This photograph received the Grand Prize Winner, Monthly Astronomy Guide (Gekkan Tenmon Guide), July 2026.
“Exploring the Star Formation Time Lag in the Andromeda Galaxy through Ground-Based Near-Ultraviolet and Hα Observations”
This technical note describes the methodology, instrumental setup, and observational techniques developed for ground-based near-ultraviolet (NUV) imaging of the Andromeda Galaxy.
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Challenging “Invisible” Near-Ultraviolet Astronomy with a Ground-Based Telescope
As noted in the previous article, acquiring near-ultraviolet (NUV) astronomical images with ground-based telescopes presents significant technical challenges. As the project progressed, it became clear that four major obstacles would need to be overcome before successful observations could be achieved. This technical note describes how each of these challenges was systematically addressed.1. Atmospheric Absorption
Near-ultraviolet light is strongly absorbed by the Earth’s atmosphere, especially by water vapor. As a result, only a small amount of this radiation reaches the ground, which makes observations difficult.
2. Optical System Limitations
The choice of telescope optical system is an important consideration for NUV observations. In refracting telescopes, the lens material can absorb much of the near-ultraviolet light. Therefore, reflecting telescopes are generally better suited for NUV observations.
3. Camera Performance
The CMOS image sensor requires adequate sensitivity in the near-ultraviolet wavelength range. It is also important to consider transmission losses introduced by optical components within the camera system.
4. Filter Selection
In summary, near-ultraviolet imaging of faint astronomical objects such as galaxies and nebulae cannot be achieved simply by making minor changes, such as replacing a filter. Instead, it requires careful optimization of the entire observational system, including the optics, detector, filters, and observing conditions. Only when all of these elements are properly coordinated does NUV imaging become practically feasible.
1. Atmospheric Absorption
The first challenge was reducing atmospheric absorption. Near-ultraviolet wavelengths are strongly absorbed by the Earth’s atmosphere, especially by water vapor. To minimize this effect, I selected observing sites at high elevations with low humidity. Most of the observations were carried out at locations I frequently visited in Koumi (Nagano Prefecture) and Numata (Gunma Prefecture), Japan.
However, a major obstacle remained: I do not live near either of these sites. Opportunities for near-ultraviolet imaging are naturally limited, and practical constraints—such as work commitments, weather conditions, and travel costs—meant that I could not simply travel whenever conditions were favorable.
Looking back, this limitation in observing opportunities was likely the main reason the project took five years to complete. If I had lived closer to the sites, the required data might have been collected within days or weeks. Likewise, a professional astronomer might have been able to observe on every clear night.
However, I consider myself more of an “astronomy enthusiast” than a professional astronomer. Astrophotography is a passion I pursue alongside my professional and personal life, with a focus on the process rather than efficiency. Although this inevitably extended the timeline, it also made each observation more meaningful. In this sense, the final image is more than just a scientific or technical achievement—it represents five years of effort, patience, and nights spent under the stars.
2. Optical System Limitations: Can the RASA 8 Be Used for Near-Ultraviolet Imaging?
The next question was whether the telescope itself could transmit enough near-ultraviolet light for practical imaging.
Although I did not own a telescope specifically designed for ultraviolet observations, I had access to a powerful instrument: the Celestron RASA 8 (203 mm aperture, f/2.0), an exceptionally fast optical system capable of collecting a large amount of light in a short time.
However, one critical question remained:
“Can the RASA 8 perform effectively in the near-ultraviolet?”
To answer this, I examined the telescope’s specifications. According to the manufacturer, its wavelength range is 390–800 nm, with the lower limit lying at the very edge of the near-ultraviolet region.
While this suggests that the system is not specifically optimized for near-ultraviolet observations, it also indicates that such imaging is not entirely impossible.
In other words, the RASA 8 falls into a borderline category—far from ideal, yet potentially capable of transmitting enough near-ultraviolet light to make further investigation worthwhile. At the very least, the specifications suggested that the idea was technically feasible.
3-1. Camera Performance
The next critical component was the camera. Because near-ultraviolet signals are extremely faint, maximizing sensitivity was essential.
For this project, I used a monochrome CMOS camera, the ZWO ASI294MM Pro. Its Sony IMX492 sensor is known for relatively high sensitivity in the blue-to-green region of the spectrum (the B and G bands), rather than being optimized for the near-ultraviolet. This suggested that it might also retain some sensitivity in the near-ultraviolet range.
However, an important limitation remained. The quantum efficiency (QE) curve published by the manufacturer extends only down to about 400 nm, leaving the sensor’s performance at shorter wavelengths unknown. Since sensitivity does not abruptly drop to zero at a fixed cutoff, it is reasonable to assume that some response continues below this point. My working assumption was therefore simple:
“At least some sensitivity must remain in the near-ultraviolet.”
