Selecting Celestial Objects

How to select appropriate astronomical objects for astrophotography and scientific research with the Nieves Observatory

There are four criteria for selecting a celestial object for imaging using the Nieves Observatory:

  1. The object can be observed from the northern hemisphere
    As the observatory is located in the Northern Hemisphere, objects in the Southern Hemisphere are blocked by the Earth itself.
  2. The object is close to the zenith
    As the light penetrates the atmosphere, it is scattered and absorbed and becomes attenuated. This attenuation is also known as atmospheric extinction. Objects that are situated far away from the zenith and close to the horizon would appear dimmer and redder than they would be if viewed at the zenith. We thus want the object that we’re taking to be as close to the zenith as possible to reduce extinction. This can be found by looking for objects that are transiting or close to the middle of the sky when you are observing.
  3. The object is brighter than the limiting magnitude of the telescope
    The theoretical dimmest object that the telescope can take is 16.5 apparent magnitudes. However, due to the long exposure time needed to image 16.5-magnitude objects, the background noise would be significant. It is best if the objects you choose are no dimmer than 11 magnitudes. Bright galaxies with magnitudes of 9 or less work very well [Note that the larger number for magnitude, the fainter the object—a funny astronomical convention!].
  4. The object is smaller than our camera’s field of view
    Sizes of astronomical objects are denoted in either arcseconds, arcminutes, or degrees. An arcsecond (”) is 160 of an arcminute, and an arcminute (‘) is 160 of a degree (°). The CCD camera has a FOV of 47′.

As there is a panoply of objects you can choose from, we’d want to be smart about choosing the celestial object that satisfies the four criteria best. There are two browser-based software that you can use to select the best objects for imaging.

1. Telescopius

Telescopius is an astronomy planner website that helps astronomers pick out the best objects for study based on one’s coordinates in the world. It helps to list objects that will be closest to the zenith for a given night, and you can search and filter celestial sources. It even has a filter to not show objects that will be close to the moon.

Step 1: Input coordinates 

If you’re not already at Aliso (like me), click on your location and “edit your location manually”:

The Nieves Family Observatory has the following coordinates:

Latitute: 33.5548 \ 33° 33′ North
Longitude: -117.7375 \ -117° 44′ West

Step 2: Search away!

Click on any of the search categories. For example, I’d like to filter my search to Galaxies and by Magnitude. Select at “find more objects sorted by” > Magnitude.

Step 3: Select an object of interest

You will end up with a search result that resembles the below. Mouseover to see what the graph means and if these objects are good for observations at the Observatory for the night.


In the case of the M31, its closest approach to zenith (82°) is at 12:55am. You will also notice that it will leave its highest point fairly quickly by dawn. It is hence best to take an image of the galaxy at a time closest to 12:55am.


M33 has an even closer approach to zenith (87°) than M31 at 1:46am. For an efficient viewing session, it is best to take images of these objects in sequence with their closest approach to the zenith. For example, from 12am to 1am we can be observing M31, and then spend the time from 1am to 2am observating M33.


The closest approach that M81 has to zenith (55°) is at 10:06am, when the sun is already out. During the night, especially during midnight, the object is very close the horizon (and remains so for most of the night), and hence is not a good object for observation for the night.

Step 4: Further consideration

Apart from the altitude of the object, you’ll have to determine if the object is bright enough and big (or small) enough for observations. To do so, you can check the magnitude and angular size of the object.

Click on object > Scroll down to “Data Sheet” on the right.

Below is an exampl of the Pinwheel Galaxy (M101).

Angular Size

M101 has an angular size of 28.8′ by 26.9′, which is smaller than our FOV of 47′. The M101, size-wise, is a good observing target.

Apparent Magnitude

M101 is also relatively bright, at 7.9 magnitude, which is much brighter (>200x) than our limiting magnitude of 16.5. In terms of brightness, M101 is a good observing target.

2. Stellarium

Another method that is more interactive and fun is using Stellarium, a star-mapping software. While in the past you’d have to download the software to your machine, they recently launched an online version of the software that works as well for our purpose. You can choose to download Stellarium or use the online version.

Stellarium works by presenting a view of the night sky based on one’s coordinates and time, highlighting the objects of the sky that are hypothetically visible to us if we look up in the sky. For example, at around 9:30pm, 26 October 2019, at Aliso Viejo, California, this is what the night sky will look to me, as predicted by Stellarium.

Here’s a breakdown on the interface of Stellarium:

Location Window

In this window you can select your ideal location. This is especially useful if you’re not working in the same location as the telescope. To see the night sky of the Nieves Observatory, search for “Aliso Viejo” in the search bar of the location window.

Date and Time

This allows you to set the view of the sky to a past or future time. You can use this tool to see what objects are closest to zenith at a particular time, for you to plan the observing run for the night.

[More on Stellarium coming soon.]

Now that you’ve selected an object for study, let’s see how we can operate the telescope.

Appendix: A brief guide to the objects of the Universe

Much has been catalogued and classified. Here’s a brief description of what the various astronomical objects are that can be viewed with the Soka Observatory.

1. Stars
2. Star Clusters
3. Galaxies
4. Comets/Asteroids/Meteors
5. Supernovae & other powerful transients

To help you get to know more about these objects, we have prepared a guide to astronomical objects in the sky – something of a mini online textbook. Enjoy!

The Nieves Observatory Guide to the Astronomical Objects