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

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

Focal length of scope Focal length of eyepiece Eyepiece apparent FOV Calculated Real Field of View

Method 2

Focal length of scope Eyepiece field stop diameter Calculated Real Field of View

back to top

CCD arc-sec/pixel & Focal Ratio

The formula for arc-sec per pixel is:

Focal length of scope CCD pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Calculated arcsec/pixel

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.

Object size in arc secs Image size of object in pixels Calculated arcsec/pixel

Solving for focal ratio, this becomes:

CCD pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Image arcsec / pixel Scope aperture (mm) Calculated focal ratio

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).

Or if you want the focal length used for an image and know the angular size of the object imaged:

CCD pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Object size in pixels Object size in arc seconds Calculated focal length

back to top

CCD Planetary Critical Sampling

Use this formula to calculate the minimum focal length required to fully sample a high resolution image with any particular CCD.

This is based on the assumption of perfect seeing and the Airy disk being the limit of resolution. For planetary imaging using 'lucky' imaging short exposure techniques this is a reasonable assumption. For long exposures the FWHM value for any particular night would be a better measure of the resolution obtainable.

I have made the assumption that adequate sampling requires the pixel spacing to be three times greater than the scopes resolution (forget Nyquist!)

Light Colour Green Red Blue user entered Wavelength (nm) Scope aperture (mm) CCD pixel size (microns) Calculated focal ratio Calculated focal length

back to top

Focal Reducers

Using focal reducers with refractors and lenses.

Use this formula to calculate the resulting focal ratio when using a focal reducer with a refractor or a simple lens:

And this one to calculate the amount of in-focus required by that set-up:

Focal Reducer Celestron f/6.3 Meade f/3.3 William Optics 0.8x ATiK 0.5x user entered FR focal length (mm) CCD-FR distance (mm) Focal ratio of scope Calculated focal ratio Calculated in-focus required*

Where a = Distance of CCD from focal reducer

b = Focal length of FR c = Focal ratio of scope

The Meade/Celestron focal reducers have the following focal lengths.

[note: Around 2006-2010 Meade manufactured some f/6.3 focal reducers with a focal length of around half what they should be making them unsuitable for use with SLR cameras or filter wheels, any marked "Japan" are OK, as are later "China" ones.]: Note: the Meade and Celestron focal reducers are called f/6.3 reducers, this is because when used as designed they result in a focal ratio of approximately f/6.3 on an f/10 SCT.

They are NOT 0.63x reducers. Meade FR f/3.3 focal length = 85 mm

Celestron/Meade FR f/6.3 focal length = 240 mm

William Optics 0.8x FR focal length = 260 mm

ATIK 0.5x FR focal length = 80 mm * 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).

back to top

Using focal reducers with SCTs with a moving mirror - !NEW!

Use this formula to calculate the resulting focal ratio when using a focal reducer with one of the 'classic' SCTs, note that the calcuations are only valid within 'sensible' ranges of focus. They are based on the FR being placed in its designed position at the back of the SCT:

Moving mirror SCTs require special treatment because their focal length changes (lengthens) as you adjust focus to compensate for the in-focus required from inserting the focal reducer in the optical path.

Telescope Celestron 8" Celestron 9.25" Celestron 11" Celestron 14" user entered Aperture of scope (mm) Focal length of scope (mm) Scope-FR distance (mm) Focal Reducer Celestron f/6.3 Meade f/3.3 William Optics 0.8x ATiK 0.5x user entered FR focal length (mm) CCD-FR distance (mm) Calculated focal ratio Calculated effective focal length Calculated in-focus required* Calculated SCT focal length

Where a = Distance of CCD from focal reducer

b = Focal length of FR

c = Focal ratio of scope The Meade/Celestron focal reducers have the following focal lengths.

[note: Around 2006-2010 Meade manufactured some f/6.3 focal reducers with a focal length of around half what they should be making them unsuitable for use with SLR cameras or filter wheels, any marked "Japan" are OK, as are later "China" ones.]: Note: the Meade and Celestron focal reducers are called f/6.3 reducers, this is because when used as designed they result in a focal ratio of approximately f/6.3 on an f/10 SCT.

They are NOT 0.63x reducers. Meade FR f/3.3 focal length = 85 mm

Celestron/Meade FR f/6.3 focal length = 240 mm

William Optics 0.8x FR focal length = 260 mm

ATIK 0.5x FR focal length = 80 mm * 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).

back to top

Eyepiece Projection

Use this formula to calculate the resulting focal length and focal ratio when using eyepiece projection:

CCD-EP distance (mm) Scope focal length (mm) Scope focal ratio Eyepiece focal length (mm) Calculated focal ratio Calculated focal length (mm) Magnification factor

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.

back to top

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:

CCD pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Imaging system focal ratio Diameter of dust shadow (pixels) Calculated distance from CCD (mm)

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

back to top

CCD Filter Reflections

Want to know where those annoying reflection disks around stars are coming from?...

To calculate this

CCD pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Imaging system focal ratio Diameter of reflection (pixels) Calculated distance from CCD (mm)

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

back to top

Visual Limiting Magnitude

A rough formula for calculating visual limiting magnitude of a telescope is:

The photographic limiting magnitude is approximately two or more magnitudes fainter than visual limiting magnitude.

back to top

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

back to top

Size of the Airy Disk

A formula for calculating the size of the Airy disk produced by a telescope is:

and

or

where: D = Diameter of Airy disk in mm

λ = Wavelength of light (in mm here, normally in nm)

FR = Focal Ratio of system

A = Angular diameter of Airy disk in arcsec

fl = Focal length of telescope in mm

d = Diameter of telescope in mm

Note that this formula gives the diameter of the Airy disk, the resolution limit of the optics is generally taken to be the radius of the first minimum, i.e. half this angular value.

Light Colour Green Red Blue user entered Wavelength (nm) Telescope Diameter (mm) Imaging system focal ratio Calculated size of Airy disk (mm) Calculated angular size of Airy disk

back to top

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.

back to top

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:

Focal length (mm) Exposure time (secs) Declination Pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Star trail length

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

Guide scope focal length (mm) Mount guide rate (fraction of sidereal) Declination of guide star Auto Guider pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Guide rate

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



Light Colour Green Red Blue user entered Wavelength of light Focal Ratio Camera pixel size (microns) Pixel binning 1x1 2x2 3x3 4x4 Critical Focus Zone CCD Focus Zone

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 under sampling ratio) the CCD focus zone value defined above. For small focal ratios the CFZ gives a misleadingly small figure for imagers.

back to top