As one might imagine, taking a picture of a galaxy on the other side of the observable universe takes some planning. A LOT of planning. The whole shebang starts with one simple question: What do I want to take a picture of?
Choosing a Target
Choosing what I want to shoot is possibly the hardest step in the whole process of astrophotography. I get the chance to go to a dark site maybe three or four times a year. This means that I need to make the few trips I do have count. I’ll take you on the journey of planning an image for the Golden State Star Party (GSSP). GSSP is in late July this year, so the first thing I need to do is find out what the sky will look like in late July at GSSP. To do this I have a free program called Stellarium. Stellarium is basically a map of the sky. I can tell it a time and place, and it will show me what the night sky will be like. Here’s what the sky will look like at GSSP around 10pm.
That yellow line going from North to South is called the Meridian. I’ll get into why it’s a problem later.
OK, so we now know what the sky’s going to be like. This is where a good base of astronomy knowledge comes in. The thing on the left is the Milky Way. In fact, that bright patch in the south is the center of the Milky Way. This is great news for us, because that means more targets. See, all nebulae and star clusters lie inside the Milky Way. If the Milky Way is rotated so that we are looking in to it, we will naturally see more of the galaxy, and thus more targets.
So now we know we want to be shooting in the Milky Way. This presents a problem. Remember that Meridian line? The meridian line is the line where my Equatorial mount ceases to function properly. I can track an object up to the line (starting from the left) but once I reach that line, the tracking will go bonkers. This is because as I approach the line, the counterweight bar becomes more and more horizontal. One I pass the line, the counterweight bar is rotated into the air, with the scope hanging underneath it. This is bad. While the scope is fastened safely, and wont fall, it will hit the tripod leg eventually. Scopes hitting tripod legs make unhappy astronomers.
Luckily, there’s an “easy” fix. Once I’m near the line I can stop the tracking and flip the ra axis and dec axis 180deg. This will in theory put me back to exactly where I was, just on the other side of the meridian line nice and safe. Unfortunately, it’s easier said than done. It takes me about half an hour to successfully flip the whole the rig and get the target re-centered to exactly where it was. After such a huge movement with lots of changes to the weight distribution I also usually have to refocus the scope. All this means that when I chose a target, I’m going to have to find the exact time the object transits the meridian line, and be ready for it. No sleeping for me.
Back to the point: Targets in the Milky Way. Because I only shoot a few times a year, I want to shoot something with a “WOW” factor. I don’t want to shoot some mundane open cluster full of white stars. I want colorful gas, depth, and lot’s of stars. After a bit of searching I’ve found my target. Rho Ophiuchi
Gathering Data and Planning the Image
I shot Rho Oph last year with a 100mm lens. The above picture is the result. As you can see, it meets all my criteria for what I want to shoot. Colorful gas, depth, and lot’s of stars! Now that I have the target, I need to find some basic data on it. I’ll need to know the Meridian transit time, apparent magnitude, apparent size, and because it isn’t a very common object, I’ll need to know it’s coordinates.
Now that we have our data, we can see the first problem. That red box is the field of view of my camera. We can see immediately that the object is too big to fit. Oh boy! Mosaic time!
In astrophtography there are two ways to deal with massive objects. Either shoot them with a shorter focal length (less magnification), or shoot a mosaic. Because telescopes have fixed focal lengths, I’ll need to shoot a mosaic.
A mosaic in astrophotography is where one shoots multiple images in the same region with a slight overlap. In post processing, the photographer combines the different pictures into one larger picture of the whole area. Kind of like how your phone takes a panorama. Except this panorama takes a week to shoot. And it’s a pain in the ass.
If I’m going to shoot a mosaic for this target, I’ll need a lot more info. First off, I’ll need to actually map out the mosaic, so I know where to point the telescope for each frame. There’s no room for error when making a mosaic, so I really need accurate measurements. To plan the mosaic, I’ll use a program called Asimoplan. Asimoplan will let me enter in the number of frames in the mosaic, and it will project a simulation of my field of view into Stellarium. If that doesn’t make sense, here’s a picture.
Now we’re cooking with fire! As you can see, with 4 frames we cover the whole nebula. I’ve overlapped the frames 20% in order to avoid any mistakes when I go to combine the frames. Better safe than sorry.
So now we have a plan. We’re going to shoot Rho Ophiuchus in a four panel mosaic. I know the orientation of each frame, and where to center them. It’s time to plan the exposures.
Planning the Exposures
Rho Ophiuchus is faint. Really, really, really faint. The fainter the object, the longer exposure I’ll need. Long exposures mean that the tracking will need to be accurate. In order for the tracking to be accurate, I’ll need a good polar alignment. I’m going to plan to spend about an hour on the alignment, and then another hour testing it. This means the start time for the first exposures on Rho Ophiuchus will be about 11pm. That’s good because by 11pm, Rho Oph will have crossed the Meridian line into safety.
Planning the exposures is where the astronomy begins to blend with the photography. I know that I’ll need long exposures, and that I’ll want a high ISO. The higher the ISO, the more sensitive the camera is to light. The longer the exposure, the more light I’ll physically receive. From a couple tests, I know that my camera only becomes more sensitive only to a point. After about ISO 800, the camera begins brightening data that’s already there instead of adding new data. I’ll set the ISO to 800 for this shot, right at the cutoff. We call this the camera’s Unity Gain. Any ISO higher, and I’m just adding noise not data.
Because high ISOs and long exposures create noise in the image, I’ll be stacking multiple exposures to average out the noise. I’ll cover Stacking in another post, but basically, the more exposures I take the better the image will be. Also the more exposures I take, the more insane and sleep deprived I will be. It’s a trade off, just like everything else in astrophotography.
I don’t value my sleep very much apparently, so I’ll be shooting 30x7min for each frame. That will be a total of 14 hours for the whole mosaic. That’s a lot of shooting. I’ll plan to do half over a long weekend in June, and the rest at GSSP. this brings me to the next section. Finding a location.
Finding a Location
Or, Where are There Not Bears?
The first thing to think about when planning an imaging site is safety. Where do you feel comfortable being outside alone at 3am? Chances are there aren’t a lot of places. This is why I do most of my shooting from either my home, a club-owned dark site, or star parties.
Besides safety the one thing that all good sites have in common is darkness. Light pollution is the bane (well one of them, there’s a lot) of an astrophotographer’s existence. It’s the reason I don’t shoot from home a lot anymore, and it’s why I drive 8 hours to GSSP every summer. I use this MAP to find dark sites. Black is the best, and White is the worst. The hierarchy goes White<Red<Orange<Yellow<Green<Blue<Grey
Altitude is also a very important factor when choosing a dark site. Not all grey zones are the same. The higher up you go, the less atmosphere there is, making the image crisper. At low altitudes, images can suffer from blurring and discoloring. GSSP and Glacier Point are both at altitude so I’m set there. All that’s left to do is pack the car and go!
Astrophotography is the ultimate practice in planning. Every step and image must be planned and thought out. It isn’t easy to plan for an astronomical image, but it’s harder to wing it. Astrophotography is ultimately a game of tug of war. Every action has a reaction: lengthen exposures, get more noise/ better pictures, less sleep.
Three weeks from publishing this post, I will be taking the first two frames of my Rho Oph mosaic. Everything included in this post were real screenshots of my process. I’ll post an update after taking the frames. Let’s hope this planning pays off!