Telescopes

haddockhellskitchenUrban and Civil

Nov 15, 2013 (3 years and 6 months ago)

45 views

Telescopes


Overview



Aperture: Diameter of primary lenses



Focal ratio: also the “speed” of the telescope

o

Higher f
-
ratio ~ slower speed, but gives larger magnification with the same
aperture and eyepiece. Also the large f
-
ratio will make potential problems

eg.
coma less noticeable

o

Lower f
-
ratio ~ faster speed, gives a wider field and brighter image, better for
astrophotography. However due to sharply converging light, very good quality
eyepieces needed to eliminate potential problems of coma


Telescopes


R
efractors



Refractors are the “usual” type of telescopes



People will usually think of refractors when they think telescopes



Consist of one lens and the receiving eyepiece, in a straight tube



Uses lens to work



Easy to D.I.Y.


Telescopes


Refractors
-

Long t
ube vs. Short tube



Short tube advantages: wide field of view, portability



Long tube advantages:
smaller spherical aberration, longer focal length



Short tubes now have focal length correction and spherical aberration correction


Telescopes


Refractors
-

Ap
os and Achros



Apo ~ Apochromat



Achro ~ Achromat



Difference? Achromat minimises effect of CA, apochromat totally eliminates CA

o

How? Apochromats use different coatings

o

Disclaimer! Lousy apochromats still may have CA…



The Apochromat has highest cost per inch
among all telescopes


Telescopes


Newtonians



Newtonians are the only design of telescopes that use the open
-
tube design, ie the
tube is not sealed



This is a problem to Newtonians especially when air
-
movement in the tube is
great, thus affecting the qualit
y of the image



Due to the exposure to the outside surroundings, they are also the most expensive
in terms of maintenance cost



Uses mirrors to work



Support that holds up the secondary mirror


spider

o

Usually 4 sided, due to image quality and wave motion



DIY

also possible



Advantages:

o

Low cost per inch due to use of mirrors instead of lenses



Disadvantages:

o

Open tube design allows air flow, image degradation

o

High maintenance cost


Telescopes


Newtonians
-

Types of newts



Tube design



Truss
-
dobsonian design



T
elescopes


Newtonians
-

Long tube vs. Short tube



Same as above in refractors



Telescopes


SCT aka Schmidt Cassegrain Telescope




Uses both mirrors and lenses



Primary lens is odd
-
shaped



Advantage:

o

compact even at big apertures



Disadvantage:

o

Very expensi
ve, second after apochromats

o

Very heavy

o

Suffer from collimation problems


Telescopes


MCT aka Maksutov Cassegrain Telescope




Uses both mirrors and lenses



Primary lens is a thick meniscus



Advantage:

o

compact even at big apertures



Disadvantage:

o

Very expens
ive, second after apochromats

o

Very very very heavy due to thick meniscus

o

Suffer from collimation problems


Telescopes


Schmidt
-
Newtonian, Maksutov
-
Newtonian



Combines the advantages of cassegrains with Newtonians, cancelling
disadvantages





Problems


C
hromatic aberration



In lenses, different wavelengths of light bend at different angles
.
Therefore, even
at sharp focus, there will be a halo of blue/violet light around the object



Different coatings / lens type will try to solve this problem, example:

o

Doub
let lenses (in achromats)

o

Triplet lenses (in apochromats)

o

Fluorite coating (in apochromats)






Problems


Spherical aberration



Light reflected from the mirror's edge gets focused to a slightly different point
than light reflected from the center



C
orrected for most scopes, only noticeable in lousy ones


Problems


Miscollimation



Refer to article on miscollimation


Problems


Coma



At the edge of FOV of the telescope, the images become blurred


Problems


Astigmatism



If the mirror is not ground symmet
rically about its center, star images focus to
crosses rather than to points


Problems


Thermal equilibrium



Only in maksutov models



Thick lenses in the telescope takes a long time to reach thermal equilibrium with
the surroundings



Only when thermal equili
brium is reached, image quality will be best


Problems


Central obstruction



Refer to article on central obstruction



Accessories


Eyepieces



Refer to article on eyepieces


Accessories


Finderscopes / Viewfinders



Normal finderscope



Red
-
dot finder

o

Why re
d???




Telrad finder





Accessories


Barlow lens



Barlow lenses give x times the magnification with the same field of view



However people complain about Barlow lenses because they give a “detached”
view of the objects, ie. The observer feels to “far” awa
y from object



Barlows come in different magnifications

o

Most common: 2x, 3x, 2.8x




Shorty” Barlow more expensive, but more convenient in “Barlow stacking”



Long tube Barlow less expensive


Accessories


Diagonals



Tilt the view to a more comfortable position
to view through the telescope



Prism and mirror diagonal



Upright diagonal


Accessories


Focal reducer



Reduces focal length of the telescope without physically modifying it



Usually only for catadioptrics


Accessories


Lens cleaning kit



Cleans lenses? Duh.


Accessories


Mount



GEM


German Equatorial Mount

o

Good for astrophotography due to its design, only one
-
axis drive needed

o

Complicated for first
-
timer

o

Due to the counterweights needed, it is very big and heavy



Fork Mount

o

Modification of GEM, not much easie
r to use than GEM



Altitude
-
Azimuth

o

Easier to use

o

Light and portable, since no counterweight needed



Dobsonian

o

Used for Newtonians

o

Uses the alt
-
azi design



Accessories


GOTO



Automated motor to point the telescope at celestial object with pda/computer



Some
GOTO have GPS, so no need to do alignment.


Accessories


Dew shield



Protects the telescope lenses from dew in the course of observation
Calculations


Focal length



Focal length(mm) = Telescope aperture(mm) x Focal ratio of telescope


E.g.: A 5”, F/10 te
lescope yields a focal length of (5x25.3125) x 10 = 1265mm



Calculations


Magnification






Focal length of telescope (modified)

Magnification =
--------------------------------------------------------





Focal length of eyepiece


E.g.:

1.

A telescope of

focal length 2000mm coupled with a 20mm eyepiece gives:




Magnification = 2000/20 = 100x


2.

A telescope of focal length 2000mm with a 2x Barlow and 20mm eyepiece gives:



Magnification = (2000 x 2) / 20 = 200x


3.

A telescope: 8”, F/10 telescope with a focal redu
cer of F/6 with a 25mm eyepiece
gives:



Magnification = (200 x 6) / 25 = 48x ,

OR



Magnification = (2000 / 25) x (6/10) = 48x


4.

A telescope: 8”, F/10 telescope with a focal reducer of F/6 with a 3x Barlow and
25mm eyepiece gives:



Magnification = (200 x 6 x 3)

/ 25 = ??? ,

OR



Magnification = [(2000 x 3) / 25] x (6/10) = ??? ,

OR



Magnification = (2000 / 25) x (6/10) x 3 = ???




Calculations


Field of view





Apparent FOV of eyepiece


Real FOV =
---------------------------------------




Magnification



E.g.:



A 50° eyepiece with a magnification of 100 yields:

o

FOV = 50 / 100 = 0.5° OR 30’