- Explore Scientific, 16 inch / F 4.5 Truss tube Dobsonian
- Celestron PowerSeeker 70AZ Telescope ($10 Scope)
- Orion EQ-26 Mount Review
- Review of Explore Scientific First Light 8
- Rebuilding my CGE Pro
- COUNTING SUNSPOTS WITH A $10 OPTICAL TUBE ASSEMBLY
- Hubble Optics 14 inch Dobsonian - Part 2: The SiTech GoTo system
- iStar Optical’s Phantom FCL 140-6.5 review
- Who’s Afraid of a Phantom: Istar Phantom 140mm F/6.5, that is?
- SHARPSTAR 94EDPH APOCHROMATIC REFRACTOR
- My Losmandy G11T review
- FIELD TEST: THE NOH CT-20 ALT-AZ MOUNT
- SkyTee-2 Alt/Az Mount Review
- SharpStar Askar ACL200 200-mm f/4 astrographic telephoto lens
- A review of the Unistellar EVscope
CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.
Innovations Foresight On-Axis Guide and Starlight Xpress SXV-AO-LF Review
Discuss this article in our forums
Innovations Foresight On-Axis Guide and Starlight Xpress SXV-AO-LF Review
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.
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.
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.
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.
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.
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.
-- 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.
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!
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.
- 1E1HFPPE, bsavoie and deepsouthernskies like this
What an amazing review! I immediately went to their website but then sticker shock hit me! Do you know another off-axis guider that comes in second place that you would suggest?
Walter, this is an ON-Axis Guider not off-axis. In any case, I think (but am not certain) that Innovations Foresight is the only company that makes such a unit (at least for amateur astrophotographers).
the Innovations Foresight ONAG is indeed a bit steeply priced, which motivated me to make my own:
However "my own" is much less robust and less flexible than the ONAG. I eventually gave up on it (a better mount made OAG unnecessary).
The ONAG is a trade mark and unique patent pending technology from Innovations Foresight.
It is designed and made of high quality components to insure diffraction limited images. Custom high performance dichroic mirrors are expensive to make, they use many inferential coating layers, which is a much more complex process than standard optical AR and reflection coatings (used for traditional lenses and reflector mirrors).
The devil is on the details, as usual, and the ONAG is the results of many years of experience on this matter.
As with any new technology cost is higher initially, however good quality OAGs are close in price range, yet we are working hard to offer the ONAG at economical feasible and competitive price, without compromising its performances and unique features.
The ONAG does not require any rotator which usually add some extra load, back focus and cost in the setup, also the associated patent pending real time auto-focus (SharpLock) technology provides an unique solution to handle critical focus without any interruption of the image session.
A good mount is always better, but more pricy too.
With a high end mount, on a permanent setup, and a good pointing/tracking model such as Tpoint/ProTrack you can achieve unguided exposure for few minutes, depending of the target location, seeing and setup.
Longer exposure times, such as 10 to 30 minutes are more challenging and usually require some active guiding, even professional astronomers are guiding.
As a rule of thumb you want to keep your rms tracking error at, or below, a quarter of the current FWHM seeing, which means that good seeing could require rms tracking errors as low as 0.2" to 0.3".
I must add to my previous post.
I am currently not using the home-made on-axis guider. There were too many problems with it, e.g. no way to move the guide camera, there was astigmatism even in the main camera, a horrendous amount of vignetting.. and adjusting focus on both the guider and main camera was very problematic.
In short - it was good as an intellectual exercise (and to exercise my ATM DIY skills), but not really viable for anyone serious about imaging.
Astronomy is wide open.. there is always a place where reality can be better appreciated. That is the fun of it. Astronomy is where hardware, software, and beauty connect.
I read your home-made on-axis guider whey you were first interested in it.. I am also happy that you do not give up. Your lessons learned will be key to some future approach that will be successful. Yesterday I bought a ADM Accessories MAX-M Guidescope Aiming Device. I am going to wade into this branch of astronomy that uses cameras.. In long - it is a good ongoing intellectual exercise. The future will break the bondage of high costs for all of us.
great information, I see you have a z-EQ6 pro, I am very interested in purchasing a mount like that, you could help me by giving me a review of this mount also want to check that you can work in less than 3 ° latitude, and if I could send photographs would appreciate it! most of all in which is the minimum latitude, greetings and best wishes !!
Thanks to everyone for your feedback about this article, much appreciated!
