Solar Observation with the Seestar S50 Telescope 

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An Educational Illustration of Field Rotation and the Heliographic Grid 

Warning: Using the Seestar S50 for solar observation requires the use of a solar filter (provided by the manufacturer). 

Presentation of the Seestar S50 

The Seestar S50 is a compact, fully automated telescope developed by ZWO, designed for intuitive astrophotography. Lightweight (around 3 kg including its carbon tripod), it’s highly portable and controlled via a smartphone or tablet using an ergonomic mobile app. Key features include:

• A 50 mm apochromatic lens (250 mm focal length)

• A Sony IMX462 color digital sensor (1920 × 1080 pixels)

• A motorized altazimuth mount

• A removable full-aperture solar filter for safe solar imaging

• An electric focuser, anti-dew heater, and automatic guiding system

Thanks to its user-friendly design and automation, it’s an ideal tool for quick observations—particularly of the Sun—allowing extremely short exposure times.

Limitations of the Altazimuth Mount

The Seestar S50 operates by default with an altazimuth mount, which means it tracks celestial objects in azimuth and altitude while accounting for Earth’s rotation—but without maintaining the orientation of the field of view. This leads to a phenomenon well known to astronomers: field rotation, which affects long-exposure images.

This drawback can be corrected by switching the Seestar to equatorial mode, but this requires a different alignment setup: the instrument must be tilted according to the observation site's latitude and precisely aimed toward the north celestial pole. This process is more technical and requires appropriate mounting.

Illustrating Field Rotation in Solar Observation

When observing the Sun with the Seestar in altazimuth mode, one notices that the image gradually rotates throughout the day:

• In the morning, before solar noon (meridian crossing), the angle to the Sun is negative

• At solar noon, this angle becomes zero

• In the afternoon, it turns positive

In the northern hemisphere, this rotation is retrograde—that is, clockwise. This phenomenon is invisible to the naked eye, but clearly noticeable when comparing morning and afternoon images.

Comparison of Annotated Solar Images

Thanks to our program, two images have been annotated and compared, each displaying a heliographic grid showing:

• Solar latitudes from –90° to +90°

• Heliographic longitudes from –90° to +90°, centered on the central meridian

Note: Unfortunately, in the example above, the two images were not taken on the same day; the demonstration would have been more compelling otherwise. Nevertheless, the orientation of the heliographic grid clearly illustrates the field rotation.

This grid allows us to observe how sunspots appear to shift and rotate depending on the time the image was taken — solely due to field rotation, not actual motion on the Sun.

It’s worth noting that this phenomenon disappears entirely with a well-aligned equatorial mount, which naturally compensates for Earth’s motion.

Heliographic Parameters Used

The heliographic grid relies on three key parameters defined by celestial mechanics:

• P: The angle between celestial north and solar north, measured positively from celestial north

• Bo: The latitude of the center of the solar disk, measured positively toward solar north

• Lo: The longitude of the center of the solar disk relative to the Carrington meridian

📌 The Carrington meridian is an arbitrary reference that defines solar longitudes from 0° to 360°, in the prograde (east-to-west) direction, allowing for consistent tracking of the Sun’s ~27.3-day synodic rotation.

Extrait_Sol_Seestar Program

The Extrait_Sol_Seestar program is written in Python and, upon launch, presents the following interface:

The Application Interface

The application is highly intuitive. During the first use, users are prompted to select their preferences and save them.

Image Selection and Display Options

Once launched, the program lets you choose the image to display by selecting one of the options at the top of the interface. Important: Image files must follow the naming format: YYYY-MM-DD_HH-MM-SS.jpg — for example: 2025-07-25_12-55-00.jpg. The separators (–, _, or space) are interchangeable.

Checkbox Options

• "Show Grid + Legend"

  • Unchecked: Displays the original image

  • Checked: Displays the image with the heliographic grid and parameters: P, Bo, Lo, and Carrington meridian

    • The meridian line appears as a solid line on the visible side and as dashed on the hidden side

• "Remove Field Rotation"

  • Unchecked: The image includes natural field rotation

  • Checked: The Sun is shown with a vertical solar north-south axis, unaffected by field rotation or the P angle

• "Show NS Axis"

  • Checked: Displays the celestial north-south axis in blue

  • Unchecked: The axis is hidden

Saving the Image

The “Save Image” button exports the current display view. The filename includes suffixes indicating selected options. Example: soleil_S_grid_NS_2025-07-28_09-06-00.png

• S → field rotation is present

• grid → heliographic grid applied

• NS → celestial north-south axis shown

Some Examples of Output Images :

Raw Seestar Image

Image with grid, field rotation not corrected 

Image with grid, field rotation corrected 

Image with grid, field rotation corrected, celestial NS axis shown 

Conclusion

The Seestar S50, although a modest instrument, enables a spectacular and precise illustration of phenomena observed in altazimuth mode. By combining modern technology with rigor, it becomes an effective educational tool for raising awareness of solar geometry and heliographic coordinates.