Starting with a Camera Tracker

by Don Selle

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On occasion, I have talked or traded emails with a new HAS member who was interested in learning astrophotography but was hesitant to get started. The one reason most often given for the hesitation has been that the cost of the equipment is prohibitive. This is followed closely by “if I get into it, I know I will go all-in”.

While I can’t help much with the second reason (after all -- there’s always an upgrade!) there are some ways to get into astrophotography without totally breaking the bank. One of them is to start by using one of the more advanced camera trackers which are currently on the market. 

If you are already into photography, especially landscape and wildlife, adding a camera tracker can open the door to both nightscape and wide field astrophotography. If you already have a camera, sturdy tripod, and a good wide field lens (for nightscapes) and a good telephoto lens (for wide field astrophotography), a camera tracker is the next logical step. You can find used units in the $2-$300 while a new one with most of the bells and whistles will set you back a bit over $500, with a little bit of practice, your results can look really high end. 

I was fortunate to have won a SkyWatcher Star Adventurer Pro as a door prize at the 2019 Okie-Tex star party. I had been using it off and on for nightscape imaging but had not really put it to work doing wide field astrophotography until recently after I had my trusty Canon 6D astro-modified.

This past June, I had a successful imaging session with the Star Adventurer Pro, my 6D and a Canon 300mm f/4 L telephoto lens. So to help you decide if this entry point into astrophotography is for you, I thought I would walk you through my outing. But first, a few things you need to consider before taking the plunge.

Nightscape and Time Lapse – Tracking the night sky is not just for wide angle astrophotography, tracking the night sky with your camera can also do wonders for nightscapes and time lapse videos of the night sky. The major camera trackers have several different tracking speeds and programs to help you improve in these types of photography too.

No Computer Required – If you are using a DSLR, you can do everything using only the view screen on your camera (this is not true with a OSC astro only camera). Using a smart device like a phone or tablet with a camera remote control app can make things much easier though and you will likely have one or both with you.

Camera Payload – 11 lbs. seems to be the standard camera payload for camera trackers. This is not too surprising, as the two major brands are probably manufactured by the same company in China. My Canon 6D plus 300 mm f/4 prime lens weigh about 8.5 lbs. Small aperture refractors with modern CMOS OSC cameras should also get under this weight limit (TAAU! - there’s always an upgrade!).

Tripod Required- As the camera tracker itself weighs in at about 9 lbs. including the counterweight, you will need a tripod which will be stable with 20 – 25 lbs. attached and remain stable. The trackers themselves will replace your tripod head and will thread on to the photo tripod standard 3/8 inch stud. You can also purchase a tripod designed specifically for these trackers if you don’t have such a sturdy tripod.

Star Hopping Required – All camera trackers are motorized only on the RA (right ascension) axis which means that once they are polar aligned, the RA axis rotates at the rotation rate of the Earth, and thereby, your camera will stay on the same point in the moving sky. Getting to that point requires you the operator to know where it is on the sky, and for you to rotate both the RA and Dec (declination) axes to point your camera at your target. No GoTo here, but not to worry, finding your target is not too difficult as I will explain below.

TAAU – remember, there is always an upgrade! Typical upgrades are exchanging your camera lenses for a small aperture short focal length refractor telescope, astro modifying your DSLR or exchanging it for a OSC CMOS camera. Oh, and both SkyWatcher and Ioptron have introduced lightweight GoTo mounts branded the same as their camera trackers, and that’s just the start!

I will be describing step by step how I acquired the data to complete the image shown at the end of this article. My assumption is that the images will be taken unguided. I have had very good results this way when using 200mm to 300mm camera lenses after I have carefully polar aligned the tracker and autoguiding requires additional equipment and adds more complexity. 

I will also make some suggestions about auxiliary items you may want or need to make things work for you more reliably. While I will do so in the context of setting up at a dark site, I would encourage you to set everything up for a session in town first. Wide field astro imaging (like most other styles) is best done from a dark sky location, however, getting familiar with your equipment, and learning how to use it in town is time very well spent. So lets get started on our imaging session!

chart.jpgPlanning- First you should pick your target. If you are starting out, make sure that the targets you pick are on the brighter side. With your target in mind, simulate your image in your planetarium software. To do this, in addition to knowing where your target is, you will also need to know how to create a field of view indicator for your telescope & camera. This makes things very easy.

