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Somnium
29-10-2015, 09:17 PM
SBIG are the major players in the amateur AO business but is it really worth it? i know a few people on the forum are using them but from what i gather about how they work, it may be difficult to get full field correcting for variable seeing. it seems that they are best suited to managing guiding and small issues within the mechanics of the mount. would multi star guiding be just as effective? interested in getting some thoughts

Peter Ward
29-10-2015, 09:53 PM
There has been some considerable discussion about AO's elsewhere on IIS ...so I'll be brief.

I have both an AO8 and AOX. They simply work. I have never seen them make images worse.

...but there are a couple of caveats.

You need a brightish guide star...which is much easier with the new SBIG FW-G systems...but can be tough, to nigh-impossible with NB filters on SBIG STX and legacy self guide cameras due a lack of bright guide stars (this is why the "Pros" create their own guide stars with sodium lasers...)

You also need "slow" seeing....i.e. the sort of seeing that doesn't turn airy disks into fuzzballs.

The best you can expect is around a 30% improvement in FWHM's and about the same with intensity gain.

As to whether they are worth the money?....it's rather like getting an extra inch of aperture for free (well OK, not free, but on a good night, for the cost of the AO )

As I said...I have two ;)

billdan
29-10-2015, 10:42 PM
Gooday Aidan,

As far as multi-star guiding goes, on Google Groups under the PHD2 forum, they have been experimenting with multi star guiding.

News is not too good, If you only had mediocre stars to guide from then multi-star was better than a single star, 20% better FHWM, however a single decent guide star was better than multi-star.

But its early days with their experiments.

Just remembered you had to use a guide scope for multi-star, cannot use OAG, not enough separation of stars

Regards
Bill

Atmos
30-10-2015, 07:09 AM
It really depends on your FOV but I would imagine that if someone was to go down the path of AO they're likely to be working at a longer FL anyway. Fundimentally, multi star guiding is there to average out stellar motion across a field so as to eliminate seeing induced motion so that you're not chasing seeing. AO by definition is to actually chase seeing in and effort to counteract it which is why it needs to run many times per second.

Somnium
30-10-2015, 07:25 AM
clearly large FOV would not benefit from AO because there is too much variability across the field, definitely talking about sub degree FOV



interesting, i haven't read about that, i will have to look into it.



i have read those threads, i was really wondering if there had been any updates from people who made the decision to buy AO, have they been happy with the purchase, do they no longer image without it?

it is interesting because FLI make high quality gear that a lot of people trust but they don't have AO. if it is such an advantage then why would they even be in the consideration set ?

Atmos
30-10-2015, 08:19 AM
Depending on your back focus requirements you could always use the Starlight version.
http://www.sxccd.com/sxv-ao-lf

Have no idea how it compares to the SBIG AOX or AO8 but they fundimenylalu work the same.

Shiraz
30-10-2015, 08:37 AM
There is no simple answer Aidan - depends on too many interacting variables. However, although the SBIG marketing guys are gung-ho on AO, their tech guys are more realistic. "Based on our experience and knowledge, AO-L and AO-7 rates help only slightly with reducing ground layer atmospheric turbulence, but are quite effective in reducing guide errors and wind buffeting"

As I see it, the big problem with AO is that the major amateur implementation is being marketed as though it offers some of the ability of the professional systems to cut through seeing - when in fact it simply cannot do so when used as recommended. It helps correct some mount errors and is likely to be effective in good seeing and with mounts that are normally used with slow guiding updates (or that have inherently lower quality). In average to poor seeing it can certainly have a detrimental input to the guiding, but whether that will over-ride the advantages from improved mount correction will depend on other factors in the guide system.

It would be interesting to see what difference AO makes in a comparison where the mount+AO is compared to mount-AO at the same update rate. There is an assumption that mounts cannot correct at high rates, but I cannot find any data that shows if that is a reasonable assumption.

Peter.M
30-10-2015, 09:13 AM
After looking at Paul's results with the 12 rc first hand. Seeing both raw subs with and without AO guiding, I'm sold. Do I care if it's cutting through seeing ? Or fixing mount corrections? Nope. Images look sharper. When you print your images to put on the wall you can no longer use statistics to guesstimate if one is better than the other. The final test for me will always be looking at an image.

Peter Ward
30-10-2015, 09:46 AM
Couldn't have put it better.

Attached is some real world data from my system captured within a few minutes of each other, using pretty good mount (a PME ) that had about +/- 2 arc sec Periodic error...the telescope is inside a Dome, hence I don't think wind buffeting or poor tracking of the mount are factors here.

I typically guide at 1-2 second intervals without AO, so would only expect 1/60th of an arc sec error between corrections.

