What is collimator in X ray

Camera manufacturers sometimes list these equivalents because some photographers are more familiar with 35mm cameras and they want to make it easier to understand. It also gives us a standard of reference for all the different format sizes. They may also list the multiplier factor. For example, the APS-C multiplier is around 1.6x.  So a 32mm lens on an APS-C camera (like the Nikon D3200) would act like a 50mm lens on a 35mm film camera. Does focusing affect the focal length?

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The focal length number tells us how much of the scene is captured in the picture. The lower the number the wider the view, and the more we can see.  The higher the number, the narrower the view, and the less we can see.   This is illustrated below – where the camera is stationary and the focal length (in white numerals) changes:

Now comes the most critical step. Adjust the primary mirror to ensure that the reflection is headed into the center of the focuser. On the back of the mirror cell are typically three adjustment screws (or six push-pull adjustment screws). With the laser on and in the focuser, you want to adjust the primary so the beam returns to the place it’s emitted from.

Above we mention that focal length is related to focus distance.  Focal length is the principal distance of a camera when it is focused at infinity. In photogrammetry we are interested in the camera’s internal geometry at the time photos were taken – so it is the principal distance that we want to know precisely in photogrammetry.

My personal preferences for Newtonian collimation are both a Cheshire and a laser collimator. And if you’re collimating a 2″ focuser, I strongly recommend using a 2″ laser collimator. A 1¼”-to-2″ adapter can misalign the laser.

Collimating mirrormeaning

What’s going on? No matter what you do, you simply can’t get a clear image. You can’t see Jupiter’s belts. Saturn is a teacup with two handles. And lunar craters blur out. Is your telescope a dud? More than likely it just needs to be collimated.

In many cases though, the advantages of using focus (i.e. crisp targets and distinct features) with subtle effects on principal distance/focal length, outweigh the advantages of keeping focus and principal distance constant (i.e. potentially causing blur in some photos taken at a different distance).

Laser collimator: Even though a laser collimator has advantages and disadvantages (the optics can be slightly out of alignment and yet look perfect), it’s always been my preferred “quick” method. It sits in the focuser and shines a beam that ideally reflects onto the hole it came from.

Collimation cap: A collimation cap, or sight tube, is a plug that fits in your reflector’s focuser. It has a small central hole. Although it’s mainly used to ensure that your secondary mirror is aligned with respect to your focuser, it can also be used to ensure correct alignment of the secondary and primary mirrors.

Cameras can have fixed lenses (sometimes called ‘prime’ lenses) which have just one focal length, or zoom lenses which allow the focal length to be varied (for example between 18mm-55mm, or 55mm-200mm). For high accuracy photogrammetric work in PhotoModeler, a fixed (or prime) wide lens (such as a 20mm lens on an APS-C frame camera) is recommended as the primary option, but different applications may require different focal lengths, and cameras with adjustable zoom lenses can still be used to achieve very good results with some extra procedural care over the focal length.

Let’s say you take a picture of an automobile with two cameras, a 35mm film camera and a smartphone camera. You stand in the same spot and take two photos, one with each camera. In both cases you want to take a photo of the automobile that fills the frame. If the 35mm film camera lens has a 50mm focal length, the digital camera’s focal length might be 4mm. So even though they are very different numbers they produce the same result because of the size of the imaging surface. So the “equivalent 35mm focal length” for this smartphone camera at 4mm is 50mm.

Collimating mirrordiy

Step one is to ensure that the secondary mirror is centered under the focuser. Insert the Cheshire and inspect the position of the secondary. It may need to be moved forward, backward, or even rotated slightly. You may also need to adjust the spider mount (the thin metal vanes that hold the secondary) to slide it side to side or up and down. When correctly positioned, the secondary should appear centered and circular. You should need to do this only once. The rest you’ll do every time you set up.

In a reflector, three things need alignment: the primary, the secondary, and the focuser (ultimately the eyepiece). Glance down the focuser in the daytime and identify each of these items in the reflection of the secondary.

A camera typically has focal length in a range of 10mm to 500mm.  Different types of camera can have different ranges and speciality lenses can extend outside this range as well.  A 10mm focal length would be a very wide lens (capturing a lot of the scene), and 500mm would be a very narrow lens (capturing only a small part of the scene – giving a large magnification like binoculars or a telescope).

Star test: A star test is free, easy, and works with any type of telescope. The only disadvantage is that not all nights have good enough seeing (atmospheric steadiness) to ensure that the diffraction rings are centered around the star.

Collimatinglens

Next, the laser collimator comes into play. While you could do this step with a Cheshire, a laser makes it far simpler. Insert it into the focuser and adjust the secondary mirror, using its adjustment screws, until the beam falls on the spot you previously marked on the primary mirror. Be careful that the reflected beam doesn’t hit you in the eye.

