What You Will Do

• Observe the Moon with a robotic telescope*
• Build a mosaic image of the Moon
• Create a colour image of the Moon using data collected through three colour filters
• Interpret the Moon’s physical characteristics based on your data products

* If you do not have robotic telescope access, the observing instructions can be skipped and you can process sample images to create a colour image of the Moon and analyse its physical characteristics.

What You Will Learn

• Technical: You will build intuition about:
  • Telescopes’ fields of view,
  • Observing targets that move through the celestial sphere
  • Appropriate exposure times for bright objects such as the Moon,
  • Narrowband filters,
  • How to stack and mosaic a large astronomical target
• Scientific: 
  • How colour is used to draw inferences about the mineralogy and composition of the lunar surface

Introduction

In this module, you will create a colour picture of the Moon using images collected with robotic telescopes. Specifically, the robotic telescopes we will provide instructions to use for this task, as with the rest of the projects described in this book, are telescopes in the Skynet Robotic Telescope Network and those in the Las Cumbres Observatory global telescope network. If you do not have access to observing time with these robotic telescopes, you can still complete this module (as well as all the later modules in this book) using archival data sets we have provided within Skynet’s astronomy image processing software, Afterglow, which these instructions will teach you to use. Or you can collect your own images by other means, e.g. using other robotic telescopes or even your own backyard telescope, though if your equipment is quite different from the systems described here the instructions may be of limited use.

As you work through the steps to observe the Moon and process your images, you will learn techniques for imaging targets that are larger than a telescope’s field of view, and later processing a mosaic image, which are useful skills you may apply in other projects, e.g. when studying star clusters, nebulae, or even large galaxies. This first module will therefore help you to start building skills that you’ll apply in later exercises, with appropriate modifications that apply to these different applications. And in the end, you will produce a colour image of the Moon that you can use to identify and explore the sizes of surface features, and learn about the varied mineralogy across the lunar landscape.

Note: This observation is best done between a few days after 1st quarter moon and a few days before 3rd quarter moon. If you have time to delay submitting this observation until the Moon reaches the appropriate phase, it may be useful to wait. Please refer to this lunar calendar for more information.

Observing the Moon with Robotic Telescopes

Imaging the Moon may seem rather straightforward first task—after all, it’s is big and bright and nearby—and it may even seem like a bit of a boring task—after all, the Moon doesn’t have much colour; it’s mostly just grey—but there are a number of complicating factors that make this a rather interesting first exercise to work through, and a great way to start learning a lot of skills and start building knowledge that you’ll apply throughout the rest of the modules in this book. And in the end you’ll find that you can exaggerate the colours in your final image and learn what they tell us about lunar mineralogy, which is really quite neat.

Actually, the facts that the Moon is big and bright and nearby all make the moon trickier to observe with Skynet and LCO telescopes than, say, a typical galaxy or a star cluster—i.e. apart from the ones that are big and bright and nearby.

Let’s start with the fact that the Moon is big—roughly a half degree in size on the sky. There are few telescopes on Skynet that are big enough to capture the Moon in a single image, so with most Skynet data we will use Afterglow’s mosaic image processing tools to create a single frame out of a patchwork of smaller images—and before that, we will use Skynet’s dithering feature when scheduling the patchwork of images we want to collect. With LCO data, we will provide instructions to use telescopes with fields of view that are larger than the Moon, which can therefore be imaged in a single frame. If you will be collecting your own LCO data, you may still find it useful to process the sample images collected with Skynet telescopes to learn how to create mosaic images in Afterglow, as these tools can be used with LCO images to create mosaics of large regions such as the Large Magellanic Cloud.

The fact that the Moon is very bright means that our colour images must either be taken with VERY short exposure durations through broadband filters or more reasonable exposure durations (though still relatively short by most standards) using narrowband filters. Since Skynet telescopes are all equipped with CCD cameras with shutters that necessitate exposure times greater than 0.1 second (otherwise, the shutter interferes with the image), though the LCO 0.4-m telescopes are equipped with CMOS cameras that can take shorter exposures, our instructions will stick to using a combination of optical narrowband and ultraviolet broadband filters. (Note that the latter can be used because our atmosphere blocks most UV light, so the Moon is actually relatively dim in the ultraviolet.

