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Autoguiding: As Simple as “Push Here Dummy”? By Craig Stark, Ph.D. In his article on Starizona’s Hyperstar system ( ATT April 2007, p. 28), Scott Tucker described astrophotography as something that, at least historically, require a “certain borderline lunacy.” Astrophotography is typically not for the impatient, casual amateur, nor is it typi- cally for those without fat wallets. Take, for example, the relatively simple, unavoidable fact that the earth rotates. The result of this inescapable fact is that the stars move slowly across the sky. “Slow” is a relative term, of course, and if you use a large amount of magnification (e.g., by using a high-power eyepiece), you can see the stars race across the field of view. The solution to this is, of course, quite simple and well-known. With a single motor turning the telescope about an axis that is parallel to the earth’s axis of rotation (i.e., that points at the celestial pole), we can counteract that motion and keep the target from drifting away. For visual observation, this is all we need. It matters little if, over the course of 10 min- utes, the target rocks back and forth in the eyepiece by a half an arcminute. At 200x in a Plossl eyepiece, a half an arcminute corresponds to under 4% of the field of view. You’re losing sleep over observing, not over this slow, small, wobble of the image. To an astrophotographer, this slow, small, wobble of the image is a serious problem to lose sleep over in and of itself. If we have a camera exposing this entire time, the wobble will make the stars become lines rather than points. For example, if we image at a fairly typical image scale of 1.5 arcseconds/pixel, instead of the star being perhaps 4 pixels in diameter, it might be 4 pixels high and over 20 pixels wide. The solution to this problem is also quite well-known. If, in addition to expos- ing our main image we carefully watch a star, we can use this “guide star” to see the error the mount is making and correct for it as it happens. As the star starts to drift to one side, we send a signal to the mount to either reduce or increase the speed of the motor briefly so that the star will be pulled back to the proper position. We can do this either manually, by physically watching a star and pressing buttons (manual guiding) or automatically, by having a separate camera watch the star and send commands to the mount (auto- guiding). This sounds simple. Where’s the problem? Why doesn’t everyone guide their mounts and take pictures with nice round stars? Historically, there have been a lot of problems, hurdles, and myths that have kept guiding out of reach of many astrophotographers. For example, many are told that astrophotography cannot be done on basic, mass-produced mounts. Bigger and more precise mounts have not only less error overall, but error that is “smoother” and more easily fixed with guiding. These are also priced out of reach of many amateurs. If you must spend $3,000 to get a mount that is considered entry-level for astrophotography purposes (and still need to guide), many will shy away. Even if you have a suitable mount, you may be presented with a bewildering array of parameters. For example, you may be asked to enter in the number of arcsec- onds/pixel, the RA and Dec aggressive- ness, their hysteresis, the amount of back- lash in milliseconds, which way north is in the guide image, whether it is mirrored N- S and/or E-W, how much error to tolerate, etc. While some of these are easily deter- mined, others are not and with so many to set, the chance of user error is high and finding the source of the error can be chal- lenging. I remember not-so-fondly trying to use several systems and becoming so frus- trated that I abandoned guiding. Astronomy TECHNOLOGY TODAY 47 AUTOGUIDING Sometimes I’d hit “guide” and the star would race off 10x faster than the slow drift it had shown. Other times, the pro- gram would lock onto a hot pixel and insist my mount was perfect (funny how they don’t move) or would work for a short while and then degrade. My “favorite” night was when it was working well in RA but after about 5 minutes of joy, drift in declination began to build up and the program and I had a disagreement as to which way “north” was. Its correc- tions only made the error bigger and the star promptly shot off the chip. I can still remember the literal pain in my neck manually guiding was and despite a num- ber of valiant efforts, I never managed to produce round stars (it didn’t help that I too would forget which axes I should keep in the same layout as the buttons and which should be mirrored). I abandoned guiding in favor of the ease of unguided imaging and lived with the noisier images my shorter exposures produced, always wanting a better way. In early 2006, I had already written and released Nebulosity, a program designed to both capture and process astrophotog- raphy images. Its mantra was to be power- ful, but easy to use. I know first-hand that much of the time when doing this, I’m standing