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The Crab Nebula, OIII finally added

MartinB

By MartinB
in Imaging - Deep Sky

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MartinB

MartinB

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At Kelling 2008 I got some close up Ha data at 2500mm focal length. In Dec 08 I was intending to add some OIII but used a CLS filter by mistake. Have now had a revisit at a slightly shorter focal length and got the belated OIII with a little more Ha. I was intending to use AO but Maxim V5 didn't want to co-operate so I used the unit as a conventional off axis guider.

Scope:10" LX200 ACF

Camera: QSI 532, SX active optics being used for the F10 Ha luminence

Filters: Astronomic 13nm Ha for the Ha luminence, Baader 7nm OIII and Ha for colour and background.

Exposures: 6x15mins and 10x10mins Ha @ F10 (26th Sept and 6th Dec 2008)

4x20 mins Ha and 10x20 mins OIII @ F10 (6/7th Jan 10)

I used the F7 data as the colour layer and the F10 Ha was used as a luminosity layer. For the colour I used Ha for red and used the OIII for both the green and the blue channels. Sometimes I find this works better than creating a synthetic green from the blue and red data.

Interesting to see that the FWHM for the 2500mm data using active optics was signifcantly less that the 1750mm data from conventional guiding. AO is a battle but worth it!

I'm very happy with how it has come out.

post-12794-133877419163_thumb.jpg

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MartinB

MartinB

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Thanks everyone. Great to have the feedback:) I've got some plain luminence data (gathered by mistake) and wondered about adding that in to give some more diffuse light in the centre but I've decided I prefer the filamenty look.

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Tim

Tim

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Now thats a bit special! Superb detail in there Martin, very crisp.

When you say AO is a battle, in what way? I hope to extend my OAG use into AO one day, seems a natural progression :)

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MartinB

MartinB

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Now thats a bit special! Superb detail in there Martin, very crisp.

When you say AO is a battle, in what way? I hope to extend my OAG use into AO one day, seems a natural progression :)

Thanks Tim

The AO has been a bit of a saga for me as anyone who was in my general vicinity at Kelling in 2008 will tell you!

Problems I've had

i) The expected learning curve - off axis guiding but with the added need to find a guide star that is bright enough to allow the rapid guide cycles needed to make the AO pay.

ii) When I changed to my QSI the connection for the guide cam was too short and the standard extension tube too long. SX kindly supplied me a bespoke extension tube free of charge.

iii) The big problem, the tearing out hair and smashing fist into face at 2.00am in the morning problem. You have to calibrate the AO, this is best done on a bright star and is a one off excercise. Easy, no problem with that. You also have to calibrate the mount. The AO has a limited range of tilt, and once it reaches it's limit it needs to adjust (bump) the mount to centralise itself. It does this on the fly and, because of the speed of the guide cycle this doesn't cause any trailing. The fly in the ointment is that the AO driver doesn't allow simple dark calibration of the guider. My Lodestar has hot pixels and Maxim loves to use the hot pixels in preference to the calibration star. This is frustrating enough but worse still is when it appears to calibrate correctly but has actually used a mix of calibration star and hot pixel. Unlike the normal calibration in Maxim, the AO software doesn't show a calibration track so it isn't obvious that things have gone wrong. The result is that the mount bumps in the wrong direction. It took me a long long time and many frustrating hours and lost images to suss out what was happening.

The solution is easy. Calibrate on a nice bright star in the general vicinity of the target. Alternatively, in Maxim 4 you can calibrate the mount as if for conventional guiding and transfer the guide star co-ordinates into the AO software for tracking (I've never had to do this but I have since heard of others who do)

iv) Dodgy guide cable - this was my problem at Kelling. The mount just refused to calibrate. I couldn't get the mount to respond. Michael for SX was there and neither of us could sort out what the problem was. After a couple of wasted hours I spent the rest of the night sat in front of the computer manually bumping the mount every 20 minutes or so.

v) Maxim 5 This week I tried to use the AO for the first time since last Spring. Since then I have changed computers and also moved to Maxim v5 The driver is there but just didn't want to work. I eventually discovered that you mustn't have the guider selected in Maxim, only use the AO window. In v4 they work together. However, having sussed this the software wont run a full calibration - takes one shot and then stops. So I will be off to the Maxim Yahoo group to explore further. I've tested in Maxim 4 since I have this loaded up as well and things seem to be fine here on daylight testing.

By last spring I seemed to have things sussed and was finally having trouble free use of AO. I use the FOV indicators in The Sky 6 and they really help. I reckon you need a Mag 10 or brighter star to really make it worthwhile. It's wonderful to see it working properly. There is the sound of frantic activity coming from the unit and you think it must be vibrating the whole set up but then the images come out with tiny round stars. On M51 there is a Mag6 star to guide on an my old computer was managing 10-12 guide cycles per second, you can really start to have an effect on poor seeing at this sort of speed. My next target is a close up Horsehead, I've checked in The Sky and with things properly framed Alnitak should be smack in the middle of the guide chip! Will be interested to see what the max achievable guiding rate is.

