Optical Instrumentation Flashcards
What are some examples of third order/monochromatic aberrations?
- spherical aberration
- coma
- astigmatism
- curvature of field
- distortion
- chromatic aberration (refracting elements)
What causes aberrations?
Imperfections in optical elements and rays hitting the elements beyond the paraxial approximation.
What is a spherical aberration?
Image points formed by rays from the same object point will appear to come from different positions due to reflection/refraction from different points of the imaging element.
How can you rectify spherical aberrations?
Stopping down the system to restrict the cone of rays or use a combination of positive and negative lenses to cancel out the aberration.
What is chromatic aberration?
Due to different wavelengths of light experiencing different refractive indices passing through an element, the focal length becomes wavelength dependent.
How can we reduce chromatic aberration?
Use mirrors or multiple refractive elements of opposite powers.
Reflecting telescopes use …
mirrors
Refracting telescopes use…
lenses
What type of telescope uses both mirrors and lenses?
catadioptric
What type of telescope is the Keplerian/astronomical and what type of image does it produce (how)?
- refracting telescope
- uses two lenses and produces inverted image
What type of telescope is the Galilean and what sort of image does it produce (how)?
- refracting telescope
- uses a negative focal length eyepiece lens to produce an upright image
What allows a real image in the object plane to be produces by Keplerian and Galilean telescopes?
Large objective lens collecting ~ parallel rays from distant objects
How do telescopes often correct for chromatic aberration?
Making the objective lens a doublet
The intermediate image for the eyepiece is the image formed by the objective. What type of image is it for Keplerian and Galilean?
Real for Keplerian
Virtual for Galilean
What is the formula for angular magnification for a telescope?
M = fo/fe
What sign of M (angular magnification) gives an upright image?
negative
What is the formula for telescope length?
L = fo + fe
What is the exit pupil for telescopes and where should it be?
The image of the entrance pupil (objective lens) in the ocular)
It should be far enough from eyepiece for comfortable eye relief
Where is the exit pupil Galilean
behind eyepiece
Where is the exit pupil Keplerian
just outside ocular
Where would a reticule be placed for an astronomical telescope?
in the real intermediate image
What is the formula for linear magnification of a telescope?
me = Dex/Dobj = -fe/fo = 1/M
Why are reflecting telescopes favoured for larger installations?
- don’t require large heavy lenses
- maintain optical quality
- remove chromatic aberration
Name the 3 basic reflection telescopes
Newtonian, Cassegrain, Gregorian
How does a Newtonian telescope work?
- parabolic focusing mirror, all rays to focal point
- plane mirror intercepts rays, diverting them to a point near the eyepiece
- parabolic mirror removes spherical and chromatic aberrations
How does the Cassegrain telescope differ from the Newtonian?
- it uses a hyperbolodial mirror instead of a parabolic mirror
- this focusses light through a hole in the primary mirror
- allows perfect imaging between primary and secondary mirrors
What type of secondary mirror does the gregorian telescope use?
concave ellipsoid
Describe how the Schmidt telescope works
- catadioptric
- refractive component is a correcting plate ensuring spherical aberration from the concave mirror used to form the image is minimised
- this modifies the optical path
- it is a modification to conventional reflection telescope
What differs about the rays coming into a telescope and a microscope?
the rays coming into a microscope are coming from nowhere near infinity
where does a microscope form real and virtual images?
a real image is formed within the microscope tube
intermediate image is just inside the eyepiece focal length
the observer sees an inverted, magnified, virtual image
How can you give a hard edge to the field of view of a microscope?
Place an aperture by the intermediate image to act as a field stop
What is the formula for the angular magnification of a microscope?
M = 25(cm)/feff(cm) = 25(fe + fo - d)/fofe
What is the image to object distance ratio for a microscope?
so’/so = (d-fe-fo)/fo
where so’=d-fe (d is dist between lenses)
(in words) what is the total magnification for a microscope?
the product of angular magnification of eyepiece at infinity and linear magnification of objective
What is the formula for total magnification of a microscope?
M = (so’/so)(25/fe) = -(25/fe)(L/fo)
standardised L=16c,
What is the lateral magnification?
m = hi/ho = L/fo
where L is dist between objective image and second focal length
How are modern microscopes infinity corrected?
They incorporate a tube lens between the objective and eyepiece to create infinity space between tube and objective
What happens to the focal length and diameter of the objective as the magnification increases?
They decrease
(in words) what is the numerical aperture
The ability of a lens to collect rays
What is the formula for NA
NA = nsinθ where θ is the half angle of the cone of rays from an object to the edge of the lens
What is the maximum value for NA in air
1
How does a microscope system overcome the limit of the numerical aperture?
using an immersion oil improves the NA, matching to the refractive index of the coverslip to increase NA by noil
What is total internal reflection fluorescence microscopy?
images a very thin section of a sample above coverslip, requires very high NA
