Review: Innovations Foresight On-Axis Guider with Starlight Xpress SXV-AO-LF (1/2)

[gaston (Gaston)]

Hi Peter,

The Palomar AO system makes a huge improvement, the FWHM (near the guide star) goes from 1.2" down to 0.2", 6 times better.
This type of AO systems usually correct for tilt/tip and higher order wavefront errors, at a very fast rate, amateur versions cannot come close of it anyway.
We should remember here that the all image is only 40" by 40", the AO improvement decreases slowly but surly with the angular distance from the guide star already.
Comparing to the central performance the drop is significant at the C position, but of course, relative to our experience, it is still a quite good FWHM, we would dream to achieve this, wouldn't we?
Yet the star shapes are ugly, so what would be the point (beside science) to have tight elongated/distorted stars?
Now with our scope FOVs we are talking about at least a fraction of a degree, like 30', which means 1800", not even close to 40".
The image quality that far from the isoplanatic patch will not be better, likely worse with the AO enabled, than the left image.
Using an AO unit at fast tracking rate will only improve the image in a very tiny area (the isoplanatic path) and than spread the guide star local seeing noise (the isoplanatic path seeing) all over the image, results to a worst situation than without the AO.
In short at fast rates you chase the seeing across the all atmospheric path, making better only the image in few arc seconds around the guide star, blurring more everything else.
This is why you should consider using an AO at low rate to avoid chasing the seeing, at least for us with small scopes and large FOVs.
In my opinion the true value of amateur AO is image stabilization for mount tracking issues (much faster than PE) and in some conditions for low frequency local seeing (near ground) effects.
I would recommand not to go faster the 0.5s, with most mid range mounts the AO will do a good job around one second or 2. Which is also a good choice since you do not need bright guide stars anymore.
I had very great success with an AO-8 and AO-L units and my old Celestron CGE mount, now with a PMX I do not need this help anymore, I can guide with typicality 30" to one minute exposure time, before atmospheric refraction and OTA/mount flexure come to play.

I hope this made my comments a bit clearer.

[gregbradley]

I used a 25mm 720nm IR filter and a 760nm IR filter. I see the ONAG uses 850nm so there's a difference there.

Your mention of the UV is interesting as well. So yes my test is not matching the specs of the ONAG so perhaps there's nothing to be concluded from it.

The imaging camera has these NIR and UV bands filtered out by the dichroic mirror of the ONAG - is that right?

That could also be useful for me in that I find my Trius 694 being so sensitive to IR and probably UV that I get star bloat in some images from brighter stars which per the theory here on ONAG and different types of stars output points the finger at too much NIR and UV affecting my images at times.

If I understand this thread correctly then the ideal setup would be:

1. An ONAG guider.
2. A high sensitivity and larger guide camera ideally a Sony ICX style chip which seem to be the highest QE lowest noise CCDs at the moment.
3. An AO unit before the camera.

So the advantages of the ONAG then are:

1. Better guiding.
2. More guide stars as your guide camera is able to see the same scene as your imaging camera so if you select a guide star closest to the object you are imaging you minimise the seeing differences between patches of sky.
3. Reduces IR/UV that is getting through to your imaging camera also resulting in potentially tighter stars.
4. Ability to select brighter guide stars to enable faster AO corrections thus optimising AO performance.

So an ONAG with a small ICX674 based guide camera, an AO unit like an SBIG or Starlight Express, the continuous focusing system could result in nailing down 2 of the more difficult and troublesome areas of astrophotography - the seeing / guiding as well as obtaining critical focus.

The disadvantages of an ONAG are:

1. Cost.
2. Extra weight.
3. Takes up a lot of backfocus.
4. Having your camera vertical (does this cause balance problems on heavy cameras?) possible cable tangle.
5. Does it reduce the amount of light that arrives at your imaging camera? (I though dichroic mirrors had some loss like the Sony translucent mirror technology they use in their A900 camera - that loses 10%).

By the way this is an excellent thread and we could be talking here about the next step forward in an imaging system which I know I am ready for.

Greg.

[PRejto (Peter)]

Originally Posted by gaston

Hi Peter,

I hope this made my comments a bit clearer.

