The ONAG® is a device used by astronomers that includes a special beam splitter. This beam splitter allows a specific type of light called near-infrared (NIR) light to pass through it. The guiding system in the device uses this NIR light. Using NIR light for guiding has significant benefits because it helps reduce the negative effects of the Earth’s atmosphere, resulting in better and smoother tracking of celestial objects. Let me explain this further.

Based on the theory of black body radiation, we can describe the spectrum of starlight (the amount of light at different wavelengths) based on the star’s temperature. Stars are like “black bodies” that emit a lot of energy in the near-infrared (NIR) region, as shown by their spectra. This means that stars emit a significant amount of energy in the NIR range.

The Hertzsprung–Russell diagram is a chart that shows the relationship between a star’s temperature and how bright it appears. It helps us understand different types of stars. Additionally, I’ll provide a table that shows the different classes of stars and their proportions.

When astronomers want to guide their telescopes at a relatively fast rate, like when using an f/10 or faster setting and taking short exposure times, they usually have many stars to choose from. This is because there are numerous stars within the chosen field of view, giving astronomers plenty of options to select suitable guide stars.

To understand how near-infrared (NIR) guiding works, we need to consider a few factors: the efficiency of the guider sensor, the performance of the ONAG® device, and the spectrum of the star’s radiation.

The efficiency of the entire system* is determined by multiplying the quantum efficiency (QE) of the sensor chip with the star’s spectrum (for a given surface temperature) over the ONAG® bandwidth (roughly 750nm to 1000nm).

The provided plot shows the difference in visual magnitude between the ONAG® and a typical silicon sensor, depending on the surface temperature of the guide star (measured in Kelvin). By adding this difference to the visual magnitude of a star, we can determine the equivalent visual magnitude of a star observed through the ONAG®, considering the full spectrum. The offset value depends on the star’s surface temperature.

For example, let’s consider a star with a surface temperature similar to the Sun (~5800K, class G) and a visual magnitude of 10. When observed through the ONAG®, its equivalent visual magnitude would be 10 + 1.33 = 11.33. This means that, in terms of sensor signal level, the star seen through the ONAG® would have the same signal level as a star with a visual magnitude of 11.33 but without the ONAG® (using the full spectrum). Conversely, to achieve the same sensor signal level in NIR with the ONAG®, we would need a star with a visual magnitude of 10 – 1.33 = 8.67 (approximately), assuming the initial magnitude without the ONAG® is 10.

Similarly, for a star with a surface temperature of 3700K (class M) and a visual magnitude of 10, the corresponding visual magnitude offset is 0.76. In this case, a star with the same surface temperature and a visual magnitude of 10 – 0.76 = 9.24 would produce the same sensor signal level with the ONAG® in NIR.

It’s important to note that we have neglected factors such as atmospheric extinction and optics efficiency in these calculations, as they have a lesser influence compared to the dominant factors considered here.

*The term “overall system efficiency” refers to the efficiency of the entire system comprising the ONAG® and a typical silicon sensor chip.

Indeed, the level of signal received from a star depends not only on its temperature but also on its absolute energy output (luminosity) and its distance from Earth (relative magnitude). In practical terms, with current sensor technology, it is generally feasible to guide at f/10 using stars ranging from 9th to 10th magnitude and exposure times of a few seconds. Therefore, finding a suitable guide star within this magnitude range is usually not a significant limitation or concern.

The ONAG® has a large field of view (FOV), especially when using its integrated X/Y stage or guiders with large chips. This wider FOV allows for a greater selection of guide stars within the field, increasing the chances of finding suitable options for guiding.

The plot below illustrates the electromagnetic spectrum, spanning from ultraviolet (UV) to mid-infrared (IR) wavelengths. The wavelengths are measured in microns (µm) or nanometers (nm). The visible band, represented in yellow, ranges from 350nm to 750nm. The NIR range used by the ONAG® (750nm to 1000nm) is depicted in red. The plot also shows the power density spectrum of stars with surface temperatures ranging from 3000K (reddish) to 7000K (beginning of the blue).

From the plot, it’s clear that the black body radiation representing the star’s spectrum extends significantly into the infrared (IR) region. As the surface temperature of a star increases, the peak of its spectrum shifts towards shorter wavelengths. It’s worth noting that very hot stars (>7000K) emit a considerable amount of energy in the ultraviolet (UV) band, which is not easily accessible for most silicon-based sensors (such as CCD or CMOS). These sensors typically have limited quantum efficiency (QE) within the wavelength range of 350nm to 1000nm.

