Target Visibility

Before attempting to observe an object you should think about its visibility: when is the object in the sky, is it viewable from any of LCO's sites, and how long is it is visible. The visibility is dependent of the right ascension (RA), the declination (DEC), the day and time, the observatory longitude and latitude and the airmass limit.

Worked example using Visibility Tool

Because LCO is not a conventional observatory, having 7 observing sites located around the globe, this provides more opportunities for an object to be observed. We have developed a visibility tool to visualize which sites can be potentially used for observing your requests.

It has 2 modes, seasonal visibility and daily visibility, which we will discuss below.

Daily Visibility example.

In this example, we will assume that the observer is interested in observing the Galactic Bulge (RA2000 : 17:00:00, DEC2000: -30:00:00) tomorrow (i.e the 9/05/2018). The observer does not need very good data quality and can set the airmass limit to 3.

Using the Visibility Tool, enter the target coordinates. Here we are interested in the daily visibility only. Entering the desired date will result in a graph similar to the one below:

Visibility of the various LCO sites for the chosen day

Visibility of the various LCO sites for the chosen day

This graph shows how the height of the target above the horizon, changes throughout the night. Each color represents an LCO site, solid lines represent time where the target is visible. Dotted lines represent times when the object is not visible because of day time, being too close to the horizon or an area of sky the telescope physically cannot point at.

Our example target (the bulge of the Milky Way) is in the Southern Hemisphere, so it is not surprising that the southern sites (Cerro Tololo, Siding Spring and Sutherland) are better suited, with a longer observing windows at lower airmass.

Observing Portal

The daily visibility tool is embedded in the LCO observing portal. When you make a request, the observing portal performs a set of checks on the values you enter. One of those is making sure that your target is visible during the time window you have asked for. The graph you are shown is the same as the daily visibility graph, but spanning multiple days (if appropriate).

Seasonal visibility

Let’s assume that the observer wants to observe a target regularly for a period of 10 days. What would be the best season to do so? The seasonal visibility option is the one to use. Using the same target as the previous example, we can see the seasonal visibility below.

Visibility of the various LCO sites during all the season.

Visibility of the various LCO sites during all the season.

The plot now describes the day visibility in UT vs the year day. Again, we can see that the Galactic Bulge is mainly visible during the summer time. The optimal time to monitor our target is between the 1 May and 1 July, where the visibility at the various LCO sites overlaps the most.

Factors Affecting Visibility

Airmass

Airmass is a useful quantity when calculating the visibility of a target. Airmass is a measure of how thick the atmosphere appears to an observer. The higher the number, the thicker the atmosphere and the less sharp your image will be.

Looking at the "Daily Visibility" graph above, you could set the airmass limit to 1.5, which will eliminate the northern sites. This reduces the number of places your observation can happen but will increase the chance of your having sharper images.

However, if your object is close to either north or south celestial pole, your target may never be visible, unless you increase your airmass limit.

Take the example of NGC188 (RA: 0:44:00, Dec:+85:20:00). Using the default airmass limit of 1.6, means the target will be impossible to observe on 1 July (as shown below).

ngc188-am1.6.png

However if the airmass limit is relaxed to 2.0, the target is observable from 2 sites on 1 July, albeit for a short time (as shown below).

ngc188-am2.png

Position of the Moon

In the observing portal you may encounter gaps in the visibility. This may be due to your object being too close to the Moon for LCO to observe it. This is more common with Solar System objects, i.e. planets and minor bodies.

Time of year

The shape of the solid, coloured regions on the Seasonal Visibility graph is due in part to when objects are obscured by the Sun.