How to Build a Solar Imaging Telescope

Building an automatic tracking solar imaging telescope

Video Timeline

00:00 Experiment Introduction

01:15 Telescope Overview

01:54 Choosing Lenses

03:56 Selecting ND Filter OD

04:34 Assembling the Telescope

07:28 ND Filter Sequence

08:40 Electric Tracking Function

13:15 Python Control Software

15:28 Solar Imaging and Tracking Demonstration

Telescope Design and Component Selection

The basic optical path device of the solar imaging telescope is shown in the figure below, sunlight is imaged on the camera sensor through the neutral density (ND) filter and lens. We introduce how to choose each component in the order numbered in the figure.

① is a monochrome camera, ② is the imaging lens. The diagonal length of the camera sensor is 1/2.9 inches, with a length and width of 4.97 and 3.73 mm respectively, which determines the upper limit of the lens focal length to ensure the image does not exceed the sensor.

1/2.9 inch camera sensor

Using thin lens imaging and magnification formula:

Solar imaging schematic and parameters

Since the length of the sensor is greater than its width, the image size is limited by the width. We set 80% of the sensor width as the maximum image size. In this configuration, the image is inverted, so the image height is negative.

The maximum image height and estimated image distance are:

Organizing the imaging formula can derive the maximum focal length:

For this, a lens with the closest focal length can be chosen, here a 300 mm focal length achromatic lens is used to reduce the focal length variation for different wavelengths. For low-light applications, it is recommended to use a larger diameter lens, but this telescope also needs to attenuate light, so a Ø1 inch lens is chosen to keep the system compact.

③ and ④ are fixed and adjustable lens tubes respectively, the total length of all tubes is determined by the focal length. These tubes connect the lens and camera through SM1 threads and provide sufficient length adjustment range to produce the clearest solar image.

⑤ is the neutral density (ND) filter. Too low optical density (OD) may damage the sensor. The best practice is to start with an OD that exceeds expectations and gradually reduce it until a sufficiently fast exposure time is achieved. Here, it starts from OD8, which will not saturate the camera, and then when reduced to OD6, the exposure time reaches within 10 ms.

Assembling the Telescope

After selecting the components, assembly can be completed quickly. First, several sections of tubes are connected, and the camera is installed at the fixed end. When installing the camera, the entire device should be tilted downwards, as tightening the two anodized mechanical parts through threads may generate particles, and tilting the device down reduces the risk of particles entering the camera. The imaging lens is installed at the adjustable tube end.

It is easier to complete rough alignment before adding the filters. For this, the telescope can image distant objects (preferably more than 10 meters away), adjusting the tube length to make the object image clear on the camera. After rough alignment, install three filters with a total OD equal to 6.

Solar imaging telescope assembly completed

Camera – Lens Tube – Lens – Filter

Here, three absorbing ND filters are used. Using reflective filters will increase multiple reflective surfaces, which may reduce imaging quality. Additionally, along the direction of incoming light, the filters should be arranged from low to high OD to avoid a single filter bearing too much thermal stress. If the filter with the highest OD is at the front, most of the energy will be absorbed by this filter. In high-energy system applications, filters may fail, and the remaining optical density may not be sufficient to protect the camera. Although here it only attenuates unfocused sunlight, not causing thermal stress that leads to device failure, this issue needs to be noted in other devices.

Correct Order

Incorrect Order

Although it is now possible to manually track the sun, the sun will move out of the sensor in one minute, making operation very laborious. To achieve this, two electric rotary displacement tables can control the telescope, implementing automatic tracking through the elevation-azimuth coordinate system.

Azimuth Axis

Elevation Axis

Automatic Tracking Solar Telescope

Install the telescope on two orthogonal electric rotary displacement tables, and during installation, ensure the lens is in front of the 180° scale of the displacement table, while the camera is behind it, so that the positive angle indicates the elevation angle pointing upwards.

Assembling the Electric Telescope

After assembly, use K-Cube software for testing to ensure the device operates normally: the positive azimuth angle causes it to rotate clockwise, while the positive elevation angle causes it to rotate upwards. Then modify the settings of the displacement tables as shown in the figure, disable backlash correction, and check Persist Settings to the Device.

Change Backlash to 0, check Persist Settings…

Python Software Control

The Python script and usage method can be downloaded from the Thorlabs GitHub page. The script uses the Thorlabs serial communication protocol and does not require .NET drivers, so it can be used on Linux operating systems like Raspberry Pi.

Thorlabs GitHub application examples are continuously expanding

When running the script, users need to input whether to track the sun or moon, longitude and latitude, time zone, and the serial numbers of the two controllers, as well as create virtual COM ports for each device in the device manager. After the setup is complete, run the GUI and tracking script in sequence, allowing the displacement table to reset and move to the sun’s current position, then press enter to start tracking.

Parameters users need to update

When operating outdoors, the telescope needs to point north after resetting. If the telescope is aimed at the sun but there is no image on the camera, the position offset can be modified through the GUI script interface, and the direction of movement can be judged by the shadow of the telescope: the lens tube aimed at the sun should not have a shadow.

Adjust the position offset to make the solar image appear

Users also need to select automatic exposure in ThorCam software to avoid saturation. If the sun is not fully focused initially, continue to adjust the tube length, using visible sunspots to determine if the image reaches the highest contrast.

The following image shows the entire process of tracking a solar eclipse.