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Old 02-09-2013, 03:55 PM
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Autoguiding using NIR

This is an interesting article. You can reduce the effects of seeing by using NIR in this on axis guider.

http://www.innovationsforesight.com/NIRGuiding.htm

I am wondering if you put a filter to allow only NIR through on your guide camera would it have a similar effect. I think I'll try that out and post the results.

Greg.
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Old 02-09-2013, 04:15 PM
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Hi Greg,

That would be very interesting! I have the ONAG. There is somewhat less light to guide on in the IR. Your test would be very interesting if you can do a comparison by somehow using the exact same guide exposure and comparing sub frames for FWHM. I suppose, however, if you had to use even longer guide exposures and got better FWHM that would also constitute proof of some sort. Looking forward to seeing your results!

Peter
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Old 02-09-2013, 04:25 PM
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I might be showing my ignorance here but this seems silly. I understand that the NIR signal is not going to be refracted by the atmosphere as much as conventional OAG stars which could lead to better tracking. But surely the signal that you are collecting is not NIR therefore it is going to move more like a conventional OAG star and therefore be blurred. AO attempts to mimic this movement in fast iterations to combat the seeing. What they are suggesting is that having bad seeing is going to make the mount make corrections that are not actually star movement, which is why we use longer exposures to average out the seeing.

Following the center of a blurred star really well is still going to take an image of a blurred star.
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Old 02-09-2013, 05:10 PM
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I call shenanigans on the two example images on the bottom of page:

* The OAG image has trailed stars (many people have imaged successfully at low altitude with an OAG and achieved round stars).

* The OAG image is brighter overall than the ONAG image - they're not identical exposures. (Looking at the faint non-saturated stars, you can see that the OAG stars are brighter than the ONAG despite being trailed.)

* The ONAG image appears ridiculously sharp - as if it were imaged very high in the sky. You'd also expect a bit of blurriness in an L exposure due to low altitude refraction varying between wavelengths. Upon looking closer, you can see sharpening/deconvolution artefacts in the bright stars (dark spots in the centre), but the OAG image looks like it has only been screen stretched.

It's a very dishonest comparison...

Last edited by naskies; 02-09-2013 at 05:30 PM. Reason: "at low altitude"
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Old 02-09-2013, 05:27 PM
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Quote:
Originally Posted by naskies View Post
I call shenanigans on the two example images on the bottom of page:

* The OAG image has trailed stars (many people have imaged successfully with an OAG and achieved round stars).


It's a very dishonest comparison...
I too picked up on this too, if you're going to make a fair comparison at least get the OAG unit to guide properly
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Old 02-09-2013, 05:48 PM
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Yes I thought the example was pretty extreme. But the point remains that traditional autoguiders in poor seeing can easily chase the seeing. So a correction is issued to the mount only because the seeing shifted the star not PE from the mount.

It may mean less seeing related corrections and more corrections that are PE based. Its not the guide exposures I am interested in its the more accurate tracking in poorer seeing for the imaging camera that I am interested in. It could be hogwash hence the test.

Anyways. I'll conduct a test and post the results. I have to get a filter first.

Greg.
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Old 02-09-2013, 06:52 PM
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Quote:
Originally Posted by gregbradley View Post
But the point remains that traditional autoguiders in poor seeing can easily chase the seeing. So a correction is issued to the mount only because the seeing shifted the star not PE from the mount.

It may mean less seeing related corrections and more corrections that are PE based. Its not the guide exposures I am interested in its the more accurate tracking in poorer seeing for the imaging camera that I am interested in. It could be hogwash hence the test.
It would be interesting to see how this compares with adaptive optics, i.e. chasing the seeing with the fast AO plate and driving the (slow) mount to correct major periodic error.

Quote:
I'll conduct a test and post the results. I have to get a filter first.
I look forward to seeing your results.
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Old 02-09-2013, 11:29 PM
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ONAG and M83

