Sunday, July 16, 2017
Issue #544: To PEC or not to PEC
|Me and my trusty Ultima 8 PEC circa 1995...|
If you read the last installment of the good old Astro Blog, you know I am a proponent of shorter (60 – 120-second) sub-frame exposures for astrophotography. When appropriate. Like from a light polluted site. There are times when you want to go longer, to 300-second or 600-second or longer sub-frames, however. Say when you are at your dark site and want to pick up as many details in the target object as possible.
Some of you, especially cheapskates like me who use inexpensive mounts like my Celestron Advanced VX German equatorial mount (GEM), are afraid of longer exposures. How can you break the 300-second barrier without getting trailed stars? There are ways to do that fairly easily. Good polar alignment is one. Spending time tweaking the settings in your auto-guide program is another. One other thing that is often overlooked and unused, but which can maybe get you the last step on the road to longer subs, is PEC.
Yeah, PEC, aka “PPEC.” You know what that is doncha? You don’t? Well, PEC, “periodic error correction,” was an idea that came out of the early 1990s, when microprocessors and memory chips not only came down in price, but began to appear in telescopes. I don’t know who originally came up with the idea, but it was a good one.
In those days, we amateurs were still guiding manually. You’d monitor a guide star in the guide scope or off axis guider with a cross-hair reticle eyepiece. When the star wandered away from the center due to the inevitable “periodic” errors in the mount’s gears, or drifted north or south due to polar alignment error, you pushed a button on the hand control (which we still called a hand “paddle”) to move that pesky star back to the cross-hairs. You did this over the duration of a long deep sky photographic exposure, which in the days of film was likely at least half an hour.
The good idea represented by PEC was this: What if there were a way to record your button pushes? Record your corrections for that periodic error and play them back? That would, if nothing else, make guiding easier. Thanks to the recurring—periodic—nature of the gear error in worm gear sets, which is what most of us were using by then, it sounded like PEC could indeed work.
Again, I am not sure who came up with the PEC idea, but the first people to implement it in a commercial telescope were Celestron, the old American based (though Swiss owned) Celestron out of Torrance, California. They brought PEC to the market initially with their top of the line C8 Schmidt Cassegrain, the Ultima 8, in a new version of the telescope, the Ultima 8 PEC.
It just so happened that not long after the final and best version of that telescope was released, the 9-volt battery powered Ultima 8 (PEC), I was in the market for a slightly upscale SCT. Well, as upscale as penny-pinching me ever goes. By the spring of 1995, I finally had an Ultima of my own, and after buying a few rather expensive (I thought) accessories like a declination drive motor (optional in those days) and a counterweight and rail for this fork mount scope, I undertook to do some deep sky imaging. I was curious to see if PEC really took the pain out of guiding.
|Phd2 guiding graph: VX with auto-guiding + PPEC...|
What I found was that it helped. You certainly could not expect to go unguided with the C8 at 1500mm (at f/6.3) for over a minute or two—not long enough to accomplish much with film—but it did make guiding less arduous. You still had to watch the guide star, but if you did a good PEC recording, you would usually be OK if you looked away from the reticle for a moment or your attention wandered. The periodic error was still there, but its magnitude was lessened.
Celestron’s PEC implementation was certainly not the end of manual guiding. Naturally, it only recorded your east-west corrections. Any errors in declination are not periodic; they are due to polar alignment error or seeing, not periodic error. Too bad I was always rather lazy about polar alignment in those days. That meant I still had to monitor the guide star attentively.
Also, Celestron’s version of PEC left a little to be desired. Mainly because you had to do a brand-new recording every single night. It was a shame you couldn’t save an especially good PEC track for future use. Turn off the scope at the end of the evening, and your PEC recording disappeared into the ozone.
Nevertheless, I used PEC to the end of the film days, and it did improve my photos, no doubt about that. But when electronic cameras and auto-guiding came in, I forgot all about PEC. Yes, there had been improvements in it—Meade and, finally, Celestron had equipped their mounts with PPEC, permanent periodic error correction. With PPEC, your recording was preserved through power cycles. But why worry with PEC since you had a guide camera and a computer watching that guide star now?