Once you have the image framed in software, check out where the brightest stars are located in the FOV, and remember that pattern. You will use the pattern and position of those stars to ensure you are on your target.

 

 

chart-detail.jpgNow zoom out to find the brightest stars (3rd magnitude or brighter) at the scale that best represents your naked eye view. Take note of what constellations they are in and where these bright stars are relative to where you will image. This will help you point your camera close to where you need it to be. 

You might even want to rehearse your star hop from the brightest star near your image, following less bright naked eye stars until you are centered on your target. This is star hopping. Later in the field, you will use a red dot finder to do this star hop on the sky for real.

Ready Your Equipment – Speaking of red dot finders, how can I connect one to my camera? It turns out that there are extremely inexpensive adapters which fit into the hot shoe on the top of your camera that present a ¼” 20 threaded stud (get 2 or 3).  I use a small metal plate I had which had the ¼” threads and was  also drilled for a finder scope shoe. The red dot has a dovetail which fits well into it. If you decide you want to add autoguiding, guidesope rings with dovetails drilled for ¼” 20 studs are also available.

Bhat.jpgYou should also purchase a Bhatinov mask with a diameter that matches the size of the filter threads on your lens. I have one that is 77mm in diameter for my 300mm lens. It snaps into a 77mm UV filter which threads into the lens. A 77mm to 67mm reducer ring lets me use the same Bhatinov mask on my 70-200mm zoom lens.

Either you can use your camera batteries, or an external battery pack designed to power it. Camera battery replacements are available which allow you to power the camera from a 5v power tank. 

The camera tracker will run on AA batteries. I use rechargeables here and they typically will run the tracker all night. I have two very large power tanks, one to power the camera, the second as a backup to power the camera tracker or my tablet if needed.

If your camera has an external intervalometer (which is used to automatically take a given number of long exposure subframes), make sure that you change its batteries as well. And finally, make sure that your smart device is charged, as you can use it to remotely monitor the camera.

Make sure you charge your batteries during the day you plan to head out to the field. After your equipment is set up and you are imaging, there is nothing more disappointing than having everything come to a grinding halt because your battery power ran out.

Set Up Early –  Pick a spot that is fairly level with a view of the norther horizon (we need to see Polaris) and a hard surface for your tripod legs, then set up your tripod. When you set up your tripod, extend the legs only high enough so that you can point and work the camera from a sitting position behind the camera (a folding chair of observing stool is a must for your back!). 

Fit the tracker on to your tripod using the 3/8” stud on the tripod, and face the tracker polar scope so that it points over a single leg of the tripod. This leg will point north, and leave lots of space for your chair and legs behind the other two tripod legs. Please note that bumping the tripod once you are polar aligned can be disastrous for your imaging session. By adjusting the leg height, level the tracker using the bubble level on the base of it.

Make sure that the cover is off the front of the polar scope. Put your camera and red dot finder on your tripod, then center something a long distance away in your camera and adjust the direction your red dot points so the two are aimed at the same object. THIS IS CRITICAL. The red dot finder is how you will get to your target.

Take the opportunity to autofocus your camera on a very distant object, then keep track of where the focus point is on your lens. You will be refocusing your camera around this point during the night. 

Turn off the autofocus. It will not work well (or not at all) pointing at the dark sky. You also may want to use some painter’s tape ( I keep multiple rolls of red colored tape around) to tape the focus ring on your lens in place. It is easy to remove and replace when refocusing, and helps prevent your lens from being bumped out of focus.

white.jpgTake Your Flats and Bias Frames – Hopefully you have arrived at the site early enough so that when your equipment is set up, there are still a couple of hours before the sun sets. You can use this time and the sunlight to efficiently take your flat frames (see last month’s AP Corner “Lets Get You Calibrated”) and bias frames.

