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 apparent FOV 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 apparent field of view of all of your eyepiece becomes 100% of the screen, and others are scaled down accordingly. That is to say, if the eyepiece with the largest apparent field of view is an Ethos with a 100° aFOV, that eyepiece (and any other 100° aFOV eyepieces) will draw using 100% of the screen. If you have a Nagler eyepiece, which has an aFOV of 86°, that eyepiece will have a circle on screen that is 86% of the screen size. This is very handy when you want to compare eyepiece and telescope combinations, to see which provide the best view of a perspective target.
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 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.
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.
Scaling the eyepiece view
By default, the view drawn on your computer screen when in Ocular mode 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 anything 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 is 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 86°, 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 the eyepiece view help? The plugin will find the eyepiece you have with the largest aFOV. This aFOV becomes 100% of the computer screen. This, any other eyepiece will has 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 represent the view through the eyepiece will take up 100% of the screen. Then, if you an eyepiece with an 86°, it's view will be scaled to 86% of the screen, and a 60° 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 foal length, but its magnification is nearly identical).
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).