Note: This page requires Javascript for its functionality.
Magnification = Objective Focal Length / Eyepiece Focal Length
Field of View = Eyepiece Apparent Field / Magnification
Exit Pupil = Objective Diameter / Magnification
1. Enter the Scope Parameters. All fields must be filled in. You can click the Clear Form button to clear all entries in the form if desired.
2. Select the Object to be observed. The Objects available in the form are the brightest and best in each category of Deep Sky Object (DSO).
For Astromartin's full list of Best DSOs, click the Navigation button at the top of the page.
3. Select the Sky Transparency (Darkness) for your observing site. M45 (Pleiades) is very bright and will be well seen even in an urban sky.
NGC104, a globular star cluster visible only in the southern hemisphere, is moderately bright, but will appear diminished in a bright sky.
M57 (Ring Nebula) is a faint object, but due to its small angular size it shows good contrast and will be visible in a bright sky.
M31 (Andromeda Galaxy) has a bright central core that will be visible in an urban sky, but the fainter outer regions will only be seen in a dark sky.
4. Click the Compute button.
5. See the results in the OUTPUT box.
In addition to the input Scope Parameters, you will see the Magnification, Field of View and Exit Pupil.
6. Click the Draw Object button to display a simulated image of the selected Deep Sky Object (DSO) as it will appear in the eyepiece of your telescope set-up.
If the angular size of the selected Object is larger than the Field of View, the full extent of the object will not be seen.
The size of the image in relation to the Field of View is accurately displayed, just as it will appear in the telescope.
7. You can change the Eyepiece Focal Length and Apparent Field of View in the Scope Parameters form.
If you then click the Compute and Draw Object buttons, the new OUTPUT data and simulated image will be displayed.
8. You can select a different Object (DSO) and a different Sky Transparency at any time.
If you then click the Draw Object button, the image displayed will reflect the new selections.
9. Enjoy!