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. Any number of telescopes and eyepieces can be configured, to help you choose the best eyepiece for a given object. This makes eyepiece - or telescope - comparison easier.
NOTE: this describes the plugin as-of the 0.10.5 release of Stellarium.
Here is a shot showing an active ocular with an active CCD sensor.
Using the Ocular plugin
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 ocular is selected. So if you hit the hot key, and nothing happens, make sure you do not have one of the oculars active.
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°.
- Define some eye pieces and telescope (see below).
- Select an object to view (i.e. a star, planet, etc.)
- Click the tool bar button for toggling the Ocular mode, or press Command-O (control-o for non-Mac users).
- Swap between Oculars and Telescopes to see how the view changes.
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.
This is the General tab. Currently the only option is to scale the images based on exit-circle or not. 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 telescope you define. The largest exit-circle based on all of your equipment becomes 100% of the screen, and others are scaled down accordingly.
This is the tab used to enter your own eyepieces. But default, a sample one is added; feel free to delete it once you've entered your own.
The fields on this tab are:
- A free-text description of the ocular. 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 only used in calculating the exit circle for scaling the image. If you do not know what it is (not all manufacturers provide this value) just leave it the default zero.
Once you change a value, please press the Update Ocular button. The next version should do this automatically, but currently, if you do not press the button, the value will be lost.
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 ocular view, showing what the CCD will capture. Note that old versions will draw this bounding rectangle as gray, which is difficult to see. Version 0.10.6 has this changed to red.
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. But default, a sample one is added; feel free to delete it once you've entered your own.
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 in mm
- Horizontal flip
- If the view through this telescope should flip horizontally.
- Vertical flip
- If the view through this telescope should flip vertically.
Once you change a value, please press the Update Telescope button. The next version should do this automatically, but currently, if you do not press the button, the value will be lost.
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.
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.
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).