Thanks to the helical focuser on the finder/guider scope in Vernet, we can accurately tell the difference in focus position for various eyepieces etc as follows.
Furthest in is the USB camera I’ve adapted so call that position 0
- Svbony graticule +6mm
- Altair ( when inserted up to the end of the blue part ) +10mm
- Meade cheapo 25mm eyepiece +13mm
- Pentax zoom ( varies slightly with focal length ) +16mm
What’s also worth knowing is the field of view available from everything.
That depends on the telescope too of course
But if we know the flt of the telescope, the fle of the eyepiece and the apparent size of the view in degrees avdeg, then fov = avdeg * fle / flt
For a sensor, of course, that doesn’t work, and we need the alternative:
- fov = 2 * arctan (S / 2f),
- where S is the diagonal size of the sensor in millimeters, and
- f is the focal length of the lens in millimeters.
Of course for a telescope arctan is more or less the same as angle in radians, so the simpler formula
fov = S/f * 60 is good enough