As promised, I will be road-testing the ONAG SharpLock™ auto-focus feature in the coming weeks, so stay tuned for that.
Although the AZ-EQ6-GT has been on the market for a couple of years now, a few people have asked me to review this excellent mount and I will endeavor to do so in the coming weeks. Unfortunately I can't answer your question regarding the operation of that mount below 3 degrees latitude, that is something you would want to ask Skywatcher.
Looking forward to your AZEQ6GT review!
I understand about the advantages of On Axis Guiding as far as the flexure issues etc, but why can't I just place a near infrared filter on my external guide scope to guide in the near infrared, with a camera that's IR sensitive?
That would seem to help any guiding system not just the ONAG based on the comments in the article.
Fundamentally this is correct. The NIR seeing effect (around 850nm) is close to 70% less than in the visible (550nm).
Here few comments:
There is a drop of the star signal level using only NIR versus using the visible + NIR (the all sensor spectral range).
This is a function of the star surface temperature (class) and the guider chip sensitivity versus wavelength.
Class M stars have surface temperatures, lower than 3700K therefore they radiate a lot of NIR. They are also quite common, since more than 76% of the main sequence stars are from class M.
With a classical Sony ICX429AL chip (used in the lodestar, the X2 version has a new generation of the Sony Exview chip, whcihis twice more sensitive now, chip QE improves all the time driven by the security camera market), the above signal level drop is about 50%, or near -0.8 magnitude. However it is not has bad as it may seem since there is less seeing induced guide star star wander in NIR your star signal level, relatively speaking, is usually better since the starlight is not as much as spread in NIR than in visible under seeing limited conditions.
Also with an ONAG you have wide FOV to find a guider star, which significantly increases the probability to find a suitable one. You are closer the the optical axis where the optical aberrations are minimum, those features help to offsets the signal drop.
An OGA by nature looks far of axis, especially with large ships, it is not uncommon to experience some coma, field curvature, ..., depending of the scope, and also some diffraction issues from the small pick up prism. Using a NIR filter maybe more challenging in term of SNR with an OAG.
Guidescopes do not have such limitations but they have others, beside differential flexure, they exhibit a smaller aperture D, 80mm is common, and this is an important figure of merit in term of SNR.
Since a star is a point source (a plane wave), the level of energy collected is a function of D squared, not a function of F/# like for an extended object.
Therefore a 11" (D=280mm) scope collects roughly (280/80)^2=12x more starlight than a typical guidescope (80mm), this is a +2.7 magnitude gain in guide star selection for the same exposure time and SNR. This is valid for an OAG, ONAG, or self-guided cameras.
Also to benefice of a NIR filter for guiding you need to have a steep filter transition at the cut-off wavelength. This usually requires using interference filters, not die based filters.
But beside the above arguments, using NIR for guiding will help by reducing the seeing effect on the auto-guiding process.
Seeing a is a complex beast. Usually it is made of various temporal and spacial components coming from different layers of the Earth atmosphere. The guide star wander due to the seeing is only correlated with the other part of an image in a very small area surrounding it, few arc-second across, known as the isoplanatic angle, or patch.
Therefore it is not uncommon for the auto-guider and related software to "chase" the seeing while using short exposures (few seconds or less).
If such short exposures are needed indeed, using NIR will significantly help. Another approach is to use longer guider exposures, every time it is possible, to average out the seeing. You have to watch your polar alignment, and you may need to use a model of your setup to correct in real time the tracking rate (King's rate), such as Tpoint/ProTrack.
Thanks for your detailed synopsis. I fortunately have a good mount and optical system. I image with an AP1200GTO, C14 Hyperstar f/2, QHY12, Orion 120mm dia + QHY5LII external guidescope. My main/guide scope are are rigidly attached with a bulletproof monster Andy Homeyer cradle.
I have a remote observatory in the dark skies of New Mexico with good seeing. I typically get 0.02 - 0.03 arc/secs RMS guiding with my system using MaximDL. I know this system pretty well and have been imaging with the mount and HyperStar since 2007, and with the mount/C14 since 2000. I've recently started using Astro-Physics APCC with atmospheric tracking correction and good modeling. I've been acquiring 10 minute unguided images at 600 mm FL down to low declinations that look better than my guided images sometimes. (poor seeing) I'm sure as I tweak my polar alignment, I will get longer duration unguided images at the lower declinations. Just goes to show you the uncertainty of accurate guiding in less than perfect conditions. (Why I would like to use NIR guiding)
Many successful refractor imagers use external guidescopes successfully. These days it's fairly easy to mount them without a large amount of flexure and the main scopes are also fairly short focal length systems too which makes guide requirements easier for automated imaging instead of using a pick off mirror OAG etc. (This depends on image train deflection too) Of course my system suffers from mirror flop, but I've made major modifications to minimize.
