[Eden (Brett)]

Introduction

Being less knowledgeable than many of the regulars here at IIS, I have had reservations about doing equipment reviews in the past but this is something that I really wanted to share with the amateur astronomy community and I hope you find it to be informative or at the very least, an opportunity for discussion.

Astrophotography for me has always been much more than just taking photographs of the night sky, it's also a chance to learn about the technology which makes it all come together at the end of the night. I'm also a firm believer in supporting the people at companies like Starlight Xpress and Innovation Foresight, people who are just as passionate about astrophotography as you are and who work hard to make advances in the field which ultimately benefit the community.

With the exception of mounts at the highest end of the market, auto-guiding is an essential component in getting great results and when the opportunity to improve on it is there, one should consider it a priority to do so.

My personal experience with the traditional auto-guiding method of using a guide scope has been satisfactory at best and an utter disappointment on the worst of nights. Unexpected problems (including wildlife!) have all thrown a spanner into the works and it is all the more frustrating when you live in a location when imaging opportunities are at a premium.

Experimenting with alternatives such as the humble OAG proved to be less than satisfying on my particular system. Finding guide stars can sometimes be tedious and is a job which is ideally suited to a rotator, the cost of which is difficult to justify purely for the ability to rotate the imaging train.

Hearing about the ONAG from other forum members last year, I simply could not resist the urge to do away with the OAG coupled to the SXV-AO-LF and to try and marry the AO body with an ONAG.

This results in a system which provides the entire field of view for guide star selection compared to the small off-axis field imposed by the OAG.

This review applies to the standard ONAG, which supports CCD sensors up to 28mm diagonal size. An XT version is available for full-frame sensors, which features a larger aperture and user-adjustable dichroic mirror.

All tests were done using a Skywatcher AZ-EQ6-GT mount, Skywatcher ED120 APO Refractor and the cameras mentioned herein.

How It Works

For those who are unfamiliar with the concept, the ONAG implements a beam-splitting dichroic mirror which reflects the visible portion of the light (< 750nm) received by your telescope to your imaging camera and the near-infrared portion (> 750nm) to your guiding camera.

Aside from giving you the entire field for guide star selection, the distorting effects of atmospheric disturbance are reduced due to the guide star image being composed from light at longer (near-infrared) wavelengths. This results in a steadier guide star image and directly translates to a measurable improvement in guiding accuracy.

Why Near-Infrared?

Around 75% of the stars in the main sequence are of spectral class "M" (Red Dwarf). These stars at the lower right-hand side of the Hertzsprung-Russell diagram emit most of their energy towards the near-infrared end of the spectrum, their output peaking between 750nm and 850nm. The abundance of these older, cooler stars makes the concept of guiding in the near-infrared not only viable, but logical.

Many monochrome CCD cameras have good relative sensitivity at this end of the spectrum, making it possible to image these stars in the near-infrared at similar exposure times to what you would normally use in visible light. By contrast, younger and hotter stars emit less energy at this end of the spectrum and are less suitable for guiding in the near-infrared.

The flexible design of the ONAG also enables the user to reverse the orientation of the imaging and guiding camera, turning the system into a near-infrared imaging platform. Long exposure images at near-infrared wavelengths penetrate gas and dust, revealing stars and galaxies which are obscured by nebulae.

Real-time Auto-focus

Possibly the most innovative feature of the ONAG is the ability to perform real-time auto-focusing.
By taking advantage of how the guide star is presented to the guiding camera in near-infrared light, Innovations Foresight have developed a software solution that is able to monitor your guide-star images for any changes and send fine-grain adjustments to your focuser as you are imaging, ensuring that you remain in focus right throughout the session rather than having to wait until the end of an exposure or until a filter change occurs.

Through the ONAG, guide stars take on the approximate appearance of a cross when correctly focused (See attached image). If and when ideal focus is lost, the guide star will appear either more elongated in the horizontal or in the vertical, depending on which direction focus shift occurs.

The supplied SharpLock software works in real-time to maintain the symmetry of the guide star, thus ensuring that your main imaging camera is always in perfect focus and maximizing the time during which your imaging camera is collecting photons.

For anyone who has experienced focuser shift due to temperature change will certainly appreciate the benefits of this technology, as will those who take longer exposures and postpone focus adjustments until a filter change occurs.

The improved steadiness of the guide star in near-infrared light ensures that SharpLock will not make unnecessary changes to system focus in response guide star distortion.

An in-depth look at SharpLock will follow in the near future.