There was also an old wives’ tale making the rounds. That PEC and an auto-guider would FIGHT each other. That the auto guider and PEC would conflict, one wanting to correct this much and the other that much; one wanting to go in this direction, and the other in that. Some of those old wives were pretty sharp, but this particular tale doesn’t really make a heck of a lot of sense when you think about it. Nevertheless, for a while it was the conventional wisdom astrophotographers embraced. I just sort of accepted it—when I thought about PPEC at all, which was seldom. I was auto-guiding and that always seemed to be enough.
|M13: 300-seconds at 900mm f/l...|
Well, I thought it was enough, anyway. Auto-guiding with my Advanced VX GEM with my QHY-5L II camera and PHD2 resulted in an RMS error around 2” usually. That was good enough for my most used telescopes, my 80mm and 120mm f/7 ED refractors and my DSLRs. There were some occasional R.A. spikes, though, and at times the error would climb above 2”.
What could I do to improve on that error figure? One thing I’d avoided doing since buying the VX: spending some time fine-tuning those blasted PHD2 brain settings, the settings in the program that modify the guiding algorithm. I’d pretty much left them on their defaults other than just increasing the guide-step size to accommodate my fast 50mm guide scope.
So, I finally buckled down and took one whole night where I did absolutely nothing but tweak PHD settings. The result? My guiding was now smoother. The spikes were gone, and I rarely had an error above 2”; usually it was 1.5” or just a bit more. The elimination of the spikes meant I could now do longer exposures without having to throw out more than a few subs.
And then I got to thinking. Why should I stop there? If I could get that error just a little lower, it would make imaging with my Edge 800 (at f/7) easier. But how to do that? I’d worked hard on those PHD settings, and had also begun using Sharpcap’s polar alignment tool in lieu of the less accurate ASPA polar alignment in the NexStar HC—that really brought the declination error down.
What else could I do, though? Well, how about PPEC? If it helped A-P’s fancy mounts, which come with factory recorded PPEC to lower their already impressive error figures, why couldn’t it help my plebian (like me) mount?
I decided to find out. Luckily, we were undergoing a strangely cloud-free pause in the usual summer evening thunderstorms. My VX along with my 120mm SkyWatcher ED refractor, Hermione, was still set up in the backyard, polar aligned, goto aligned (hibernated), and ready to rock following the night of my Yearly M13. Since the evening didn’t look that good for imaging—haze and also unsteady seeing in advance of the next storm front—why not devote it to PEC? After all, I’d spent a whole evening getting my PHD2 settings tuned to the VX.
OK, so how exactly would I make a latter-day PEC recording? I’d taken a brief look at the short set of instructions in the VX manual and concluded there really wasn’t much to it. The process wasn’t much different from back in the day except for the fact that PHD2 Guiding would be doing the “button pushing” and not me (thank God). Since I guide via the mount’s auto-guide port with an ST-4 cable, the set up was simple. No need to worry about ASCOM or anything like that.
While the book instructs you to use a bright star for guiding when PEC recording, there’s no reason for that with today’s sensitive guide cameras. I pointed the scope at the field of M57 and there were dozens of good guide star candidates. Some folks will also tell you that for best PEC results you need to make the recording using a star near the Celestial Equator. That is also untrue. It really doesn’t make any difference.
Step one, it appeared, was “indexing,” allowing the mount’s PEC routine to find the worm gear “index,” a marked point on the worm that is picked up by a sensor. For PEC to work, the hand control has to match the gear to the recording every time you use PEC, the gear and the recording must be synchronized. Indexing does that. Anyhow, I selected PEC in the Utility menu, and hit enter. The mount then indexed, which only took a second or two (if the worm has to rotate far to bring the index mark to the sensor, the mount will move slightly). Time to record.
