Carey mask
Encyclopedia
A Carey mask is a focusing aid
for astronomical telescopes. The mask is in the form of a thin card or sheet that is placed over the front aperture
of the telescope. There are four series of slits in the mask which form a diffraction
pattern in the image plane.
In this example the two sets of slits on the left are angled at 12 degrees to each other. Those on the right are angled at 10 degrees to each other. Different telescope and imaging combinations may require slightly different angles.
The diffraction
pattern caused by the left hand slits will be in the form of an 'X'.
The right hand slits will also form an 'X' shape, but the lines forming the 'X' will cross at a narrower angle.
When perfect focus is achieved the two 'X's will be superimposed and be perfectly symmetrical.
Any slight error in focus will cause the 'X's to be offset, and this is very noticeable to the naked eye.
In the example images below, focus error
is obvious in the first two images. The third image is very close to perfect focus as shown by the equal spacing between the elongated spikes on the left and the right.
A negative image can sometimes show the diffraction
spikes more clearly.
The enlarged view below shows the left hand spikes to be slightly further apart than those on the right. This is an indication that the focus knob needs to be rotated slightly anti-clockwise. If the right hand spikes were further apart, a clockwise rotation would be needed. The mask should always be placed over the aperture with the same orientation
. If the mask was rotated 180 degrees, then the focus knob directions would be reversed. The operator soon becomes familiar with the rotation directions needed for a given setup.
This indication of the direction of focus change needed removes much of the trial and error that can often be encountered when attempting astrophotography
.
Focusing screen
A focusing screen is a flat translucent material, usually ground glass, found in a system camera that allows the user of the camera to preview the framed image in a viewfinder. Often, focusing screens are available in variants with different etched markings for various purposes...
for astronomical telescopes. The mask is in the form of a thin card or sheet that is placed over the front aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
of the telescope. There are four series of slits in the mask which form a diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
pattern in the image plane.
In this example the two sets of slits on the left are angled at 12 degrees to each other. Those on the right are angled at 10 degrees to each other. Different telescope and imaging combinations may require slightly different angles.
The diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
pattern caused by the left hand slits will be in the form of an 'X'.
The right hand slits will also form an 'X' shape, but the lines forming the 'X' will cross at a narrower angle.
When perfect focus is achieved the two 'X's will be superimposed and be perfectly symmetrical.
Any slight error in focus will cause the 'X's to be offset, and this is very noticeable to the naked eye.
In the example images below, focus error
Focus error
In photography, a focus error is an aberration of the autofocus ....
is obvious in the first two images. The third image is very close to perfect focus as shown by the equal spacing between the elongated spikes on the left and the right.
A negative image can sometimes show the diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
spikes more clearly.
The enlarged view below shows the left hand spikes to be slightly further apart than those on the right. This is an indication that the focus knob needs to be rotated slightly anti-clockwise. If the right hand spikes were further apart, a clockwise rotation would be needed. The mask should always be placed over the aperture with the same orientation
Orientation (geometry)
In geometry the orientation, angular position, or attitude of an object such as a line, plane or rigid body is part of the description of how it is placed in the space it is in....
. If the mask was rotated 180 degrees, then the focus knob directions would be reversed. The operator soon becomes familiar with the rotation directions needed for a given setup.
This indication of the direction of focus change needed removes much of the trial and error that can often be encountered when attempting astrophotography
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
.