Instead of relying only on the published specifications, I decided to test this experimentally and evaluate the camera’s actual performance under real observing conditions.
Another complication arises from the optical path. Even if the sensor itself is sensitive to near-ultraviolet light, incoming photons must first pass through the camera’s protective window and other optical elements. In other words, sensor sensitivity is only part of the story—the transmission of all components in front of the sensor is just as important.
However, as the investigation progressed, another major limitation became apparent: the protective window in front of the CMOS sensor.
While this optical element performs well for standard visible-light imaging, a closer look at its transmission characteristics revealed a serious drawback for near-ultraviolet observations. Its transmission drops off rapidly at wavelengths below about 400 nm—exactly where the near-ultraviolet region begins.
As a result, even if the sensor itself retains some sensitivity below 400 nm, a significant portion of the incoming near-ultraviolet light may be blocked before it ever reaches the detector.
This finding clearly showed that sensor performance alone is not enough for near-ultraviolet imaging. The transmission properties of all optical elements in front of the sensor must also be carefully considered.
3-2. The Protective Window Became a Critical Bottleneck
Even if near-ultraviolet light could be successfully collected by the telescope, there was still a significant risk that much of it would be lost at the very entrance to the camera. This limitation was caused by the camera’s protective window, whose transmission properties effectively blocked near-ultraviolet wavelengths.
“This is a serious limitation.”
If the protective window absorbs a large fraction of the incoming near-ultraviolet photons, then the efforts made to reduce atmospheric absorption and optimize the optical system would be greatly undermined.
To overcome this problem, I decided that a more fundamental modification of the camera was necessary.
To overcome this limitation, I commissioned Shibuya Optical Co., Ltd. (Saitama, Japan) to manufacture a custom protective window made from fused quartz, a material known for its excellent transmission in the near-ultraviolet region. By replacing the standard AR-coated window with a fused-quartz one, the goal was to maximize the transmission of near-ultraviolet photons to the sensor.
This modification effectively removed one of the main bottlenecks in the imaging system, allowing more efficient use of the near-ultraviolet photons that had already passed through the atmosphere and the telescope optics.
In retrospect, this modification proved to be one of the key factors in the project’s success.
The protective window is a component that most astrophotographers rarely consider in standard visible-light imaging. However, in near-ultraviolet astronomy, it proved to be critically important.
A component that may seem insignificant in a typical imaging system can become a major bottleneck when working at wavelengths near the limits of atmospheric and instrumental transmission. By improving the transmission of the camera’s entrance window, a much larger fraction of the limited near-ultraviolet photons could reach the sensor.
Therefore, replacing the protective window was more than just a hardware modification—it was a key factor that enabled ground-based near-ultraviolet imaging of the Andromeda Galaxy.
4. Filter Selection
The final obstacle was the filter.
In near-ultraviolet imaging, the filter must transmit the desired wavelengths while effectively blocking unwanted radiation. However, at the time, only a limited number of filters suitable for astronomical near-ultraviolet observations were available. In practice, the options were limited to just two:
* OPTOLONG Venus-U Filter (1.25″)
* IDAS UV-372-80
Uncertain which would be more suitable for this application, I consulted the technical staff at Nature Shop KYOEI Osaka (Osaka, Japan).
Their advice proved highly informative. Although the OPTOLONG filter is marketed for ultraviolet imaging, it does not transmit only near-ultraviolet wavelengths. Instead, it also allows some visible light above about 380 nm to pass through, raising concerns about contamination that could reduce image contrast and compromise spectral purity.
To examine this issue more closely, I reviewed the transmission curves. As expected, the OPTOLONG filter shows strong transmission in the near-ultraviolet region. However, its transmission extends into the visible range, and there is also a small but non-negligible leakage in the near-infrared.
For bright targets such as Venus, these characteristics may not be a serious limitation. In this study, however, the target was the Andromeda Galaxy (M31), a diffuse and extremely faint object.
For a study aimed at tracing star formation through near-ultraviolet observations, these concerns were far from trivial. Ensuring that the recorded signal primarily came from the intended wavelength range became a critical requirement. As a result, filter selection emerged as one of the most important decisions in the entire project.
With this decision, the major technical challenges were effectively resolved:
- Selection of high-altitude observing sites to minimize atmospheric absorption
- Utilization of the RASA 8’s near-ultraviolet transmission capability
- Replacement of the camera’s protective window with a custom quartz element to improve throughput
- Adoption of a near-ultraviolet filter with minimal visible and near-infrared leakage
By addressing each obstacle systematically, the system evolved into a configuration capable of true near-ultraviolet imaging.
At last, the preparations for imaging the Andromeda Galaxy (M31) in the near-ultraviolet were complete.
The next step was simple in principle—but far more demanding in practice: to point the telescope at the night sky, begin collecting photons, and determine whether all of these efforts would ultimately succeed.
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