I was with Dr. Motta when we imaged M83, very low near the horizon for testing purpose.
In each cases (OAG and ONAG) the adaptive optic unit AO-L from SBIG was used.
Exposures were the same, likely stretching was different, but beside there is not post-processing, such as deconvolution as suggested. To be precise those images are not taken exactly at the same time. However they were both very low on the horizon to maximum seeing problems. The light travel has traveled a large chunk of the atmosphere leading to bad seeing anyway.
It is fair to say that there was no seeing monitor to record the exact values, therefore seeing may have been somewhere different for both images, but it was below average in any case.
I like to point out that the images posted in our website are in the JPEG format with medium compression for download speed. Standard JPEG compression creates some inevitable artifacts which could be seen sometime on the background (noisy patterns), also some stars may look like having a darker center, or a strange shape. This has nothing to do with any post-processing.
Attached a zoomed section (bit map) of the M83 ONAG XT image (as downloaded), you can clearly see that stars does not exhibit dark centers.
Guiding with NIR decreases seeing effects on the guider camera but not on the imager camera. However the auto-guiding algorithm (depending of the settings used) may start "chasing" the seeing. Since the isoplanatic patch is quite small, it is very likely that guide star seeing is not correlated with the target image (much wider FOV).
Therefore correcting for the seeing will make matter worse, excepted in a vicinity of the guide star (10" at most). This will be great if you try to resolve close binary stars, but for usual targets it does not work, it makes things worse.
AO does not work outside the isoplastic patch FOV, they are too slow to correct for seeing as well. You need at least a 100Hz correction rate to do so, way faster that you can afford in real situations (units not fast enough, guide stars not bright enough). AO unit could improve the quality of tracking when mounts are not able to deliver the expected performances.
Auto-guiding is always a compromise. In a perfect world we should use long guide star exposures to average out seeing and correct slow mount (an other) drifts. However a lot of mounts exhibit short term ("high" frequency) errors, beside the PE, and require correction every few seconds, or less. At this rate seeing effect on guide star becomes more an issue and NIR guiding will help (even a 23% reduction).
Dr Motta's telescope uses a large fork mount. Although it is truly a beautiful piece of equipment it is hard to expect his mount to behave like high end ones.
In his set-up the OA-L is used as an image stabilizer device (correction rate around 3Hz). AO units have a limited range of correction. When near their limits, it is typical to defer bigger corrections to the mount itself ("bump" the mount), until the AO is re-centered.
In the OAG M83 image we may have experienced some artifacts (trails) from "bumping" the mount, since the AO-L faced a more challenging seeing.

I do understand that our comparison is not a perfect science, there are a lot of parameters to account for when auto-guiding especially with AO unit in the loop. But I do believe it is a fair enough demonstration of the benefice of NIR guiding. It is quite possible touchy differences in the system and software settings may have improved the OAG image as well, but in this case they were not needed, and I think it is a valid point for itself.
Attached Thumbnails
Click for full-size image (M83_ONAG_Zoom.jpg)
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Old 02-09-2013, 11:46 PM
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Thanks for posting and clearing that up. A very informative post.

Greg.
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Old 03-09-2013, 01:34 AM
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Ao

To be clear in my previous post and comments on AO I was referencing to amateur AO units, not to professional ones.
Although if some units in the market claim they can reach correction rates as large as 50Hz, in reality it is not enough for seeing compensation and sledom possible unless you have a very bright star (or laser made artificial star) in your scope FOV and for that matter a large aperture scope.
Even then isoplanatic patch size will limit seeing corrections to at most 10" or so near the guide star. Further away you start to make things worse.
Above 1 meter scope aperture tilt/tip seeing correction is not enough, and you need to locally change the scope mirror to account for wavefront deformation across the all aperture.
In my opinion amateur AO units should be called image stabilizers instead, this will be much closer to the reality.

Seeing is reduced in NIR. 23% or so does not seem a lot but extreme deviations from average become less frequent. This is not a linear effect either.

Most mounts are difficult to correct fast accurately, due to mechanical limitations (such as inertia) and/or communication protocol time lags and quantization effects (from legacy or poor designs, the right setting does matter here).
Problem is most mounts exhibit "fast", some time erratic, (one second time scale) mechanical errors. Those could be large enough to impact the image quality and require "fast" correction too, which is difficult to do with guide star seeing on your way.

AO will help in such case and reducing guide star seeing is always a good idea. Algorithms and processing can do so much, raw data quality always will matter.

At the end auto-guiding and other astro-photography tricks are challenging tasks, it is where science and "art" meet. Human experience does matter and this is not a bad thing, at least from my point of view:-)
The ONAG(R) technology has been designed to improve the process but certainly does not solve and fix all the issues alone.

Our customers feedback talked for themselves, however if there is somebody here who wants to design and conduct an experiment in a even more scientific matter I would be very happy to help in such quest.

I am always opened for criticism, constructive comments, suggestion, ideas, ..., for improving products and end user experiences, nothing is perfect. After all we all do share the same passion.
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Old 03-09-2013, 07:02 AM
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Hi Gaston,

Thanks for taking the time to explain and clarify - an interesting read and much appreciated. The ONAG is definitely an interesting technology, and I know there's at least a couple of satisfied users of it on IIS.
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Old 03-09-2013, 11:21 AM
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Onag

Hello Dave,

Thank you for your feedback,

I have to say that our NIR seeing comparison could have been more sophisticated and completed. Dr. Motta did this test at the end of a two long nights testing the new ONAG XT while challenging it to its limits (see his comments on our website). We did not sleep much and I am still recovering from those, it was quite exhausting:-).
Also I have to give him credit for the images and support. Both are one single frame and therefore could be subject to some variations. His scope is quite an amazing piece of equipment I hope to have more opportunities to do more imaging with such a beast.
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