Originally posted by Collinb I would like to figure out how to use it. Can't find instructions anywhere.
It has a 4-division reticle inside with each division having 10 divisions.
It appears that these may represent angles, with the outside edges being the full 6.2 degrees. But I could be wrong on that.
I have one of these. It is designed to be both a monocular telescope (for distant things) and a low-power microscope (for viewing small things). There are two optical pieces: the monocular itself, and a clear plastic stand which contains a lens and a focusing ring.
To use it as a monocular, you simply look through it like any monocular, ignoring the measuring reticle. The eyepiece focuser allows you to focus on whatever distant object you are looking at. In this mode, it is 8 X magnification and has a 6.2 degree field of view.
To use it as a microscope, you screw the threads on the upper end of the stand into the front end of the monocular, forming a single longer unit. While looking through the eyepiece, you adjust the eyepiece focuser to make the reticle sharp. Then you place the plastic stand on something flat (e.g. a piece of paper with printing on it, a slide mount with film in it, etc.) and use the focusing ring on the stand to bring the object under the stand into focus. In this mode, the magnification is about 25 X, comparable to mid-power on a stereo microscope.
The reticle is intended for use in microscope mode. It can be used to measure the size of objects under the microscope in mm. The reticle covers 4 mm in 0.1 mm steps. Try putting the microscope on top of a metric ruler and see how accurate the graduations are. On mine, they are very accurate.
Although the reticle was intended to measure distances in microscope mode, it can also be used to measure angles in monocular/telescope mode. (The lens in the stand creates a virtual image of the object being examined at infinity, effectively converting distances on the object into angular separation in the image. Then the monocular focuses on this virtual image and converts angular distance back into linear distance on the reticle.) To use the reticle to measure angles, you just need to know what the reticle scale means in angular terms.
By calculation: I assumed that the overall magnification of the devices in microscope mode is 25 X. Then the magnification of the lens in the base is 25/8 = 3.125 X. This means the lens focal length is 250/3.125 = 80 mm. A 1 mm distance on the microscope's subject becomes an angle difference of 1/80 = 0.0125 radians when placed at the focus of an 80 mm lens. So, 1 mm distance on the reticle scale (10 divisions) represents an angle of 12.5 milliradians. (If you aren't familiar with milliradians, one milliradian is the angular size of a 1 foot long object viewed from 1000 feet, or a 1 mm long object viewed from 1 m).
Then I tried measuring the size of a door in my house using the monocular reticle, and measuring its width and distance directly with a laser distance measurer. Using this method, I calculated a scale factor of 12.7 milliradians per mm on the reticle. That's close enough to 12.5 mrad/mm to be nothing more than experimental error. So I think 12.5 mrad/mm is a good value to use when measuring angles.
An example of how you might use this: You sight a tree some distance away, and measure its trunk diameter as 2 mm on the reticle. That is an angle of 12.5 x 2 = 25 milliradians. This means that the diameter of the real object is 25/1000 of the distance to the real object. If the tree is 10 m away (10000 mm), then its trunk diameter is 250 mm. On the other hand, if the tree is 40 m away, the trunk is 1 m in diameter.
Similarly, if you know the size of something (a human standing up, a golf hole flag, etc) and you measure its angular size in milliradians, you can calculate how far away from it you are.
- Dave