Over at the PC, I started PHD2 guiding on my pre-selected guide star. I gave it a little while to settle down, returned to the mount, and hit “record.” That began the ten-minute process of making the actual PPEC recording. Unlike the old days, there wasn’t anything for me to do. Assured PHD2 was guiding with its usual alacrity, I headed for the den to cool off from the hot and muggy backyard.
The rest was rather anti-climactic. I returned outside ten minutes later, and could see from the HC that the recording was done. Just like with a tape recorder, you don’t just record, you play back. I selected the playback function and let her rip, beginning a 600 second exposure of M57 with the main camera, my Canon 400D.
What were the results? Error wise, they weren’t like night and day, but there was a difference. As above, my normal error level with good PHD2 settings and a good polar alignment had been around 1.5”. With PEC playback on, the RMS R.A. and declination error declined to around 1” to about 1.10” with the actual R.A. error almost always well under 1”. While I was slightly out of focus with the main telescope, the stars in the ten-minute exposure were decently round, as were the stars in a 300” exposure of M13 despite the fact that it had crossed the Meridian and the mount was not balanced properly in R.A. for imaging in the west. My verdict? Recording PEC had been nearly effortless and certainly worth it for a noticeable improvement.
Over the last several months, my easy improvements: PHD settings, polar alignment, and PEC have taken my AVX mount from an average total guiding error of 2 – 2.5” to 1”. While I was getting by before, I am certainly doing better now. And the fact that my guiding is now consistently smooth with no excursions mean I am much, much more able to undertake longer exposures, 300-seconds and above, when appropriate. My sense is that as long as balance is reasonable, the mount will guide at the above error level for as long as I want to go.
So, am I at the end of my improvements? Maybe, and maybe not. I could certainly leave the AVX alone now. But there is one further “easy” improvement I could essay. My single PEC recording improved the error figure. But averaging several runs and uploading that resulting smoothed curve to the mount might make it even better.
Celestron still offers a free program to do that, “Pec Tool” (even though it hasn’t been updated or publicized in a long time). I may be reaching the point of diminishing returns with the VX—1” RMS error is pretty good for a mount in this class—but it might be worthwhile to take this one last step. I’ll let you know how it goes if I decide to do that (ain’t broke/don’t fix it) and if clear skies ever return this summer. It is, yes, raining hard now.
Sunday, July 09, 2017
Issue #543: My Yearly M13 (from the backyard…)
One of my traditions is that each year, sometime over the course of the summer, I take a picture of star cluster Messier 13. Why? Well, it’s tradition as Tevye said. But it also ensures I get out at least once during the hot, humid, hazy, and usually stormy Gulf Coast summer and take a few deep sky pictures.
I’ll admit these days I am not sanguine about braving sweat and mosquito bites trying to get images from skies that look like milk. If I lay off until fall, however, I get out of practice. And as complex an endeavor as deep sky astrophotography is, you do not want to get out of practice.
Usually, I do my portrait of the Great Globular in Hercules from my dark site in the wilds of northwestern Mobile County. Not this year. With June already segueing into July and hurricane season threatening to get started in earnest, I thought I’d better get my M as soon as possible. The conditions were just lousy, though. So lousy that I had no intention of loading a ton of gear and driving half an hour to the dark site only to sit under clouds hoping for sucker holes while providing dinner for hordes of six-legged fiends. The good, old, backyard it would be.
Can you get decent photos of deep sky objects from the backyard? Yes, you can, and not just of the brighter objects, either. You’ll notice in the shots here that M13’s little “companion,” the near 12th magnitude galaxy NGC 6207, shows up readily and even gives up its nebulous disk. M13 itself and similar bright clusters are really no challenge. But whether you’re trying easy or hard from the back forty, what will lead to success is the understanding that imaging the deep sky from brighter skies is a battle.
