Astronomical Seeing - How Good are the Observing Conditions?

Have you ever wondered why you see the stars in the night sky more clearly on some nights than on others? You are about to measure quantitatively how the Earth’s atmosphere affects the quality of sky images, and thereby imposes fundamental limitations to ground-based astronomical observations.

As a first task, you will acquire two sets of images with a robotic telescope.

As a second task, you will then pick 3 stars in one of the observed fields, and compare how these look on the different images that you have acquired.


The Las Cumbres Observatory (LCO) global network comprises telescopes with primary mirror diameters of 2 m, 1 m, and 0.4 m, with the 0.4 m telescopes.Professional astronomers no longer directly look through an eyepiece of their telescope, but use arrays of electronic light-sensitive detectors to deliver a digital image to their computer, which can then be viewed on a screen or further processed. In fact, the camera chips used are not that dissimilar to what is being found in mobile phones and consumer digital cameras.

Moreover, astronomical telescopes can now be controlled robotically, where not only the telescope positions automatically for the target to be acquired, but even the optimal scheduling of a list of targets to be observed is determined by computer software. This operating model means that a telescope user defines what he/she would like to observe, submits a request to the telescope scheduler, receives notification about when and if the requested observation has been carried out, and is being sent the acquired data.

Astronomical observatories have also established databases of past observations that can be queried as “virtual observatories”.


You are going to use data from LCO robotic telescopes. You will need to acquire two identical images taken on different nights, everything from the target object, the exposure time, the filter and the telescope class must be the same, the only difference between your observations will be the date in which they were acquired. The table below includes some potention observation targets, along with the ideal filter and exposure time. There are two options for collecting observations:

[1] If you have an LCO observing account, you can submit an observation request and collect your own observations.  A guide on how to do use the LCO observing interface can be found here:

[2] Otherwise use data from the LCO archive, a guide on how to search for data on the LCO archive can be found here:

Target name



Exposure time

Telescope Class

Archive data (image A)

Archive data (image B)


Orion Nebula (one of the brightest nebulae)


30 s





open cluster


120 s




NGC 1232

spiral galaxy


300 s




NGC 1501

planetary nebula


180 s





Triangulum Galaxy (spiral galaxy)


120 s




NGC 346

open cluster


60 s





Pinwheel Galaxy (spiral galaxy)


300 s




Centaurus A

giant elliptical galaxy


180 s




Acronyms of catalogues of astronomical objects

  • M: Messier [1781]
  • NGC: New General Catalogue (of Nebulae and Clusters of Stars) [1888]
  • Note: the term “nebula” originally referred to any diffuse astronomical object, including galaxies other than the Milky Way

1. When you have downloaded your observations, note the following in your student worksheet for each of the images:

  • the file name of the reduced image,
  • the date and time of the observation,
  • the corresponding filter.

2. Your data will be stored on your computer in the form of a Zip archive. You will need to use a standard software tool to uncompress Zip archives in order to receive the image files which will have a filename ending in “fits.fz”.

3. If one of your image sets looks rather awkward (see below for an example), you might want to discard it and take another set.

Background: The ground-based telescope rotates with the Earth, so that the positions of stars change slightly during the exposure. By “guiding” the telescope is made to follow. If this fails, one sees stars trailing over the image rather than appearing circular.


1. Download and install the latest version of Aperture Photometry Tool (APT) software from

APT is a tool for professional research, not watered down for educational purposes. Consequently, it has a larger number of features than you are going to use.

Troubleshooting: On Macs with newer OS X versions, the following error may be displayed upon attempting to install the software: “Aperture Photometry Tool” is damaged and can’t be opened. However, the problem is with the security settings rather than downloaded file. See additional instructions at the end of “Task 2” for fixing this problem.

2. Start APT. A window with buttons, sliders, and display areas will open. The exact look will slightly differ depending on whether you use Windows, MacOS, or Linux.

3. Click on Open Image and select one of your images from Image Set I. Its filename will end in “fits.fz”. Note the filter of your chosen image in your student worksheet.

4. You might see a dialogue box, reporting that the selected file contains two HDUs (Header Data Units) with image data. Click Cancel to proceed.

5. You will now see your image being displayed on the right hand side of your window. You might need to use the sliders to view all of it.

