You never quite know when skies are going to cooperate on any given evening. Tonight, the stars (err… solar system objects) were in alignment. Checkout these shots of this evenings conjunction involving The Moon and The Planet Mercury. I’d actually forgotten about this alignment, and had to go home to get my camera and tripod. Around 8:30pm or so, I saw the crescent moon coming into view. I knew that Mercury couldn’t be far behind, but where would it appear. A quick peak at a sky map provided by Dr. Tony over at Spaceweather.com was all I needed. Back down the road (to my less obstructed western horizon) and 15 minutes later there was the first rock sitting just below and to the left.

I took a few shots with each lens to make sure that I’d have a good enough exposure to work with. I figured the wider field of view of the first shot made for a more neighborly feel. These are our nearest celestial neighbors, after all. The narrower field of view (I’d hoped) would show a bit more Earthshine. Alas, I needed a better view to the west to allow skies to further darken to capture this more distinctly. Oh, well….

Astro Photo Details:

* Designation: Conjunction: The Moon & Planet Mercury

Astro Photo Processing Details

* Canon Digital Rebel XTi
* Image #1 - wide view 1/8sec @ f5.6 ISO400 70mm focal length
* Image #2 - narrow view 1/10sec @ f/5.6 ISO800 200mm focal length
* Adobe PhotoShop CS2 Curves & Levels
* Neat Image

We’ve all seen amazing astro photos created by ‘amateur’ astronomers. Some are on par with the finest works of professional observatories just a few years ago. What’s their secret? Well, there are a number of secrets… more than can go into a single blog post. First, there have been amazing breakthroughs in CCD technology in the last several years. Combine this with dramatic price reductions and high end astrocams are within reach of many an aspiring astrophotographer. But, guess what? The finest CCD camera on the market all by itself isn’t enough to catapult you to the top tier of the astrophotography world.

Secondly, the degree of control available to astrophotographers through the latest image processing software programs is unrivaled. With an off the shelf computer and off the shelf image processing software, aspiring astrophotographers can coax an almost unimaginable amount of detail from raw images. Additionally, many kinds of ‘mistakes’ can be cleaned up with these programs. Still, with just the latest copy of Photoshop, you won’t find yourself among the top tier of astrophotographers.

I’ve saved arguably the best and most important component of high-end astrophotography for last. Polar alignment. Without a precise polar alignment, your hopes of taking the spectacular images that’ll end up in Sky and Telescope or on the APOD simply aren’t going to happen. Sure, you can get lucky and shoot some phenomenal passing event. I know, I’ve been lucky a few times I’ll certainly take luck over skill any day. But, that said… if you want to take seriously amazing deep sky images, you must have an absolutely spot on polar alignment. Skip this step and you’re doomed to ‘nice’ shots… but not truly amazing.

Polar alignment involves adjusting your telescope’s mount such that the polar axis of the mount is pointing at the north celestial pole. Huh? Ok, its not as complicated a concept as it might sound. Let’s think this through. We all know that objects in the sky (sun, moon, stars) seem to rise in the east and set in the west. This phenomenon is caused by the Earth’s rotation. Earth is spinning on its axis and astronomical objects (not bound to the Earth) are seen to move because of this. The polar axis of your mount (when aimed properly) corresponds to the axis upon which the Earth turns as it rotates. Simple, right? When properly polar aligned, your mount only needs to make use of one of its two motors to keep the image centered. The RA (Right Ascension) gears compensate for the Earth’s rotation and keep your image centered in the eyepiece or in the camera’s field of view. If your alignment is off, the mount needs to make corrections on two axis. This is almost impossible to do with incredible precision. For visual use, this really isn’t even a critical issue. Simply eyeballing the telescope’s direction as north is ok. For lunar or planetary images you might even stop at this point. However, for deep sky images, you’ll need to go the extra mile to get your scope precisely polar aligned.

Ok, so how do we actually go about precisely polar aligning your mount? First, get the scope roughly polar aligned. Simply pointing the mount’s polar axis to the north star (Polaris) is a good first step. Next, you’ll want to make use of the ‘drift method’ of alignment. The drift method is a technique whereby you point at a couple of specific stars and watch the star drift out of the center of your eyepiece over time. Hence, the name. Adjustments are then made to the mount itself to bring you closer to alignment. This processes is repeated until there is no more drift. Note, that as you fix one axis, you can be slightly messing up the other, hence the iterative nature of this adjustment.

Azimuth Adjustment

To proceed, choose a star near the intersection of the meridian and the celestial equator. The closer the better. I’ve seen references say you should be within 1/2 hour of Right Ascension and 5* of declination. I’m not sure how these tolerances were picked, but I’ve tried my best to follow them. You’ll find that at times, this is easier said than done. Sometimes, there simply aren’t any bright stars in those areas. A goto scope and some good astronomical software will be helpful to find these points (you’ll need a pretty accurate pointing model to find these). To get the stars perfectly centered, you’ll need an illumated reticle eyepiece. I have a non-illuminated one at the moment and usually have to use a red light to help me see the lines that crisscross the eyepiece. Accuracy here is pretty important. Additionally, the higher the power your setup the better. So, either get a very small focal length eyepiece… use a barlow… or both. The higher the power, the faster you’ll notice the drift and the more closely you can watch for improvements.