This battle is between the target object and the bright background. While it is much easier to pull a washed-out object out of the light pollution today with electronic cameras and digital processing, it’s still best to minimize light pollution induced background brightness to the extent you can.
|Pac Man Nebula with "Imaging" LPR filter...|
One thing you can do to accomplish that is use a relatively slow telescope. Why? Have you ever tried a wide-field image from light pollution? If you have, you know it’s pretty hopeless. After little more than a minute—or maybe even less—the image appears to be of the daytime sky. Most (fixed focal length) camera lenses are so fast, f/2 or faster, that the background blows out in a hurry, before many details in the object you are wanting to image are recorded. So, slow it down. I like f/6 or, better, f/7 from the backyard.
How about filters? I’ve tried them, mild “imaging” LPR (light pollution reduction) filters, and it’s a mixed bag. I do find them helpful in capturing fainter nebulae. A filter allowed me to get a respectable image of the Pac Man Nebula from my yard on a not so good evening. There is a penalty, however—color shift. While the nebula was easy enough to color balance, when it was just right the stars were a distinct reddish hue due to the presence of the filter. On the other hand, I was able to get a better picture of the Pacmeister by far with than without the filter. I use a filter only when there is no alternative.
In the interest of keeping the background glow a little lower and not burning out—overexposing—the cores of globulars and similar objects with bright centers, I generally set my DSLR’s ISO no higher than 800. That is more than adequate to bring home faint nebulosity, and in addition to keeping the background less overpowering, it reduces the noise in my frames. Stacked ISO 800 frames are visibly less noisy than stacked ISO 1600 ones.
The big question, though? How long should your subframe exposures be and how many should you take? The latter is easy to answer: “As many as possible.” Each additional subframe added to a stack decreases noise and makes processing easier. Certainly, you shouldn’t keep exposing when the object reaches problem areas like the Meridian (for some mounts) and the horizon (for all mounts). But the more good subs you can get the better the results will be. Don’t be shy about throwing out poor subframes, of course—ones with trailed stars or aircraft or satellite intrusions. If you take lots of subs, it won’t be as painful if you have to delete a few.
How long should the individual exposures be? That’s harder. Longer exposures pick up more details and are less noisy than shorter ones. Remember, no matter how many frames you stack, no details not present in a single subframe will be visible in the finished, stacked image. So, the basic requisite is that you must expose long enough for desired details to be visible in individual frames.
|In a 1-minute exposure the background is brown...|
At a dark site, go as long as necessary, or as long as you and your mount can stand it exposure wise. In the backyard, though, you will be limited. Expose for much over a minute or two and the sky background will become incredibly bright and color shifted as in the picture below, a two-sub 300 second exposure with my f/7 120mm ED refractor, Celestron AVX mount, and Canon 400D. Processing can bring back a passable final result, especially when it comes to darkening the background, but fixing the light pollution caused color shift is a more serious and difficult problem.
As you can see in my final 300-second x two subs picture in the comparison shot below, M13 is noticeably (too) blue. I got the background unreddened using the “background color offset” function in Nebulosity, but that left M13 with a cool tinge. That can be fixed as well, but it takes more work and more skill.
While the 300-second sub picture shows more stars, frankly I think the 60-second x 10 image actually looks better. 60-seconds isn’t long, no, but NGC 6207 is just as visible in the shorter sub-stack. It was also much easier to process with a less bright background and not as much color shift (the background was more on the order of brown than red).
Conclusion? In a light polluted backyard, shorter, more numerous subs are often better, or at least easier to process, than longer subs no matter how many longer subs you take. What your exposure limit should be depends on the degree of light pollution and the current sky conditions.
For me, 300-seconds is a good subframe exposure on a dark(er), dry winter night when I have a zenith limiting magnitude of 5.0 or so. On a spring or summer evening when humidity scatters light pollution, 1 – 2-minute subs are what I do. On this summer’s night, ten 60-second subs were definitely preferable two two 300-second subs. And more 60-second subs would have been better still. So why did I stop with ten? Ah, on that hangs the short tale of this annum’s M13…
|300-seconds and the background is a bright pink-red...|
As July came in, the question became not “When will I get M13?” but “Will I get M13 at all?” There had been precious few opportunities to take deep sky pictures all spring long. And not that many this past winter, either. Summer was thus far shaping up to be as bad if not worse. So, when Accuweather’s Astronomy Forecast on the web and my Scope Nights and Clear Sky Chart apps on the iPhone began to look slightly favorable, I got my rig set up in the backyard tout suite despite temperatures climbing well past 90 (try “feels like 101F”) and high humidity.
