Performance indicators are: pixel scale, FOV, extended object irradiance, point object irradiance, etendue and signal.

Version 1.0

Source code at https://github.com/d33psky/compare-telescopes/

This is the web version of the Python CLI program.

Pixel Scale, or pixel resolution, is the solid angle that is projected on a single pixel.
It is measured in arcseconds per pixel, `["/pixel]`

.

Formula:

```
pixel scale ["/pixel] = 206.265 [k"] * pixel size [μm/pixel] / focal length [mm]
```

With 206.265 the amount of arcseconds per radian / 1000 .
And `arcseconds per radian = (360 / (2 * pi)) * 60 * 60 = 206264.80624709635515795...`

FOV, Field Of View, is the solid angle that is projected on the camera sensor.

Formula:

```
angle_x ["] = camera_pixels_x [pixels] * pixel scale ["/pixel]
angle_y ["] = camera_pixels_y [pixels] * pixel scale ["/pixel]
```

FOV is displayed in arcminutes `[']=["/60]`

.

Extended Object Irradiance is the radiant flux (power) received by the sensor per unit area of an extended object.
Extended Object Irradiance is measured in `[W/m^2]`

.
We do not compute the irradiance itself because the ratio suffices and that varies as the inverse square of the focal ratio.
**Aperture size alone does not matter for Extended Object Irradiance, only focal ratio does.** (Aperture size does matter for Point Object Irradiance).
An extended object is anything that is not a point source, where a point source can be a star or anything else close to the size of the angular PSF projected onto the sky.

Formula:

```
Extended_Object_Irradiance_ratio = 1 / (focal ratio of ota 1/focal ratio of ota 2)^2
```

The Extended Object Irradiance is also known as the Speed of a film camera where an f/4 is twice as fast as an f/5.6, meaning you need only half the time.

Point Object Irradiance is the radiant flux (power) received by the sensor per unit area of a point object.
For point objects such as stars the image irradiance varies as the aperture area ratio and the inverse square of the focal ratio.
**Aperture size matters for Point Object Irradiance, as well as focal ratio.** (Aperture size alone does not for Extended Object Irradiance).

Formula:

```
Point_Object_Irradiance_ratio = (ota 1 aperture area/ota 2 aperture area) * 1 / (focal ratio of ota 1/focal ratio of ota 2)^2
```

Pixel Etendue represents a measure of the size and angular spread of a beam of light onto a pixel.
Etendue is a system property of the OTA, `G = aperture_area * pi * NA^2`

. It is translated to a single pixel here.
Etendue is also known as light-gathering or light-collecting power.

Formula:

```
pixel_etendue = aperture_area [mm^2] * pixel_scale^2 ["^2/pixel]
```

Classically we should use `aperture_area * pi * (pixel_scale/2)^2`

instead. But for the ratio this does not matter.

Pixel Signal is the Pixel Etendue corrected for the sensor Quantum Efficiency and total optical system Transmittance losses.

Formula:

```
pixel_signal = pixel_etendue * QE-factor * Transmittance-factor
```