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What say I lay off the nostalgia for awhile? Oh, rest assured, you’ll eventually get more of the exciting adventures of Li’l Rod, Wayne Lee, and Miss Jitter, but not right now. Go to the old memory well too many times, and you start dredging up more bad than good. Most of my memories of my early days as an amateur are good ones, but, like nearly everybody else’s, my early adolescent days had plenty of downs in addition to ups. Time to depart-off and let some more good memories rise to the surface.
So, then, what’s the subject for this morning? One I’ve covered before. But additionally one I keep getting questions about. Numerous questions. And it’s a confusing subject. What I’m a-talking about, brothers and sisters, is autoguiding. What hardware do you need to purchase? How about software? How do you hook everything up? How within the H-E-double-L do you get it all work?
You are a sopping wet behind the ears newbie and ain’t got a clue what I’m occurring about? Well, listen up, and your old Uncle will edumacate you. Let’s say you wish to take a picture of the night sky, of the deep sky. You want an in depth close-up of a nebula, a galaxy, or a star cluster. In other words, you need to take pictures through your telescope, not just through the lens of a camera riding “piggyback?on your telescope. What does that require?
Back when I was a youngun, I used to see advertisements for stuff like guide scopes and drive correctors in Sky and Scope, but for the life of me I couldn’t work out what you probably did with the stuff. In the event you wanted to take a picture of, say, M42, you just mounted a camera in prime focus position in your telescope, plugged within the mount’s drive (no battery clock drives in them days) and let her rip. Open the shutter and come back in a half-hour or whatever. Sure, I knew you did need a great polar alignment, but that was it, right?
That’s what I assumed till I got my first C8, a Celestron Orange Tube, and decided to get serious about deep sky picture taking. With careful polar alignment and balancing, the C8’s drive could maybe have delivered OK 30-second images. I let it go for 30-minutes while I watched the latest episode of Mork and Mindy. After i developed my picture, I got, yep, STAR TRAILS. Bad ones. What the?!
Slightly asking around at the club and a little reading of the few books on astrophotography I could find ‘splained what was occurring. While I knew poor polar alignment could make the stars trail because of “declination drift,?I hadn’t heard about something called “periodic error,?which, it appeared, was the cause of my star trails.
The thing is, campers, most drive gears are usually not perfect, not even close. Take a gear, size it to where it’s appropriate for a normal, portable amateur mounting, drop the value to something acceptable to most of us, and also you darn sure are gonna have imperfections. The gears will do an exquisite job for visual observing, but will stumble in relation to imaging. Nothing is more demanding of a mount and its drive train than deep sky imaging. The smallest bump, the smallest irregularity in the gears, will cause the mount to hurry up or slow down a small amount.
That’s periodic error; it’s “periodic?because it repeats every time the bad spot on the gear rotates back around. The effect in the camera is a slow east-west back and forth of the target. In modern drives this deviation from proper tracking is usually lower than 30 arc seconds, lower than the diameter of Jupiter.
But that is enough. Even that small amount of periodic error will cause stars to trail in an extended exposure. They may at best be oblong rather than perfectly round, and there’s not much tolerance for that on the part of most astrophotographers. Even just slightly off-round stars look horrible to most of us. What to do? You have three choices: live with it, throw money at it, or guide.
The simplest solution, and one that’s maybe recommended for beginning imagers is “live with it.?Almost any current mount, even an humble CG5, can track for a good length of time without error. If the mount can expose for 30-seconds to 1-minute at reasonable focal lengths (say, less than 1000mm) and produce good stars, you may get started.
I discovered my CG5 was capable of do good 30-second exposures at 800mm of telescope focal length most of the time. Yes, I’d get occasional frames where the stars would be oval or worse, but I just threw those pictures out. The rest I “stacked?in the computer to supply the equivalent of 1 long exposure. I could capture just about any deep sky object I wanted on this fashion with my C8, CG5, and Meade DSI camera.
I was pleased with this “unguided imaging?solution while I used to be learning (finally) the difficult art of CCDing. Before long, though, as amateur astronomers always do, I wanted More Better Gooder. What’s the issue with stacking unguided short exposures? For one thing, I got uninterested in throwing out 1/3rd to 1/4th of my images. I needed to spend more time with each target than I should have. There was also the noise problem.
