Calculators Back | Up| Next

Calculators have data already in them. Some are examples from my telescope, a C11 SCT with 279.4 mm of aperture and 2,794 mm focal length at f/10.

Highlight the numbers already entered in the boxes and delete them and enter your own.

For large numbers, do not use commas.


1. Focal Ratio

Focal length of scope in mm
Aperture in mm
Focal Ratio


1a. Aperture from Focal Length and Focal Ratio

Focal length of scope in mm
Focal Ratio
Aperture in mm


2. Linear Diameter of the Airy Disk

Focal Ratio
Wavelength in nanometers
Linear Diameter


3. Angular Diameter of the Airy Disk

Scope Aperture in millimeters
Wavelength in nanometers
Angular Diameter


4. FWHM Angular Diameter of the Airy Disk

Scope Aperture in millimeters
Wavelength in nanometers
FWHM Angular Diameter


5. FWHM Linear Diameter of the Airy Disk

Focal Ratio
Wavelength in nanometers
FWHM Linear Diameter


6. Image Scale per Pixel

Focal length of scope in mm
Pixel size in microns
arcseconds per Pixel


7. Focal Length and Focal Ratio Needed For Critical Sampling

Aperture of the scope in mm
Pixel size in microns
Sampling Factor based on seeing
Average = 3, Good = 4, Excellent = 5
Focal Length Needed
Focal Ratio Needed


8. Focal Length Needed for a Desired Image Sampling

Desired Image Sampling in arc sec / pixel
Pixel size in microns
Focal Length


9. Angular Size Based on Distance
Linear Size and Distance should be in the same units.
Linear Size
Distance
Angular Size
Cassini's division is about 4,800 kilometers wide.
Encke's gap is about 325 kilometers wide.
Saturn is about 1,200,000,000 kilometers distant at opposition.


10. Linear Size Based on Distance

Angular Size in arcseconds
Distance in Kilometers
Linear Size
The Moon is 1,800 arcseconds in diameter on average.
The Moon is 403,635 kilometers distant from the Earth on average.


11. Effective Focal Length from an Object's Size in an Image

Object Size in Pixels
Pixel Size in Microns
Object Size in arcseconds
Effective Focal Length


12. Eyepiece Projection

Scope Focal Length in mm
Scope Focal Ratio
Eyepiece Focal Length in mm
Eyepiece Distance to Camera Sensor
Effective Focal Length
Effective Focal Ratio
Magnification


13. Afocal Projection

Scope Focal Length in mm
Eyepiece Focal Length in mm
Camera Lens Focal Length in mm
Scope Aperture in mm
Effective Focal Length
Effective Focal Ratio
Magnification


14. Barlow (Negative Lens) Projection

Scope Focal Length in mm
Scope Focal Ratio
Barlow Focal Length in mm
Barlow Distance to Camera Sensor
Effective Focal Length
Effective Focal Ratio
Barlows have a negative focal length, but input it here as a positive number.


15. Maximum Exposure Length for Planetary Rotation

Smear Allowed in arcseconds
Planetary Diameter in arcseconds
Planetary Rotation Period in minutes
Maximum Exposure Time
Jupiter is about 47 arcseconds in diameter at opposition.
Jupiter rotates in 595.5 minutes.


16. Field of View

Scope Focal Length in mm
Frame Size Long Side in mm
Frame Size Short Side in mm
Field of View Long Side
Field of View Short Side
Canon's 60D, 60Da, 7D, T2i (550D), T3i (600D), and T4i (650D) all have sensors that are 22.3 mm x 14.9 mm in size. Canon and Nikon sensor sizes are listed here.


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