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Visibility Modeling

Deep-Sky Planner includes mathematical models that predict whether an object can be seen, given a set of equipment and sky conditions. In the case of extended deep-sky objects and single stars, it means whether the object can be detected visually. In the case of a double star, this means whether the pair can be split visually.

These models are the product of several years of research and field testing by the author, and published research by other scientists. These models are intended to provide guidance rather than an absolute answer to the question of whether an object can be seen.

The models require various input parameters in addition to object data. The table below describes the data required by the various models. Important: A model cannot be employed if any of the required input data are missing. Some of the input parameters are objective in nature while a few are rather subjective. If the visibility model is yielding results that seem incorrect given instrument, observer and sky conditions, these subjective parameters might be adjusted for improved results. In particular, you might adjust (in the order of greatest significance): observer's experience level, instrument's cleanliness or observer's dark-adapted eye pupil size.

Object Type	Sky Conditions Required	Equipment/Observer Information Required
Extended Deep-Sky Objects, Larger Planets	Darkness and transparency	Instrument aperture, optical system transmission, eyepiece apparent field or binocular true field, dark-adapted eye pupil size
Double Stars	Seeing	Instrument aperture, obstruction (if any) and cleanliness (if any), observer's dark-adapted eye pupil size and experience level
Single Stars, Carbon Stars, Asteroids, Smaller Planets		Instrument aperture and cleanliness (if any), observer's dark-adapted eye pupil size and experience level
Variable Stars		Instrument aperture and cleanliness (if any), observer's dark-adapted eye pupil size and experience level

The result of the models' computations are shown below. These results appear on reports in the Vis Model column.

Object Type	Vis Model	Comments
Extended Deep-Sky Objects, Larger Planets
	Easy	The object is bright enough to be seen without difficulty.
	Moderate	The object is bright enough to be seen with some difficulty.
	Difficult	The object may be bright enough to be seen. Try to observe this object at its highest altitude in the sky at a dark location, and be sure to use averted vision.
	Improbable	The object is probably too dim to be seen. Try to observe this object at its highest altitude in the sky at a dark location.
	Too dim	The object is too dim to be seen.
	Too low	The object is low in the sky; the model becomes very inaccurate at low altitudes.
	Down	The object is below the horizon
	<blank>	The object can't be modeled due to missing data.
Double Stars
	Easy	Both stars in the pair are bright enough to be seen, and they may be resolved. The two stars appear to be widely separated.
	Moderate	Both stars in the pair are bright enough to be seen, and they may be resolved. The two stars may appear to be fairly close, but they appear to be clearly separated.
	Difficult	Both stars in the pair are bright enough to be seen, and they may be resolved. The two stars may appear to be touching or they may be separated by a hair-line gap.
	Improbable	Both stars in the pair are bright enough to be seen but the pair is so close that it is improbable that they can be resolved. The two stars may appear to be a rod-shaped or figure-8 shaped star.
	No split	Both stars in the pair are bright enough to be seen but the pair is too close to be resolved.
	Sec. dim	The primary is bright enough to be seen but the secondary is not.
	Both dim	Both stars in the pair are too dim to be seen.
	Unmodeled	The stars fall outside the modeled parameters.
	Down	The stars are below the horizon
	<blank>	The stars can't be modeled due to missing data.
Single Stars, Carbon Stars, Asteroids, Smaller Planets
	Visible	The object is bright enough to be seen.
	Too dim	The object is too dim to be seen.
	Down	The object is below the horizon
	<blank>	The object can't be modeled due to missing data.
Variable Stars
	Visible	The star is bright enough to be seen throughout its magnitude range.
	Maybe	The star may be seen when it is at its brightest magnitude, but not at its dimmest magnitude; thus, it may be visible depending on where it is in its light curve.
	Too dim	The star is too dim to be seen even when it is at its brightest magnitude.
	Down	The star is below the horizon
	<blank>	The star can't be modeled due to missing data.