In the vast expanse of the night sky, capturing the beauty of celestial objects through astrophotography is a rewarding pursuit. However, aligning and identifying these objects precisely can be a daunting task. This is where plate solving comes to the rescue, offering a solution to accurately pinpoint the exact location of stars, galaxies, and nebulae in your astrophotos. In this guide, we’ll delve into how you can harness the power of Astrometry.net, coupled with the capabilities of a Raspberry Pi 5, to effortlessly plate solve your astrophotos.

This article describes a procedure to install software on a Raspberry Pi 5 with 8G RAM running Raspberry Pi OS (64-bit) (Debian v.12 bookworm) booted directly from a 1TB NVMe SSD PCIe drive. This procedure may work for other configurations, so please do let us know if it does, or tell us about any issues you faced and your configuration. Thanks!

What is Plate Solving?

Before we delve into the technical aspects, let’s briefly discuss what plate solving entails. Plate solving is a technique used to determine the celestial coordinates of a captured image. It works by comparing the stars in the image with a reference star catalog to accurately identify their positions. This information is then used to align the image with a star map, enabling precise identification of celestial objects.

Online Plate Solving with Astrometry.Net

The Astrometry.net online plate solving service is a powerful tool designed to help astrophotographers and astronomers precisely identify celestial objects in their images. Unlike the locally installed version which requires setting up and configuring on your own system, the online service offers a convenient and user-friendly platform accessible via a web browser. Here’s a detailed look at how the Astrometry.net online plate solving service works and its key features:

How it Works:

1. Upload Image: Users begin by uploading their astrophotographs to the Astrometry.net website. This can be done through a simple file upload interface.
2. Plate Solving: Once the image is uploaded, the Astrometry.net service analyzes it using advanced algorithms to identify the positions of stars, galaxies, and other celestial objects present in the image.
3. Comparison with Catalogs: The service compares the stars and other features in the image with known star catalogs, such as the USNO-B, 2MASS, and UCAC catalogs, to determine the precise celestial coordinates of each object.
4. Annotate Results: After plate solving is complete, the service generates an annotated version of the uploaded image. This annotated image overlays the celestial coordinates of identified objects, making it easier for users to interpret and understand the results.
5. Output Data: In addition to the annotated image, the service provides users with detailed information about each identified object, including its celestial coordinates, magnitude, and other relevant data.

Key Features:

1. Accuracy: The Astrometry.net service boasts high accuracy in plate solving, allowing users to precisely identify celestial objects in their astrophotographs.
2. Ease of Use: The online platform is designed to be user-friendly, with a simple and intuitive interface for uploading images and accessing results.
3. Speed: The plate solving process is typically fast, providing users with results in a matter of minutes, depending on the size and complexity of the uploaded image.
4. Accessibility: Since the service is web-based, users can access it from any device with an internet connection, making it convenient for astrophotographers of all skill levels.
5. Free to Use: The Astrometry.net online plate solving service is offered free of charge to users, making it an accessible resource for the astronomy community.

Limitations:

1. Image Size Restrictions: While the service can handle a wide range of image sizes, there may be limitations on the maximum file size or dimensions that can be uploaded.
2. Internet Connection Required: Users must have a stable internet connection to access the online service and upload their images for plate solving.
3. Privacy Concerns: Since the service operates online, users should be mindful of the privacy implications of uploading their astrophotographs to a third-party website.

Offline Plate Solving with Astrometry.Net Installed on a Raspberry Pi 5

The Astrometry.net offline plate solving service with a locally installed copy offers astrophotographers and astronomers the ability to perform precise plate solving directly on their own systems, including Raspberry Pi 5, without relying on an internet connection. Here’s an in-depth overview of how the locally installed Astrometry.net service operates and its key features:

How it Works:

1. Installation: Users start by downloading and installing the Astrometry.net software package onto their local system.
2. Index Files: Once installed, users need to download the necessary index files, which contain data about the positions of stars and other celestial objects. These index files are essential for the plate solving process. Due to the size of these index files, we recommend extending the storage of you Raspberry Pi 5 using NVMe SSD drives. If you haven’t already done so, check out our guide Unleashing Raspberry Pi 5 Power: Installing NVMe SSD via PCIe.
3. Configuration: Users configure the Astrometry.net software to point to the correct directory where the index files are stored. This ensures that the software can access and utilize the index files during the plate solving process.
4. Plate Solving: With the software properly configured, users can then input their astrophotographs into the Astrometry.net software. The software analyzes the image, identifies stars and other celestial objects, and compares them with the data in the index files to determine the precise celestial coordinates of each object.
5. Output Results: After plate solving is complete, the Astrometry.net software generates output files containing detailed information about the identified objects, including their celestial coordinates, magnitudes, and other relevant data. These results can be used to accurately align and annotate astrophotographs.

