Wilmslow Astro
Astronomy from a Cheshire suburb
Useful Formulae
Here are some formulae that I find useful from time to time. If you have any corrections or others that may be useful please send them to the email address on the contact page.
Eyepiece apparent vs real field of view
CCD arc-sec/pixel and focal ratio
Focal reducers
Eyepiece Projection
CCD Dust Shadows
CCD Filter Reflections
Visual limiting magnitude
Signal to noise ratio
Size of Airy disk
Mount Periodic Error
Star trail lengths
Auto Guider rates
Critical Focus Zone & CCD Focus Zone
Eyepiece Apparent vs Real Field of View
Two methods, the first is an approximation but is often the easier to work out due to lack of information about field stop sizes.
Method 1
Method 2
CCD arc-sec/pixel & Focal Ratio
The formula for arc-sec per pixel is:
or
Alternatively you can measure the size of an imaged object in pixels, and divide that objects known size in arc-seconds by the size in pixels.
Solving for focal ratio, this becomes:
You can use this formula to get a reasonably accurate focal ratio from any image where you know the angular size of an object (or angular distance between two stars).
Focal Reducers
Use this formula to calculate the resulting focal ratio when using a focal reducer:
And this one to calculate the amount of in-focus required by that set-up:
Where
a = Distance of CCD from focal reducer
b = Focal length of FR
c = Focal ratio of scopeThe Meade/Celestron focal reducers have the following focal lengths:
FR 0.33x focal length = 85mm
FR 0.63x focal length = 285mmWilliam Optics 0.8x FR focal length = 260mm
ATIK 0.5x FR focal length = 80mm
* Note that the in-focus figure assumes that the FR-CCD spacing is added to your physical imaging train length (as when using the Meade/Celestron FR's with spacing tubes). If you are using a FR like the ATIK that is fitted internally, then you have to add the FR-CCD spacing to this figure (to make it smaller). If you are using a FR like the Meade/Celestron then you will have to subtract the depth of the FR itself to this figure (to make it a larger negative number).
Eyepiece Projection
Use this formula to calculate the resulting focal length and focal ratio when using eyepiece projection:
Where:
epid = Distance of CCD from Eyepiece
epfl = Eyepiece focal length
Note: This only gives an approximate value for the resulting focal length and focal ratio. The distance from the eyepiece is hard to measure, and the nodal point of the eyepiece (the point of its effect - which is usually internal to the eyepiece) is normally unknown.
CCD Dust Shadows
If you are troubled by dust shadows on your CCD images you can calculate the distance that the dust particle is in front of the CCD with the following formula:
Where:
Dist = Distance from CCD surface in mm
p = CCD pixel size in microns
f = Focal ratio of imaging system
d = Diameter of dust shadow in pixels
CCD Filter Reflections
Want to know where those annoying reflection disks around stars are coming from?...
To calculate this
Where:
Dist = Distance from CCD of reflection surface in mm
D = Diameter of reflection disk in image in pixels
P = CCD pixel size in microns
FR = Focal ratio of imaging system
Visual Limiting Magnitude
A rough formula for calculating visual limiting magnitude of a telescope is:
![VLM = 7.5 + (5 * Log(Aperture [cm]))](images/formula15.png){kind=small}
The photographic limiting magnitude is approximately two or more magnitudes fainter than visual limiting magnitude.
Signal to Noise Ratio
A simplified formula for calculating the signal to noise ratio in an image is:
where:
S = total nebula signal
B = total background signal
D = dark current
RN = read noise from bias frame
n = number of sub-exposures
Size of the Airy Disk
A formula for calculating the size of the Airy disk produced by a telescope is:
and
where:
D = Diameter of Airy disk in mm
λ = Wavelength of light in nm
FR = Focal Ratio of system
A = Angular diameter of Airy disk in arcsec
fl = Focal length of telescope in mm
Calculating Mount Periodic Error
To calculate the periodic error of your mount using a CCD or webcam, you will typically put some numeric deviation data into a spreadsheet and create a graph. The data is normally in the form of a pixel offset and a timestamp. To convert this into an error in arc seconds you need to know how many arc seconds per pixel the images were captured, and the declination of the star used. The formula to plug into your spreadsheet is based on:
where:
D = Deviation of star from base position in pixels
R = Resolution of the camera in arc seconds per pixel
Dec = Declination of the star
Of course you can remove the requirement to use the declination in the calculation by using a star at (or close to) declination 0 degrees - the celestial equator.
Star Trail Lengths
You can use the calculator to estimate how long (or short) the star trails will be in a fixed camera image of the sky:
where:
F = Focal length of lens scope (trail length in same units as focal length)
E = Exposure length
T = Length of sidereal day in same units as exposure
D = Declination of the star
Or, for the CCD imagers:
Auto Guider Rates
Calculate how many pixels per second your auto-guider will move when the mount is being guided. Note that this assumes your auto-guiders axes are aligned with RA and Dec directions.
where:
str = Sidereal Tracking Rate (15.04 arcsecs/second)
gr = Mount Guide Rate (fraction of sidereal)
aspp = Autoguider arcsecs/pixel
dec = Declination of the star
Critical Focus Zone
Calculate the length of the zone in which the focused image of a star is smaller than the size of its Airy disk.
which simplifies to:
For CCD cameras, if we take a 2x sampling ratio:
where:
λ = wavelength of light (Red ~700, Green ~530, Blue ~470)
Note that because at low f/ratios the size of the Airy disk becomes significantly smaller than typical CCD pixels sizes I have introduced a value for the CCD Focus Zone. The value for the CCD focus zone takes the larger value of the CFZ, or where the Airy disk is half the effective pixel size (2x over sampling ratio) the CCD focus zone value defined above. For small focal ratios the CFZ gives a misleadingly small figure for imagers.