However, I'm always looking for guiding upgrades. Since I already use an external guide scope with fairly decent aperture, I'm getting very good FOV for guiding and use CCDAutopilot for unattended imaging. Only on rare occasions in guide star poor regions do I have an issue. Obviously using an NIR ONAG with a larger Cass or RC would be better with NIR ONAG guiding. When I start to image at prime focus on the C14 again, it seems very logical. However I'm currently in the same boat as many refractor imagers with smaller apertures that would suffer from reduced signal with a NIR ONAG solution. I have to use an external guide scope because of the HyperStar system and my camera combo.
With my external guide scope configured with an NIR filter, it seems to me that I will obviously loose a fair amount of stars for guiding. Since I perform completely automated imaging runs it seems I might have to make some target selection restrictions to prevent guide star acquisition failures during an automated run. With my current system, I'm successful 95% of the time. I originally chose the larger 120 mm external guide scope system of guiding instead of camera pickoff OAG to get better automated guide star selection and performance in the first place. Based on your calculations ((120mm/80mm)^2 = 2.25), my 120mm is already acquiring 2.25 times the light as a typical 80 mm guidescope system.
Question: Am I correct in thinking that since there is an approximate loss of 50% signal level based on your comments, that my external 120mm guide scope with an NIR filter would translate close to guiding with an 80mm refractor (no nir filter) with the advantages of guiding in the NIR. ((2.25/2) = 1.125)
The apparent guidescope aperture D (assuming 120mm nominal), for a 50% drop of signal would be given by 120*sqrt(2) = 85mm.
Since (120/85)^2 = 2, the dependency is on the squared root of the drop.
The guide star FWHM and HFD due to the seeing (star wander) decreases with longer wavelengths. The net result is that the central peak and HFD carry more energy (higher amplitude, better Strehll's ratio), if you are seeing limited which is usually the case with large aperture scopes (>3").
You may have a gain in SNR up to 50% in NIR, depending of your local seeing, exposure time, and scope aperture.
From experience part of the NIR 50% signal drop is offset by a better NIR seeing and a resulting tighter guide star.
Most of the ONAG users have experience very little change in their ability to find a guide star.
That's encouraging for my setup. Good point about central peak and HFD carrying more energy. I'm kind of excited about trying NIR out. At first with an external guider, and then maybe with a Foresight NIR ONAG system when shooting at prime focus with the secondary in place. Most of my improvements over the years have been incremental.
Thanks again for the info.
If you have a good mount and tracking using a model as I understand you have, I would recommend to try using long guiding exposures as much as you can, like 30 seconds or more. Basically your maximum exposure time will be limited by the left over drift, since no model and mount are perfect, but it could be very small, depending of the position of the target above the horizon.
With long enough guiding exposure you are averaging out the seeing (in NIR and/or visible), which means that the guide star centroid is essentially seeing free. Of course you still have the starlight energy spread from the star wander, this is where NIR may be useful. Yet with long exposures finding a suitable guide star with a good enough SNR is seldom an issue for most targets and apertures any way.
Many people try to guide with too short exposures to deal with the guide star drift in my opinion, they would be better off, with most today's mount (and PEC), with longer ones and a good pointing/tracking model, at least for a permanent setup.
Making a good model takes some time and commitment, but it pays back. Too short guiding exposures increases the risk of chasing the seeing, especially in the visible wavelengths. Even when using NIR this remains a concern since the guide star wander isoplanatic patch is usually quite small, few arc-seconds across.
On the other hand using a multi-star (constellation) guiding strategy offers an unique interesting and promising alternative. You will average out the seeing by using several stars across the scope FOV, which also improve the SNR, the resulting guiding information is essential correlated with your setup error, not the seeing anymore, at least not its contribution related to small isoplanatic angles (upper Earth atmosphere effects).
Maxim DL version 6 offers such capability, other are coming soon. The only way to use constellation guiding with the same scope than imaging is the ONAG technology using a guider having a large chip to access a wide FOV without any part in motion.
