Collimation radiology

Collimating an SCT is simply a matter of adjusting the three screws on the secondary mirror. This changes the tilt of the mirror and aligns it with the (fixed) primary mirror. The tilt of the mirror is tested by viewing an out-of-focus star image through the telescope.

It is very important to let the telescope thermally stabilize before collimation. A scope that is still cooling down to ambient temperature will produce a heat spike as warm air radiates off the optics. This can distort the star image and make the telescope appear out of collimation when it is not.

Collimation telescope

Defocus the star to produce a donut shape. The hole in the donut is the shadow of the secondary mirror. If the hole is offset from the center of the star, the collimation must be adjusted.

Choose a fairly bright (1st magnitude) star for the test. It is important that the star be centered in the field of view when testing collimation. A star at the edge of the field may be distorted, especially at lower powers, and could make the telescope appear out of collimation when it is not.

Collimation lens

For the star test, use a relatively high power eyepiece. A 10 or 12mm is a good choice for most SCTs, providing a magnification of 200-300x.

Collimation cap

Also, analysis applications where for example (spectral) transmission data of translucent materials is important for managing production processes. The advantage of collimating lenses attached to light sources and/or spectrometers is that light enters the sample under one specific angle only, with almost no variation. Again, this contributes to stable and repeatable measurement results.

If tightening one screw makes the collimation worse, return that screw to its starting position and try tightening the other two screws. The most important thing is that the screws end up snug in the end. Try not to loosen a screw without tightening the others to compensate. Leaving the screws loose can cause the collimation to be lost when moving the telescope.

Collimating lenses can also be used in lighting measurement applications. In particular when measuring diffuse surfaces of light sources such as OLED panels. Similar parameters including spectral power distribution, color and flicker can be determined depending on the measurement device.

Unlike collimating a Newtonian, there are no special tools required for an SCT. However, you will need to test the collimation on a star, so it must be clear and dark. Otherwise, all you will need is a screwdriver to adjust the screws on the secondary mirror.

Collimation binoculars

Collimation is critical to obtaining the best performance from your telescope. Aligning the optics of a Schmidt-Cassegrain telescope (SCT) is much easier than collimating a Newtonian telescope and can easily be learned by any user. However, there are some tricks to doing it right, and some things to avoid. If done right, collimation should only be necessary every few months. If you find it necessary to collimate your telescope every few weeks, the mirror is probably not being locked down properly after adjustment.

A collimating lens system is typically made up of a tube with one or more lenses. By selecting the right properties of the lens and focal distance, light can be collimated with high accuracy. A collimating lens can either be attached directly to the measurement device or via a fiber connection for remote sensing.

In recent years new display technologies have emerged. OLED displays, TFT’s with quantum dot technology or laser projectors offer brighter and more saturated colors than possible before. To achieve consistent colors across different displays, colorimetric calibration of the displays is essential.

Collimation in laser

Typical applications for Admesy products with collimating lenses are found within display measurement applications. Collimating lenses are for example extremely useful when measuring displays, which includes color analysis, white point adjustment, flicker, response time, gamma, etc. The optics are designed in such a way that alignment of the measurement spot can be optimized with stable measurement data and high repeatability as a result. This all contributes to the development and production of high-quality displays.

Collimation in surveying

Note: In our experience, the original screws on an SCT secondary mirror are much better to use for collimation purposes than the aftermarket thumbscrews that can be added. Thumbscrews cannot be turned as precisely, making accurate alignment difficult. Also, thumbscrews do not hold the mirror as tightly, increasing the need to collimate more often. Thumbscrews also tend to make people "collimation happy," tending to collimate a scope far more often than necessary. Under normal use, you should be able to go months without collimating a telescope.

The star should focus down to nice point, with no asymmetric flaring. The overall sharpness of the star image will depend on the magnification and seeing conditions, but it should be symmetrical and perfectly round.

‘Collimating’ is the process of accurately aligning light or particles in a parallel fashion. For light measurement, this ensures that the light has minimal spread as it propagates. Collimating lenses are for examples attached to spectrometers, colorimeters or light meters to ensure that the light that enters the instrument is parallel and covers the relevant measurement area.

Admesy supplies a large range of sampling accessories for measurement devices such as spectrometers, colorimeters, and light meters as well as light sources. Within this range, there are various collimating lenses available depending on your application requirements. Lens systems are either connected directly to the measurement systems, or they can all be connected to Admesy’s robust custom optical fiber. Our lens accessories provide flexibility when using a measurement device for research and development, or when integrating it into production lines with limited space for positioning the equipment.

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Collimation X ray

After adjusting the screws, be sure to return the star to the center of the field of view by adjusting the position of the telescope. Once the adjustments have been completed, the screws should be snug and the image of the defocused star should appear perfectly concentric, as shown below.

Now the trick is figuring out which screw to turn. The low-tech trick is to reach up in front of the telescope and stick a finger in front of theaperture. You will see the shadow of your hand in the star image. Move your hand around until it reaches the narrowest (or widest) part of the donut. Take a look at the secondary mirror and see what screw your finger is nearest to (or opposite from). It doesn't matter whether you use the narrow or fat part of the donut, or whether your finger ends up next to a screw or across from one. The only difference will be whether you tighten or loosen the screw.

Whether you tighten or loosen the screw you have found, depends on whether the image is inside or outside of focus. The usual method is to try tightening first and see if the star image improves. Also, note that turning a screw the correct direction will cause the entire star image to move toward the fat part of the donut (upper left in the diagram above). Begin by turning the screw about 1/8th of a turn.