Before discussing why it matters that the Moon is nearby, which will lead into our first activity of the module, it’s worth adding some clarification about the filters discussed in the last paragraph. When we make colour images with digital cameras, we use colour filters that block out all light that falls outside a specific range of wavelengths. Thus, a red filter blocks all light that isn’t red, a blue filter blocks all light that isn’t blue, etc., so we see only the red light (or blue, or whatever colour our filter happens to be) from the Moon when we image it through a red filter. Then later on we can combine the varying levels of red, green and blue light to create a colour image. Actually, this is very similar to how the colour camera on your smartphone works, except in that case your camera uses an array of microscopic filters to capture levels of red, green and blue light all in one shot, then processes light levels from adjacent pixels with different filters to create a colour image.

Finally, we need to discuss why it matters that the Moon is nearby. It’s actually not the proximity so much as the fact that the Moon is orbiting Earth (because it’s so close and gravitationally bound to our planet). Because the Moon orbits the Earth once a month, it does not exist at fixed coordinates on the Celestial Sphere that we can give to a telescope and tell it to go observing. The Moon moves nearly 13 degrees across the sky every day! Now, Skynet actually calculates the Moon’s position so you can tell it “observe the Moon!” and whenever Skynet manages to get to your observation (Skynet uses a queuing system, so the observation time vary’s depending on conditions at the various observatories) it will do a quick calculation to determine where the Moon happens to be at that time and will send the telescope there. LCO, on the other hand, does not have a built-in Moon observing mode. Furthermore, LCO observations aren’t simply placed in a queue to be completed when the robots decide. LCO uses a block scheduler, with observing windows set for each observation to be made at set times, if observing conditions happen to be favourable. Therefore, it’s a bit different imaging the Moon with LCO.

Given this important background information, we will now work through the steps to observe the Moon. Click the appropriate arrow below to expand the instructions for observing the Moon with either Skynet or LCO. If you will be processing archival images in Afterglow’s Sample images folder rather than collecting your own, note that those images were collected with Skynet’s Prompt5 telescope following the procedure described in the “Observing the Moon with Skynet” instructions, so you should familiarise yourself with that procedure before moving on to the next section.

Processing your Images to Create a Colour Picture of the Moon

Once you have either collected your own images of the Moon or decided to proceed with the sample images, you are all set to create your own tri-colour image of the Moon. The procedure for doing this is split into two parts: Image post-processing and mosaicing, and tri-colour image creation. We will work through both parts first, using the sample images provided in Afterglow, before providing instructions for processing images you may have collected with Skynet or LCO telescopes.

Note: for the best experience, image post-processing and mosaicing should NOT be done in a group setting. The steps involved in this procedure are computationally expensive and the amount of time Afterglow takes to complete them scales with the number of simultaneous users. For the best experience, we recommend completing this part of the procedure asynchronously.

Image post-processing and mosaicing

1. Login to Afterglow and close any images you may have open in your Workbench from a previous session.
2. At the top-left, click Open Files, then navigate to Sample > MWU > Module 1 – optical moon, then select all images and Open. Don’t worry that some of the images missed the Moon.
3. The next step is normally cosmetic correction. The Cosmetic Correction tool is the fourth from the bottom on the right-hand tools list. If you click through a number of images, you may note that they do not have any particularly noticeable defects due to bad pixels or columns in the camera. If you do not think it’s worth running cosmetic correction (may take 5-10 minutes), you can skip this step. If you want to try, simply select all images using the square button next to the files drop-down, leave the default parameters, and click Submit.
4. Next, select the Aligner tool (third from bottom, below Cosmetic Correction). Click the square “select all” box next to the image selection drop-down. Click Mosaic Mode. Disable rotation, scale and skew. Set the Mode to “Features”. Change Ratio Threshold to 0.3, and leave the remaining inputs with default values. Click Submit.
   Note: A Ratio Threshold of 0.3 works best for 5×5 grids, as in the case of the sample images. If you collected your data with a 3×3 grid, you should set the Ratio Threshold value to 0.4 when aligning. These things only matter when running features-based alignment, as we need to do with the Moon. When mosaicing star and galaxy fields which have WCS solutions, WCS-based alignment should be used.
5. When alignment is complete, click on the Stacker tool (second from the bottom, below Aligner). You need to stack images per filter. The easiest way to do this is to type e.g. “Halpha” in the “Filter list” dialog above your files list, so only the images collected with the H-alpha filter are displayed. Then close the top two files, so the first image in the list is one that contains the Moon, as this will be the basis for your stack. Then click the square select all button next to the files drop-down in the Stacker panel. Enable Multiplicative, Additive and Global Equalization and set Equalization order to 0. Click Submit.
6. Repeat the previous step, filtering your Files list for “OIII” and “U”, closing the top two images in each set.
7. When all three mosaics have finished stacking, type “stack” in the file filter list so you see just these three images. Select all three images and save them to your Workspace, inside a new “Moon” directory.
8. Delete “stack” from the filter list, select all images, and close them, discarding all changes.

