astrotools

This is a set of Python 3 scripts that I use to aid in low focal-length DSLR astrophotography with my Linux laptop.

The project was started because from my backyard, Polaris is obscured by a building, and I wanted a means of polar aligning my mount that was more convenient than drift alignment. The result of this are the two scripts capture.py and analyze.py, which use libgphoto to take pictures through a DSLR attached to the computer and pointed at the sky. These pictures are then plate-solved to determine the exact pointing-position in the sky, and a simple rotational model is fitted to the data to obtain the exact alignment of the mount axis in a local alt-azimuthal frame. This location is then compared to the theoretical polar axis given by the local latitude, and a recommendation for adjusting the mount axis is given.

While the mount model is very simple and does not take into account complicating factors, I have been happy with the results, at least up to a focal length of 200mm and with exposures of 2-3 minutes. I have not tried to push the system to its limits. Your mileage may vary. I am happy about feedback that would allow me to improve the model.

Requirements

• Python >= 3.6
• astropy
• rawpy
• imageio
• exifread
• local install of astrometry.net (in particular, the solve-field and plotann.py tools must be callable)
• libgphoto2 and python bindings

Polar alignment with align.py

Make sure your mount is roughly polar aligned. Attach your camera via USB and set the desired exposure length. I have found 6s at f/4 and ISO 200 on a Nikon D7100 to work well. You may also want to shoot in JPEG instead of RAW to speed up the plate-solving process.

Point you camera at a region of the sky close to the celestial equator and somewhere halfway between the horizon and the meridian, such that errors in altitude and azimuth alignment are about equal in magnitude.

To obtain, e.g., 3 measurement exposures with 20 seconds of wait time between them and get the mount axis estimate, run

./align.py --lat your_latitude --lon your_longitude --height your_height --wait 20 3

The resulting files will be put in a temporary folder and automatically plate-solved. Make sure that your camera's time is set correctly, since EXIF data will be used to convert the plate-solved coordinates into a local frame.

A full example input might look like

./align.py --lat 42.358991 --lon -71.091619 --height 9 --wait 20 3

Here, latitude and longitude are given in degrees and height above sea level in meters. The script will take exposures, automatically plate-solve them, and tell you its estimate of the mount axis alignment, alignment error, and estimated pixel drift. After making adjustments to the mount, run the script again until satisfied.

Aiming your camera using aim.py

After successful polar alignment you want to aim your camera at your target. To make your that your pointing is really correct, you can use aim.py to plate-solve a camera image and display an overlay with any deep-sky objects in the field of view. Again, this is just a convenience script which uses astrometry.net's solve-field and plotann.py internally.

Make sure your camera is plugged in and desired exposure settings are set. Again, 10s seems to work well. Then, run

./aim.py

to take an exposure, plate-solve, and display an overlay.

Mathematical model

The model used for fitting is described in the file MathematicalModel.ipynb.