in a dark field that is either freez- ing cold or infested with mosquitoes. On top of that, I’m generally tired and it does- n’t take a cognitive psychologist or cogni- tive neuroscientist to tell you that under these conditions, you don’t work or think your best. (If you do feel you need a cog- nitive psychologist or cognitive neurosci- entist to tell you this, consider yourself so- told.) It’s at times like these that one real- ly appreciates a clean, simple, user inter- face that offloads as much as possible from the user’s brain to the computer. After one night of not being able to image as long as I’d wanted, I decided to write PHD Guiding, taking the same approach I’d taken in Nebulosity. The name comes from both its attempt to be intelligent (or at least well-informed and an expert in a particularly small domain) and to be “Push Here Dummy” simple. ScopeStu f Telescope Accessories & Hardware FEATURING ITEMS FROM: TeleGizmos Covers - Astrozap Dew Shields Dew-Not Dew Heaters - Peterson Engineering Antares - Telrad - Rigel Systems - Sky Spot Starbound Chairs - Smart Astronomy David Chandler - Lightwedge - Baader ScopeStuff Piggyback & Balance Kits Rings, Rails, Dovetails, Cables, ATM, Eyepieces, Filters, Diagonals, Adapters Green Lasers - And MUCH more! www.scopestuff.com 512-259-9778 EZ BINOC MOUNT KIT 5-axis outdoor mount handles all sized binocs. View standing, sitting or reclining. Kit contains all parts requiring welds, machining, drilling & tapping, plus specialized parts. You supply inexpen- sive pipe & fittings readily available locally. Put yourself in the driver’s seat for $99.99 PETERSON ENGINEERING CORP. TEL: (888) 712-9332 or (401) 245-6679 WWW.PETERSONENGINEERING.COM Hardware Before we cover how to autoguide with something like PHD, we need to cover the bits and pieces of hardware you need for guiding. There are three basic things you need: 1) a second camera chip, 2) a means of projecting an image onto that chip (e.g., a guide scope), and 3) some way to have your computer tell your mount which way to move. We’ll take these each in turn. Our 20th Year! Khan Scope Centre Guide Cameras The same pixels you’re using to take a long-exposure image of a DSO cannot be used for guiding. Sorry, it’s true. Once you dump the charge from the CCD wells to read off an image, that charge is gone and cannot be replaced to let the image build up more. 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Piggyback guide setup. Here, a small refractor (William Optics ZS 66 SD) with a guide camera (CCD Labs QGuide) ride atop the main imag- ing scope (Vixen R200SS). The guide scope is held in place by adjustable rings that are affixed to the main imaging scope. problem. With their Star-2000 system, Starlight Xpress used half of the lines (e.g., the odd lines) to build up an image while the other half (e.g., the even lines) were read off for guiding purposes. But, in nei- ther setup are the same pixels being used for guiding and imaging at the same time. If not using one of these systems, we need a separate guide camera. Numerous choices exist for a guide camera since the quality of the image is not paramount. That said, some cameras work better than others. In general, the best guide cameras will be a) monochrome and b) capable of true exposures of at least a second or so. It is true that you can guide using a simple webcam, but the color filter array over the sensor and the short exposures typically available on webcams conspire to place real limitations on how faint a star you can guide on. A one second exposure captures 30 times as many photons as a 1/30 sec- ond exposure and a filter designed to pass only red, green, or blue light passes less than a third as many photons as no filter. Put these together and a monochrome camera with a one second exposure cap- tures about 100 times as many photons as a webcam running at 1/30 second. While you can stack short exposures on the fly to get closer to a long exposure (and PHD Guiding does this), your’re facing a differ- ence of five star magnitudes. What this means is simply that the right choice of guide camera will make finding a suitable guide star far easier. Off-axis guiders have several advan- tages. First, you don’t need any other tele- scope since your main imaging scope feeds both cameras. Second, you don’t need to worry about things like mirror-flop or flex in the mounting of your guide scope, so you’re always sure the main camera and the guide camera stay pointed in the same direction. That said, off-axis guiders do have their own issues. First, they take up a bit of focus distance (you need to make sure you have enough inward travel in your focuser to make up for the thickness of the OAG). Second, you have to find a guide star at the focal length you’re imag- ing at and in the small part of the sky cov- ered by the pick-off prism. Difficulty with this aspect is what Guide Scopes and Off-Axis Guiders We need some way to form the image on our guide camera. One method is to use an off-axis guider (OAG) – a device that uses a small prism to pick off a por- tion of the light from your scope that is headed for an area just outside the main camera’s sensor. By directing this light to your guide camera, you can use any star that’s available here as your guide star. 