Tri-colour image creation

1. Login to Afterglow and close any images you may have open in your Workbench from a previous session.
2. At the top-left, click Open Files and open the mosaic images you created in the image post-processing and mosaicing procedure above by navigating to Files > Workspace > Moon.
3. Select all images, then above the file filter list, select the three vertical dots (between the refresh button and select all check box), then group selected files. This creates a combined image with all three layers set to “Screen” mode so the features in each show through equally. You can preview this combined layer by clicking the top level.
4. Right-click on the H-alpha layer and set Color map to Red. Right-click on the OIII layer and set Color map to Green. And right-click on the U layer and set Color map to Blue. Once the colours are added in, the Moon should appear pretty much grey, as expected, but with hints of colour throughout.
5. Click the Display tool (top of the right-hand panel). At the top-right, select Color Composite Tools > Link All Layers (Percentile). This will link the levels in each image so you can adjust one set of brightness values and all colours will scale evenly, preserving the colours introduced by setting equal exposure times in each filter.
6. Set Stretch Mode to Midtone, then click the Default Preset. Raising the Background level will darken the background, and lowering the Midtone level will brighten the Moon (and raising Midtone level will darken the Moon). For example, setting background 20 and midtone 80 works well with the sample image set.
7. When you are done adjusting brightness levels, at the bottom-right of the image click “Export Image as JPG” (not Download Viewer Snapshot, which will take a screenshot of the display rather than exporting a full size jpeg) and save the file someplace you can access it later. Click the Save button next to your top image layer to save the grouped image with final settings to your Workspace.
8. Open your image in any generic picture viewer. Google images of the Moon to use as references and rotate your image into the correct orientation. Then, increase the colour saturation and the sharpening/clarity.  Be careful with both of these!  It’s very tempting to boost both of these by, say, 100% (or more!)  However, this will leave you with an over-processed look – you’ll come to recognize it in time.  Until then, try increasing both by ~50%.  The former can really bring out the moon’s subtle colours.  The latter can really bring out its craters. Lastly, adjust the brightness/light level one last time, and crop/reframe the image to your liking. Save your image.

Exercises

1. Post your final image to social media and share your creation with your family and friends. If you don’t have any social media accounts, send an email to someone you care about.
2. Go back to Afterglow, open the tri-color fits. Use a map to identify features and measure them. Since the pixel scale of the camera is known, you can measure angular diameter. Open Stellarium and go to File Info for the time the image was created, then open Stellarium, set the date and time and get the distance to the Moon. Use the small angle formula to calculate the sizes of features on the Moon. What are some terrestrial features/countries/etc that are the same size.
3. Lunar mineralogy stuff.

**Blue text = Mae’s version

Background

Let’s start our investigation of the Solar System with our nearest neighbor–the Moon. You’ve probably seen the Moon countless times in the sky at different times of day and night.

Observing the Moon

If you’re going to take your own observations of the Moon, this will be a guide on how we recommend you should do that. There are some general things we will cover about observing the Moon, then walk through the example of imaging the Moon using Skynet telescopes. If you’re not imaging the Moon and are using some archival data to make your picture, the ‘General Tips & Tricks’ section is still worth reviewing, as it will cover why we need to do some of these steps.

General Tips & Tricks

In this section, we’re going to cover some information that applies to most telescopes you will try to observe with, though the specifics will vary based on properties of the individual telescope. There are a few things we need to understand about telescopes first:

