Using the Virtual Micrometer

Setting up the defaultParameters file

Before you use the virtual micrometer for the first time, you'll need to enter your eyepiece focal lengths and field sizes in arc seconds into your defaultParameters file. Here are the entries for my C-8:

# List up to 9 eyepieces for double star observation here.
Eyepiece focal length mm / field arc seconds:2.8/189 
Eyepiece focal length mm / field arc seconds:6/434
Eyepiece focal length mm / field arc seconds:9/639
Eyepiece focal length mm / field arc seconds:12.5/866
Eyepiece focal length mm / field arc seconds:24/2676

A rough estimate of field size is given by:

F = Afe * Ef * 3600 / Tf


A more accurate estimate is given by how long, in seconds, it takes a star to drift across the Center of the eyepiece's field. Once you have this time, the field is given by:

F = S * cos(d) * 15.04017


Using the virtual micrometer

The virtual micrometer is invoked when the finder chart for a double star is displayed. For example, assume we're observing STTA 73:

STTA 73 finderchart

If you right click anywhere on the chart, the virtual micrometer will come up:

The left panel will contain the micrometer, the right panel contains:


Estimating the position angle

Clicking the left hand panel brings up the WDS position angle in degrees and separation angle in arc seconds. The field of view shown is scaled to the first eyepiece in the defaultParameters list:


Click the PA button to begin your position angle measurement.

Perhaps the best way to estimate position angle is to use a reticle (crosshair) eyepiece that is accurately alligned along the north south axis. The vertical line in the micrometer will correspond to the north south reticle line.

However, this might not be very practical with the short focal length eyepieces required for accurate double star measurements. A 3x barlow might help increase the power.

Another method, that only works with an equatorial mount, is to slew the telescope in declination. This will give a clear indication of where the north - south line is.

Clicking anywhere on the circle changes the position angle to that position. In this way, you can transfer what you see in the eyepiece to the micrometer. Note that the position angle measurement label changes to reflect your input.

Position Angle Measurement

Estimating the separation angle

Once you've got the position angle set, click the "Sep" button to begin the separation procedure. A line perpendicular to your position angle is drawn. This reflects the fixed and movable position angle reticle lines in a mechanical bi-filar micrometer. Initially both lines are atop one another reflecting zero are seconds separation.


Clicking anywhere along the position angle line moves the movable reticle. Move it until the separation between the two reticle lines matches the difference between the centers of the two stars in the eyepiece.


Once you're satisfied with your measurement, you can click the Add observation button. This brings up the regular double star observation form, but with the position angle and separation angle filled in.


Record any notes your want to add in the large text box, and then click "Add the observation" to save it.

You can then cancel the Virtual Micrometer for this star.