Continuous Real Time Autofocus (SharpLock)

Focusing:

Achieving and maintaining accurate focus is essential for astrophotography. However, factors such as mount motion and temperature changes can lead to focus errors over time. Additionally, temperature gradients within mirrors and differential thermal inertia between secondary and primary mirrors can introduce subtle focus issues that are challenging to address, resulting in recurrent refocus interruptions.

Common imaging software packages offer periodic refocusing capabilities to address these focus challenges. The traditional procedure involves slewing the telescope towards a bright reference star, running an autofocus (AF) utility like a V curve focusing algorithm, and then reacquiring the target. However, this process is time-consuming and interrupts the imaging of the target. Moreover, there can be additional problems if the mount struggles to accurately reacquire the target after the focus routine. Moving away from the target not only consumes valuable imaging time but also increases the likelihood of encountering issues. It is not uncommon to observe poor Full Width at Half Maximum (FWHM) in the first frames after target reacquisition due to mechanical settling time associated with slewing.

Our patent-pending SharpLock technology offers a superior and unique solution for dealing with focus changes through a Real-Time Auto Focus (RTAF) approach. SharpLock continuously monitors and maintains critical focus without interrupting imaging operations. It eliminates the need to slew the scope for refocusing. Unlike traditional autofocus software that assesses focus based on star size using a V curve approach, SharpLock analyzes the shape of the star itself. By analyzing both the focus quality and any movement of the focuser (direction and amplitude), if any, SharpLock enhances focus accuracy. All of this is achieved while you continue imaging your target, providing a seamless and efficient focus control solution.

Auto-guiding and real time auto-focus:

The SkyGuard (SKG) with SharpLock technology is a groundbreaking solution that combines auto-guiding and real-time autofocus (RTAF) capabilities while using an ONAG®. This innovative technology ensures continuous optimal focus during normal imaging of the target object.

Unlike conventional autofocus software, SharpLock’s advanced algorithms analyze each guide star image in real-time, taking into account their history, to evaluate focus quality and determine the necessary focus correction. Importantly, these focus corrections are calculated without physically moving the focuser mechanism. Instead, the controller computes the required motion (number of steps and direction) for the focuser, enabling real-time autofocus capability.

With a typical auto-guiding and RTAF rate ranging from 1 to 30 seconds, depending on the guide exposure settings, the focus corrections applied by SharpLock are very small (a few microns). These minute adjustments have negligible impact on image quality. Additionally, any potential movement introduced by the focus correction is corrected by the auto-guiding function before it becomes visible, ensuring optimal imaging performance.

By integrating auto-guiding and real-time autofocus functionalities, the SkyGuard with SharpLock technology streamlines the astrophotography process, allowing for continuous high-quality imaging while maintaining precise focus at all times.

How does SharpLock work?

SharpLock utilizes the foundational technology of the ONAG® to enable its functionality. The ONAG® incorporates a dichroic mirror that plays a key role in the operation of SharpLock. The mirror is positioned at a 45-degree angle, allowing it to reflect visible light (<750nm) towards the imaging camera while transmitting near-infrared light (>750nm) to the guider camera. This configuration ensures that the guider camera receives the necessary near-infrared light for guiding purposes, while the visible light is directed towards the imaging camera for capturing the primary images.

Due to the optical configuration of the ONAG®, the guider camera views the guide star through a “tilted window,” which introduces a slight but useful distortion to the image. This distortion can be observed in the right image below, which shows the same star captured by the guider camera in near-infrared (NIR) light. In contrast, the left image displays the star as seen by the imaging camera in visible light, where no distortion is present due to the perfect reflection of light.

While both images have similar half flux diameter (HFD), which is the essential factor for auto-guiding, the NIR guider image exhibits some degree of astigmatism caused by the tilted window effect. Despite the astigmatism, the guider camera can effectively utilize the distorted image for accurate auto-guiding, focusing on the HFD as the primary guiding metric.

The distortion of guide stars doesn’t affect the algorithms used for auto-guiding, which mainly rely on calculating the center of the star’s image. However, this distortion presents an interesting opportunity for real-time auto-focus (RTAF). When the telescope’s focus changes, even by a small amount, the shape of the guide star seen by the guider camera changes in different ways depending on whether the focus moves inward or outward. The diagram below summarizes the theory of operations for SharpLock, our auto-focus system.

SharpLock Basic Theory of Operation

The two images below clearly illustrate the impact of moving the focuser by a significant amount (up to +/- 400 microns or 0.4mm) on the shape of the star. However, in practice, the RTAF controller does not wait for such extreme distortion to occur before taking action. This example serves to highlight the potential range of distortion, but even a few microns of focus motion are sufficient for accurate detection of changes in the guide star’s profile and subsequent auto-focus correction. These small changes are not visible to the naked eye in the guider image and do not visibly affect the target image. However, by processing and analyzing a sequence of guide star images, these subtle shape changes can be detected and used for precise and reliable auto-focus adjustments.