Every CCD camera produces a certain amount of noise due to varied electronic gremlins. Some varieties of noise are more prevalent in shorter exposures than in longer ones. While stacking multiple frames together tends to scale back this electronic noise, the result isn’t quite as noise-free as longer exposures . This was especially problematical with my DSI. As much as I liked and still just like the infant-shot color camera, it was by nature noisier than its more expensive and (especially) monochrome cousins.
What then? I could have gone the “throw money at it?route. If you’re willing to shed 10k dollars to your mount, you possibly can begin to do longer unguided exposures. Maybe several minutes. My problem with this approach? In addition to my naturally stingy—err?”thrifty”—nature, living where I do, it’s impossible for me to have an observatory and permanently mounted telescope. If you are going to spend a lot on a telescope mounting for unguided imaging, you actually do need to have the polar alignment dialed in in addition to possible and the telescope precisely balanced so as to maximise the mount’s unguided potential. Having to set up on the club dark site every time I wanted to take pictures does not lend itself to either of this stuff.
That left autoguiding. I was no stranger to guiding per se. In my second go-round with deep sky astrophotography back within the late 80s and early 90s, I guided loads of pictures. That was well before the advent of the autoguider—for amateurs anyway. Which meant that for the duration of an exposure, which with film was usually ½ to one hour, I had to squint through a crosshair eyepiece and keep a dim star centered. That blamed star would wander, and I’d push a button on my hand paddle to place it back in the crosshairs. Not fun.
Things are different now. Theoretically, at least, you must be capable of guide without tearing your hair out and assuming the old astrophotographer’s 1000-yard stare. Guiding has been automated. Computerized, actually, like everything else. A CCD camera watches the guide star instead of you. It pipes its images to a computer program that can tell when the star moves and issues guiding commands to the telescope drive to move it back where it oughta be.
That is way-oversimplifying autoguiding, after all. In the actual world there are just so many things to go wrong: camera, guide scope, cables, computer programs, etc., etc., etc. It is possible to conquer the assorted demons that may most assuredly bedevil you, but before you possibly can begin troubleshooting/tuning your system, you must have a system, and you must make some decisions before you get one.
The primary of which is “Guide scope or off axis guider??There are just a few cameras that may guide and image at the identical time through the identical telescope. But generally you’re going to need to either provide a separate telescope for a separate “guide camera,?or a technique of picking off a few of the main telescope’s light and sending it to that guide camera.
The simplest solution is a separate guide scope mounted to, riding piggyback on, the large scope. The usual choice is a small refractor, a 60mm to 80mm rig, maybe. Back in the times of manual, visual guiding, it was necessary to use as high a magnification as possible to make sure accuracy. You had to be able to note the tiniest “excursions?of the guide star. That is no longer necessary. A CCD camera can do perfectly well with a brief focal length guide scope like a short Tube 80 refractor or one of the ubiquitous 66mm ED lens-scopes.
Naturally, you’ll need to mount the guide scope to the main scope, and the way you do that is critical. You want “as sturdy as possible.?If there’s any potential for flexure in its piggyback mounting, the guide scope may move slightly over the course of an exposure. If the guide scope moves independently of the imaging telescope, that can cause stars to trail within the image regardless of how well the guiding actually worked. “As sturdy as possible?is the reply.
Sometimes the fault isn’t with the guide scope, but with the principle scope. If you’re an SCT fan, you may have probably heard of “mirror flop.?Since the telescope’s mirror moves back and forth to focus and isn’t securely mounted, there’s the likelihood it might move a bit of during an exposure. That can, like guide scope mounting flexure, cause the stars to trail, since, in essence, the main scope has moved independently of the guide scope—its image has, anyway.
Solution? Some modern SCTs, like the Meade ACFs and the Celestron Edge HDs, have mirror locks to fasten down the primary mirror, preventing flop. Your CAT ain’t got ‘em? Some mechanically inclined imagers have fabricated their very own locks, usually with bolts threaded through the rear cell. That’s not an option for the all-thumbs brigade headed by your old Uncle, though.
Actually, I’ve rarely had problems with mirror flop over time, since I’m careful to do one thing and avoid another. I always finish focus “uphill,?counterclockwise, which leaves the mirror in a stable position. I’m also careful not to image anything crossing the meridian, for the reason that change within the telescope’s attitude when tracking across the Local Meridian is when flop often happens. If you’d like to make certain of eliminating mirror flop, no ifs ands or buts, there is only one real fix, however.
