Innovations Foresight On-Axis Guide and Starlight Xpress SXV-AO-LF Review

Innovations Foresight On-Axis Guide and Starlight Xpress SXV-AO-LF Review
By GazingSkyward
February 2015

Revision 1.1

Introduction

Being somewhat less experienced than many of the amateur astronomers here, I have had reservations about doing equipment reviews in the past but this is something that I really wanted to share with the 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 Innovations 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 proverbial 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 can prove to be less than satisfying.  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 last year, I simply could not resist the urge to do away with the OAG coupled to my 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, with the added benefits of near-infrared guiding.

The ONAG also gives SXV-AO users  the freedom to use a guide camera other than a Lodestar or Lodestar 2x.  This is an attractive option for owners of monohrome cameras with larger sensors, such as the Sony ICX 825 (found in the new Atik 414EX), or other cameras which are highly sensitive in the near-infrared.  In putting together the system featured in this article, I found that a TEC-cooled ICX618 camera represents excellent value for money, high sensitivity, low noise and a pixel size suitable for a range of optical configurations.

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, user-adjustable dichroic mirror and corrective optics in the guide camera focuser.

The test platform emplyed in this article is a Skywatcher AZ-EQ6-GT mount fitted with a Skywatcher Black Diamond ED120 refractor of 900mm focal length.  The mount has not been corrected for periodic error and a basic polar alignment using the Synscan hand controller was performed prior to testing.

How It Works

For those who are unfamiliar with the concept, the ONAG implements a beam-splitting dichroic mirror which sends 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 (seen on the left side of the diagram above) emit more ultraviolet light, which is highly susceptible to atmospheric distortion and this ultimately makes them a much less suitable candidate for guiding.

The flexible design of the ONAG also enables the user to reverse the orientation of the imaging and guiding cameras, 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.  Maffei 1 and Maffei 2 are examples of objects which can only be imaged using this technique.

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.  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.

Sharplock is not covered in this article, but an in-depth look at SharpLock and it's features 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. 

Click image to launch full resolution version

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 millimetres.

Individual M42-threaded filters or filter wheels can be connected between the ONAG and the imaging camera.  Alternatively, filters which do not block near-infrared light (such as the Astronomik CLS) can be connected between the telescope focuser and the ONAG.

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 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 near-infrared guiding is more than just theory and that the ONAG was 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 OpenPHD.

Auto-guiding using the ONAG yielded a total RMS Error of 0.34 pixels, a considerable improvement over the RMS Error of 0.5 to 0.75 to which I am accustomed and very promising results for a "first light" test.  By refining the polar alignment and using longer guide camera exposure times (further averaging what little atmospheric interference remains), an even greater improvement in guiding can be achieved.

Click image to launch full resolution version

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 (Sony ICX424), Orion Starshoot (Aptina MT9M001), QHY5L-II (Aptina MT9M034), QHY IMG0H (Sony ICX618) and Starlight Xpress Lodestar (Sony ICX429), the last two of those five cameras proved to be the most sensitive in the near-infrared.  The Atik Titan, IMG0H and Lodestar all presented very low noise.  The sensitivity results were a surprising find given the larger pixel size of the much heavier Titan (7.4 micron) compared to the IMG0H (5.6 micron), 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 $250USD 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, shorter focal length systems should avoid larger pixel sizes.

ONAG with AO -- The ultimate combination?

Finally, with the help of OpenPHD, I added the SXV-AO-LF to the ONAG and put the two to the test.

It should be made clear: depending on what you're expecting to get from your AO, this combination may be better suited for larger aperture telescopes given that longer exposure times are sometimes needed in the near-infrared and shorter exposure times are often preferred for AO operation.   Larger aperture telescopes have the luxury of being less discriminant when it comes to guide star availability and will obviously be able to extract a higher SNR from fainter M-Class targets at shorter exposure times.  Results and exposure times used will also differ across focal lengths and aperture sizes.

Attempting to run the SXV-AO on my system at a rate of 5-10Hz required a reasonably bright guide star and even with the improved guide star stability in the near-infrared, the resulting image quality was degraded somewhat because I was "chasing the seeing".  It is important to note that this neither a fault of the SXV-AO nor the ONAG and is simply due to inherent limitations of consumer AO technology in general, which can only provide rapid corrections to a very small area of the total field of view, restricted to a region local to the selected guide star.

Increasing the AO update interval (and ultimately the guide camera exposure time) to 500ms and then to 1s and 2s improved image quality and increased the range of available guide stars with suitably high SNR. 

I eventually settled on 2s exposure time and later tested on another target using 3s and 4s.  In spite of the AO making less frequent corrections, the adjustments made by the SXV-AO tip/tilt element as opposed to physically moving the mount drive resulted in slightly better image quality through tighter stars, as would be expected from a consumer AO system.  In turn, the AO naturally benefits from the improved seeing conditions provided by the ONAG.

In the interest of factual integrity, I have opted not to post "AO vs Non-AO" image comparisons in this review, as I feel that more testing is required, proper FWHM analysis is needed and other factors including transient changes in seeing must be ruled out before being able to accurately quantify the improvement over the ONAG alone.  Observations made so far are highly indicative of improved guiding in a ONAG + AO system over one which employs either AO or ONAG alone.

Considerations

-- Because of insufficient inward focuser travel on my system, I am not able to use a focal reducer in conjunction with the ONAG and certainly not with the ONAG and SXV-AO-LF.  Although my OTA could be modified to support a focal reducer, on most telescopes this would not be a problem.

-- Without modification, some Newtonian telescopes lack the sufficient backfocus required for the ONAG to operate.  Innovations Foresight have indicated that they are working to develop an ONAG solution for Newtonian systems.

-- Due to the increased load (~800 grams) on the imaging train, a good quality after-market focuser may be required and is definitely recommended on low and mid-ranged telescopes.  A motorized focuser is obviously required to take advantage of SharpLock auto-focus.

-- ONAG requires 66mm backfocus to the imaging camera and 90mm backfocus to the guiding camera.  The ONAG XT requires 68mm and 92mm respectively.

-- Because of the orientation of the imaging camera (vertical, as opposed to the typically horizontal), effective cable management is important to ensure that the telescope can slew without cables getting tangled up or caught on any equipment.

-- The ONAG XT works out to be the most equitable of the two models available; I purchased the standard ONAG and found that my CCD sensor (28.4mm diagonal effective area) only just fits into the illumination area. This is only a minor inconvenience but worth noting if your APS-C sized sensor is approaching this limit or if you anticipate purchasing a larger CCD sensor in the near future.

Conclusion

Would I recommend the ONAG?  Absolutely.  The benefits of near-infrared guiding on-axis are not simply theoretical, you can see them right there in your guide camera images and in the results that you take home at dawn. 

Compatible with the majority of systems out there, in my opinion the reasons in favor of switching to ONAG far outweigh those against it.  Whether you are imaging close to the horizon or simply wish to improve the accuracy of your auto-guiding generally, guiding in the near-infrared is clearly superior to guiding in visible light.

I'm glad to be rid of my guide scope!

Acknowledgements

Dr. Gaston Baudat from Innovations Foresight for spending a great deal of time patiently answering my many questions and for kindly assessing my results.  His dedication, service and diligence are second to none.

Terry and the team at Starlight Xpress for their work on the SXV-AO and Lodestar, their friendly service and for promptly supplying me with the parts that were missing from my SXV-AO kit at purchase.

QHY-CCD for their excellent range of cameras, including the QHY12, IMG0H and QHY5L-II units which were used in this review.

Clear Skies!