Single (Defocused) Star Collimation Method

Defocused star collimation method is new in CCDInspector 2.1.0. The main differences between the previously available collimation method in CCDInspector and the Single Star Collimation (SSC) are:

• Only one, centered bright star is needed for SSC
• The star must be sufficiently out of focus, so as to fill out a circle of at least 40-100 pixels radius
• The star must be positioned close to the image center
• The star must be be exposed well enough to get a good signal-to-noise ratio

This method should look familiar to anyone who has done visual collimation through an eyepiece. The main advantage of using CCDInspector is that unlike the conventional methods, you can keep your camera attached to the telescope, your hands and eyes next to the collimation screws and get a continuous and precise numerical read-out of collimation error and direction.

Quick Start

The following describes all the steps necessary to start collimating. With practice, this process can take as little as few minutes, even when the scope is far out of collimation.

1. Point your telescope at a bright star, well above the horizon. A magnitude -2 or higher is recommended, but larger aperture telescopes may use dimmer stars. A dim star can still be used, but may require a longer exposure.
    
2. Start taking continuous images of the star using a focus routine of your image acquisition software:
   
   - Make sure the image acquisition software is properly selected in Settings->Camera Control Software menu of CCDInspector
   - you can bin the chip 2x2 or even 3x3. Binning increases S/N ratio, and increases download/processing speed, so it's definitely recommended
   - use automatic dark subtraction, if this option is available in the acquisition software
   - for bright enough stars, an exposure of one second can be plenty, especially with a binned chip
    
3. Start the collimation viewer from Real-Time->Collimation->Defocused Star Collimation Viewer menu in CCDInspector
    
4. Start  the image viewer from Real-Time->Image Viewer menu
   
   
    
5. Position the image viewer window and the collimation viewer side-by-side, so you can see both at the same time
    
6. Defocus the star so that it occupies about the size of the circle near the center in the image viewer. Exact size is not very important, but it should be greater than about 20 pixels radius, and not so defocused so as to become too dim. CCDInspector can handle stars the size of about 90% of the whole chip, and as small as 20 pixels radius. If the star is too dim, or too small, CCDInspector will display a brief error message indicating the problem.
    
7. Center the star as best as you can using your telescope's movement controls. If it's not exactly at the center, no big deal. But, if it is really far away, the error size and the direction will no longer be accurate.
   - The circle at the center of the image viewer is positioned exactly in the middle.
   - The cross marks the center of the defocused star.
   - Ideally, the cross should be positioned near the center of the circle (in the image above, its away from the center, to the top right)
    
8. Take a few images and watch the Collimation Viewer update. Each time the window is updated, it will flash red for a few seconds, the number in the center will change, and the direction arrow will be adjusted
    
9. Your goal is to turn a collimation knob so that the image of the star moves in the direction that the collimation arrow is pointing. The star movement doesn't have to be exactly in this direction, but should be close. You can determine which knob to turn by trial and error.
    
10. After you turn the correct knob, you will see the image of the star shift in the direction the arrow is pointing. In order to read the updated collimation values, first, re-center the star using your telescope controls
     
11. Wait a few seconds after making the adjustment for the telescope vibrations to die down, and then watch the result shown in the Collimation Viewer. The error number should decrease
     
12. As the number gets smaller, the amount of knob adjustment needed also gets smaller. With error value less than about 5, a very light touch to the knob in the right direction is all that may be needed.
     
13. Note: If the direction arrow keeps pointing in different directions all by itself, your optics are already collimated to the best possible level under the current seeing conditions: it's not advisable to proceed any further
     
14. Repeat steps 8-14 until the value in the middle is as low as possible, and while the direction arrow does not jump around to completely different directions
     
15. Under good seeing conditions, it should be possible to reduce the error to near zero.

Collimation Viewer

The same Collimation Viewer that is used for the multi-star collimation method in CCDInspector is also used for One Star Collimation method. The viewer is a real-time display that provides an intuitive guide to the corrections necessary to achieve perfect collimation. This tool uses innovative image processing and image recognition algorithms, and shows the result in a form suitable to real-time collimation:

Collimation Viewer can be started by clicking on the Real-Time->Collimation->Defocused Star Collimation Viewer  menu in CCDInspector.

Collimation viewer shows the three-screw, tip-tilt collimation arrangement  that is commonly used on SCT's, RC's, Newtonians, and other styles of telescopes. This display is for mnemonic purposes, to help you keep track of which knob does what.

Important: In order not to loosen the secondary mirror completely, avoid repeatedly turning any one knob in the counter-clock-wise direction. Turning a knob counter-clockwise is equivalent to turning the other two knobs clock-wise.

The Viewer display shows:

• The magnitude of the collimation error in the center of the display. This number is unit-less, and doesn't change with image scale or the amount the star is defocused

• The direction of the collimation error is shown by a line from the center of the viewer display. The line only shows the direction of the error, not the error magnitude

Your goal is to get the error number as low as possible. On a night of good seeing, this value can be lowered to nearly zero.

Note that the Collimation Viewer window can be sized to be better visible from a distance: simply click and drag on the bottom-right corner of the window to increase or decrease the display size.

Image Viewer

While the Collimation Viewer is displayed, the Image Viewer display will overlay the image of the star with some additional graphics:

The Angle and Error display at the top show the collimation error numerically: the error value is the same value as shown at the center of the collimation viewer, while the angle is the angle in degrees of the error direction (negative degrees indicate clock-wise direction, while positive indicate counter-clock-wise).

The cross indicates the center of the star image, while the line circle indicates the center of the CCD chip. As you make adjustments to collimation, re-center the star by moving the cross closer to the center of the circle. The extra line coming out from the center of the circle is the collimation direction arrow. It points in the same direction as the arrow in the collimation viewer.

Helpful Hints

• Make sure to re-center the star if it moves more than about 20 pixels away from the camera center. The accuracy of the measurement will be affected in some optical designs (for example, SCT) if the star is far away from the center
   
• You can use a sub-frame mode, but leave a background border of about 20-50 pixels all around the defocused star. Make sure that the subframe you use is centered on the chip, otherwise the collimation accuracy will be affected
   
• A very bright star may require shorter exposure and/or no binning. If the exposure is too long, there could be bright blooming and diffraction spikes around the star that can affect accuracy. To see if this is the case, decrease the display range of the image to make the star very bright on the display. If you see solid rays emanating from the top and the bottom (sometimes left and right) of the star, then the exposure is too long
   
• Don't make the exposure time too short, either. Less than 0.5 seconds is not recommended, as that will often be affected by seeing changes in the atmosphere. Best results will be achieved with an exposure from 0.5 to about 2 seconds.
   
• Pay particular attention to the collimation error direction arrow: if it keeps jumping by more than 30-40 degrees between consecutive measurements, then you have reached the limit of what can be done in the current seeing conditions, regardless of the value of the actual error. Under good seeing conditions, I have been able to lower the collimation error to less than 1. Under poor seeing conditions, the error direction started getting jumpy at around 5.