A study was conducted to understand the practical impact of using the near-infrared (NIR) band (750nm to 1000nm) of the ONAG for guiding, compared to the full spectrum accessible to current silicon sensors (which includes the visible and NIR range). The study utilized the IRSA-PPMXL all-sky catalog data, which contains information on approximately 900 million objects. This catalog provides magnitude measurements for different electromagnetic bands, including the NIR range of 750nm to 1000nm.

By performing appropriate calibrations and statistical analysis using the visible (B, R) and NIR bands, the study assessed the magnitude offset introduced by the ONAG. The figures below present the results of the analysis from three different perspectives, providing valuable insights into the study’s findings.

When using an ONAG compared to utilizing the full spectrum, there is typically a signal drop observed, resulting in an average magnitude offset ranging from 0.67 to 1.53. This corresponds to a factor of approximately 2x to 4x decrease in signal. It’s important to keep in mind that the specific silicon sensor’s quantum efficiency (QE) being used may require an additional adjustment of around 0.2 to 0.3 magnitude.

To put this into perspective, employing a 2×2 binning on the guide chip already increases the signal-to-noise ratio (SNR) by a factor of 2, which is equivalent to a magnitude offset of 0.75. Additionally, increasing the guiding exposure time from 1 second to 2.5 seconds (a factor of 2.5x) allows for a one-magnitude increase in the limit of the guide star magnitude.

These considerations demonstrate the trade-off between signal strength and exposure time in guiding. It highlights that there are various factors that can be adjusted to optimize the guiding process based on specific requirements and constraints.

Selecting the appropriate exposure time for auto-guiding is crucial, and it is generally recommended to use the longest exposure time feasible for a given mount and setup. Several factors influence this choice, including mount quality, the use of periodic error correction (PEC), polar alignment accuracy, flexure, and the utilization of point/speed models.

The main objective is to avoid continuously adjusting for variations in the atmospheric conditions (known as “chasing” the seeing) and to ensure a suitable signal-to-noise ratio (SNR) for guiding. A key consideration is to keep the guiding exposure time within a range that prevents noticeable drift in the position of the guide star.

In modern mounts with a permanent setup, it is common to use unguided exposures of 5 to 10 seconds or even longer. For portable or “grab and go” configurations, the typical range is around 2 to 5 seconds.

The lower limit of the exposure time depends on the prevailing seeing conditions and the SNR of the guide star. By using exposures in the range of a few seconds, the effects of star wander (tilt/tip component of seeing) can be significantly reduced, particularly when combined with NIR guiding, as mentioned earlier.

The provided plot illustrates the increase in the limit magnitude of guide stars when the exposure time is extended beyond one second. It demonstrates that with an exposure time of approximately 2.5 seconds, there is already a gain of one magnitude. Furthermore, with an exposure time of around 4 seconds, the NIR efficiency of the ONAG compensates for class F stars. This emphasizes the advantages of using sufficiently long guiding exposures to optimize the performance of guiding.

Using near-infrared (NIR) for guiding can enhance tracking accuracy by reducing the impact of atmospheric turbulence, known as seeing. To assess the performance of auto-guiding under challenging conditions, black and white images of M83 were captured with a relay telescope at a low elevation angle of 14 degrees above the horizon. The guiding process involved comparing two strategies: visible + NIR guiding and NIR-only guiding (wavelengths above 750nm). These images were taken as an extreme case to evaluate the guiding performance in poor seeing conditions.

The comparison clearly demonstrates that using the NIR guiding strategy with an ONAG XT and an adaptive optic AO-L unit leads to improvements in the full width at half maximum (FWHM) and overall image quality. The adaptive optics system was employed to compensate for residual mechanical errors in the custom mount, rather than attempting to correct for the seeing, which is challenging with a large field of view.

By implementing NIR guiding, the effects of guide star wander caused by poor seeing were significantly reduced. This resulted in smoother auto-guiding tracking and ultimately led to better image quality. The elongation of stars observed in the M83 image captured using the full spectrum (visible + NIR) indicates the difficulties faced when guiding under adverse seeing conditions, especially at a low elevation angle.

These results highlight the benefits of integrating NIR guiding with an ONAG and adaptive optics system. They underscore the potential for enhanced tracking performance and improved image quality, particularly in challenging seeing conditions.