No.2 - Getting Started in EAA, Part I

I can't count how many times I've heard some variation of the questions: "What exactly is EAA?", and then the inevitable follow-up, "Well, how do I get started?" In the first blog issue last month, I tried to give my perspective on what distinguishes EAA from both  serious ( i.e. multi-hour ) astrophotography, and more casual AP,  sometimes called "AP-lite". Short answer: Both serious and casual AP involve post-processing saved data or images, while EAA doesn't.

 It's also interesting to note that a lot of people starting out with EAA just want a nice picture that they can share with their friends. To these folks it doesn't make a difference whether the picture is a straight capture of an EAA live stack,  or if they spent a little time post-processing to make it appear more aesthetically pleasing and/or to bring out hidden details not visible in the original capture.  The point here is that if your goal is to get a "good" final image, rather than just saving an accurate reflection of your EAA live stack, you are probably heading in the direction of casual imaging rather than EAA. Nothing wrong with that, but it's a different perspective....

This blog may not hold as much interest for those interested primarily in AP-lite or casual imaging, as I'm not going to touch upon the  post-processing tools that are available to help folks create a great final image from a saved EAA capture.  My intent is to keep it simple, and  just stick to whatever processing can be done on-the-fly while your camera live stacks EAA captures in real time. As part of the EAA software discussion, I'll certainly talk about the basic tools that are available to EAAers for that on-the-fly processing, but I won't delve into post-processing - there are plenty of other places to get info on that. One thing to keep in mind is that as EAA software has improved over time, it's certainly possible to capture attractive images purely with EAA and no post-processing. I expect that trend to continue as EAA software continues to improve.

Expectations and EAA Targets 

In the last issue I included examples of EAA captures of a couple of DSOs - the globular cluster Omega Centauri, and the Horsehead Nebula, a dark nebula silhouetted against a bright background emission nebula. EAA is good at showing most of the DSOs that visual observers may struggle with, because modern astro cameras are much more sensitive than either the human eye or pre-digital camera sensors, such as photographic emulsions. Even under severe light pollution (LP) a modern astro camera can be used to show targets that may be invisible to visual observers under the same sky. However, it's important to realize that there is no free lunch here - under darker skies (less LP), a given camera/scope combination  will typically show more detail in many targets than an identical total exposure using that same camera/scope under more light polluted skies. This applies to many (but not all targets), a point I'll discuss a little later. 

 

NGC6946 (Bortle 8); 15 x 15s

As an illustration of the effects of LP, take a look at this almost 4 minute EAA capture of the Fireworks galaxy (NGC6946) from a few years ago. It was taken under heavy light pollution (Bortle 8 on the Bortle scale of 1-9) from my back yard in a NYC suburb when I lived in NJ. Some of the spiral arms are faintly visible in the image (on the left) from my C8 reduced to f/4, using an old, but sensitive, mono CCD camera, a Starlight Xpress Lodestar X2M. Visually, I couldn't see any of  NGC6946's spiral arms  from my back yard, and so EAA can indeed bring into view deep sky objects that are invisible under heavy LP without a camera.

NGC6946 (Bortle 4); 12 x 15s

Moving forward a few years,  I took the 3 minute capture to the right using the same camera and scope, but under Bortle 4 skies from my back yard in Georgia. What a  difference going from Bortle 8 to Bortle 4 makes: The spiral arms are clearly more pronounced in the image, and the fainter spiral arm to the northwest (approximately the 2 o'clock position), that is almost invisible in the Bortle 8 image can be better seen in the Bortle 4 image. Unfortunately, LP is one factor (like the weather) that is beyond an EAAer's control, and it doesn't matter whether you are using an older CCD sensor (like the Lodestar X2M) or a newer CMOS camera - LP can still be a killer.

You might also notice from comparing the Bortle 4 and Bortle 8 images that stars appear much less affected by LP than extended objects, like a galaxy's spiral arms. The background sky glow from heavy light pollution has a more severe effect on the visibility of faint extended objects, like nebulosity, than it does on point sources like stars when doing EAA. Although the faintest stars (relative to the sky background) will be lost under heavy LP, the loss is not as severe as it is for visual observation where software can't be used to subtract a bright sky background. If you live under strong light pollution, or the moon is getting close to full, then EAA views of open star clusters or globular clusters will be much less affected by LP than faint nebulosity. Star clusters are a good EAA target under such conditions. 

What do you do if you really want to use EAA to view nebulae or faint galaxies, but suffer severe light pollution? That is a good discussion topic for another post, but in short, if you want to look at emission nebulae (like the Horsehead Nebula) or planetary nebulae, where most of the signal from the target is in a narrow discrete part of the electromagnetic spectrum, usually the Hydrogen alpha (H-alpha) or  Oxygen III (OIII) emission lines, then a narrowband filter can help a lot. These filters will allow through only specific emission lines, like H-alpha or OIII, and reject everything else, including most, but not all, light pollution. This allows a target with strong emission in H-alpha or OIII to literally shine through.  For example, the Horsehead Nebula shown in the last blog post was taken through a ZWO Duo narrowband filter, and the dominant H-alpha signal that comes through in that image was a result of the relatively narrowband of transmission around the H-alpha line, and the suppression of LP across the spectrum.  Unfortunately, narrowband filters don't work their magic on broadband targets like galaxies, or reflection nebulae. They only work if the target selectively emits most of its signal at discrete wavelengths, like Hydrogen alpha. In a later post I'll try to address what other types of filters can help with galaxies. 

In the next blog, I'll start to cover equipment choices - think cameras, mounts, telescopes - to consider when getting started in EAA, and how those choices might differ from someone just doing visual, or hard core astrophotography.