I have tried to stay with 4 second guide exposures through the years with my system in order to not chase seeing. I have not experimented with long exposures with APPC, but will give it a try. I have a significant FOV with my external guide scope system and have tried multi-star guiding as implemented in MaximDL6, but have not found any consistent improvements to guiding so far. In some cases it appears to perform worse and others better. This could be due to the implementation by MaximDL and/or the combination of my system. I was excited when MaximDL6 implemented multi-star guiding, but have not kept up to see if others are having better success. Since Maxim's algorithm is proprietary and feedback is limited, it's difficult to analyze the issues with it.
The biggest advancement for me as an AP mount user seems to be with APCC's refraction correction and a good pointing model for my system. I just turn off guiding when the seeing is not great. Six minute subs are no sweat unguided at most declinations tested so far. I have tested up to 10 minutes a few times with no problems. Of course, guiding at the lower declinations is a harder test and most difficult usually due to the air mass refraction errors, but with APPC tracking correction, that part is minimized. It really makes you appreciate how tentative auto guiding is and that we are mostly chasing long period mount tracking errors and not seeing like a lot of folks think. The seeing issues just get in the way of good auto guiding while trying to correct for mount tracking errors. I guess that's why the new generation of quality mounts with hi-res shaft encoders used with programs like APPC and ProTrack are so incredible.
I think your suggestion of long guide exposures like 20-30 seconds with APPC enabled may be a good strategy in order to correct for any polar mis-alignment drift or mount mechanical errors in long exposures. I do not have shaft encoders on my 15 year old AP1200GTO so it is an open loop system. Of course AP's new series of mounts have that as an expensive option and integrates feedback into their adjusted rates. A new mount with shaft encoders are above my dollar limit. APCC can only tell my mount to track at the corrected rates, but with no closed loop polar and dec shaft encoder feedback, it can't guarantee positional accuracy. (I think its close) I don't think APCC checks the Dec and Polar drive motor's encoder for position feedback, but I could be wrong about that. And I guess there is always the image rotation component that can not be corrected with guiding or APCC. I'm not sure how significant that would be. I guess it could be pretty small. My mount was aligned with PEMPRO, but believe it could be adjusted better by performing alignments on both sides of the meridian, although Dec and RA alignment is always a compromise.
Loved that article - it's always fun when you do it on your own. When are you going to strip down a Canon ultrasonic lens and use those motors to build your own AO?
I have been using an ONAG on my TEC140 and TEC180 for some time now. It is an impressive product and now even better with the new helical focuser for the guide camera. I'm using an ATIK 414EX to guide. An expensive solution but gives an impressive FOV. Also, very recently I have auditioned SharpLock using MaximDL. It works as advertised once set up and the system understood. In the future I think I will be using SL and the new Optec solution FocusLock (essentially SL engine that allows for a large number of capture programs like TSX). If anyone is interested I can publish some of my observations using SL.
Following Peter's post I'll like to update on the FocusLock status, it is now live.
You can download and try the beta version till August 31 2015.
Any comments and feedback are most welcome.
In case anybody is interested, this is a demo video I put together for a talk I gave last week to the imaging group of the New South Wales Astronomical Society on Optec's FocusLock which I used entirely for my image of NGC 1532. It's a bit long mostly because I was taking long guide exposures. If short guide exposures are used it is considerably faster. In reality it is of no consequence as camera focus changes are quite slow. It only matters for the purpose of demonstrating. The exciting thing is that this focus system is coming to the Lodestar guide camera + OAG, so no ONAG required. This will be released in April at NEAF.
I took this image (ca 26 hours) entirely with FocusLock:
The background music is performed by my partner, Bernadette Harvey, playing the Australian composer Ross Edwards' "Night Mantras."
I think the youtube video link is not correct.
Now it is, I have updated the link in this post as well, thanks Peter
Let me provide some further information on this and related matter:
OPTEC and Innovations Foresight are working together for providing a retrofit-kit for Lodestar owners which will allow the SharpLock (SL) real time auto-focus technology (implemented in FocusLock software) to be used with some OAG models.
We expect to launch this new patent pending accessory during NEAIC/NEAF 2016. Its is an interesting option for setups having limited back focus, such as Newtonian, for which the ONAG solution may be challenging to mechanically fit in.
Also new development are coming for providing full frame auto-guiding and real time auto-focus (SL) capabilities while using an ONAG and a large guiding chip, since all ONAG devices now features a guider port supporting sensor diagonals up to 28mm (APS-C size).
In this approach auto-guiding will not required the selection of any guide star anymore, details to follow... This may be specially useful for robotic/remote operations.