Sometimes a target is larger than the field of view of the telescope you are using. To capture such large targets, you need to point the telescope at different parts of the large target so that you end up with a series of images that come together to create a larger image. This process is common in astrophotography and the whole thing is referred to as a Mosaic. More accurately, the finished image is said to have been created from a mosaic because several smaller images are combined into one larger image.
Planning a mosaic can be complex because you have to know how large you want your mosaic to be. For example, you could create a 1×2 mosaic such that you have two mosaic images to create one larger finished image. You could also create a 2×2 mosaic where you have four images, two stacked over one-another, to create the larger image.
The number of mosaic tiles you use depends on the size of your target and depends on how complex you want your post-processing to be. Generally, the greater the number of mosaic tiles, the more complex your final processing becomes.
This article discusses planning and booking a mosaic. I discuss some background details and highlight an online mosaic planning tool called Astro Mosaic.
Challenges in Planning a Mosaic
Planning a mosaic can be complex because you have to know a number things:
- Your telescope’s location
- The field of view of the telescope you plan to use
- The time and date of your observation
- Whether your target is visible
- The separation between the moon and your target
- The size of your target to determine the number of mosaic tiles you need
- The amount of overlap between mosaic tiles for a good result
Using planetarium software such as TheSkyX along with details of the telescope you plan to use, can help you answer the preceding questions. Even if you own TheSkyX, you have to configure it correctly to be able to use it for planning your mosaic.
Fortunately, a free online tool called Astro Mosaic by Jarmo Ruuth can help you to easily plan your mosaic.
An Overview of Astro Mosaic
Astro Mosaic makes it easy to plan Slooh.com missions to create a larger final image made up of smaller mosaic images.
Astro Mosaic is configured for all of the Slooh.com telescopes, so you don’t have to know the settings for each telescope. Astro Mosaic lets you choose any target or helps you select a suitable target based on the Slooh.com 500 list of great targets. In addition, Astro Mosaic helps you plan your mosaic by showing you what your image will look like before you commit to taking your images, thereby saving you a lot of time and taking the guesswork out of planning your mosaic.
Walkthrough Using Astro Mosaic
In this section I walk you through taking a mosaic of IC 1318, the Butterfly Nebula. I show you how to make the appropriate selections and what to do with the information Astro Mosaic gives you.
Start by pointing your browser here: https://ruuth.xyz/AstroMosaic.html
Across the top of the screen, on the first line are settings you can change – this is where you set things like the telescope you want to use, the date, and details about your mosaic. On the second line are filters: the first is a selectable list of Slooh.com 500 objects, a filter for the altitude of your target, a field for the time of your observation, and a field where you can select the moon’s separation.
Astro Mosaic starts by showing you what an observation of the Eagle Nebula on the Chile Two telescope might look like.
For this example, we’re going to create a mosaic of IC 1318, the Butterfly Nebula. The nebula is visible from the Southern Hemisphere so we’ll use one of the Canary telescopes. We’ll take a look at our options for how many images we want to use to make up our mosaic.
First ensure we’re using the correct telescope: select T2 from the Telescope dropdown. Next, ensure that the date is correct – use a date August 8, 2020 – enter the date (2020-08-08) to the left of the ‘View Image’ button and press Enter on your keyboard to commit the change.
Next, select how high the target should be during your imaging session. Targets at a higher altitude are preferred because there’s less air between the telescope and the target, leading to a better overall image. Select 45 degrees from the Filter drop down.
Next, we want to ensure that the moon is far away from our target so select 90 degrees from the moon dropdown.
Each time you make a selection, Astro Mosaic updates the Slooh 500 list with targets that meet the criteria we are setting.
Now that our criteria are set, locate the entry ‘Emission Nebulae: Butterfly Nebula IC 1318’ in the Slooh 500 list and select it. Note as you scroll through the list – these are all of the objects in the sky that meet the criteria.
