Uncle Rod's Astro Blog

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 12^th 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.

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.

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…

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)...

That out of the way, I used Celestron’s StarSense camera to do the mount's goto alignment, sent the scope to Vega so I could focus up, powered on the camera and, at the PC, started Stellarium and StellariumScope, PHD2 Guiding, and Nebulosity (my camera control program; I always tether my DSLR to the laptop). Focusing was a snap with a Bahtinov mask and the full screen display furnished by Nebulosity. I went on to Neb’s fine-focus tool, too, and noted that seeing was OK but not great, surprising for a humid summer night. Focus done, I sent the mount to M13, centering the cluster in 2-second exposures using ASCOM’s little onscreen HC.

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.

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.