Absolutely! I knew my question was not fully thought out. The blanks are now very well filled in! Thanks!!

Peter

[PRejto (Peter)]

"That could also be useful for me in that I find my Trius 694 being so sensitive to IR and probably UV that I get star bloat in some images from brighter stars which per the theory here on ONAG and different types of stars output points the finger at too much NIR and UV affecting my images at times.

Greg"

I just looked at the ONAG web pages. It looks like the ONAG transmits from 370nm. The problem I experience with my TEC140/Trius combination also shows up using Baader RGB filters which if memory serves correctly transmit from 380nm. I see clear improvement once I start clipping from 410nm. Thus, I would conclude (with my scope) that the ONAG would not make an improvement in UV bloat (if indeed that is what you are seeing). Now that you have sold your TEC180 are you seeing bloat with other scopes of yours? I've always felt that two issues were involved here. With a refractor there is possible blue haloing from unfocused UV, but this would vary from APO to APO depending on the lens figuration in blue. As best I can tell the Televue NP127/trius combination is immune. But general bloat could very well be due to overflow from the tiny wells. And that I would guess has little or nothing to do with UV. Anyway this is a bit OT for this thread!

Peer

[gaston (Gaston)]

Hi Greg,

I used 850nm for my calculation because it is the average value on the NIR range accessible for most guiding cameras.
This wavelength range starts around 750nm (the ONAG's cut off wavelength) and ends around 950nm, the silicon sensor upper wavelength limit.
For the same reason 550nm is usually used as the average wavelength for the visible range.
This is just a first, but good, order approximation, since for more precise values we would have to integrate (weighted average) the seeing effect with the sensor quantum efficiency versus wavelengths.
By definition the UV range starts below 400nm , down to 10nm. The lower end of most silicon sensors (CCD/CMOS) is around 300nm, which means they can see some UV light.

The ONAG reflects typcially 98% of the light from 350nm to 750nm, above 750nm the relection is less than few percents, below 350nm it is not specified. However color one shot cameras, or LRGB filters with monochrome cameras, usuly cut most of the UV, below 370nm, and NIR, above 650nm. Although the ONAG will remove most of the light energy above 750nm, you may still want o consider removing NIR above 650nm or so, as well as UV below 400nm, for the imaging camera using filters, yet as I pointed out above most cameras or filter sets will do just that.
Using wavelength above 750nm only for guiding using the ONAG will provide smoother tracking, with a marginal lost in SNR.
Having a wide FOV for guiding at, and near, the optical axis associated with a large guider chip provides a unique opportunity for using multi-star (constellation) guiding, for superior performances, which is now available in Maxim DL 6 for instance, just a beginning though.

Although the ONAG option with a large guiding camera seems expensive at first, and I do understand that, it should be compared with other set-ups, such as a good OAG and a rotator alternative (+ a guider). Good quality mechanics and optics are never cheap, especially in relatively small quantity.
Beside the ONAG offers a very large FOV, access to multi-star guiding (with a large chip guider) using one scope, and the only unique real time auto-focus solution in the market (Sharplock), which solves yet another challenge.

Having the main camera vertical is obviously unusual. Yet it may offer a more compact mechanical solution limiting the distance from the scope visual back and the imaging camera, which impacts the gravity center and reduce torque coming for lever arm.
I never experience any balance problem related to this configuration.

[gregbradley]

Hi Gaston,

Yes definite advantages.

Does the real time focusing work with any electronic focuser or only certain types? Is it compatible with the Planewave CDK electronic focuser (which is now compatible with Focus Max)?

Greg.

[alistairsam]

Hi Gaston

Thanks for the detailed explanation, helped answer a number of questions.
but can you explain the effect altitude has on the size of the patch.
You mention that a low altitude, effects of seeing are averaged leading to a larger patch but its smaller at high altitude? Is this correct?
Finally are there any applications of thr onag on a newtonian with AO? The 66mm backfocus requirement does use up a lot of backfocus in most coma correctors including the rcc1.

As for the sharplock maxim plugin, how does it connect to the guide camera when maxim is also connected? And how is it dependent on the onag? Can it not be used with any oag provided the field is flat.

