Starlight Xpress SXV AO-LF Large Format Adaptive Optics

[KevinCornish]

Hello,

I am looking to guide an Lx200 12" with a starlight Express lodestar and noticed the AO from SX.

I read that it uses a tip tilt mirror but does it use the lodestar to measure or has it got another way of measuring position change? If it uses the lodestar would it not only be as fast as a lodestar?

Any thoughts welcome.

Thank you

Kevin

[NickK]

Not sure you'd call it a mirror, basically the plate performs minor tracking adjustments in the same way a mount does for larger tracking adjustments.

A fully loaded mount can't move at the same speed as the AO plate in relationship to small changes in tracking. This allows the image to be maintained on the CCD more accurately than just a guided mount.

Real deforming mirror active optics use a very different mechanism split the image into the sensor for sensing (wave front sensing) and then deform the mirror for the feedback using voice coils. These deal with the atmosphere "boiling".

[ollypenrice]

Can't the OA systems give corrections as frequently as 8 per second? That compares with guiding corrections of one to four seconds common in normal guiding.

Looking at forum results and at two visiting AO systems here, they do allow the long FL instruments to beat some of the seeing. I think they might be a bit fickle but they certainly improve things.

Olly

[MartinB]

You do need the lodestar to do the measuring of guide error.

With the AO system your speed is limited to the slowest link which certainly won't be the speed of movement of the mirror which takes 5msecs to readjust. You are only using a tiny part of the lodestar chip, typically a 9x9 box.

Normally your number of corrections is guide star limited however last year I was able to use Alnitak when doing a horse head image. I was averaging 12hz which I guess is the top speed of my system. Once you stat to get above 6hz the benefit of AO really starts to become apparent.

What sort of mount are you using?

[KevinCornish]

I have the giant field tripod and fork mount that came with the LX200 and have read that the PEC can be quite bad.

I want to start some deep sky imaging and think the OTA is up to the job.

I want to use a lodestar to train out the pec error and then the Lodestar and AO to improve it.

If i understand the SX-OA device correctly the lodestar will control the AO and the mount at the same time? I wonder how it would be connected in that configeration?

Also if i do progess to another OTA I can take the SX-OA with me.

Thanks

Kevin

[MartinB]

You will need to have the LX200 on a wedge for the AO to to work. It wont work in Alt Az. The RA PE is only relevant when the scope is mounted equatorially.

The AO connects to the computer via a serial connection and the Lodestar via USB. The AO also connects directly to ST4 guide port on the mount. SX supply software to control the AO device however Maxim also has an SXAO driver and I prefer to use Maxim. Lpdestar picks up the guidestar and the data is passed via usb connection to the computer software. This calculates the star's centroid and then passes a correction to the AO via the serial connection. The amount of tilt of the AO is limited. Once it is approaching it's limit a signal is sent to the mount to apply a pre determined "bump" This enables the AO glass to be centred again.

AO is great for long f/l imaging but there is an extra layer of complexity to using it compared with conventional guiding. The other challenge is finding a bright enough guide star to make it worthwhile. Best used with a focal reducer. It isn't an easy solution and I would recommend getting to grips with conventional guiding before attempting.

[KevinCornish]

Will the lodestar work with an alt-azi mount with the guider input on the lx200?

Kevin

[MartinB]

You can't auto guide a mount in Alt az mode. For long exposure images with a fork mounted scope you need to use a wedge to tilt the base to match your latitude. You must use the handset to select equatorial mode. In this mode the RA axis will track at sidereal rate. You also need to polar align.

Here is an old set up of mine using a wedge

http://www.astropixels.co.uk/equipment/NS8.jpg

[KevinCornish]

Thanks Martin I am in the process of obtaining an ultra wedge

Kevin

[MartinB]

Let us know how you get on. Here is a link to my first pic using an SX AO unit. I was using a Celestron 8" Nexstar on a wedge.

M27 - The Dumbell Nebula

[Paxo]

The other thing to bear in mind is the size of the unit on the imaging train. With the AO then perhaps a filter whel, then a camera, this may physically hit the bottom of the forks when trying toimage near the zenith for example.

I use the LF-AO with my 12" but I have it GEM mounted now instead of fork mounted, as Martin has said, the system works very well optically.

Steve

[KevinCornish]

Hi Paxo

Did you take the OTA off the fork your self? How easy would it be to put back on?

Kevin

[Paxo]

It was relatively straightforward, but not something I would do on a regular basis. There are several internet based guides which were useful.

Steve