Maksutov telescopes — a design like the SCT — tend to hold collimation well. Unlike an SCT, however, collimating one isn’t easy. If a Maksutov needs collimation, it’s best to send it to the manufacturer or contact a professional because it’s generally difficult to access and adjust the primary mirror.

A laser collimator emits a beam that bounces off the primary and secondary mirrors in a reflector and (hopefully) back onto the collimator’s target.

Optical pinholes are small circular holes for optical applications, e.g. for mode cleaners (spatial filters) in laser resonators and for pinhole cameras.

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Focal length is a number that is vital to photography and photogrammetry but often misunderstood. What is focal length?

The easiest way to check a telescope’s collimation is to wait until the optics have cooled, pick a bright star, place it in the center of the eyepiece’s field of view, and defocus slightly, first one way, then the other. These two non-focused regions are called inside and outside focus. Inside focus refers to the eyepiece’s focal point falling inside the light cone; it’s the reverse for outside focus. The diagram on p. 59 shows these points, along with perfect diffraction rings. If the rings aren’t centered, odds are good that the telescope is not collimated correctly. Certain designs — reflectors and Schmidt-Cassegrain telescopes, or SCTs — require collimation every time you set up.

use of collimator in x-ray

A strict technical definition of focal length is difficult without providing a lot of background in lens theory, so we will use a simplification. You can think of focal length as the distance between the imaging plane (e.g. the image chip in a digital camera) and a point where all light rays intersect inside the lens (the ‘optical center’). So a focal length of 20mm means that the distance from the optical center to the imaging plane is 20mm long (about ¾ of an inch). What does the focal length number mean?

There are two types of collimation: optical and mechanical. Optical collimation aligns a telescope’s optical surfaces to bring the image to the correct orientation in the focal plane. Mechanical collimation is necessary when the physical components in your scope don’t line up properly — a focuser isn’t square to the tube, a mirror isn’t centered in the tube, or a secondary mirror is misaligned. Further complicating the matter, your telescope could appear to be optically collimated without being mechanically collimated.

There is some ability to calibrate a camera (which solves the principal distance) at one focus and execute your photogrammetric project at another focus.  The actual discrepancy that is acceptable depends on your accuracy requirements and how much the focus changes.  Generally a calibration done at 2m/6ft focus distance is acceptable for projects up to infinite focus (again depending on accuracy requirements), but may not be acceptable for a project where the focus distance was 50cm/20in.

Collimation is a necessary process, but it’s not overly long or complicated. You spent good money on that telescope — get all the performance out of it that you can!

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collimator is used for?

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After everything is aligned, a star test will confirm proper collimation. This won’t always be possible because the seeing may not be good.

When you buy a digital camera you will often see the specification “equivalent 35mm focal length”. What does this mean? Most digital cameras have imaging chips that cover much less area than a standard 35mm film frame. Since 35mm film cameras were the standard for so long in photography, much of the techniques and methods were developed around them. A 35mm film camera has a negative that is about 36mm wide by 24mm high (the “35” comes from the physical width of the film stock that is exactly 35mm wide).  A ‘normal lens’ (has a field of view that appears ‘natural’ to humans) on a 35mm film camera has a focal length of 50mm.

Refractors are factory aligned, and due to their fixed lens, they hold collimation well. A refractor that doesn’t suffer an accident (like a drop) typically won’t need additional collimation over its lifetime. Some, however, may not arrive collimated, most likely due to shocks during transit. If that happens, especially for more advanced designs like triplets and Petzvals, I strongly recommend returning it for an exchange. If you purchased the scope used and don’t choose to return it to the seller, send it to the manufacturer for adjustment.

Make sure your mount is on and tracking. Insert an eyepiece that will give you moderate to high magnification, and center a fairly bright star, but not the brightest you can see. If you typically observe with a star diagonal, make sure it’s in place. If you’re not using a motorized mount and you’re in the Northern Hemisphere, you can collimate on Polaris.

Some SCTs, particularly older ones, suffer from mirror shift, where the primary flops slightly to one side of the cell as the telescope crosses the meridian. For the best results, collimate on the side of the sky where you’ll be observing. If you pass the meridian, you should check collimation again. The process might seem slow at first, but you’ll gain speed with practice.

PhotoModeler is one of the leading tools for photogrammetry (the science of generating measurements and accurate 3d data from photography).

The last — and best — recommendation I can give you for collimating your telescope is to find a friend who’s willing to help. Photos, diagrams, and articles are great, but it’s quicker and easier to collimate your scope if you have a friend to tell you if you’re moving in the right direction. Otherwise, you’ll be back to look at the laser every time you turn a screw. As a thank you, I’d suggest doughnuts. And coffee. Definitely coffee.

