Quote:
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
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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.