Physical Operation

Mechanically, the unit is extremely solid and precisely engineered. Weighing in at just shy of 800 grams, it is lighter than all but the smallest of guide scopes (and indeed the accessories needed to support them), thus decreasing the load on your mount. This is an important advantage for people who are already operating their mount at or near the manufacturers recommended weight capacity. Aside from load reduction, moving the guiding system onto the imaging train also alleviates the possibility of incidental flexure at the guide scope rings and the need for dew management on the guide scope itself.

Connection to the telescope is via either female M42 or male 2" and the imaging camera is connected to the top of the unit via male M42 thread with adjustable stopping collar. The guiding camera is connected to the rear of the unit via a specialized grooved focuser with male M42 connector, also with adjustable stopping collar. Treated with temperature resilient lubrication, the focuser glides smoothly in and out of the unit and is secured using a self-centering circular clamp which firmly grips the entire circumference of the tube. Two nylon screws, seated in a groove on the top and bottom of the tube, prevent it from slipping out of the ONAG. This focuser can be removed (as I did) for cameras which do not have an M42 thread, the securing clamp working just as well on a 1.25" nose-piece or barrel camera such as the Lodestar/Lodestar 2x or the QHY5L-II.

The guide camera can be repositioned along the X and Y using a unique dual-axis staging mechanism, for a total travel of 37mm along the horizontal and 28mm along the vertical, in the event that a guide star is not immediately available near your target. Just like the guide camera focuser, the dual-axis staging is lubricated and glides smoothly in both directions for subtle adjustments. Two nylon screws along each axis firmly lock the staging mechanism in place once the desired positioning has been achieved.

The dichroic mirror behaves similarly to a star diagonal in the fashion by which the light is reflected, decreasing the outward focuser travel needed to bring the imaging camera into focus. This further reduces the potential for droop to develop on drawtube systems by maintaining the equipment load closer to the telescopes center of gravity. The ONAG dichroic mirror does not adversely affect image quality at all and is no different from imaging on a straight-through system.

Usage

Once connected, all that remains is for the imaging camera and guide camera to be brought into focus.

The ONAG ships with a selection of high quality M42 extenders to assist with additional spacing which might be required to do this. On my system I was able to bring both cameras into focus with the aid of a single 8mm extender placed in front of the main imaging camera and only needed to rack the focuser out by a few millimeters.

If sufficient inward focuser travel is available, a focal reducer can be placed either in front of the ONAG to increase the field of the entire system or just in front of the guiding camera.

Focal reducers which are positioned in front of the guide camera alone will move with the camera if the staging mechanism is adjusted along the X or Y axis, allowing guide cameras with small sensor size to benefit from stronger focal reducers (ie, x0.5, 0.33) with less chance of introducing field curvature.

Observations

Having never guided in the near-infrared before, I did not know what to expect and had reservations about whether the small aperture of my telescope would be able to supply sufficient light in the near-infrared for the ONAG to function effectively.

I was pleased to discover that there was no shortage of guide stars available in the near-infrared and that the SNR reported by the guide camera was similar to if not slightly higher than what I would expect in visible light at the same exposure time. In some parts of the sky it was necessary to increase the guide camera exposure by a small amount or adjust the staging mechanism, but I never failed to find a suitable star upon which to guide.

At one point when preparing to commence guiding, I noted that the exposures coming from the guide camera were not changing at all, leading me to the suspect that either the guide camera had come unplugged or that PHD Guiding had crashed. In actuality, what I was witnessing was evidence that the ONAG does what it claims to do by removing the distorting effects of the atmosphere. This resulted in the best recorded guiding I have had to date and was confirmed through the statistics generated by PHD Guiding (see attached image).

Whilst most cameras used for auto-guiding today have reasonable sensitivity at 750nm+ wavelengths, a small number of cameras on the market excel in both near-infrared sensitivity and low noise.

Putting a variety of cameras to the test, including the Atik Titan Mono (ICX424), Orion Starshoot (Aptina MT9M001), QHY5L-II (Aptina MT9M034), QHY IMG0H (ICX618) and Starlight Xpress Lodestar (ICX429), the last two of those five cameras were clearly the most sensitive with the Atik Titan, IMG0H and Lodestar all presenting very low noise. This was a surprising find given the larger pixel size of the much heavier Titan (7.4um) compared to the IMG0H (5.6um), both of which have TEC cooling.

Longer focal length systems would definitely benefit from a Lodestar/Lodestar x2 but the considerably less expensive QHY IMG0H (by around $300 at the time of this writing) presents a viable alternative with potentially less noise but slightly lower resolution.

To prevent under-sampling and to ensure that SharpLock is able to perform an optimal analysis of the guide star for auto-focus operations, it is important to select a guide camera with the right pixel size for your system. Put simply, short focal length systems should avoid large pixel sizes.

(Continued in Part 2)