What is collimation in radiology

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Finally, reflectors will need frequent collimation — as in, every time you transport it to a different site, and maybe even if you don’t. I collimate my observatory-based 18-inch reflector before every session. Fortunately, collimating a reflector is simple. Once you get the process down, it takes only a few minutes.

Modern digital cameras can have imaging chips that are as small as 6mm by 4mm; some Smartphone cameras are even smaller, and then up to full 24mm by 35mm size.  A very common size is the APS-C format at 16mm by 24mm. This smaller size affects what is considered to be a ‘normal’ focal length.

Collimation is the process of aligning all components in a telescope to bring light to its best focus. All telescopes need to be collimated at some point; however, it’s easy to perform this task on some and a bit more involved for others. For simplicity, I’ll talk about the three major types (and one subtype) of telescopes and point out when you need to collimate.

types of x-ray collimator

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If the scope is way out of alignment, you might want to point the front toward the side of a building. Any non-reflective surface will do. Then change the tilt of the mirror by using the adjustment screws. Do this by moving only two of the three screws, and make only small adjustments. Leave the third adjustment screw alone unless you run into the limit of one of the others. After each adjustment, check the front of the laser to see if the beam is there. If so, you’re on target.

Before you even think about this process, I suggest you mark the center of your primary mirror. Almost every amateur astronomer who has done this has used a white adhesive binder reinforcement ring — if you’ll be using a laser (highly recommended), there must be a hole in the middle of your marker for a reflection. Correctly placing the marker is critical. Optical defects increase the farther you move from the center. This can be a significant issue, especially with fast mirrors.

SCTs are a mixed bag. Some tend to require frequent collimation; others hold it well. As they say, your mileage may vary. The good news is SCT collimation is easy.

Note: A technical photogrammetry term that you may come across is the “Principal Distance”.  Strictly, the Principal Distance is the distance mentioned above (i.e. distance from imaging plane to the lens optical sensor), and the focal length is the principal distance when the lens is focused at infinity. See below for more information on focus vs focal length. When PhotoModeler lists focal length for a camera, it is actually the Principal Distance that is shown.

Cheshire: This collimator adds a set of crosshairs to a sight tube. Many amateurs I know who own reflectors think a Cheshire is the best way to go in terms of proper collimation.

Defocus slightly until you see a doughnut of light. If your scope is out of collimation, the doughnut’s hole won’t be centered. On the corrector of most modern SCTs, you’ll find a plastic circle, which is the back of the secondary mirror’s mount. There you’ll find three adjustment screws. If you don’t see them, there’s probably a cover to remove.

All lenses have a stated or specified focal length value (or range of values for a zoom lens). This printed number is actually its nominal length or the principal distance when the lens is focused at infinity. As you focus on objects that are closer to the camera, the principal distance changes.  So for example, a 50mm lens focused on an object a few feet away might have a principal distance of 55mm lens at that time. The most extreme example of this is with a macro setting (a lens setting that allows you to focus on very close, very small objects, under 5″ in size for example). A lens that has a 50mm nominal focal length (so a 50mm principal distance when focused at infinity) might in fact have a 100mm principal distance when focused at a few inches! This is why it is good with photogrammetry (where precise geometry is needed) to calibrate a camera at the distance you will be working with.

You’ll need a Phillips screwdriver or an Allen wrench. The screws tend to be small, so be certain to use a tool that fits properly. Because using metal tools so close to optics makes me nervous, I recommend replacing the screws with a set of knobs for safer and easier adjusting. Make small corrections, one screw at a time, look through the eyepiece, and observe the change in the alignment. Continue to adjust these screws until the hole is centered in the doughnut. Once it is, focus a bit more until you can see the diffraction rings, and use them to fine-tune the collimation. Ideally, you should need to adjust only two of the screws.

There are tools that let you collimate an SCT during daylight, but they are fairly expensive. They can be a good purchase if your time is valuable, or if you are part of an observing group that can split the cost.

To make your observing stress free and fun, your telescope should produce great images. To ensure its best  performance, learn how to collimate your optics.

But if the telescope is so far out of collimation that the return beam doesn’t even hit the secondary mirror, it’s useful to point it at a nearby wall to help make the initial adjustments. In a pinch at night, I’ve aimed it at a friend. (If you do this, be certain their eyes are closed because laser light can harm vision.)

Before using any laser collimator, make sure it’s trued. Turn it on and roll it along a flat surface, projecting the beam on a wall. If it scribes a straight line, you’re good to go. If not, there’s usually a way to adjust it.

The big day’s here: Your first telescope has arrived. You’ve done your research and decided on an 8-inch Dobsonian-mounted reflector, or perhaps a 4-inch refractor, or another model that had a great review. You spend an hour or so unpacking and assembling the telescope. You can hardly wait for nightfall. You take it out in the backyard, manage to find a few targets, and…

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