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Stacking accuracy

Q: How can I get the sharpest images in my stack using Nebulosity? How does Nebulosity compare to other stacking tools?

Nebulosity has several means of aligning the images prior to actually stacking them. We can use simple translation, translation + rotation, translation + rotation + scaling, and Drizzle. I've covered Drizzle in an article for Astrophoto Insight, so I'll focus on the more traditional methods here.

The big difference between "translation" and "translation + rotation (+ scaling)" is that when doing a translation-only alignment, Nebulosity does not resample the image. It does "whole pixel" registration. This sounds worse than "sub-pixel" registration. Isn't it better to shift by small fractions of a pixel? Well, it would be, except for the fact that when you do so, you need to know what the image looks like shifted a fraction of a pixel. That means, you must interpolate the image and interpolation does cause a loss of sharpness. So, you're faced with a trade-off. Keep the image exactly as-is and shift it by whole pixels or resample it and shift it by fractional pixels.

Now, toss into this the fact that our long-exposure shots are already blurred by the atmosphere (and to a varying degree from frame to frame) and you've got a mess if you try to determine which is better from just thinking about it. So, we have what we call an "empirical problem." Let's get some data and test it.

I took some data I had from M57 shot with an Atik 16IC at 1800 mm of focal length and some wider-field data of M101 shot on a QHY 2Pro at 800 mm. I ran the M57 data through a number of alignments and Michael Garvin ran the M101 data through several as well.

Here are the images from M57 (click here for full-res PNG file). All were processed identically, save for the alignment / stacking technique.

Here are the images from M101 (click here for full-res PNG version). Again, all were processed identically. Here, the image has been enlarged by 2x and a high-pass filter overlay used to sharpen each (all images were on the same layer in Photoshop so the same exact sharpening was applied).

So what do we take from all this? Well, first, there's not a whole lot of difference among the methods. All seem to do about the same thing. To my eye, adding the "starfield fine tune" flag in Nebulosity helps a touch and using the resampling (adding a rotation component) hurts a touch, but these aren't huge effects. Someday, I'll beef up the resampling algorithm used in the rotation + (scale) version. Comparing Nebulosity's results with those of other programs again seems pretty much a tie. I can't pick out anything in their stacks that I don't see as well in Nebulosity's. Overall, these images seem to be limited more by the actual sharpness of the original data than by the stacking method.

IP4AP - Save yourself $$$$$

IP4AP - Image Processing for AstroPhotography

Why do my images suck? OK, let me rephrase that a bit. Why do my images suck less than they used to but still just don't have the same bang that those really good images have? Is it my gear? Is it me?

Well, like most people, I turn first to the gear. My scope, mount, or camera must be the culprit. Or, I don't have some cool new software someone else has. That's it... yea... If I had a new camera and got that new software that does the Zambizi Triple Datum-Redux method, I'd be all set. Break out the wallet, this won't be cheap. But, I'll get those better shots!

Nope. I've been in this now long enough to know that that's just not going to do it. Sure, it may help some, but it won't really do it. The problem is me, not the gear.

I was first introduced to this concept (that I'm the one that sucks, not the gear) in another context - car racing. Like every American male, I thought I knew a good bit about how to drive and that out on a track I could keep up with most anyone, so long as the car was capable. How laughably wrong I was. A number of years ago, I first tried autocrossing (aka Solo Racing) in an old Porsche 911 I had (as old as I was). I came in last. Dead last. It must be the car - these guys all had their cars prepped a lot more than I did. Yea. New tires, rework the engine... Break out the wallet, this won't be cheap.

Then I signed up for a course run by the local club (the San Diego PCA - great group) and found out I was by far the limiting factor. Others could make my car fly around the track and I was using habits I built up on the street that were just plain wrong. Was I stupid? No, just ignorant. Nobody had taught me how to really drive and how to really control a car -- how to use weight balance, friction, and your power to really go fast. I picked up a number of tricks and techniques there that got me going a lot faster and made me a better driver - on the track and off. The hobby became a lot more fun (steering with the gas pedal is still a serious blast), I wasn't so frustrated, and, well I didn't suck quite so badly. All for a nominal fee for the course.

Why this tangent? Well, it's the same thing here. I've been my worst enemy when it comes to data processing. I've had bad habits that have let to results that didn't do justice to the underlying raw data. The gear's not the rate-limiting factor. I am. Or, I was.

Enter Warren Keller and his IP4AP series of tutorials. IP4AP may get confused on the tongue with AIP4WIN, but the two products couldn't be further apart. Jim Burnell and Richard Berry are technical wizards with a great product in AIP4WIN and The Handbook of Astronomical Image Processing. If you want the geek / weenie stuff, they're your guys (and as a geek, I mean that in a flattering way). But, if you want to easily learn how to use PhotoShop (or AstroArt) by looking over an expert's shoulder, Warren's your man.

A few weeks ago, I got an early copy of his newly done / redone "Intermediate" set of tutorials. This is aimed at using Photoshop to handle things like gradients and vignetting issues, powerful use of the Curves tool, dealing with LRGB data, and use of DDP (this last one in AstroArt). Warren spends a lot of time himself with PhotoShop, a lot of time learning techniques from others, a lot of time devising his own techniques, and a lot of time teaching people these techniques. This all shows in the videos. The idea behind IP4AP is to let Warren separate the wheat from the chaff in techniques out there, distill them down, and then show you how they work and why. In the video format, you get to look over Warren's shoulder as he processes images. It's a really effective technique. While you don't get to ask questions right then and there (you can - you can e-mail him and even setup one-on-one sessions), you get to do watch them whenever you like and replay sections as often as you like to see just what he did to achieve a certain effect.

Warren can get a bit goofy at times, but it's part of his wry sense of humor and it serves a real purpose. While watching the videos, you're there to learn, but you shouldn't loose sight of the fact that this is a hobby and meant to be fun. The semi-random appearance of a picture of a cow will help to keep you in this mindset.

So, if you're reading this blog and aren't a contender for APOD (and by reading this blog, odds are pretty much 100% you're not) and if you're not trying to image by lying on your back with a point-and-shoot, holding the shutter down and manually tracking the stars (so that it's not the equipment's fault entirely), I'll make the following wager. Spending $40 on IP4AP will do more for your images than any purchase you could make for 10 times this cost. So, wanna break out the wallet and for that new scope, camera, or mount? Or want to part with a mere $40 (direct) and quite possibly (if not probably) get an even bigger effect on the images you're making? The choice is yours. I know what I'll be doing when he comes out with the next DVD in the series...