The webcam in astronomy acquired its letters of nobility with the planetary imagery.

It is pretty and that can be useful ...

At the cost of some modifications of electronics, it is possible to reach a little deep sky.

That is not necessarily useful but it is very funny ...

What is less known is that modified or not, a webcam permits to get very good images of double stars.
It is funny, it is pretty and that maybe useful ... at a condition : measure them.

The observers who tried to make double stars measurements know how much this activity
is demanding and sometimes frustrating whith amateur means :
- Calibration never easy
- Cost of a micrometer
- Results not always in respect with the personal and financial investment

Well, take a look at this living star

The pictures are acquired with an amateur 8" telescope and a 50$ webcam.
When you know that this couple is separated by about 1"6, it seems obvious that there is something to do ...

What are we looking for when measuring ?

The position angle of the secondary star
in relation to the main component

The angular separation between the components

We will meet two problems at the time of reduction :

We need to find the image orientation that depends of the orientation of the camera at the time of the shot

The determination of their scale (sampling)
that depends of the focal distance of the instrument

A camera fixed at the focus plane of a long focal distance instrument,
mounted on a permanent pier and with perfectly known instrumental constants seems ok but ... everybody doesn't have that in his garden!

Alas, the amateurs instruments rather look like that

Rarely in a permanent mount, some having even the troublesome tendency to possess a focal distance that is dependant of focusing (Schmidt-Cassegrain) !

Fortunately, thanks to its easiness of setting, the webcam allows us to calibrate our instrumentation very quickly at every session while using a list of standard stars.

It is one of the big advantages in relation to the classical systems that generally require long sessions of calibration.
Here, we can take the liberty to change optic combination from a session to the other.