonian telescopes show the sky upside down; refractors and Schmidt-Cassegrains with a star diagonal show it mirror-reversed. Flip the chart to match, or use a correct-image finder for the hop.
- Too much magnification: start every search with the lowest-power eyepiece.
- The sky is too bright: galaxies such as M81 need a dark sky; see what each telescope reaches.
- Dark adaptation: a white phone screen resets your night vision; use red light.