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If modest pressure on your guide camera lets you move it relative to the main scope, you will be limited in how long you can expose without your stars trailing. As the mount rotates, gravity acts on the two setups and flex will result in it acting on them differently. The guide star may therefore remain stationary on the guide camera while the image in the main camera slowly drifts. Addressing this flex in my own setup let me move from 2-minute exposures to 20-minute exposures. Back in the days of film imaging and manual guiding, there used to be a rule of thumb that the guide scope should be at least half the focal length if not the same focal length as your imaging scope. This was so that with typical reticle eyepieces, you could see the motion soon enough and react accurately enough to manually guide out the mount’s error. With CCD-based autoguiding this rule can be thrown away. Computers are far better at spotting very small movements and far faster and more accurate in their reactions. With modern guide software, motions that are small fractions of a pixel can be accurately estimated. Subpixel guiding only requires that a star’s light covers several pix- els (which, even with short focal length guide scopes and large CCD pixels, can be created with a slight defocus). Suppose, for example, that a star’s light strikes four pix- els in a 2x2 grid and that the star is placed exactly at the center of this grid. Each pixel in this 2x2 grid would therefore be equally bright. Now, suppose the star moves ever so slightly to the right – a tiny fraction of a pixel to the right. As its Airy disk’s energy is now centered up/down but is slightly off center left/right on this 2x2 grid, the two pixels on the right will get more energy and be a bit brighter than the two on the left. Move it down a bit and the lower-right would get the most energy and the upper- left the least. It is this basic notion that allows us to use short focal length guide scopes. Personally, I use a 66-mm telescope with a 388-mm focal length (William Optics Zenithstar 66 SD doublet). I know many who use scopes of similar focal lengths and have even seen excellent results from an 8x50 finderscope that had been converted into a guide scope (200-mm focal length). The SBIG eFinder accessory for their STV guider is even shorter at only 100 mm! The days of very long focal length guidescopes are over. Getting Guide Signals to the Mount At this point, you’ve got an image of the stars from your guide scope on your guide camera. You still need some software 50 Astronomy TECHNOLOGY TODAY AUTOGUIDING that will capture these images and figure out what commands to send to the mount and you need some way to get those com- mands to the mount. We’ll take on this lat- ter bit first. There are two basic ways of nudging your mount during guiding. First, if you have a computerized mount, you can use the same cable used for controlling the mount from your computer (e.g., via some planetarium package) and for updating it. Generally, this is a serial (RS-232) cable or a combination of a USB ages rely on ASCOM as a result. Individual drivers are written that translate “ASCOM- speak” into each individual telescope’s dialect and other programs just need to know how to “speak ASCOM” to then end up successfully working with a wide range of telescopes. The vast majority of ASCOM drivers are free and most mounts will be able to be controlled with the drivers included in the download. Don’t expect tech support via a toll-free call, however, as ASCOM is a collection of programmers helping the community out in their spare time. For many mounts, guiding via the seri- al port can be very effective and accurate. For some mounts, guiding via the serial port (either directly or via ASCOM) is not as accurate as one would like. The reason for this can usually be traced to the small computer present in the mount and to the nature of the commands that can be sent. These small CPUs may listen for a new command once every quarter second or so, allowing them to spend most of their time Advanced Telescope Systems www.AdvancedTelescope.com Manufacturing Quality Portable and Permanent Telescope Piers serial adapter and a serial cable. When sending commands over this con- nection, your guide software either needs to know the particular dialect spoken by your mount (i.e., the specific commands needed to nudge a Meade Autostar vs. a Celestron Nexstar vs. a Losmandy Gemini, etc.) or it needs to know how to talk to something else that knows this dialect. In Windows, the ASCOM plat- form (www.ascom-standards.org) provides this intermediary and many software pack- Toll Free (877) 96-SCOPE Astronomy TECHNOLOGY TODAY 51 [ Pobierz całość w formacie PDF ] |
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