The off-axis guider, the “OAG,?doesn’t just totally eliminate mirror flop in SCTs, it eliminates the necessity for a separate guide scope. How? It does so by stealing just a little light from the sting of the CAT’s field. An off axis guider is a really specialized prime focus camera adapter. Along with allowing you to attach your camera to the scope, it features a star diagonal-like setup. There’s a focuser tube into which either an eyepiece or a guide camera might be inserted, and there is a diagonal mirror, identical to a star diagonal. The difference is that it’s a really small mirror, and extends only into the very edge of the sunshine cone.
This mirror, the “pickoff?mirror, grabs a little of the main scope’s light and allows the stars around the very edge of the field to be delivered to an eyepiece or camera. Since it’s very small, the pickoff mirror only shows a few stars. With a purpose to locate a superb guide star, the OAG is constructed so the mirror may be rotated around the edge of the sphere. Usually, but not always, an appropriate star might be found this manner. Some pickoff mirrors could be extended a bit of farther into the sphere within the quest for stars, as well. Since it’s small and at the edge, the mirror’s shadow isn’t likely to show up in the sphere of the imaging camera.
How does the OAG eliminate mirror flop? Actually, it doesn’t. But it does allow the guide camera to see star movement attributable to mirror movement. If the guider sees the star move, whether on account of periodic error or flop, it’ll issue the suitable guide command to follow it.
I used an OAG for years, and got some good pictures, but it wasn’t pleasant. The OAG’s irritants are twofold. First, it is a pain to locate a very good guide star when you’re restricted to the small choice offered by the small mirror. Often, I’d must compromise framing of my target within the imaging camera by moving the scope to bring an excellent star in. Today’s guide cameras are way more sensitive than my eyes, though, so that isn’t as much of an issue as it used to be. Nevertheless, the guide scope is still the winner here, since it may be moved in its rings to search for more stars, and the guide camera is, of course, taking in the full field of the guide scope, not just the sting.
One thing that is still an issue with OAGs is the shape of the sphere edge stars. Many scopes, particularly SCTs and faster focal ratio refractors and Newtonians, deliver misshapen stars at the sector edge due to coma and/or field curvature. A star that appears like a blob or seagull could also be impossible to guide on. Field-flattener lenses or coma correctors can assist, and with some telescopes may be a necessity.
One last potential downcheck? The OAG comes between camera and telescope. While an OAG can potentially be used on Newtonians and refractors, those scopes may not have enough focus range to accommodate one; especially if a focal reducer is being used. The camera will be placed too far out to come back to focus.
All in all? I prefer the guide scope. It’s less painful to get going. Alternatively, once you have located a superb guide star, the off-axis guider is able to delivering better results. The Celestron f/6.3 reducer corrector I exploit with my DSLR for imaging delivers pretty good edge-of-field stars, so I’m thinking I’d dig out my old OAG (if I can find it), the subsequent time I’ve a hankering for some deep sky snapshots.
There’s a third option, too: “OAG without the OAG.?Santa Barbara Instrument Group, SBIG, the famous CCD maker, offers cameras with built-in guide cameras. There’s a guide chip arranged right at the sting of the imaging chip, and this guide chip, like the OAG, picks up stars at the edge of the field. No OAG is needed, and the very sensitive nature of the guider in these cameras means finding a star is rarely a problem.
The main limitation with Santa Barbara’s “self-guiding?cameras is edge of field star shape. I’ve had good luck imaging with my ST-2000 CCD camera with the f/6.3 reducer; not so good, variable anyway, when using Meade’s f/3.3 reducer with the C8. If you are within the marketplace for a new imaging camera, the SBIGs with built-in guiders most assuredly deserve your serious consideration. No guiding setup is closer to plug and play.
Beginners tend to shy away from SBIG’s self-guided cameras for a number of reasons: cost, perceived complexity, they simply want to use the family DSLR for astrophotography, etc. If, for whatever reason, you don’t want an SBIG ST, you will want a separate guide camera in addition to your imaging cam (self-guiding is proprietary to SBIG).
One thing I urge is that you simply not scrimp. Some new imagers waste precious time and reduce their hairlines by playing around with webcams. One can work with the suitable software, like the wonderful freeware Metaguide, but only in case you are lucky enough to have a nice, bright guide star in the sphere. Which isn’t the case. Unless modified, a webcam cannot expose for longer than 1/30 second. Even semi-webcams just like the Meade LPI or Celestron NexImage that can expose for longer are not sensitive enough for straightforward guide star acquisition and good guiding.
