Oculars plugin

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This plugin serves several purposes:

  • the primary use is to see what the sky looks like through a particular combination of eyepiece and telescope. I wanted to be able to get an idea of what I should see when looking through a physical telescope, and understand why one eyepiece may be better suited to a particular target. This can also be very useful in deciding what telescope is best suited to a style of viewing. And with the support for binoculars, you now have the ability to understand just about any type of visual observing.
  • to show what a particular camera would be able to photograph of the sky. Also to better plan what type of telescope (or camera lens) to pair with a particular camera to capture what you want.
  • lastly, with the help of the Telrad sight, understand where object in the sky are in relation to each other. This can be very useful if you have a non-GOTO telescope, and to get an idea of how to star-hop from a known location to an area of interest.

None of these activities can take the plce of hands-on experience, but they are a good way to supplement your visual astronomy interests.

NOTE: this describes the 1.0 version of the plugin, which is distributed with the 0.11.0 release of Stellarium.


Using the Ocular plugin

The plugin is controlled through a popup menu (hot keys were disabled because of a lack of global ability to edit the keys used throughout the application). By default, the hot key to display the popup is Option-O (Alt-O for non-Mac users). This can be changed in the configuration dialog. The menu will popup where your cursor is located.

What options are available on the popup menu depend on what you are currently doing. The default popup looks like the following.


The menu is navigated by either the arrow keys on your keyboard, or by your mouse. The up and down arrow keys move the selection up or down the menu, and the left and right arrow keys display or hide sub-menus. NOTE: the numbers after the menu items should be activations keys; however, I have been unable to get thee to work. I am still trying to do so.

In the default menu, you can choose to configure the plugin, activate a CCD, or activate the Telrad finder.

CCD Sensors

This is a great way to get an idea of what a particular camera will be able to capture when attached to a particular telescope or lens. For camera lenses, you must still define a telescope with the appropriate values for the lens to be used. When activate, this feature will display a red bounding box of the area that will be captured, as well as zooming in to give a better view of the surroundings. You can manually zoom in or out from there.

The default CCD will display similar to the following.


In the information area in the upper right hand corner, you can see the angular size captured by the CCD.

When a CCD is displayed, the popup menu changes to the following.


Notice that you can select what telescope to use, as well as progress to the previous or next CCD, or go to a specific CCD. You can also rotate the CCD to better capture your subject, or to see if the CCD can be rotated in such a way as to catch the area of interest.


Once rotated, the image on screen display the new orientation.


Telrad Finder

The Telrad feature can be used without defining any of the items below. As a reflex sight is non-magnifying, this feature can only be enabled when no eye piece is selected.

The three circles that appear in the center of the screen are 0.5°, 2.0°, and 4.0° in diameter. They stay centered in the screen, so move the 'telescope' (click-drag the background) to center the circles on the object of interest. I find it useful to zoom in to better see what stars are in the circles. At the default angle of 60° on my 17" laptop screen, not too much is visible. Zooming in to around 40° gives a better image. The screen shots below show this.



The top image is the default 60°, and the bottom one is about 40°.

While the Telrad finder is active, the popup menu changes to the following; this reflects that you can not activate a CCD while the Telrad is active.



  1. Define some eye pieces and telescope (see below).
  2. Select an object to view (i.e. a star, planet, etc.)
  3. Click the tool bar button for toggling the Ocular mode, or press Command-O (control-o for non-Mac users).
  4. Swap between eye pieces and telescopes to see how the view changes.

This is really the area of interest to must users. This is a great way to compare different eyepiece/telescope combinations, to see how they change the view of the sky. And now, it's easy to do so with binoculars too. To show this, I'll use a target of the M37 cluster. Through a pair of Celestron 15x70 binoculars, it would look like the following.


A very pretty sight. Now, what would it look like through my Celestron 80mm EDF finder 'scope, with an Explore Scientific 14mm 100 degree eyepiece?


Not bad at all. But, I'd like to see more. So I move the eyepiece to my C1400 atop my CGE Pro.


Very nice indeed! So for this target, the C1400 is going to be the best bet. However, if my target was the Pleiades, the C1400 with that eyepiece would not be good; the 80EDF would do much better.

When an eyepiece is active, the popup menu again changes. With a non-binocular eyepiece selected, it looks like the following, where you also have the ability to select a particular eyepiece or telescope.

PopupOcular.png PopupOcular2.png

When a binocular is active, the menu changes to the following, where you can not select a telescope, as it is not relevant. Changing the eyepiece to a non-binocular will again allow the telescope to be selected. Also notice that as my mouse cursor was very near the right hand side of the screen, the popup menus sub-menus display to the left, not the right.



As-of Stellarium version 0.10.3, you no longer need to edit the ini file. All configuration is done through the user interface in the application. To open the configuration dialog hit the alt-O key, or click the configure button on the plugin setup dialog. There are four tabs in the configuration dialog; General, Eyepieces, Telescopes, and About'. The first three are the ones we are interested in here.


The first option allows you to define whether or not to scale the images based on apparent FOV. In general, I'd recommend you not select this, unless you have a need to. It can be very useful in comparing two eyepieces, but, for general use, it can really reduce the image size on the screen. If you set this option, the image on-screen will be scaled based on the eyepieces and telescopes you define. See the section below for information os what scaling means, and why you might want to use it.

