Correction factor with Barlow/Reducer
Seeing values FWHM
Focal length Guidescope

Binning / Binning Guidecam
Pixelsize Guidecam without / with binning

Focal length mm
Apparent field of view in degrees
Field apperture in mm

Formula Vignetting


Focal length Telescope
Aperture ratio without corrector

Camera pixels x/y
Pixelsize x/y

Magnification factor
Field of view in arcmins
Exit pupil

Current filter distance to chip
Diameter of filter or bottleneck

Effective focal length Telescope
Aperture ratio with corrector

Number of pixels
Pixelsize with binning

Min. useful magnification
Max. useful magnification

Vignetting start at chip diagonal in mm
Illumination of the corners in percent

Useful focal length - Aperture ratio - Barlow
for planetary imaging

Target image scale range for act. seeing FWHM
Sampling for actual setup

min. useful wide-angle EP
max. useful tele-EP

Max. distance of bottleneck to
avoid vignetting on image

Telescope aperture in inch
Resolution of Telescope in arcsecs

Width  Height  Diagonal
Chip in mm


Limiting magnitude of Telescope
Max imaging time (sec) without tracking

Width  Height  Diagonal
Field of view in arcmins

Opt. focal length of Guidescope for current Telescope
Max. focal length of Telescope with current Guidescope

Mega-Pixel (Binning)
Subpixel correction value


Implications of under-/over-sampling:

undersampling reduces the influence of guiding errors and improves signal to noise at the expense of finest detail

oversampling will require a good mount and careful guiding. OK for high magnification solar, lunar or planetary imaging. Might cause signal to noise issues with wide-field imaging

It is important to find a good balance between over- and undersampling. Namely so, that the camera still works as sensitive as possible, but the images are good shaded. The following figure illustrates graphically this relationship.


On the left the undersampling shows almost only square stars. On the right oversampling shows many shades of gray, but the light of the stars is smeared over many pixels.

Center of the picture is an example of good sampling. For deep sky imaging, the rule of thumb for the image scale is to achieve a value of about 1.5 arcsecs to 2 arcsecs per pixel. In this rule of thumb, the normal local seeing should be included. The seeing value is simply divided by 2.67 (low) or 2 (high) if you have local seeing on average of 4 arcseconds.



Yellow rectangle = FoV of selected chhip
Red outline = Vignetting
Green outline = FoV of selected eyepiece on telescope
White circle = Apparent moon size


1 = FWHM 0-1 Dream
2 = FWHM 1-2 Perfect
3 = FWHM 2-3 Very good
4 = FWHM 3-4 Good
5 = FWHM 4-5 Average
6 = FWHM 5-6 Average
7 = FWHM 6-7 Moderate
8 = FWHM 7-8 Bad
9 = FWHM 8-9 Very bad
10 = FWHM 9-10 Unusable

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