Optional background: Your “image” is an array of integer numbers resulting from the count of electrons released from each pixel of the light-sensitive electronic detector, proportional to the flux of incident light, with a small number of further electrons adding some background “noise”. Producing detectors with low noise is a key goal of technology development.

6. Your target should be located near the centre of the image. Try to find it.

7. In order to best view your image, you will need to adjust how the numerical values reported in your image file are being represented by the visual brightness on the screen:

  • Ensure that the Stretch Type is set to “Linear Stretch”. If not, click on Stretch-Type Toggle until this is the case.
  • Next, alter the values of Lower bound and Upper Bound and observe how this affects the way the image is displayed.
  • Choose settings that give you a dark background star and a good resolution of the target features.

8. Take a screenshot of your displayed target, this will be used as part of your evaluation demonstration.

9. Now turn your attention to stars within the field of your target. Select 3 stars of medium brightness that are easy to find and look circular, keeping in mind that you will need to identify the same stars on the other image that you acquired with the same filter.

Tip: Take a screenshot or note down a quick description of your star to ensure you remember the location on your student worksheet. E.g. Two stars below the galaxy

10. To reasonably estimate the centre of the image of a star, APT needs to roughly know its size. Click on your chosen star and then on the big button on the right that by default shows 5/5/0 Alter (and different numbers if the settings have been altered).

11. A window will open with a preview box on the left showing a red ring, this is the “annulus”. For our analysis, the annulus must surround the entire star including some of the background.

12. You now need to click on Recompute Photometry (highlighted in red) for your settings  to take effect.

13. If you click on the given star again, APT will automatically identify the brightest pixel (Peak pixel) and report its photon count in the section APERTURE MEASUREMENTS. Moreover, APT will report the sky background level. Note the latter in your worksheet.

[For the example shown below, the Peak pixel value is +32905 and the sky background is +499.62]

14. Select Curve of growth to check whether the light is properly contained within your aperture. It should rise smoothly and become close to flat. If your curve looks much different, you might want to pick another object on your image (it might not be a single star).

15. Select Aperture Slice. A window will appear with a plot showing the flux per pixel within the annulus as well as the (median) sky level.

16. First we want to measure the peak brightness of the X slice (blue). Hover over the point to get maximum flux reading. Note this in your worksheet.

17. Our aim is to find the full-width at half maximum (FWHM), as a way to measure seeing conditions on the night each observation was taken.

18. Let’s start by getting the “half maximum”. Subtract the background sky level from the maximum flux and divide by two.  Note this figure down.

19. We now need to know the width of the peak at this point, we will be measuring this in pixels. You now have the FWHM for a star, note this down on your student worksheet.

Tip: Zoom in by drawing a rectangle on plot (left-click) to get a more precise reading. Draw a small line to zoom back out to full view.

20. Repeat steps 11-21 with two more stars, until you have the FWHM for three stars from this image.

21. Now you will need to compare the image from Set II that has been taken with the same filter with the image from Set I. Click Open Image and select option A. Repeat steps 4-22.

Note: It is important to compare images from Set I and Set II that have been taken with the same filter.

22. Determine the differences in FWHM for the three stars and write down the values in your worksheet.

Fixing Mac OS installation problems:

- Open the Terminal app from the /Applications/Utilities/ folder and then enter the following command syntax:  sudo spctl --master-disable

- Hit return and authenticate with an admin password

- Relaunch System Preferences and go to “Security & Privacy” and the “General” tab

- You will now see the “Anywhere” option under ‘Allow apps downloaded from:’ options

- Select the “Anywhere” option

- You should now be able to launch APT successfully.

- Finally, you can use  sudo spctl --master-enable in the Terminal app to restore to the default security setting


Compare the FWHM of stars from Image A and Image B taken with a same filter. 

  1. Would you consider the FWHM being roughly similar for the 3 stars on the same image or substantially different? What could cause small differences in the obtained values?
  2. Would the FWHM depend on the filter used?
  3. Do you see a difference in FWHM between image A and image B?
  4. Which image do you think was taken on a night with better seeing conditions? What may characterise such better conditions, if any?
  5. How can we minimise the impact of atmospheric conditions on observations?
  6. What are the direct benefits from an astrophysical point of view to work under better conditions? Explain your answer.