The other two options allow you to define the key combinations to use to activate an eyepiece, and to activate the popup menu. General.png


This is the tab used to enter your own eyepieces. But default, a few sample ones are added; feel free to delete it once you've entered your own.

Eyepieces.png EyepiecesBinocs.png

The fields on this tab are:

A free-text description of the eye piece. You could modify this to match your personal descriptions of eyepieces.
Apparent field of view in degrees in degrees
Focal Length  
Eyepiece focal length in mm
Field Stop  
The field stop of the eyepiece in mm. This is used to calculate the true field of view of an eyepiece. If you do not know what it is just leave it the default zero. Not all manufacturers provide this value; TeleVue is one that does.
selecting this checkbox tells the system that this eyepiece is binoculars; this means that this eyepiece can be used without defining a telescope.

CCD Sensors

This tab allows you to define sensors for any camera you may have. When defined and selected, this will draw a red bounding rectangle in the center of the eye piece view, showing what the CCD will capture. Because of the way floating point numbers are stored, sometimes you may see one of your defined values change (for example from 2.2 to 2.19999) but this should not effect what you see.


The fields on this tab are:

A free-text description of the sensor.
Resolution x 
the width of the censor in pixels.
Resolution y 
the height of the censor in pixels.
Chip width 
the width of the censor in mm.
Chip height 
the height of the censor in mm.
Pixel width 
the width of an individual pixel, in microns.
Pixel height 
the height of an individual pixel, in microns.

The resolution is easy to find, even for DSLRs. The chip size and pixel size may be more difficult for a DSLR, but searching the internet should turn up these values.


This is the tab used to enter your own telescopes.


The fields on this tab are:

A free-text description of the telescope. You could modify this to match your personal description.
Focal Length  
Telescope scope focal length in mm
Telescope diameter (aperture) in mm
Horizontal flip 
If the view through this telescope should flip horizontally.
Vertical flip 
If the view through this telescope should flip vertically.

Scaling the eyepiece view

I'd like to thank Al Nagler over at TeleVue for helping to set me straight on the topic of eyepieces. They are a lot more complicated than you might think!

By default, the view drawn on your computer screen when the plugin is active fills the screen. That is, there is a circle drawn to represent the view through the eyepiece, and this circle will fill the screen. For general use, this is what most people would want. There will be times that it's not.

If you are going to be observing any deep space object, it can be very import to choose the best eyepiece for that object. You will typically want an eyepiece that will magnify the object as much as possible, while showing all of the object in the eyepiece view. Getting this can be tricky, especially if you do not like changing eyepieces at the telescope. Or maybe you want to understand why one type of telescope may be better for observing what you are interested in, more than another type of telescope. This is where you will want to scale the image on screen based on your eyepiece.

Different eyepieces will generally have a different apparent field of view (aFOV). An easy way to think about this is, the larger the aFOV, the bigger the picture you see in the eyepiece. Older eyepieces would generally have aFOV in the 50° range. Today, there are massive eyepieces with 82°, and recently even 100° aFOV! These eyepieces are huge, as they require a lot of very special glass to achieve their incredible field of views. An eyepiece of the same focal length with a 100° aFOV will produce an image though the eyepiece that is twice as wide as one produced by a 50° eyepiece.

Different telescope, with an eyepiece of a given aFOV, will also produce a different true field of view. The true field of view is the actual size of the piece of sky that you see through the eyepiece. Getting these two 'just right' can be very important. It's easy to assume that you want the biggest telescope you can get, with the eyepiece that gives you the highest magnification. This is never true in reality. Depending on where you live, and especially what you like to look at, a 100-120mm quality refractor with a wide aFOV eyepiece may very well be better than a large SCT with the same eyepiece. This is something I learned the hard way.

So how does scaling the eyepiece view help? The plugin will find the eyepiece you have with the largest aFOV. This aFOV becomes 100% of the computer screen. Then, any other eyepiece will have its aFOV compared, and the image on screen will be scaled down percentage wide. These 100° aFOV eyepieces make the math here easy. If you have one, then when that eyepiece is used, the circle that represents the view through the eyepiece will take up 100% of the screen. Next, if you select an eyepiece with an 82° aFOV, it's view will be scaled to 82% of the screen, and a 60° aFOV eyepiece will be scaled to 60% of the screen.

This is easier to understand in action, so lets look at an example that uses three eyepieces all with the same 17mm focal length, so they all produce the same level of magnification (well, one has an 18mm focal length, but its magnification is nearly identical) and see how the view changes.

Example in action

Let's see what all of this means in practice. These example all use a Celestron C8 8" SCT telescope, and the target is the Great Orion Nebula.

This is an image with a 17mm TeleVue Ethos eyepice, with an aFOV of 100°. Magnification is 119.5x 17Ethos.jpg

This is an image with a 17mm TeleVue Nagler eyepice, with an aFOV of 82°. Magnification is 119.5x 17Nagler.jpg

This is an image with a 18mm TeleVue Radian eyepice, with an aFOV of 60°. Magnification is 112.8x 18Radian.jpg

We can see from these images that the target is all three images is the same size. The 100° image fills the screen, the 82° is smaller, and the 60° is smallest yet, filling 60% of the computer screen. Note that in each image, the field of view that you see changes. The larger the aFOV, the more you can see of the sky. So in this example, if you had an 8" telescope, you would want to use the 17mm 100° Ethos eyepiece to see as much of the nebula as possible.

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).

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