Thanks

AlistaIr

[Eden (Brett)]

G'day Alistair,

Originally Posted by alistairsam

As for the sharplock maxim plugin, how does it connect to the guide camera when maxim is also connected? And how is it dependent on the onag? Can it not be used with any oag provided the field is flat.

I covered this briefly in my article. SharpLock relies on the ONAG because of how the guide star appears in guide camera through the ONAG -- approximately cross-shaped (+). When out of focus, the cross will appear wider or taller depending on which direction the focuser needs to be adjusted to restore the symmetrical cross shape.

In a ordinary OAG, the star would appear very much the same as it would in the imaging camera (hopefully round), leaving SharpLock without anything to work with.

Maxim allows 3rd party programs and plug-ins to connect to and access it's cameras. (AstroTortilla makes good use of this, as does SharpLock).

[gaston (Gaston)]

Originally Posted by alistairsam

Hi Gaston

Thanks for the detailed explanation, helped answer a number of questions.
but can you explain the effect altitude has on the size of the patch.
You mention that a low altitude, effects of seeing are averaged leading to a larger patch but its smaller at high altitude? Is this correct?
Finally are there any applications of thr onag on a newtonian with AO? The 66mm backfocus requirement does use up a lot of backfocus in most coma correctors including the rcc1.

As for the sharplock maxim plugin, how does it connect to the guide camera when maxim is also connected? And how is it dependent on the onag? Can it not be used with any oag provided the field is flat.

Thanks

AlistaIr

The isoplanatic patch is a probabilistic concept based to the all atmosphere effect (including every altitude contributions). The math behind is not trivial, but with some simplifications we can guess the general idea. To explain the concept I made a simple sketch, have a look at:

http://innovationsforesight.com/TurbulanceStructure.jpg

First we should remember that the isoplanatic patch is defined by an angle.
Now let’s, for the sake of this discussion and to make this simple, assume that the turbulence cells have all the same size everywhere in the all atmosphere above the observer. Obviously this is not true, but it does not matter much here to understand the general idea.
Inside a cell the optical properties are the same, which means the cell impacts the light traveling through it, coming from any object, the same way. Of course this assumes that all rays coming from a object are traveling through that cell. Under those assumptions the AO corrections for one object is valid for another one, the only condition is that the light from both objects have traveled through that cell indeed. On the other end, two different cells impacts in a totally different way the light, this means there is no statistical correlation between them, and AO corrections made for one does not apply for the other. This is a bit a binary simplified model, in reality there is a progressive, more or less smooth variation of correlation, from one cell to the next, which is related to the coherence length, but let’s keep it simple.
I divided the atmosphere in just three layers, low (near ground), medium, and high altitude. Since we are talking in separation angle terms here we need to translate it in distances at different altitudes.
Near ground (low) the distance separating two star lines of sight would be d-low for an angular separation theta, in my sketch this distance is about a half of a cell size (just an example of course).
For small angles (which is the case in astronomy with our telescopes) we have:
d=Z*theta

Where Z is the layer altitude, and theta the angle in radian. Therefore d-low = Z-low*theta. From this it should be clear that d increases with Z.
If I use 30’ for theta, or 0.0087 radian, and for Z-low 15m (50 feet), then d-low = 15*0.0087 = 0.13m, or 13cm, or about 5 inches.
From the sketch we should conclude that in this example the cell size is 10 inches, the size of a typical amateur scope aperture. This means that if the seeing was only a function of the low altitude cell contributions my isoplanatic patch would be defined by theta (the coherence angle) indeed, and AO corrections made with the first star (let’s say the guide star), will work just fine for the second star (let’s say my target).
But there are many layers across the all atmospheric path.
For the same angle theta at high altitude, let say 20,000 feet, or 6 km, the distance d-high = 6000*0.0087 = 52m, or about 170 feet, or 2000 inches, there are now many cells involved (200)!
This means that higher altitude layers will create seeing effects which are not correlated inside my angle theta anymore, and now their contributions cannot be corrected using an AO unit across the 10 inches scope FOV. Another way to say this is the isoplanatic angle must be much smaller than theta because the higher altitude cell contributions for the same separation angle.
Now typically, for a given altitude (layer), the all cell structure moves at the speed of the wind (V) which is larger at higher altitude (the atmosphere density drops therefore wind speeds increase with altitude).
This means that the coherence time (the time for a cell to move by its length) is usually much shorter for high altitude seeing contributions than near ground (where friction with the ground decreases the wind speed even more).
In the temporal (or frequency) component of the seeing the low altitude (ground level) contribution is mainly made of low frequencies (a long coherence time). Since the cells are close to the scope, and therefore lead to a relative large coherence angle (with some luck in the order of your scope FOV), you now have some hope for improving the near ground seeing contribution using low AO correction rates. However going too fast with the AO will chase the higher altitude seeing, related to much narrow coherence angles, therefore defeating the all idea.

