Ocular plugin

From Stellarium

Contents 1 Description 2 Using the Ocular plugin 3 Configuration 4 Exit Circle Scaling 4.1 Example in action 5 How you can help

[edit] Description

The purpose of this plugin is to simulate what a user would see through their eyepiece when viewing an object. The plugin takes into account all aspects of the viewing system to produce as accurate a view as possible. Up to 10 different telescope eyepieces can be configured, to help you choose the best eyepiece for a given object. This makes eyepiece - or telescope - comparison easier.

[edit] Using the Ocular plugin

1. Select an object to view (i.e. a star, planet, etc.)
2. Click the tool bar button for toggling the Ocular mode, or press Command-O (control-o for non-Mac users).
3. Swap between Oculars using the keys 0 through 9.

[edit] Configuration

Each Ocular is defined in the module.ini file, which can be found in <user data directory>/modules/Ocular/module.ini. This file can be edited using a regular text editor.

In the setting file under the [options] heading you can add

max_exit_circle = 5.7

max_exit_circle 

The maximum exit circle - or exit pupil - produced by any of your eyepieces, in mm.

This setting may be a bit difficult to understand, and if it is, just delete it or don't add it. See below for an explanation. If not present, scaling will be disabled for all eyepieces.

Each section ocular0 .. ocular9 defines the ocular which will be selected when the 0 .. 9 keys are pressed while ocular mode is enabled. Here's an example section from the module.ini file:

[ocular0]
name = Occular 0
afov = 80
efl = 20
sfl = 2082
flip = h
exit_circle = 4.1

name 

A free-text description of the ocular. You could modify this to match your personal descriptions of eyepieces.

afov 

Apparent field of view in degrees in degrees

efl  

Eyepiece focal length in mm

sfl  

Telescope scope focal length in mm

flip 

If the view through this ocular should flip horizontally (use h), vertically (use v) or both horizontally and vertically (use hv).

exit_circle  

The exit circle produced by the eyepiece when used with the specified telescope, in mm. If not present or blank, scaling will be off for this eyepiece.

Edit these to match your needs!

[edit] Exit Circle Scaling

By default, the view drawn on your computer screen when in Ocular mode fills the screen. This can be a problem if you a) don't understand optics, or b) really want to emulate what you'd see with a particular eyepiece as compared to another eyepiece. So why is this a problem? Typically it's only an issue when comparing two different focal length eyepieces, of fairly different quality; imagine the following scenario.

Let's say you have a 40mm eyepiece and a 32mm eyepiece. Let's say the 32mm eyepiece is more expensive, and has a wider aFOV. Maybe the 40mm is 55 °, and the 32mm is 82 °. With the same telescope used, we know that the 32mm eyepiece will have a higher magnification; so when viewing the moon with the Ocular plugin enabled, you'd expect the moon to appear bigger on screen than it does with the 40m eyepiece. This is where the problem occurs.

Your computer screen is a fixed size. So each of the above eyepieces will draw the the same size circle on your computer screen. The thing is, even though the 32mm produces a higher magnification, it also has a wider aFOV. This means you see more sky with the 32mm. And because the computer screen is fixed, it actually has to scale down the view of the 32mm to make it fit. So why, when looking through the actual eyepieces, does the image through the 32mm show the moon larger, even with more sky showing? Because the exit circle produced by the 32mm eyepiece will be larger. Your eye can take the larger image, and that's fine. But the plugin must scale the images to show on the same size computer screen.

To compensate for this, as of Ocular version 0.9.0, I've added an eyepiece exit circle scaling feature. How it works is this: you set the value of max_exit_circle to be the largest value of any of your eyepieces in any of your telescopes. This will usually be the longest fl eyepiece with the highest aFOV, in the shortest fl telescope. So my 32mm eyepiece with an aFOV of 82 °s in my 80mm telescope is going to produce a much larger exit circle than my 7.5mm eyepiece with an aFOV of 55 °s in my 14inch telescope. Now, for each eyepiece, set exit_circle in the OcularX section to be the exit circle produced by that eyepiece in that scope.

Now, the plugin will scale the image displayed on screen to compensate. If max_exit_circle = 5.7, and you select an Ocular with exit_circle = 5.7, the image on screen will fill the screen. If you then select an eyepiece with exit_circle = 2.8, the the image on screen will fill half of the height of the screen.

This gives an accurate a comparison of eyepieces as possible. But it may not be what you want. You may want each image on screen to fill the screen. If that is the case, simply comment out - or delete - the max_exit_circle setting in the modules.ini file for this plugin.

What it's used for is this: the image circle produced by eyepieces differs not only by eyepiece, but also the telescope that it is used in. What is the exit circle? It is the circle of light that the eyepiece projects into your eye. If you were to hold a piece of paper up the the eyepiece, at the distance specified by the eyepieces eye relief, then the exit circle is the size of circle of light you'd see.

[edit] Example in action

Let's see what all of this means in practice.

This is an image with a 40mm EP, 43° aFOV, with a 14" telescope. Magnification is 97x.

This is an image with a 31mm EP, 82° aFOV, with a 14" telescope. Magnification is 126x.

Notice that the bottom image shows the moon as smaller on the screen, and that you see a star or two in the surrounding sky. Even at a higher magnification, the moon appears smaller. This is because no attempt at correcting for the exit circle has been made, and each image fills the computer screen. Now, lets look at the same two EP's, but with correction enabled.

This is an image with a 40mm EP, 43° aFOV, with a 14" telescope at magnification is 97x. Exit circle is 1.7mm.

This is an image with a 31mm EP, 82° aFOV, with a 14" telescope at magnification is 126x. Exit circle is 3.1mm.

Now we see that the higher magnification eyepiece does indeed show a larger image. Neither image fills the screen, as max_exit_circle = 5.7, and the larger of the two EPs used here is 3.1 You still see the background star, as you see more sky with the second image, and the greater aFOV.

I hope this helps explain this complex feature.

[edit] How you can help

A TODO list is maintained in the README file for the plugin. If you are able to help with any item in this list, please contact the Stellarium developer team via the stellarium-pubdevel mailing list.

We also welcome bug reports, feature requests and feedback through the usual channels (trackers, forums and so on).