• Field of View (FOV): the field of view is how big of an area on the sky your telescope can see. This is a product of the specifics of the telescope and the camera (or eyepiece) that you use to observe. This is a set value, so if we want to image a larger area of the sky, we need to take images in different areas and ’tile’ them together to make one image that covers a larger area.
• Mosaic: a mosaic is made by taking several images in order to stitch them together into one bigger image. Think of it like taking several small tiles and putting them all together to make a single, larger tile.
• Filters: telescope cameras only take images in black and white. Astronomers use special pieces of glass that only let in certain types of light in front of the camera to combine together to understand the color. These pieces of glass are the filters. Basically, we take all the light coming in and use the filters to only let in one color at a time. A simple way to make a color image is to have 3 filters: one that lets in blue light, one that lets in green light, and one that lets in red light. By taking an image with each of those filters, we can use the computer to color the images correctly and add them together to make one color picture. It sounds a little confusing, but we will explain in more detail in the **section name** below.
• Exposure times: this is how long the camera shutter needs to be open to take a picture. It’s just like everyday phone or digital cameras! The difference is that since we’re often looking at very faint things, we need to spend a long time pointing the camera at the object to get a bright enough exposure. For brighter objects, we don’t need to have as long of an exposure. These cameras are very sensitive to light, so if we over-expose our images, we will have to retake the images. It happens! Sometimes we think we know how long of an exposure we need, but when we get the image back we realize we need to make some adjustments.

To see the most of the face of the Moon, and get the most color, it’s recommended to observe the Moon when the Moon is at least 50% illuminated. This is somewhere between the first quarter phase, up to full Moon, and back to the third quarter phase. The crescent Moon is also very pretty, and is a fun optional observation! We also want to make sure all the observations are taken in the same night, since the phase changes significantly each day. It is easiest if all the observations are taken consecutively.

***Figure somewhere showing FOV***

The Moon is about a half-degree in size on the sky, or about 30 arc-minutes by 30 arc-minutes (arc-minutes are usually represented with an apostrophe, so it would be 30′ by 30′). Most telescopes have a FOV smaller than this, so we will need to take observations in a grid pattern to cover the whole Moon. This grid-pattern of images will make the tiles, and will be used to form the final mosaic. To know how to make the grid, you’ll need to now the FOV of the telescope you’re going to use. If your telescope has a field of view that is 10′ by 10′, you’ll need at least a five-by-five grid to make your image.

“But wait, 30′ divided by 10′ is three, not five! Shouldn’t it be a 3×3 grid?” 

Ah! Good observation. It seems like it should be three, but there’s a subtle thing we need to do. For Afterglow Access, the software we use for image processing, the tiles in your grid need to overlap a bit, so that it can figure out how all the images fit together. We recommend having roughly one-third of your field overlapping. For a telescope with a 10′ FOV, that means we would want approximately 3.5′ of overlap. If you only did a 3×3 grid, at the end the image would only cover a 19.5′ field of view, so you would be missing part of the Moon! If we use 3.5′ of overlap between our images with a 10′ field of view, then five tiles across would be 32.5′. This ensures that we could get the whole Moon in our final image.

***Figure that shows overlapping regions***

The next thing we need to consider is the filters. Typically, we would use filters that represent red, green, and blue light. A popular set of filters are the BVR filters–where B lets in blue light, V lets in green light, and R lets in red light. These filters are called broadband filters, because they let in lots of light. Since the Moon is really bright,  if we use broadband filters we will reach the minimum exposure time for the camera. Instead, we use three narrowband filters to act as proxies for the blue, green, and red filters. We suggest using the H-alpha, OIII, and U filters for red, green, and blue, respectively. Since these filters let in less light,  you can take a longer exposure and avoid that minimum allowed exposure time. What is this minimum exposure time? It’s roughly  ~0.05 seconds, the time the shutter takes to open and close. If the exposure time is too short,  you’ll have darker edges and a brighter center on each tile of your mosaic.

As is standard with astronomical imaging, we want to make sure that our calibration images exist–our bias, dark, and flat images. Make sure to use those to calibrate, or reduce, your images before moving on to the steps in Afterglow! For more information about calibration images and reducing data, see **APPENDIX**.

How We Observed The Moon With Skynet

Now we will detail how you would take these images if you’re using the Skynet Robotic Telescope Network, or ‘Skynet’. If you’re not using Skynet to place your observations, feel free to skip this section. If you want to see the specific details of what we did to guide your observations, then feel free to keep reading.

First, let’s consider what telescope we want to use for our mosaic. You probably have access to several telescopes across the globe, so what telescope is best? For these observations, we recommend sticking to one of the “PROMPT” telescopes. This can be at any of the locations: Canada, Chile, or Australia, but PROMPT telescopes are the telescopes we have the most control over and are the most consistent. **TABLE** shows some specs of various PROMPT telescopes, and we will break down all this information.

Table **Number**