In the event you need to save lots of bucks, almost any CCD camera can work. Many last-generation cameras may be had for a song on Astromart, and if you will discover guiding software that supports your camera of choice, you might be in like Flynn. One caveat: eschew really old cameras that use a serial or parallel interface instead of USB. You will have a really hard time getting them going for a number of reasons, including that modern PCs have neither parallel nor serial ports. A great choice of used camera in my opinion is without doubt one of the (now not produced) Meade DSIs. I, II, III, it really don’t matter. All are sensitive enough to make good guide cams.
Don’t have an old DSI lying around and don’t want to buy used? Today, there are numerous cameras sold specifically to be used as guiders. Many of ‘em come from the identical factories in China, and all are basically similar. Lots of us are using the Starshoot guide cams from Orion. Their attractions are that they are quite inexpensive—the base model is lower than 300 bucks—and are equipped with ST-4 guide outputs (more on that later). The Orion guide cameras do have the deficiency of using CMOS instead of CCD chips, and are less sensitive than the Meade DSIs and similar cameras. Nevertheless, I’ve always been in a position to locate a suitable guide star in any field with my Starshoot.
There’s another type of guide camera you could also be curious about; especially should you don’t like toting a computer in the field, the self-contained guiders. They don’t require a PC to operate; they have a small hand unit that takes care of all the computer work. The old SBIG ST-4 and STV cameras, long since discontinued, used this all-in-one concept, but for the longest time, there didn’t seem much call for it. For those who were using a CCD camera, you’d need a computer in the field anyway, so why not just let it handle the guiding, too?
Self contained “solitaire?guiders are back in a giant way. Things changed with the appearance of the DSLR for astrophotography. A few of us run our DSLRs with laptop computers, just like we do CCDs. But some DSLR imagers eschew PCs and use a simple remote shutter release, counting on (increasingly good) DSLR video displays for framing and focusing. These folks don’t need a PC for imaging, so it’s wonderful for them to be able to dispense with one for guiding, too. If this sounds good, look on the Orion Starshoot Solitaire, the LVI Smartguider II, the SBIG SG-4, and the Celestron NexGuide.
If you aren’t using a solitaire guider, you will operate the guide camera with software running on a pc. There are numerous CCD software packages that can autoguide. Most of the highest-of-the-line imaging programs, like MaximDL and CCDsoft guide as well as image. Be certain these programs will interface with your guide cam before you pull out your wallet, though. And make sure you need this expensive software. In case you are using a DSLR, you probably don’t.
Whether you run a DSLR or a CCD, one autoguiding program is head and shoulders above everything else, and, believe it or not, it is free: Stark Labs?PHD Guiding. “PHD?in this context don’t mean “Doctor of Philosophy,?it means “Push Here Dummy.?And that’s the truth. Hook everything up, push a few buttons, and the puppy just LOCKS ON. Hell, my first night of imaging with PHD, I had set up EQMOD wrong. My Atlas mount was tracking on the wrong speed because of this. PHD said “so what??and kept the guide star within the crosshairs anyway.
PHD isn’t for imaging, only guiding, so you’ll need a separate program on your imaging camera. While PHD works great with Stark Labs inexpensive (and great) imaging program, Nebulosity, it is just as happy alongside every other imaging program.
Another freebie that should be investigated is the aforementioned Metaguide. Not only will it do a superb job of autoguiding, it should make it easier to precisely collimate your telescope with the help of a webcam. The only problem is that it’s limited within the cameras it will possibly use, being restricted to webcams and video cameras at this time.
Hokay, you done got a guide camera, a telescope mount, and a PC (maybe). How do you hook it all together? This is where a complete lotta folks get awful confusticated. They plunk down mount, camera, and computer and start pluggin?in cables where they think they need to go and are dismayed when nothing in any respect (good) happens.
The same old first mistake is that they run a serial cable from the laptop’s serial port (more like a USB ?serial converter today) to the autoguide port on the mount. Sounds logical. You’re guiding with the pc, you run a computer cable to the mount and plug it in—where else?—to the jack labeled “autoguider.?That’s fine, aside from the fact that it won’t ever work.
The problem is that the autoguider port don’t speak computerese. More on what it does speak in a moment, but for now, let’s address “serial guiding.?On this “Way One?type of guiding, commands to move the scope are sent over the RS-232 serial interface. How do you make that work?
Very first thing you do is plug the camera’s USB cable into a USB jack on the pc. If this is the primary time, follow the camera-manual’s instructions religiously relating to installing software drivers and suchlike. This connection will convey the photographs your guide camera is taking, and it may even carry data and commands to and from the computer.
