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Inspection and collimation

This tool can help to inspect the image and get a good collimation for the telescope optic. It include the image HFD inspection and two collimation method, centering the ring of a defocused star, or examining the deformation in the corner of the image using a 9 panel display.

Before any attempt with this procedure you must carefully read the telescope documentation, Beware that doing it wrong can make the secondary mirror to fall on the primary.

Image inspection

This panel allow to measure the median HFD value, sensor tilt and curvature.

Select the method to display the result:

  • Octagon based on HFD values at the center and eight peripheral area.
  • Triangle with adjustable orientation to match the tilt corrector screw.

Click Inspect to analyze the current image.
Click Start to start a preview loop and analyze each image.

The routine will detect and annotate the stars with their HFD value and plot a tilt and Off-axis aberration (curvature) indicator in the image.

In the log the following will be reported:

Image median hfd which is an excellent indicator of the quality of focusing. The lower the value the better the focusing, the sharper the stars are. The value is also depending on the astronomical seeing and the quality of the optics.

Sensor tilt as the HFD difference between the best and worst corner median values. In addition as a graphical indication it draws a trapezium in the image based on the four median values.
There can be some variation in images of the same series, so a tilt of a few tenths looks normal but anything more indicates a camera mounting problem.

Off-axis aberration as the HFD difference between the center and the corners of the image. This is an help to adjust the optimal distance between the field flattener and the camera. But be careful to first remove tilt and get a precise focus at the image center, otherwise this value is meaningless.

9 panel image

This panel allow to display a zoomed image of the corner and center of the image to help to detect the aberration. This can be used to adjust a comma corrector, a tilt adapter, or to collimate a RC telescope.

Load an existing image for analysis and click the Set button or click the Start button to loop from the camera.
Adjust the Zoom and Margin offset to clearly see the aberration with enough stars in each box. After every change click the Set button to apply.
Return to the normal display using the button Stop or by closing this window.

Single star

This method is to adjust the final collimation of a Newtonian or SCT.
To start, point the telescope to a bright star, then select the menu Tools → Collimation.
Click the Center button.
This start a preview exposure loop. Center the star in the bull eye, defocus enough to see the concentric ring in the image, adjust the exposure time so the image is not overexposed but high enough to mitigate the seeing fluctuation.

When ready click the Start button.
This crop the image to the size defined for the focus window and draw concentric circle centered on the star.
You can adjust the Number of circles so they match the main ring in the star image.

Slowly adjust the position of the star in the image field using the telescope fine movement (guide speed) until all the circle are concentric. The program try to automatically follow the star by modifying the camera ROI but this cannot work if you abruptly move the star out. When this is done the star is in the telescope optical axis.

Now click the Center button and move the star to the image center using only the telescope collimation screw (main mirror for Newtonian, secondary for SCT). This way you make the optical axis to match the center of the image.

If the initial offset is larger than the camera FOV, place the star in the border of the image where the ring are the most concentric, process as above, then retry to finish the collimation.

If no circle are draw after you click the Start button this is probably because the focus window is to small for the size of the defocused star. In this case refocus a bit to make the star circle smaller or increase the size of the focus window.

en/documentation/collimation.txt · Last modified: 2022/04/07 17:30 by pch