Equipment Functions

There is so much that you can categorise in your equipment. I find these useful for making choices about combinations of equipment or gauging how a new piece of equipment will fit in with my existing set-up, I hope you find them useful too.

AiryDiskArcSec(F_Ratio, F_Length, [wavelength_nm]) Returns the size of the airy disk in arc seconds using the Focal Ratio, focal length and optionally the wavelength you are interested in in nano-meters, if this last option is omitted then  it will use 546nm (Greenlight) which is where the eye is most responsive.
AiryDiskmm(F_Ratio, [wavelength_nm]) As above but this gives the actual size of the airy disk in mm 
CCD_FoV(CCD dimension in mm, telescope focal length in mm) For focal length in mm and a CCD dimention in mm provide the field of view in arc minutes
To find the pixel resolution simply use the pixel size in place of the CCD size remembering the pixel size is entered in mm and pixel sizes are usually quoted in microns
CCDMinFocalLength(Aperature_mm, Pixel_Size_Microns, [Wavelength_nm]) Use this formulae to work out the correct barlow to use to optimise the resolution of your detector.Takes the aperture of the scope in mm and pixel size in microns, wavelength is optional, if omitted it will use 546nm (Greenlight) this parameter should be entered in nano-meters
It will return the minimum focal length for maximum resolution of the detector.
CCDMinFocalRatio(Pixel_Size_Microns, [Wavelength_nm]) Takes the…
Pixel size in microns of the CCD
wavelength interested in, in nm (if omitted it will use 546nm (Greenlight))
Returns
The minimum Focal Ratio for maximum resolution
Coma_CCD(F_Ratio, Focal Length in mm, CCD Dimention in mm) Calculates the coma in arc seconds given the focal length, focal ratio, and CCD dimension. Assumes the optical system is optically collimated and the CCD is placed at the centre of the field
Coma_Eyepiece(F_Ratio, Off_axis_Angle) Calculates the coma in arc seconds from the off axis angle and focal ratio of the telescope.
CriticalFocusZone(F_Ratio, [Wavelength_nm]) Takes the…
Focal ratio of the system and optionally the wavelength in nm interested in (if omitted it assumes 546nm (green light)
Returns…
the size of the focus zone for critical focus in mm
DustDonutDistance(PixelSize_microns, F_Ratio, donut_Size_pixels, [binning]) Takes the…
measured size of the dust donut in pixels
the CCD Pixel Size in microns
The focal ration of the optical system
The Binning used to take the image (if omitted assumes no binning)
Returns…
the distance from the sensor to the dust in mm
EquivalentContrastAperture( Central Obstruction, Aperture) Calculates the size an unobstructed telescope would be to deliver the same level of contrast.
Aperture is in mm
 Central Obstruction is a % obstruction for the telescope
 or
 a diameter of the obstruction in mm
ExitPupil(Scope F Ratio, Eyepiece Focal Length) Calculates the exit pupil for a given telescope and eyepiece combination
ExitPupil(Focal_Ratio_of_Telescope, Focal_Length_of_Eyepiece) = exit pupil units same as used in argument, all arguments in same units
ExposureIncrease(Mag increase) for a given increase in magnitude works out how much longer your exposure needs to be, so
ExposureIncrease(2)=39.1 to increase the highest magnitude in your images by 2 you will need to increase the exposure time by 40, this takes into account an increase in noise with exposure time.
FocalLengthfromCCDImage(Object Size pixels, Pixel size , Object Size, [binning]) Takes the…
measured size of the object in pixels
the CCD Pixel Size in microns
The actual angular size of the object in arcSeconds
The Binning used to take the image (if omitted assumes no binning)
Returns…
the Focal Length of the system in mm
FoV_by_Drift(declination of object, time to cross field) calculates the field of view of a telescope and eyepiece combination based on how long it takes for an object of known declination in degrees to cross the field of view in seconds.
FullIlluminationDueToFocuser(Aperture, F ratio, Focal Point to base, Focuser neck width) Takes the…
Aperture of the scope in mm
The Focal Ratio of the scope
The distance from the focal plane to the base of the focuser (i.e. where it meets the tube) in mm
The diameter of the neck of the focuser in mm
Returns the diameter of the disk that will be fully illuminated (past this there will be vignetting)
LightThroughput(Aperture, obstruction, No of Surface A, Surface A Efficiency, No of Surface B, Surface B Efficiency, No of Surface C, Surface C Efficiency, aperture comparison) This is a really useful function for comparing different types of telescope light grasp as it considers the transparency and reflectance of the surfaces. Calculated Throughput is the increase in light gathering the telescope provides when compared to a seven millimetre, dark adapted eye.
It is based upon the light collecting area of the telescope corrected for transmission, reflectance, and central obstruction.  Aperture is in mm
 No of surfaces type A, B and C is the number of surfaces the light path crosses, you can have up to 3 different types of surface
i.e