Once you make your selection, you’ll see your screen change with an image of what the nebula might look like through the Canary T2 telescope.
Now let’s plan our mosaic.
In the top row is a field marked ‘View’ – select ‘Mosaic Grid’. Your screen now updates to show you what the individual images of your mosaic look like. The double lines are areas of overlap between images. An overlap of 20% is recommended and this is the default for Astro Mosaic (you can change the overlap in the Mosaic Overlap% field).
The default grid of 3×3 captures a lot of the nebula; however, I don’t want to capture as much, so change the values in the Grid Size field to 2 and 2. Click View Image to update the image.
This new image is more along the lines of what I was hoping for and processing a mosaic of 2×2 tiles is not too difficult.
So now the question is, what do you do next, now that you know what your target will look like?
Scroll down past the image and you’ll see a table with some numbers in it. Those numbers are the coordinates you use to book your Slooh.com mission. The numbers correspond to the RA and Dec of the center of each mosaic tile. You use these numbers to book a coordinate mission for each tile.
Scroll a little further and note the moon phase and distance to the target, also note the graph indicating when the target is visible.
Scroll down a little further and you’ll find a graph that describes when your target is visible during the following 12 months.
Let’s go back to the graph of when IC 1318 is visible in the sky and note that it is visible from about 20:40 to 22:40 and then from about 00:00 to 04:00. Also note that the moon rises around 00:40. With this information, we can select a time to book our reservation.
Booking Missions
In this section, I provide you with an overview of what to do with the coordinates that Astro Mosaic gives you. You can find detailed directions in my book, which you can download for free here.
Point your browser to Slooh.com and select the Mission Setup option from the Telescope menu on the upper right corner of your screen. Choose the ‘By Telescope’ option and select Canary Two.
Find an available time slot in the range of the time that IC 1318 is visible.
From the top of your screen select, By Coordinates. Copy and paste the RA and Dec into the appropriate fields and select the Generic processing recipe. In the description, use something like ‘IC 1318 P#’ where the # symbol is the panel number of the mosaic tile.
Once your mission is booked, Slooh.com will execute it and provide you with the images.
What To Do With The Images
Now that you have the images, they need to be assembled into your final image. While you could use something like Photopea.com, GIMP, or Photoshop to assemble your images, there is a much easier solution. You can use a free program called Microsoft Image Composite Editor, or ICE.
ICE can automatically assemble your mosaic for you so you don’t have to worry about the placement of images, final image size, etc.
Once you have the images, download the LRGB PNG image for each mission and save it somewhere convenient – I cover this in detail in my book; however, it is just a matter of downloading the correct PNG image from each mission.
Download and install Microsoft ICE – use your favorite search engine to search for Microsoft Image Composite Editor and follow the download and installation directions.
Once you have ICE installed, import your images and follow the steps on the screen to ‘stitch’, crop, and export your image – the whole process is automatic.
This article offers details on how to use ICE to assemble a mosaic.
Once you have your final image, you might need to perform some adjustments on it. You could use GIMP, Photopea.com if you don’t want to install anything, or Photoshop if you have it. The adjustments you would need to make are a Levels adjustment where you move the midpoint of the image closer to the left side to brighten the shadows. You could also use a Curves adjustment using the classic S shape to brighten dim areas.
Result and Further Processing
You might be satisfied with the result, however, if there are some issues with the images like vignetting, uneven background, or other problems you might have edit the individual PNGs before combining them. I show you how to assemble a mosaic from PNG images here. Otherwise, for more control, you could use the FITS files from your missions and I explain how to do that using PixInsight.
Conclusion
In this article you learned about mosaics, you learned how to plan mosaics using the Astro Mosaic tool, and you learned how to assemble images from your mosaic using Microsoft Image Composite Editor.
Thank You
Thanks to Jarmo Ruuth for providing the basic directions for this article and for inspiring me to write this article.
