“Observers, have been guiding their telescopes since the dawn of long-exposure astrophotography. It’s a need that arises from a very long list of mechanical, optical, and atmospheric factors that makes it all but impossible for a simple telescope drive to precisely follows a celestial object for more than a minute or two as it moves across the sky.”

“Enter the ONAG®. Because digital detectors in today’s autoguiders are sensitive to near-infrared (NIR) light beyond the visual spectrum, the ONAG® works by sending a telescope’s visible light to the imager camera and the NIR to the guider system…”

“…Ingenious!” And it works because digital technology has given us the opportunity to easily use NIR light for guiding.”

“The guiding was remarkably accurate during all my testing”.

“A clever as the concept behind the ONAG® is, the devil is on the details, and that’s where the ONAG® really shines.

The device,… , is extremely well engineered and, more importantly, well made.” 

(Dennis di Cicco, Sky & Telescope December 2012, pages 60-63)

How does an ON Axis Guider (ONAG®) work?

The ONAG incorporates an advanced technology developed by Innovations Foresight, which is both innovative and unique. It features a special component called a dichroic beam splitter (DBS), also known as a “cold” mirror. This component acts as an inferential filter.

The main function of the ONAG’s DBS is to reflect visible light from the telescope towards the imaging camera. At the same time, it allows near-infrared (NIR) light to pass through. Although NIR light is generally not useful for imaging purposes, it is directed to the guiding camera for auto-guiding.

The ONAG is equipped with an integrated X/Y stage for the guiding camera, providing a wide field of view (FOV) for selecting the guide star. This means that the ONAG allows you to use either the same optical axis as your main telescope or an offset axis for guiding, giving you flexibility in choosing your FOV. Since there is no need to rotate the device to acquire a guide star, you can reuse your flat frames without any additional adjustments.

One of the notable advantages of the ONAG is its compatibility with large chip guiding cameras. This enables the use of multi-star auto-guiding with your main telescope instead of a separate guide scope, eliminating issues related to differential flexure. Additionally, this feature makes the ONAG a suitable choice for remote or automated setups, as it does not require frequent adjustments to the X/Y stage—simply set it on axis and lock it in place.

Furthermore, the ONAG facilitates real-time auto-focus operations, ensuring that your telescope remains in optimal focus throughout the imaging process. Thanks to the patented SharpLock technology exclusive to the ONAG, you no longer need to spend time periodically refocusing your telescope.

Both CMOS and CCD chips used in cameras are highly sensitive to near-infrared (NIR) light. In fact, many sensors are specifically designed with enhanced NIR quantum efficiency (QE) to enable low-light and night vision capabilities, found in devices like security cameras and smartphones.

It’s worth noting that a significant portion of stars emit a considerable amount of NIR energy. This means that when using our ONAG device, you can harness this NIR energy and take advantage of it. Auto-guiding software applications will be able to display stars in NIR, similar to how they display visible stars.

Typically, around 98% of the light in the visible range, spanning from 350nm to 750nm, is directed to your imaging camera. At the same time, a minimum of 90% of the light in the near-infrared (NIR) range, above 750nm, is transmitted to the guiding camera. Importantly, since the light reaching your imager is reflected, there are no optical aberrations introduced in the process, making it comparable to using a good star diagonal.

The dichroic mirror used in our device is crafted from high-quality optical-grade glass with minimal thermal expansion. This ensures that it remains undistorted even when exposed to a wide temperature range. Our utmost goal and commitment have been to never compromise on image quality. The ONAG consistently delivers diffraction-limited performance to your imaging system.

As mentioned earlier, unfiltered CMOS and CCD cameras possess sensitivity to NIR light. Considering that over 76% of main sequence stars have surface temperatures below 3700K, they radiate a significant amount of infrared energy. Consequently, these stars make excellent candidates for NIR guiding. Guiding in the NIR range presents a unique opportunity to enhance accuracy since longer wavelengths are less affected by atmospheric turbulence, also known as “seeing.” Thus, the ONAG technology improves your active tracking performance by leveraging this characteristic of longer wavelengths.

The outcome of employing the ONAG is improved auto-guiding performance, resulting in tighter guide stars with lower full width at half maximum (FHWM) and half flux diameter (HFD), as well as better signal-to-noise ratio (SNR). This offsets any potential drop in signal strength when using NIR for guiding. The majority of ONAG users have reported minimal changes in their ability to locate and select suitable guide stars.

The visible-NIR cut-off wavelength at 750nm provides full access to imaging using the H Balmer alpha band. Since filters and filter wheels are positioned in the optical path of the imager, they do not interfere with the guiding camera. Consequently, narrowband imaging will no longer cause the guide star to become too dim for practical use.

“As mentioned above, there’s an X-Y stage one the (ONAG®) guider port that allows the user to move the center of the autoguider’s view 28 mm horizontally and 23 mm vertically , so you can explore a 46 mm diameter area of your telescope focal plane to search for appropriate guide stars. This area is huge compared to what is available with most off-axis guiding systems.”
“But there’s another aspect of the ONAG® that further improves the efficiency of finding a guide star…”
“…The guide stars available to the ONAG® are closer to the telescope’s optical axis and are thus of much better quality for guiding.”

(Dennis di Cicco, Sky & Telescope December 2012, pages 60-63)

• Wide FOV for your guider with the integrated X/Y stage
• No differential flexure
• NIR guiding for reduced seeing & a smooth tracking
• No imaging camera rotation needed
• No filter in the way
• Sharp images with SharpLock, our real time autofocus
  
• No more periodic refocus
  
• The only solution for multi-star guiding using your main scope
  
• Weight only 0.8 Kg (1.8 lbs)

Innovations Foresight offers two versions of the ONAG to cater to a wide range of setups and requirements.

The ONAG SC is specifically optimized for APS-C sensor sizes, accommodating diagonals of up to 28mm. It includes a guider helical focuser and a low-profile X/Y stage. All the optical ports of the ONAG SC, namely the scope (SP), imaging (IP), and guider (GP), utilize standard T2 M42 x 0.75mm (T thread) connections.

On the other hand, the ONAG XM is optimized for full-frame chips, supporting diagonals of up to 50mm. It is an improved version that replaces the now discontinued ONAG XT. The scope and imaging ports (SP, IP) of the ONAG XM employ a 59mm dovetail system, ensuring secure connections with the scope’s imaging train (including the focuser, rotator, focal reducer, etc.) as well as imaging and filter wheel equipment. For the guide port, the ONAG XM uses a T2 (M42 x 0.75mm) male thread for connecting to a guider camera.

To fit your needs Preciseparts.com provides custom adapters for any of our ONAG.

“I feel this is the ultimate design in guide star tracking and imaging. I am a firm believer in the system and plan on solely using this type of tracking from now on. My only problem now is… I need to re-image the entire sky to upgrade my stock of sky images!”

Owner of a f/6 32″ relay telescope

Dr. Mario Motta