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Calculation formulas for OVNI-M / OVNI-B
With an OVNI-M or an OVNI-B it is interesting to understand how to calculate the magnification, the field of observation and the focal ratio.
We will take for example an OVNI-M (26mm at F/1.2), a Televue eyepiece of 67mm and a telescope of 400mm (16") at F/4 (1600mm).
1/ The magnification.
- in prime : Focal length of the telescope / Focal length of the OVNI-M = 1600/26 = 61,5x
- in afocal : Focal length of the telescope / Focal length of the afocal eyepiece = 1600/67 = 23,88x
2/ Field of view in degrees.
Whether the OVNI-M is in prime or afocal mode, divide 40 by the magnification.
- in prime = 40/61.5 = 0.65°
- in afocal = 40/23.88 = 1.68°
3/ Focal ratio
- in prime Focal ratio is very simple, since it's the focal ratio of the telescope.
- in afocal, it depends on several parameters, taking for example a OVNI-M (26mm at F/1.2), a 67mm Televue eyepiece and a 400mm (16") telescope at F/4.
Here are 3 useful formulas for calculating the focal ratio (F/) in afocal:
- Focal length of the OVNI-M objective / (Focal length of the glass eyepiece / Focal ratio of the telescope) = 26/(67/4) = F/1.55
- Focal length of the OVNI-M eyepiece x Magnification / Focal ratio of the telescope / 100 = 26x23.88/4/100 = F/1.55
- (Focal length of the OVNI-M eyepiece / Focal length of the glass eyepiece) x (Focal length of the telescope / Focal ratio of the telescope) / 100 = (26/67)x(1600/4)/100 = F/1.55
- Focal ratio of the telescope x (Focal length of the OVNI-M objective / Focal length of the glass eyepiece) = 4x(26/67) = F/1.55
4/ The exit pupil
It's no longer the user's exit pupil that needs to be taken into consideration, but directly the one of the OVNI-M / OVNI-B. This is 22mm, so there's no longer any limit to the use of a long focal length eyepiece in afocal mode.
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Comments
Calculation Formulas
By: Joseph Guth On 10/01/2023Your calculation formulas page is super helpful.
It seems to me there might be an additional -- maybe simpler -- way to look at the calculation for focal ratio in afocal.
You can rearrange the first formula to look like this:
Focal ratio (F/) in afocal = (Focal ratio of the telescope) x (Focal length of the OVNI-M objective / Focal length of the glass eyepiece).
This seems simpler to me. It reveals that all that is happening is that in afocal the effective focal length is the focal length of the scope altered by a factor equal to the ratio of the focal lengths of the OVNI objective and the eyepiece. The aperture is obviously a constant.
Replied by: OVNI Night Vision On 11/06/2023