If memory serves the AO was working at about 5Hz.....hardly a blistering rate...but clearly it is improving the intensity and FWHM of the data.

If if walks, looks, and quacks like a duck.....well...maybe it is one after all :)

Paul Haese
30-10-2015, 11:10 AM
My observations are like this:

When I first started looking into an AO I was uncertain if it would be worth the money, however I gave myself the option by buying a camera that would integrate with an AO. I don't know what other AO's by SBIG were like but the AOX seems to have the bells and whistles which comes from several generations of development.

I have good seeing often at Clayton. Most nights have good seeing and a few nights have excellent seeing and then a few nights are average seeing. I generally can tell by the charts if the seeing is going to be good on any given night. Air currents of the sea tends to produce stable seeing. Northerlies produce unstable seeing. Not that there is often any surface air movement there, but upper winds make a difference.

I purchased mounts with reliable guiding and PEC capabilities. Both PME and PMX are guiding very well now. I had some issues with getting stable guiding on the PMX and received help from members here to rectify the problem. The PMX (RC sitting on top) guides well at 2 seconds. The PME (FSQ on top) guides well at 6-10 seconds. Typical guide corrections for both mounts to maintain on the line is about 0.01-0.4". Guiding never goes over 0.4". It took a lot of tweaking on both systems to get good performance. An AO will not get good results without having guide parameters working well first in my experience. a Few other well known astrophotographers using AO's said the same to me too.

I have done a number of images with and without the AOX. I noted that images taken last year without the AOX on NGC253, whilst showing lots of detail did look a lot less sharp than recent imaging with the AOX. Seeing over the course of several nights was very similar. Both runs had nights of good seeing and nights of average seeing. The results were like chalk and cheese to my eyes.

On nights of good seeing I can see the AOX sit on 0.1 x 0.1 guiding most of the night. On nights with average seeing it will hover around 0.3 x 0.3 or slightly higher. The star moves around a lot too on bad nights and that will invariably mean data will not be as good as on good nights. That would confirm Rays opinion that an AO cannot correct bad seeing. Though I might add even on bad nights it still produces better results than using traditional guiding. Not a lot but still better. I suppose it depends on what you classify as good and bad seeing and what your location produces regularly.

With using an AOX the stellar profiles look a lot smaller and the detail looks quite sharp without any sharpening in post processing. That makes for minimal sharpening and a more natural looking image.

I don't think an AO should be seen as a panacea if you have not good guiding, PA and PEC completely sorted. It would still be struggling if a few things were not tuned properly. Precision is the order of the day here.

Even guiding at 0.5 seconds (1.7hz) produces way better images than traditional guiding. I doubt I will ever go back to traditional guiding unless I cannot find a guide star which is bright enough (so far mag 9.7 allows 0.5 second guiding with the AOX). Such has my reliance become using this guide system.

Using an AO does mean guide star selection is critical. You need a bright star to work with, especially on a system that is slow photographically. I have not been able to image with the 12" f8 any faster than 0.2" (3.6 hz) at this stage. It takes about 30-40 minutes to find a suitable guide star with the use of a rotator. Even using a field of view indicator in the SkyX (which is crap for the STXL series as they are still on the old STL indicators and have not updated them) to decide on which star might be suitable I have found I still need to go hunting for a star. However once I find one and am happy with the composition or at least satisfied with composition I can image night after night using the same star.

A rotator almost becomes mandatory for use with an AOX, especially in a remote situation.

Lastly, worth every cent I paid for it in my opinion and would go down this path again. I like the results I have been getting.

Somnium
30-10-2015, 12:25 PM
Great write up guys, definitely the most comprehensive and definitive information i have been able to read on the topic. Thanks for the input

Bassnut
30-10-2015, 04:47 PM
Geez Paul, you can edit the FOV indicator to suit STXL. Once you have the FOV indicator right, you pick a suitable star in Sky, rotate to it and its there. Not 30min...., 3 min.

Paul Haese
30-10-2015, 04:54 PM
Is that all you got out of my write up Freddo? :P That I don't know how to adjust the field of view indicator. :) As it turns out I did not know I could adjust it. Its a huge programme.

Bassnut
30-10-2015, 05:26 PM
I did like you write up Paul, it was very good. I'm torn with AO and just can't comment definitively, it's also been done to death. I have experiences that are bad and good,it so much depends on so many factors it's hard to know where to start. Suffice to say that megadata makes AO consideration and many other factors mute, or at least seen in a different way.

Paul Haese
30-10-2015, 06:05 PM
Yep there is always pros and cons. I think though the pros are more numerous.