Let me know if this explanation helped.



ONAG technology with Newtonian is challenging, since those scopes have typically limited back focus. We are working in a solution to eventually solve this problem though.
As Eden mentioned SharpLock (SL) for Maxim DL uses the Maxim DL COM server, therefore it will get the guide star images from Maxim DL directly. Both software applications are synchronized, you could see this as a plug-in.
SL does not work with OAG. SL assumes that the guide star shape changes in a controlled known way with the scope focus. This change must be different before and after the best focus as well, this is a critical point.
The ONAG creates such information for the guider, not an OAG. Also OAGs may face off-axis aberrations (such as coma) and field curvature making the SL concept of real time autofocus more challenging for OAGs, or self-guided cameras.

[alistairsam]

I had a play with the 685nm filter I ordered and I did see a noticeable improvement. http://www.edmundoptics.com.au/optic...s-filters/1918

My guiding with the AO at 3hz is usually 0.3 RMS but with the filter on, I was between 0.15 and 0.2 RMS.
I wouldn't say it dramatically reduced seeing effects but it did help as far as I could see. conditions last night were similar to those when I've tested previously, maybe I need to test on a more turbulent night.
the lodestar X2 did struggle a bit at 300ms but I realized I needed to use dark subtraction. once I did that, I had a decent number of stars with good SNR.
will keep looking around for the right filter.

Cheers
Alistair

[gaston (Gaston)]

Hi Alistair,

This seems about right.

With most scopes (aperture around 300mm, or 12") the main seeing contribution is tilt/tip. Roughly half of the seeing is due to the near ground layer, the other half comes from the mid to high atmosphere.
Using 300ms will freeze some of this, especially the near ground contribution.

If you would go even faster, let's say around 50Hz the improvement in FWHM, therefore guide star wander, would be around a factor 2 to 3 in the visible, or NIR, versus long exposure time (few to 10 seconds). But this improvement will only applied to the isoplanatic patch now.

Using NIR, above 700nm or so is a good strategy which does not require tracking too fast.
With your filter, you should improve (as mentioned before) your guide star seeing (wander) by up to about 60% to 70%
Which means a 0.3 rms becomes about 0.1 rms, or so, consistent with your observation I would say (considering the model incertitude and local conditions).

The turbulence theory provides a nice model, but it is just a model with its assumptions, approximations, and limitations. We should consider its prediction with some degree variability in practice (like the weather forecast).

For instant near ground the turbulence structure could be much more complex and subtle than the model. Be in a valley, or near structures (building, roads...) may impact the results quite significantly.

I understand that your filter uses plastic substrate, how good is the optical quality?

[multiweb (Marc)]

Thank you very much, Gaston, for all the illustrations and explanations in layman's terms. Fascinating stuff.

[g__day (Matthew)]

Great thread guys! An ONAG is on my Christmas list this year.

A simple question - is the oval or X shaped stars you see in the guide star camera any issue for centeroid calaculation in say PHD or MaximDL?

[gaston (Gaston)]

The ONAG guide star astigmatism is quite small at best focus. It is not a concern for auto-guiding and centroid calculation. As a mater of fact using OAG may lead to much more off axis aberrations, such as coma, yet without a significant issues on centroid, unless the aberration becomes extreme.

Fo auto-guiding the right figure of merit is the half flux diameter (HFD), not much the FWHM. It expresees how the starlight energy is spread around the central peak.
With an ONAG the guide star HFD diameters seen by the imager and guider cameras are very close, no worries.