Key Features:

1. Accuracy: The locally installed Astrometry.net service boasts high accuracy in plate solving, allowing users to precisely identify celestial objects in their astrophotographs.
2. Independence: Unlike the online service, the offline version does not require an internet connection to perform plate solving, providing users with greater independence and flexibility in their astrophotography workflows.
3. Customization: Users have greater control over the plate solving process and can customize various parameters, such as the search radius and minimum star detection threshold, to suit their specific needs and preferences.
4. Speed: Plate solving is typically fast, especially when running on a dedicated system with sufficient processing power. This allows users to quickly obtain results and continue with their astrophotography sessions.
5. Privacy: Since the service is locally installed, users have greater control over their data and can ensure the privacy and security of their astrophotographs.

Limitations:

1. Resource Intensive: The plate solving process can be resource-intensive, especially when dealing with large or complex astrophotographs. Users may require a system with sufficient processing power and memory to handle the analysis efficiently.
2. Initial Setup: Setting up and configuring the locally installed Astrometry.net service may require some technical expertise, particularly for users who are less familiar with software installation and configuration.
3. Index File Updates: Users need to periodically update the index files to ensure that they have the latest data on celestial objects. Failure to do so may result in less accurate plate solving results.

Integrating Astrometry.Net with Other Applications Installed on Raspberry Pi 5

Astrometry.Net, when installed locally, can be used directly from the command line, or by directly integrating with other applications running on your Raspberry Pi 5. Some of these applications will even install the base Astrometry.Net package for you during their own installation, but you will still need to download the index files before use. We provide several guides for installing applications that make use of Astrometry.Net‘s plate solving capabilities.

Installing Astrometry.Net on Raspberry Pi 5

There is a good chance you already have Astrometry.Net installed if you installed other astronomy application on your Raspberry Pi 5. You can easily check with this command:

sudo apt -qq list astrometry.net

astrometry.net/stable,now 0.93+dfsg-1+b1 arm64 [installed,automatic]
astrometry.net/stable 0.93+dfsg-1+b1 armhf

If not already, it can be installed with this command:

sudo apt install astrometry.net

Installing Astrometry.Net Index Files on Raspberry Pi 5

Before Astrometry.Net can be used for plate solving, you must first download the index files that contain the various star catalogs, such as the USNO-B, 2MASS, and UCAC catalogs. The specific catalogs you need will depend on the size of the field of view in the images you are looking to plate solve. You should take care when selecting the catalogs to download as they can be very large!

For most amateur astronomers, the 4100-series, built from the Tycho-2 catalog with scales 7-19 available, are best for images wider than 1 degree. For narrower images, the 5200-series LIGHT version, built from Tycho-2 plus Gaia-DR2 with scales 0-6 available. Other indexes are very specialized, and probably wouldn’t be needed.

Once downloaded, the index files should be moved to the default directory /usr/share/astrometry.

ls -l /usr/share/astrometry

total 347244
-rw-r--r-- 1 ian ian 164995200 Apr  5 15:40 index-4107.fits
-rw-r--r-- 1 ian ian  94550400 Apr  5 15:39 index-4108.fits
-rw-r--r-- 1 ian ian  49772160 Apr  5 15:36 index-4109.fits
-rw-r--r-- 1 ian ian  24871680 Apr  5 15:39 index-4110.fits
-rw-r--r-- 1 ian ian  10206720 Apr  5 15:39 index-4111.fits
-rw-r--r-- 1 ian ian   5296320 Apr  5 15:38 index-4112.fits
-rw-r--r-- 1 ian ian   2733120 Apr  5 15:38 index-4113.fits
-rw-r--r-- 1 ian ian   1382400 Apr  5 15:52 index-4114.fits
-rw-r--r-- 1 ian ian    740160 Apr  5 15:52 index-4115.fits
-rw-r--r-- 1 ian ian    408960 Apr  5 15:38 index-4116.fits
-rw-r--r-- 1 ian ian    247680 Apr  5 15:52 index-4117.fits
-rw-r--r-- 1 ian ian    187200 Apr  5 15:52 index-4118.fits
-rw-r--r-- 1 ian ian    144000 Apr  5 15:38 index-4119.fits