Now the sticky part: that serial cable. In case you intend to guide via a serial cable, you don’t connect it to the autoguide port. You connect with the serial port on the telescope mount, usually on the base of the hand controller. You also don’t use the guide cable that came with the guide camera, if one came with it. You connect using the serial cable designed to your mount. The identical one you utilize to send the scope on go-tos via a planetarium program running on the laptop.
In serial guiding, when the computer needs to move the mount to follow the guide star, it sends a computer command not much different from what gets sent from a planetarium program to move the telescope. Don’t worry about congestion on the RS-232 freeway in case you intend to make use of your planetarium software while imaging. Cartes du Ciel (for example) and PHD Guiding (for instance) will coexist peacefully. When you’re guiding, not an excessive amount of is occurring with the planetarium software, anyhow.
Naturally, before you may begin guiding through the serial port, you may have to inform the software that’s what you intend to do. In PHD, that involves using the (free) ASCOM telescope interface software. Once ASCOM is installed, it’s not much of a hassle. Just select your telescope from the “chooser?that pops up when you begin the connection process.
Some imagers don’t like to guide serially. There are quite a lot of reasons for that. One is that some think the autoguide port on the mount is healthier, that there is less of a time-lag between issuing guide commands and telescope response. That may be, but I don’t think there’s enough difference to make much difference. I’ve used both serial and autoguide port (“ST-4? guiding and have never noticed much difference. It is true, however, that if the mount and camera both both have autoguide ports, it can save you a bit cable tangle by eliminating one wire going to the PC.
What do you need for ST-4 “Way Two?guiding? In addition to an autoguide port on the mount, you need the appropriate cable. That is not a serial cable, but an ST-4 compatible cable. What’s an ST-4? That was SBIG’s famous early imager/guider. One among its breakthroughs was that it inaugurated a simple relay/switch closure guiding system. Yep. What the autoguide port on the mount understands is not fancy computer commands, but simple switch closures. Nowadays, the switches/relays are virtual, electronic switches, but what’s happening continues to be the straightforward opening and closing of connections. Similar to pushing east/west/north/south buttons on the HC.
So, to get started you will have an ST-4 type cable, which is wired differently from a serial one. If the guide camera features an autoguide output, you were likely provided with an ST-4 cable. How do you hook up for ST-4 guiding? If the camera has an ST-4 output, it’s simple. Plug the ST-4 cable into that and into the autoguide port on the mount. You will still need to attach a USB cable between the camera and computer for image downloading and camera control, of course. What do you tell the guiding software? In PHD, select “on camera?for the guiding interface. You do not need and won’t use ASCOM for autoguide port/ST-4 guiding.
There’s also a Way Three. Let’s say you want to use your mount’s autoguide port, but are using a camera, like a DSI, without an ST-4 guide output. Does that make ST-4 guiding impossible? No, it doesn’t, thanks to somewhat company called “Shoestring Astronomy.?They provide something called the “GPUSB.?It is a small module that plugs into USB ports. It allows the guide software to send ST-4 switch closure commands out a USB port and down an ST-4 cable that plugs into the mount’s autoguide port as per normal. Assuming the software will accommodate a GPUSB, you’ve got to tell it about it. In PHD, just choose “GPUSB?on the “mount?menu.
Is there a Way Four? Yes there may be you probably have a solitaire style autoguider. The exact connections will likely be peculiar to the particular guide camera, so read the manual, but there’ll be a cable (or cables) between camera and the solitaire’s handbox. More often than not there’ll even be a traditional ST-4 output either on the camera or on the handbox to connect to the autoguide port on the mount. Read the instructions, though, since these systems tend to be more involved than simple PC-operated guidecams.
So that’s all there is to guiding, huh? Not hardly. We’ve Only Just Begun. We’ve selected a camera and got all of it hooked up. But that’s when them dadgummed gremlins rear their heads, if you actually start trying to make use of the junk. Exterminating those buggers and getting everything working in optimum fashion is somewhat long story, though, and it will have to be in a “Part II,?which I’ll provide you with-all Real Soon Now.
Next Time: By the time you read this I will probably be back from the 2010 Almost Heaven Star Party within the hills of West Virginia. It’s one of many country’s great star parties, muchachos, and you will have to listen to all about it and you’ll bet your bippy I’ll tell you about it next Sunday.
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