Said rig? My SkyWatcher 120ED refractor, Miss Hermione Granger, Celestron AVX GEM, and old Canon 400D. Why was I using the lighter mount rather than the Celestron CGEM? I was a wimp. An astro-wimp. I couldn’t face the prospect of lugging the 40-pound plus CGEM head out into the backyard in the heat.
By the time I finished cabling up everything—camera to computer, mount to computer, guide scope to computer, shutter control cable to camera, dew heater, mount power cord, hand control, etc., etc. etc.—I was wet with sweat and just this side of being overheated. Seeing as how it doesn’t get dark till way past 8:30 in these days of daylight savings time, however, I had sufficient time to cool off before starting the run.
When the stars finally began to wink on, I got the VX polar aligned. As I mentioned some time ago, I no longer use Celestron’s All Star Polar Alignment routine (in the hand control) to do my polar align. I find Sharpcap’s polar alignment tool, which uses the guide scope and guide camera is easier and more effective. My declination error with a Sharpcap polar alignment is noticeably lower than it ever was with ASPA, even given two ASPA iterations.
|60x10 (top) and 300 x 2 (bottom)...|
When I was satisfied with my composition, I switched to PHD2 and got its guiding calibration out of the way, clicking on a bright, but not too bright field star. PHD2 calibrated readily, and when that was done began guiding. I always give the auto-guiding a few minutes to settle down, and, so, walked back inside to enjoy the cool for a few minutes. Returning outside, looking at PHD2 revealed the RMS guiding was about 1.5” or lower, more than good enough for my 900mm focal length refractor and APS-C size chip. That being the case, I returned to Nebulosity, and instructed it to take 25 60-second exposures.
A great thing about Nebulosity and PHD2? They are rock solid. If I wanted, I could have just sat inside and let them do their thing without me. I got bored with channel surfing however, and returned to the laptop on the deck before long. PHD2 was guiding great, and the frames coming up on Nebulosity looked good. I noted little NGC 6207 immediately. All was well. Until...
Just as I began to wonder whether I should go back to the den and see if there were something good on Netflix, my iPhone just about gave me a heart attack with its alert tone. The issue? “A line of severe thunderstorms is headed your way.” Rut-roh, Raggy! Looking to the west, I realized that what I’d thought was distant fireworks was actually lightning.
Hmmm. Should I wait and see? I’d only accumulated ten subframes so far. Unfortunately, the phone insisted the weather would arrive by 11:45, and it was already past 11:30. Deciding discretion was the better part of valor, I turned off the AVX, covered Hermione and the mount with my Telegizmos cover (recommended), disconnected the computer, and scurried inside.
I was a little miffed, but back in the blessedly cool den, I realized that out in the heat and humidity I had begun to get dehydrated without realizing it, so mesmerized by PHD2’s tracking graph I had been. I re-hydrated with a Gatorade and called it a night. I was tired enough that I didn’t even deign to look at the year’s M13 on the laptop.
|My yearly M13 2017...|
Next morning, I stacked and processed my shots—which I thought were pretty pleasing and far from the worst annual M13 I’ve ever done—and strategized about the coming night. The storm had come and the storm had gone, so I would be able to get out for a second summer night in a row (!) it seemed.
What would I do? I had two things to accomplish. First, I wanted to take some longer subs of M13, 300-second subs, for the comparison above. I also wanted to do a little experimenting with the PEC function on the AVX, something I had not previously gotten around to despite having owned the mount for four freaking years.
And so, I hit the backyard once again. My experience with PEC and long(er) subs on the AVX? That, my friends, is a subject for next week.