Telescope calculator
The telescope calculator uses the telescope primary focal length, primary aperture, eye piece focal length, barlow lens and focal reducer lens values to calculate expected magnification, focal ratio and apparent focal length. The table calculator requires an initial focal length to be entered.
INPUTS
The telescope calculator uses the telescope primary focal length, primary aperture, eye piece focal length, barlow lens and focal reducer lens values to calculate expected magnification, focal ratio and apparent focal length. The table calculator requires an initial focal length to be entered.
INPUTS
- Focal length: Distance from the primary mirror or lens to the image that it forms.
- Eye piece focal length: An eye piece is placed near the focal point of the primary objective image to create a magnified image . The eye pieces focal length is inversely proportional to the magnification. However there will be a limited practical magnification for any telescope.
- Telescope Aperture: The aperture is the diameter of the lens or mirror that lets light in. The bigger the aperture, the more light and information that is captured.
- Barlow lens: A barlow lens increases the effective focal length of an optical system and hence the magnification. creasing the magnification of a target will also reduce its brightness.
- Focal reducer lens: A focal reducer is an optical element used to reduce the focal length of the optical setup. It reduces the magnification but increases the targets brightness.
- Magnification: This is the focal length of the primary lens or mirror divided by the eye pieces focal length.
- Focal ratio: Focal ratio is a measure of a telescope light collecting capabilities. An F4 telescope would collect a lot of light (fast) and be suitable for fainter targets.. While an F12 would collect a lot less light (slow) and be more suitable for planetary work.
- Apparent focal length: This gives the new focal length calculated from barlow/reducer lens usage.
Field of view calculator
A telescopes field of view is a measure of how much sky you can see using a given optical configuration. It is generally given as an angle and expressed in terms of degrees, minutes and seconds. The true field of view in this example is calculated by dividing the eye pieces apparent field of view by the telescopes magnification.
Tfov = Afov/Magnification = Afov x (eye piece focal length/telescope focal length).
Tfov can also be calculated using the eye piece stop diameter but this has not been considered.
The field of view calculator has been interfaced with the Aladin Lite Interactice Sky Map. This will give a selected target at the calculated field of view. The initial image has been defaulted to M1 the crab nebula at an 0.5 deg fov. If you want a different target you must select it using the search field in the Aladin interface. The image will be returned at the fov specified in the table. Enter a value into the table to initialise the calculations. An optional circular fov overlayer has been included. If you change the target during use the circular fov overlay can be reinstated by turning it off and on again.
INPUTS
A telescopes field of view is a measure of how much sky you can see using a given optical configuration. It is generally given as an angle and expressed in terms of degrees, minutes and seconds. The true field of view in this example is calculated by dividing the eye pieces apparent field of view by the telescopes magnification.
Tfov = Afov/Magnification = Afov x (eye piece focal length/telescope focal length).
Tfov can also be calculated using the eye piece stop diameter but this has not been considered.
The field of view calculator has been interfaced with the Aladin Lite Interactice Sky Map. This will give a selected target at the calculated field of view. The initial image has been defaulted to M1 the crab nebula at an 0.5 deg fov. If you want a different target you must select it using the search field in the Aladin interface. The image will be returned at the fov specified in the table. Enter a value into the table to initialise the calculations. An optional circular fov overlayer has been included. If you change the target during use the circular fov overlay can be reinstated by turning it off and on again.
INPUTS
- Focal length: Distance from the primary mirror or lens to the image that it forms.
- Telescope Aperture: The aperture is the diameter of the lens or mirror that lets light in.
- Eye piece focal length
- Eye piece apparent field of view as supplied by manufacturer.
- Barlow/Reducer lens degree of magnification.
- Apparent focal length: This gives the new focal length calculated from barlow/reducer lens usage.
- Focal ratio: Focal ratio is a measure of a telescope light collecting capabilities.
- Magnification: This is the focal length of the primary lens or mirror divided by the eye pieces focal length.
- Eye piece true field of view: This is the actual field of view as seen by the telescope.
- Exit pupil: The diameter of the image produced at the eye piece. Rather subjective depending on users eye sight and targets being viewed. Typical values can range between 2 and 7 mm.
Telescope Equations
It can be confusing understanding how to work out what you are seeing through a scope, so knowing what magnification and field of view you are getting really helps. Below are some basic equations which will allow you to work this out for yourself:
M = Magnification
Fs = Focal length of Scope
A = Aperture of Scope
Fr = Focal Ratio of Scope
Fe = Focal length of Eyepiece
AFoV = Apparent Field of View of Eyepiece
TFoV = True Field of View of Eyepiece
FS = FieldStop of Eyepiece
EP = Exit Pupil
M = Fs / Fe
TFoV = AFoV / M
= Fe x AFoV / Fs
= 57.3 x FS / Fs
EP = A / M
= A x Fe / Fs
= Fe / Fr
M = Magnification
Fs = Focal length of Scope
A = Aperture of Scope
Fr = Focal Ratio of Scope
Fe = Focal length of Eyepiece
AFoV = Apparent Field of View of Eyepiece
TFoV = True Field of View of Eyepiece
FS = FieldStop of Eyepiece
EP = Exit Pupil
M = Fs / Fe
TFoV = AFoV / M
= Fe x AFoV / Fs
= 57.3 x FS / Fs
EP = A / M
= A x Fe / Fs
= Fe / Fr