Plate Solving from the Command Line

Most of the time, you will use Astrometry.Net through another application you have installed, and which is already configured to interpret and present the data extracted from your astrophotos. Nonetheless, Astrometry.Net can still be used for plate solving directly from the command line, by providing an image in any of the supported formats:

solve-field --overwrite --downsample 2 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.fit'

At the very minimum, you should use the –overwrite option to overwrite any intermediate files created during previous plate solving attempts, and use –downsample 2 to greatly improve star detection in your image. If all goes well, the solving takes just a few seconds:

Reading input file 1 of 1: "Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.fit"...
Extracting sources...
Downsampling by 2...
simplexy: found 10601 sources.
Solving...
Reading file "./Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.axy"...
Field 1 did not solve (index index-4119.fits, field objects 1-10).
Field 1 did not solve (index index-4118.fits, field objects 1-10).
Field 1 did not solve (index index-4117.fits, field objects 1-10).
Field 1 did not solve (index index-4116.fits, field objects 1-10).
Field 1 did not solve (index index-4115.fits, field objects 1-10).
Field 1 did not solve (index index-4114.fits, field objects 1-10).
Field 1 did not solve (index index-4113.fits, field objects 1-10).
  log-odds ratio 96.0876 (5.37405e+41), 10 match, 0 conflict, 12 distractors, 13 index.
  RA,Dec = (311.336,30.222), pixel scale 2.31956 arcsec/pix.
  Hit/miss:   Hit/miss: +++--++-+----+++-----+(best)------------------------------------------------------------------------------
Field 1: solved with index index-4112.fits.
Field 1 solved: writing to file ./Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.solved to indicate this.
Field: Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.fit
Field center: (RA,Dec) = (311.336877, 30.222717) deg.
Field center: (RA H:M:S, Dec D:M:S) = (20:45:20.850, +30:13:21.780).
Field size: 2.6709 x 1.8212 degrees
Field rotation angle: up is -60.461 degrees E of N
Field parity: neg
Creating new FITS file "./Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.new"...
Creating index object overlay plot...
Creating annotation plot...
Your field contains:
  The star 52 Cyg
  NGC 6960 / Veil Nebula

You will also now have a bunch of files containing the various types of data resolved from your image:

ls -l 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026'*

-rw-r--r-- 1 ian ian   175680 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.axy'
-rw-r--r-- 1 ian ian    11520 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.corr'
-rwx------ 1 ian ian 23397120 Aug 11  2022 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.fit'
-rw-r--r-- 1 ian ian 11647139 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026-indx.png'
-rw-r--r-- 1 ian ian     8640 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026-indx.xyls'
-rw-r--r-- 1 ian ian    17280 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.match'
-rw-r--r-- 1 ian ian 23408640 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.new'
-rw-r--r-- 1 ian ian 11489679 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026-ngc.png'
-rw-r--r-- 1 ian ian 11516883 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026-objs.png'
-rw-r--r-- 1 ian ian    11520 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.rdls'
-rw-r--r-- 1 ian ian        1 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.solved'
-rw-r--r-- 1 ian ian    11520 Apr  5 18:07 'Cygnus Loop-3_TRIBAND_gain_120__300secs_1x1_0026.wcs'

There are many more options available, so feel free to experiment:

man solve-field

Conclusion

Plate solving astrophotos using Astrometry.net on a Raspberry Pi 5 opens up a world of possibilities for amateur astronomers and astrophotography enthusiasts. With the powerful combination of open-source software and affordable hardware, precise astrophotography is now within reach for everyone. So, grab your Raspberry Pi 5, venture into the cosmos, and unlock the secrets of the universe, one astrophoto at a time. Happy stargazing!