With your camera pointed west and slightly above the horizon, and set your camera on shutter priority, the ISO to where you will take your subframes (for my Canon I use ISO 1600) and set the shutter speed to a the fastest speed on your camera. Then with a shutter release or your smart device with the camera remote control software app running, hold a standard camera white point card in front of the lens. Make sure that there is absolutely no shadow on the white card. Take a test frame and review it to make sure that your have the white card close enough to the lens that none of the background leaks through (aiming at tall trees helps). 

Once you have everything lined up take at least 13 flat frames. The bonus is that you can use one of these frames to set a custom white balance for your camera.

Now cover the lens, and if your camera is a DSLR with a standard optical viewfinder, tape it over to ensure that no stray light gets into the camera. Cover the lens, then take at least 30 bias frames using the same shutter speed you used for your flats.

Sit back now with a cold beverage and a snack (its going to be a long night after all) and wait for Polaris to pop out of the twilight.

Polar Align and Initial Focus  Once Polaris is in view, use the manufacturer's instructions to polar align your scope. My preference is to set the position of the weights to the point the manufacturer specifies where the reticle in the scope has the 12 o’clock and 6 o’clock position vertical. I then use an app on my phone which uses my GPS position and time to calculate and show me where on the reticle I should put Polaris. Sometimes a little red light in the front of the polar scope can help with this positioning. 

Use the altitude and azimuth adjustments to move Polaris to the specified point on the reticle. If your tracker is level, this should go fairly quickly. Your final adjustment of the azimuth should be to tighten the push screw against the other screw. For your altitude, make sure the final adjustment is to lift the weights up, and make sure you lock the altitude using the lever on the base.

You are now ready to use your red dot finder to point the camera at the brightest star near your target. Loosen the clutches on both axes and move the camera carefully by hand. Gentleness is a good thing as it helps ensure you do not mess up your polar alignment!

spikes.jpgThread the Bhatinov mask part way into the front of the lens. If you taped the focus ring, un-tape it. Using a 4 to 10 second shutter setting, take an image, review it at high magnification and check what the bright star looks like. Using your tablet or smartphone will be a big help here.

When you have the star with an X of spikes through it, and another set of spikes centered between the arms an legs of the X you are focused. The difference can be subtle (see the above example) so be sure to examine the bright star carefully. A sharp image is desired and the closer to center the middle spikes are, the better the focus. Once you are focused, remove the Bhatinov mask and retape the focus ring on the lens.

Using the red dot finder, point the camera between the bright stars you identified as helping you frame your target. Continue to take 4 to 10 second exposures while you position the bright stars in your target field of view where they need to be to ensure that your target is where you want it to be.

Now take a long unguided exposure. I have found that 2 to 3 minutes works with my tracker, so I typically take 2 or 2 ½ min subs. Check the resulting image to make sure your target is where you want it, and that the stars are nice and round.

Taking Your Darks

Put the lens cap back on your lens and take at least 5 dark frames with the same exposure time and ISO setting as you will use for your sub frames. You will take an additional 5 or 6 darks before you refocus. This helps to average out the effect of any temperature change which results in a change in the amount of thermal noise in your subs.

 After completing your darks, make sure you put the lens hood on the lens as this will work just like a dew shield on a telescope. Set your intervalometer to take 1 or 1 ½ hours of sub frames, and make sure that you leave 5 to 10 seconds between each sub frame. This will ensure that the camera temperature has a chance to stabilize after the image is read and stored on the camera’s memory card. Sit back and relax while your sub frames are take automatically.

Before you refocus, you may want to review a few of the subs you have taken to see how they are coming out. Be aware of any bright halos that appear around stars that did not have them when you started. This can be a sign of dew forming on your lens. If needed use a lens tissue to blot it off. 

When you re-focus, take the lens hood off, and fit the Bhatinov mask back on the lens. You may need to point your camera back at the bright star you focused on initially. Refocus, reframe if necessary, then take Bhatinov mask off and take an additional 5 or 6 dark frames. Rinse and repeat.

With a little practice, you will soon be able to get good data from which you can develop a nice image. There are several fairly simple software packages that will help you calibrate your subframes and assemble them into a master image frame you would be happy to share with friends and family. 

Final processing of your image is a technique that you will need to learn, and how you process an image can make a large difference in how it looks. We will delve into image processing in future columns. 

For now, here is the big reveal. What do you think? – Let me know.

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