AFOV is the window
A wider apparent field lets you keep the same magnification while seeing more sky around the target.
Astro Lab
Visual observing simulator
Eyepiece View Simulator
See how much sky an eyepiece really shows. Change magnification and apparent field of view, then compare the Moon and planets at a physically scaled true field of view.
1. Telescope
Enter aperture and native focal length. No Barlow or reducer is assumed here.
Primary mirror or objective diameter
Native telescope focal length
Focal ratiof/6.0
Approx. useful ceiling400×
2. Target
Planet sizes change with distance, so you can adjust the apparent angular diameter.
Moon varies by a few arcminutes through its orbit.
31.1′
3. Eyepiece view
Magnification controls object scale; AFOV controls the width of the apparent window around it.
Equivalent eyepiece
10.0 mm
True field of view
0.57°
Exit pupil
1.67 mm
Quick eyepieces:
What the simulator is showing
The circular field is scaled from the target's angular size and the eyepiece's calculated true field of view.
Power enlarges, not resolves
Magnification makes the image larger. It cannot create detail that the aperture, optics and atmosphere did not resolve.
Exit pupil controls brightness
Very small exit pupils make extended objects visibly dimmer; very large ones can waste aperture if they exceed your eye pupil.
Seeing still matters
Even a mathematically sensible high power can look soft on a turbulent night. Check the Planetary Seeing Forecast before pushing magnification.
The simulator uses representative spacecraft/telescope imagery as a visual reference, not a promise of photographic detail at the eyepiece. Image sources: Moon — NASA Scientific Visualization Studio / LRO; Jupiter — NASA, ESA, STScI, Amy Simon (NASA-GSFC); image processing: Joseph DePasquale (STScI); Saturn — NASA/JPL-Caltech/Space Science Institute; Mars — NASA, ESA, Zoltan Levay (STScI). Planetary apparent diameters vary with distance, which is why the target-size control is adjustable.