Visibility Guide

Getting started

1. In “1. Time period”, select the year and ISO week; the read-only fields show the corresponding month and days. If needed, shift the period by a few days to match an observing trip or a long weekend.
2. In “2. Filters”, choose a constellation or keep “All”.
3. Select the altitude threshold and the minimum time the object must remain above it.
4. Choose objects that transit the meridian to the north, to the south or in both directions.
5. Choose “Gallery” or “Pending”, and view the results as “View” or “List”.
6. Select “View chart” to open the selected night directly, or “Plan” to analyse the object from a location of your choice.

From Visibility to Planning

Visibility is the discovery view: it compares many objects for a given week from the reference location in central Catalonia and displays times in local time.

Once you have chosen an object, “Plan” opens the technical analysis and automatically transfers the object, week, year, day shift and altitude threshold. There you can change the location and compare all seven nights using UTC data.

Local time and UTC

The cards, best time and night chart use local time in Catalonia, as stated in the header. Daylight saving time is applied automatically when appropriate.

The annual chart is the exception: it shows the object’s altitude at 00:00 UTC and labels this explicitly.

Week, year and day offset

The selector uses ISO weeks, from Monday to Sunday. The week number and the dates in the period are shown separately.

“Day offset” moves the entire period up to three days earlier or later without changing the selected week. This makes it possible, for example, to study Saturday through Friday. The offset returns to zero when the week or year changes.

The year is particularly important when calculating the Moon’s position and phase.

Astronomical filters

“Altitude threshold” lets you filter at 30°, 35°, 40°, 45°, 50°, 55° or 60°. “Minimum hours above threshold” keeps only objects that satisfy both criteria during the selected period.

“Meridian transit” separates objects according to whether they culminate north or south of the zenith from the 41° N reference location. Both options are selected by default; you can keep only “North” or only “South”, but you cannot disable both.

• 2 hours: broad selection.
• 3–4 hours: balanced selection.
• 5–6 hours: more demanding selection.

Views and report

“View” displays visual cards, while “List” presents the same information in more compact rows. Both “Gallery” and “Pending” use these formats and allow you to open the full chart or send an object to Planning; pending targets are identified by the absence of a photograph.

“Download TXT” creates a text-only list. “Download PDF” creates a report with the same results, including the period, active filters and useful data for each object. A tick confirms that the action has completed.

Astronomical night

Astronomical night is the period when the Sun is below −18°. It is the darkest part of the night and is generally the most suitable time for photographing faint deep-sky objects.

Its duration changes throughout the year: astronomical nights are longer in winter and shorter in summer.

Annual chart

The annual chart shows the object’s approximate altitude at 00:00 UTC over the weeks of the year.

• The best time of year.
• The weeks when the object reaches its greatest altitude.
• When it is above or below 30°.
• The overall evolution of its visibility.

Chart for one night

“View chart” opens the “Selected night” tab directly. The “Annual” and “Selected night” buttons remain available for switching views, and the Monday-to-Sunday buttons let you study any night in the period.

The altitude axis uses references at 0°, ±30°, ±60° and +90°; the 20° reference is not used.

• The altitude of the object and the Moon.
• The horizon at 0°.
• A thin blue dotted line marking the selected threshold.
• The general yellow 30° reference when the selected threshold is different.
• The beginning and end of astronomical darkness.
• The time when the object reaches its maximum altitude.

The object, Moon and threshold

The green curve represents the object’s altitude, the reddish dashed curve represents the Moon, and the blue dotted line identifies the selected threshold.

When the lunar curve is below 0°, the Moon is below the horizon. If it remains high for much of the night, it can reduce sky contrast, especially when its phase is advanced.

Moon phase

Moon phase and altitude should be considered together: a bright Moon below the horizon does not affect observing, whereas a high Moon can.

• New Moon.
• Waxing crescent.
• First quarter.
• Waxing gibbous.
• Full Moon.
• Waning gibbous.
• Last quarter.
• Waning crescent.

Returning to the selected object

In “Gallery” cards, select the image itself to open the photograph page. It preserves the week, year, constellation, threshold, day offset and view type. When you return, Visibility restores that selection and centres the object you were viewing.

The same applies when you open “View full visibility” from an object page: closing the chart with ×, by clicking outside it or by pressing Esc returns you to the object’s card or row, which is briefly highlighted.

What it does not account for

The tool helps with planning, but it does not replace checking actual observing conditions.

• Weather.
• Cloud or humidity.
• Terrain and horizon obstructions.
• Local light pollution.
• Atmospheric transparency or turbulence.