For this star, you’ll only be making adjustments in azimuth. If the star drifts to the south, adjust your polar axis to the west. If the star drifts to the north, adjust the polar axis to the east.

I like to try to use a consistent time period for the drift, say 2 minutes. This way, you’ll start to get a feel for how quickly your improving things. If you drift out of the eyepiece in 2 minutes the first time, and then only 1/2 way out of the eyepiece the second time, you know that you’re making good adjustments and should have things nailed shortly.  If you use widely divergent intervals, its much harder to gauge how you’re progressing. You can still do it, but it starts to feel much more hit or miss. I usually like to get 3 or 4 iterations on this star before doing an about face.

Additional Resources:

Celestron

Jerry Lodriguss

Proper polar alignment is one of the most critical issues for deep sky astrophotographers. If your mount is not spot-on as far as alignment goes, you’re going to have a heck of a time taking the long exposures needed to snag the elusive faint fuzzies that inhabit the night sky. Sure, you can take a number of short exposures and stack them. This will only get you so far though. At some point, your signal to noise ration simply isn’t high enough to reveal the details that will allow you to separate yourself from the masses. Depending on the focal length of your telescope, a poor polar alignment might limit you to exposures in the 15-30 second range, perhaps less. Beyond this, stars will elongate and quickly begin to look like footballs, or worse, lines… not the round points of light that they should be. As your stars begin to elongate, the faintest details in your exposure will simply disappear, the faint light they emit being spread so thinly that it gets lost in the background of your image. You’ve now removed any hope of creating the spectacular image that everybody is talking about.

Polar alignment, balance, mount stability and tracking smoothness will either work for you harmoniously or work against you at every opportunity. There are things you can (and will have to) do as you progress as an astrophotographer to improve each aspect of your imaging. At some point, you’ll fix one glaring issue only to reveal the next, more subtle, layer of issues. Don’t take this as a bad thing. This is a desirable progression. Eliminating the worst defects and working to remove the lesser defects, until all that remains is a spectacular image. One that you can be proud of, and one that will make other astro-imagers jealous.

The next couple of posts will focus on what it takes to get a good polar alignment. The first will simply describe the polar alignment process in detail and walk you though it. Master this, and you’ve got alignment licked. The second, will describe how some available tools can be used to improve your alignment. Specifically, we’ll look at the Gemini GoTo System and its Polar Alignment features. Other mounts may have similar features that you can use.

Ok, I purchased a new digital camera in late September. I chose the Canon Digital Rebel XTi for a number of reasons. First up, I’d had a film version of the Rebel years ago and it was awesome. I also still had the lenses from this camera, so I was able to save a bunch of money by just getting the camera body. Additionally, the newly updated Rebel now sported over 10 mega pixels. This would (hopefully) allow me to get some large scale prints down the line. I’ve wanted a full on CCD astro-cam for years. However, even as the price of these has dropped considerably, I figured that it was better to get started with the digital slr now than wait to get the camera of my dreams later. Given how much I need to learn, the sooner I get started the better.

The first few images got me really excited. Sure, they contained a bunch of issues. Most of these initially were the result of improper alignment of the mount. I’d been using the mount for visual observations and it was aligned well enough for that, but… for longer exposure astro-photos… it just wouldn’t cut the cake. I even had a bunch of vibrations coming through the mount in into the image. (click on the image to see the two distinct times that Jupiter (roughly 90lbs) ran by the mount). But, these were (for the most part) easy things to fix. I could align the mount (we’ll discuss this in greater detail in a future post)… and I could keep my puppy Jupiter from running around on the deck while I image. I could also isolate the entire imaging system from the deck. This slow moving project will find its way into a number of subsequent posts. What really got me excited was simply the amount of raw data that I was able to capture. The image shown here was a simple 2 minute exposure with no image processing whatsoever. I was salivating just thinking about what I could do when I’d addressed the first couple of sets of issues…. and started stacking many, many 2 minute (and longer) exposures. I was also itching to get seriously under the covers with PhotoShop. I knew instantly that I’d be taking some pretty spectacular shots in the not too distant future. All I needed to do was to fix a few things here and there

Ok, you’re probably wondering… and I’m certainly dying to show off a shot that did come out nicely. The following shot was another 2 minute exposure on M42, The Orion Nebula. This was taken after I’d put Jupiter inside… and after I’d gotten myself properly situated. I made sure to take very shallow breaths so as to not vibrate the camera again Again, no processing of the image has been done on this. I simply shrunk the image a bit to save on bandwidth. A quick critique would say that this was an ‘OK’ shot. A bunch of details within the nebula were revealed, it nicely framed, but… where’d the Trapezium go? In case you’re not familiar with this, the Trapezium is a grouping of stars at the very core of the nebula. These are completely lost inside the overexposed core of the photo. This can be remedied in the future via Photoshop… though I’ll likely get a better series of photos and stack them before I get into this. And so the journey continues…