Focus Versus Time Analysis using SharpLock

With the combined use of SharpLock and ONAG® technologies, it becomes possible to analyze and track the changes in focus of your system in real time. This provides valuable insights into the benefits of managing focus continuously. In a recent experiment, we utilized a 10-inch aperture Ritchey-Chrétien (RCT) carbon fiber OTA telescope operating at F/8, equipped with an ONAG® XT and controlled by SharpLock.

During the experiment, we recorded both the temperature (in degrees Celsius) and the absolute position of the focuser (in millimeters) over a period of 70 minutes. This recording commenced after an initial one-hour cooling period, with the data collection divided equally before and after the meridian passage, without any mount flip. The graph below illustrates the relationship between time (in minutes) and both temperature and focuser position.

Temperature and Focus Shift versus Time

In the provided figure, two important observations can be made. Firstly, the temperature of the system (shown in the top plot, measured in degrees Celsius) decreases steadily over time in a nearly linear manner. Secondly, the absolute position of the focuser (depicted in the bottom plot, measured in millimeters) required to maintain the best focus does not exhibit a similar linear trend.

Initially, within a span of 12 minutes, the focuser moved inward by approximately 170 microns. Subsequently, in the following 10 minutes, it moved outward by around 200 microns before eventually returning inward. It is worth noting that these values should be compared to the critical focus zone of the telescope, which, for an F/8 configuration, is approximately +/-80 microns.

Without the application of SharpLock technology, frequent interruptions in imaging would have been necessary to perform refocusing. The rapid temperature drop at a rate of approximately 3 degrees Celsius per hour contributes to this behavior. The complex nature of this effect makes it challenging to create a correction strategy based on predictive open loop techniques.

Fortunately, SharpLock technology overcomes these difficulties by continuously evaluating actual images in real time and compensating for focus changes. As a result, your images will remain sharp and clear from frame to frame, eliminating the need for frequent refocusing and ensuring optimal image quality throughout your observation.

SkyGuard (SKG) Software

With the integration of our patented SharpLock Technology, our SkyGuard system offers a user-friendly focusing solution that works seamlessly alongside guiding operations. Thanks to the unique properties of the ONAG cold-mirror, the guide image experiences a subtle astigmatism. This astigmatism serves as a valuable indicator for determining whether the camera is currently positioned on the near or far side of optimal focus.

Using this information, SkyGuard swiftly adjusts the focuser to the ideal focus position, ensuring optimal image quality. Once achieved, the system can maintain focus for extended periods of time, ranging from weeks to even months. This means you can focus your attention on capturing stunning images without the need for frequent manual refocusing, allowing for a more convenient and hassle-free experience.

What is SkyGuard?

Unlike traditional focus methods that rely on models based on physical parameters, such as backlash compensation and temperature compensation, SKG takes a unique approach by utilizing real-time feedback. While these traditional methods can partially correct for focus changes within their modeled parameters, they are limited and cannot account for changes outside of their models.

SKG, on the other hand, introduces a subtle astigmatism into the guide image. This allows the system to detect focus drifts in real time, regardless of the source of the poor focus. By continuously monitoring and analyzing the guide image, SKG is able to make immediate corrections to maintain optimal focus. This means that any factors affecting focus, even those that have not been specifically modeled, can be effectively addressed by the SKG system. As a result, you can expect consistently sharp and precise images without being restricted by the limitations of traditional focus models.

Periodic Re-focusing versus SharpLock Side by Side

The traditional focus strategy often requires regular interruptions in imaging to perform refocusing tasks, typically every 30 minutes or during filter changes. These interruptions can consume valuable imaging time and introduce potential issues related to mount slewing between the target and a reference star used for refocusing.

In a notable comparison test, the effectiveness of the traditional method using “Focus Max” (FM) for periodic re-focusing was compared side by side with the uninterrupted focus provided by SharpLock. The test was conducted at the same location and time, using two identical Hyperion 12.5 F/8 scopes and PME mounts, all focused on the same target, M82. The credit for this comparison goes to Frank Colosimo of the Blue Mountain Vista Observatory in New Ringgold, PA, USA.

The test aimed to evaluate the performance of SharpLock, which combines MaxIm-DL application with CCD-autopilot (CCDAP), in maintaining focus without any interruptions. This allowed for continuous imaging without the need for regular refocusing intervals. The comparison provided valuable insights into the efficacy of SharpLock in preserving optimal focus throughout the imaging session, highlighting the potential benefits of this technology for astrophotography.