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The SteadyStar Adaptive Optics Guider
The SteadyStar Adaptive Optics Guider

A tack-sharp image with pinpoint stars is what we want and expect to see in our astro-images. Excellent tracking, guiding, and focus are all essential, but we sometimes find ourselves taking these key components for granted. As the advanced astro-imagers venture into the ever-increasing variety of longer focal length telescopes, they will find themselves taking these essential elements far less for granted. Even when every meticulous step is done "right," there are hindrances beyond our control, such as atmospheric seeing, wind gusts, or the mechanical limitations of the mount. Consequently, the ambitious plan to shoot a high resolution image of NGC 7331 in all its galactic splendor can fall short with disappointing results.

As we at Orion have progressed further into the imaging world, our demand for good tracking with longer focal length telescopes has increased. While keeping our modesty in tact, we must admit how pleased we have been with the performance/dollar of our Atlas EQ-G and Sirius EQ-G mounts. Their raw performance has been proven in the field and they have earned themselves a place as the intermediate work horse of choice for many astro-imagers. All that being said, there is only so much a mount can do when it is holding 30-40 lbs of astro-imaging gear. Long focal length systems (1600mm or longer for our purpose) are acutely sensitive to bad seeing, wind, periodic error, or an autoguiding hiccup where recovery may take several seconds. Mounts simply cannot react to those problems fast enough. By the time any of those problems occur, it is too late for that particular exposure, so get ready for a disappointing image download with oblong stars. What to do? Beef up your mount?

There are many choices for the next performance level of equatorial mounts. Doubling, tripling, or even paying seven times more for your mount will certainly get you more payload capacity, mechanical beauty, and usually better tracking. But even so, your astro-imaging potential can depend on intrinsic limitations of the mount. The inertia of the mount typically limits your autoguiding to more than one second per correction, thus you can only correct for low frequency tracking errors. Periodic error can sometimes spike faster than the autoguider and mount can correct for. As for menacing wind gusts and turbulent seeing, the mount and autoguider are all but helpless. That’s where Orion has stepped in with the need for an active optics system. The Orion SteadyStar "Adaptive Optics" Guider is a mount upgrade, without having to actually purchase a new mount. It will not increase a mount’s load capacity, but it deals with the challenges of tracking and autoguiding that go beyond the normal capability of an equatorial mount or other tracking platform. As much as we have emphasized long focal length telescopes, the SteadyStar also applies to shorter refractors on small mounts. Why upgrade to a bigger mount for your 4 inch refractor when all you need is a SteadyStar unit to turn your mount into a stellar imaging platform?

Since the launch of the SteadyStar at the end of 2009, we have added two other versions, one with a one with a larger 50mm optical window to support larger format CCD cameras, and the other with a built-in field rotator. Thus SteadyStar is no longer limited to equatorial mounts, opening up the world of alt-az telescopes to high performance imaging.

The SteadyStar applies refractive correction by tip-tilting an optical window in the light path in front of the CCD camera. With a 6mm thick glass window, it doesn’t take much tilt to shift the image plane enough to cover most tracking errors, including the periodic error on your mount. The corrections are made at four points around the optical window, driven by stepper motors. Each step in the SteadyStar deviates the light path about 2 microns. Should there be a larger correction needed that exceeds the tip-tilt range of the SteadyStar, it will send a traditional autoguide correction to the mount. In most cases, you can count on more than 95% of the corrections being dealt with entirely by SteadyStar. It’s easier to move a 2 ounce piece of glass than it is to move a 40lb telescope!

The SteadyStar unit (left). With cover removed to show the inside components (right).

Instead of making one cumbersome correction every second or longer with an autoguider, the SteadyStar can achieve 5, 10, 20, even 40 Hz by moving its fast-acting optical window. This all assumes your guide star is bright enough and your guider and PC can keep up. In most practical applications with a reasonable 9th or 10th magnitude guide star, you can sustain 10 Hz.

This fast correction can deal with poor tracking, periodic error, wind gusts, and some effects from seeing. Low frequency scintillation can actually be corrected by SteadyStar, but more importantly, it does not overcorrect the effects of seeing, so you tend to obtain better results under turbulent skies than you would with regular autoguiding. SteadyStar makes a correction within about 2 milliseconds of detecting the guide star movement. With a regular autoguider, the mount reacts to something that happened 1 or more seconds ago, typically more like 2 to 3 seconds in the past.

Optical window placed in the light path in front of the CCD camera.

After identifying that we wanted an active optics unit to add to our astro-imaging arsenal, we sought to make this accessory work universally with as many imaging CCD and DSLR cameras and budget autoguiders as was reasonably possible. We paired the ever-popular Orion StarShoot AutoGuider (SSAG) with the SteadyStar. SteadyStar uses an off-axis guiding system that requires an autoguider camera to see what is happening. The autoguider isn’t used to send corrections to the mount in this case, only to image the guide star for SteadyStar to make the actual corrections.

To make this happen, we had to squeeze out every drop of performance that an 8-bit, CMOS, uncooled autoguider could deliver. Other limitations inherent to the camera added difficulty: it does not bin 2x2 or create a RoI (region of interest) at the hardware level, and several dark frames are needed to provide consistent frames to override most of the hot pixels. After overcoming those obstacles, we found the little CMOS camera really could measure up to the task. The SSAG also proved to be quite fast, even though the RoI was only performed at the software level. We have been able to exceed 30Hz when using the SSAG with the SteadyStar—faster than many still-frame CCD cameras designed for autoguiding can reach.

NGC 7380 taken under turbulent suburban skies with the SteadyStar. SteadyStar and control software correcting at over 13 Hz using the StarShoot AutoGuider.

We are continuing to evolve the software, drivers, and firmware for SteadyStar. We have imagers using the SteadyStar around the globe with a range of cameras, from the Orion StarShoot and Parsec cameras, to SBIG, QHY, and QSI series CCD cameras. Our current goal is to fully integrate SteadyStar with MaxIm DL to enable scripts and the use of SteadyStar with automated imaging programs such as CCDAutoPilot and DC3 Dreams.

Editor’s Note: For several years I (Russ Genet) have dreamed of seeing low cost, portable, meter-class, alt-az telescopes combined with a field derotatored autoguide camera with an off-axis pickoff and tip-tilt corrector. This seemed to me to be the most economical way to give large-aperture, out in the open, alt-az telescopes the performance needed for many tasks. With John Ridgely and three students at California Polytechnic State University, I was pleased to assist Orion in the early stages of the development of the derotator/pickoff portion of SteadyStar. Working with Bryan Cogdell and others at Orion was an enjoyable and useful learning experience for all concerned.

Details
Date Taken: 03/29/2011
Author: Bryan Cogdell
Category: Astrophotography

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