  • in a standard newtonian this is 2 one for each mirror
  • in a air spaced tripplet this is 6 (two per element)
  • in a LX200 it is 2 for the corrector plate and 1 for the mirror, so you would use type A and type B 

Surface type A Efficiency (and B and C)  is an integer representing the type of surface as follows…
 2 = Uncoated air-to-glass transmission (per surface)—96%
 3 = Multicoated air-to-glass transmission (per surface)—99%
 4 = Reflectance of a protected aluminum mirror—87%
 5 = Reflectance of an enhanced aluminum mirror—94%
 6 = Reflectance of a dielectric mirror (typical diagonal)—99%
 7 = Reflectance of Hilux mirrors (Orion Optics UK)- 97%
 or
 an actual value <=1 representing the actual reflectance/ transmission of the surface

 comparison Aperture if omitted uses 7mm otherwise uses the value entered
 Obstruction a % obstruction for the telescope
 or
 a diameter of the obstruction in mm

MagLimit(diameter, “Author”) for a given diameter telescope in mm will return one of 4 mag limits based on different predictions by different authors, ‘Clark‘, ‘North‘, ‘Schaefer‘ or ‘Standard‘ if ‘author’ is set to anything else it will return the valid inputs
NewtSecondaryOffset(Primary Diameter, Fully Illuminated Field Diameter, Secondary Diameter, Focal Length) Takes the…
primary mirror diameter
fully illuminated field diameter, i.e. the diameter of the expected detector you will use
secondary mirror diameter
focal length of the scope
calculates the secondary off-set required, this is the amount that the secondary should be moved away from the focuser AND towards the primary for a fully illuminated field
NewtSecondarySize(Focal Ratio, Secondary to Focal Plane, [Illuminated Diameter]) Takes the …
focal ratio of the scope
secondary to focal plane distance (from the centre of the secondary to the focal plane)
The desired fully illuminated diameter (If the illuminated diameter is ommitted it will calculate the smallest possiible secondary that will fully illuminate the centre of the field. )
Returns the optimal secondary diameter. 
StarTrail(Focal Length , Exposure Length , [Dec of Object]) Takes the…
Focal length of  the optical system in mm
exposure length in seconds
Declination of the object in degrees, this is optional, if excluded it is taken to be 0
Returns the length of the star trails in mm
StarTrailPixel(Focal Length, Exposure Length, Pixel Size , [Dec of Object]) Takes the…
Focal length of  the optical system in mm
exposure length in seconds
pixel size in microns
Declination of the object being imaged, in degrees this is optional, if excluded it is taken to be 0
Returns the length of the star trails in pixels
RealFoVfromEyepieceFieldStop(Focal Length of Scope, Eyepiece Field Stop) Takes the…
Focal length of the scope in mm
and the eyepiece field stop in mm (you will need to measure this)
Returns the real field of view in degrees
RealFoVfromEyepieceFoV(Focal Length of Scope, Focal Length of eyepiece, eyepiece Field of view)  Takes the…
Focal length of the scope in mm
Focal length of the eyepiece in mm
and the eyepiece field of view in degrees (you need to look up)
Return the real field of view in degrees
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