If someone had given me the above info when I started using the AO I would have been spared so much grief! AO is definitely the way to go for long f/l imaging though.

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Tim

Tim

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Crikey, I see what you mean. Still, i'm very interested in taking that route, it seems that while the stars will be so much better, also there will be more and better focussed light delivered for the details in galaxies, edges of shock areas in nebs, that sort of thing.

What does the AO unit actually move then to keep up with the seeing? The whole camera assembly?

I have decided to take up a new career as a safecracker, the hours are short, if unsociable, and the pay is good. Shouldn't be too long before I can afford the AO unit. Now if only there was any money around in the safes in this country:p

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MartinB

MartinB

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You can always set your crow bar against tax.

The AO has an optical pane of glass which tilts. It takes 5ms to reposition itself so in theory it is capable of 200 corrections per second. Of course the real restrictions are download speed and processor capability. Once you get a find a star you can use less than 1/4 second exposures on it starts to be worthwhile but the dividends really start to come in with exposures 0.1 second or less. The lodestar is a big help. It has a bigger than average chip and is very sensitive. You just need to be wary of those hot pixels!

The good thing about the SX AO unit is that the off axis prism is set in front of any filters which helps a lot when getting a bright enough star. They have now increased the aperture to cope with large format cameras although mine is fine with a QHY8.

I've attached an M51 luminence from last year. Its just had a histogram stretch, no sharpening, no deconvolution, no star reduction. The focal length was around 1750mm

post-12794-133877419267_thumb.jpg

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barkis

barkis

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How does the unit measure the scintillation, or whatever data it needs to know in order to make the fine correction Martin?

Major observatory Adaptive systems use a laser do they not?

I'm quite sure I shall never progress to a level where I would contemplate AO. I'm just curious is all.:)

Perhaps you answered the question in your last reply to TJ. I'm not sure.

Ron.

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Tim

Tim

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Ron, there is a layer in the atmosphere, around 90km up, and a laser at certain wavelength will bounce off that and give an accurate representation of the seeing. They say for conventional AO systems, as above, the results are better in the near IR range, but that is on the VLT's that are around. I cant see us amateurs ever using a laser system, we'd probably get accused of trying to bring planes down.

If Martin has a lower FWHM figure for 2500mm than 1750mm under similar conditions, then the unit speaks for itself really :)

I would imagine that the image from the guidecam is taken, a "proper" position of the star is determined, then the glass manouvered to maintain the star in that position. Does it have it own software to view/monitor the image?

I guess that same image would have to be taken as the overall guiding star though, and at those short exposures I can image the choice being very limited, especially at long FL.

You could almost do with TWO outputs from the guider, one with very short exposures for the AO, and one with the normal 2-5 second for the mount. If you ever want to try a tandem OAG system, I have just the OAG :)

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MartinB

MartinB

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You're right Ron. Large professional scopes shine a laser into the sky and monitor the deflections caused by atmospheric effects, this feeds back to mirror controls which distort the mirror (large scope mirrors are made up of many individual panels which can be individually adjusted). This, for many people, is true adaptive optics. SBIG developed a system for amateur use using a tilting mirror. By providing very rapid corrections to guiding errors, to some extent the tracking can keep pace with the scintillations. They called it adaptive optics. SX chose to call their system active optics which I think is more appropriate.

Seeing effects are quite interesting. Some seeing effects are rapid and effect the whole field of view. There are other, slower effects which can effect a small part of the fov. John Winfield has developed a plug in for Maxim which allows guiding using a number of guide stars at the same time. Theoretically this should average out slow seeing changes resulting in more accurate guiding. I downloaded the plug in some time ago but have never got round to giving it a try, I have enough trouble during a typical imaging session as it is!

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barkis

barkis

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Thanks both. It is unlikely as I said, that I will progress to that level. Anyway, I'm still a lot of the basic stuff to digest.

I have not been able to glean much in the way of the numbers who could be using AO from the Maxim Group, but it would hardly be accurate anyway, as there is software other than Maxim which imagers are using.

How much of it caters for Active Optics though I wonder.

Anyway, I do like Maxim, I'm busy putting it to work with my 1000D, Checking Exposure times etc, and the reliability of power supply for the camera. Home made with bits I got from Psychobilly. All is good so far.

Ron.

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RichieJarvis

RichieJarvis

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Thanks for posting this image and such a detailed explanation Martin. I've often wondered about the benefits of the SX AO system, but now I can see them 'in the flesh' so to speak!

I was tempted before, now I am even more tempted! So thank you... I think!

Cheers,

Richie

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