Tips on Getting the Most out of Prime Focus Astrophotography

In a prior article, we went over the things you absolutely need to make prime focus long exposure photography possible. If you have an equatorial mounted telescope that can move automatically with Earth’s rotation, plus the camera with the proper adapters then you’re good to go!

Certain Objects Require Different Focal Lengths

As stated before, the telescope’s focal length will determine how wide or narrow your image field of view is (the higher the number = more narrow and higher magnification).

dslr sensor1

Stellarium can actually help you determine how wide your camera’s image will be just by entering the specs of the sensors and the focal length of the camera lens (or telescope in this case). This can help you see if you can fit the desired object within the frame, or if a wider focal length is needed.

Using a 1000 mm focal length as an example, large and wide objects like the Andromeda Galaxy are too big to fit within the frame while the Pleiades and Orion Nebula are just big enough. While objects as big as the full moon (~30 arc minutes across) still appear impressive and show detail, sights that are less than 10 arc minutes across are too small, and need a longer focal length to show detail.

Polar Alignment is Everything!

The better your equatorial mount is polar aligned, the longer your exposures can be. While the procedures to do this can be time consuming, getting your mount within 15-20 arc minutes of accuracy is necessary, especially if you want as little error or gear backlash as possible. If you’re off by more than a degree in either your elevation (too far north or south of the celestial pole) or azimuth (too far east or west) then your exposure times and images will suffer!

Even if you successfully perform the required star alignments for a GoTo telescope, the pointing accuracy will be off if your polar alignment is poor.

Some mounts come with, or are compatible with a polar alignment scope, and some computer telescope mounts have software features that help you get your polar alignment as perfect as possible. There are also polar alignment cameras if you want to save the hassle.

Get the Telescope Well Balanced!


As you’re adding a camera and possibly an autoguider to the mix, you need to make sure that the tube is balanced.  If the balance is off, even slightly, then you get much more backlash in the gears, more vibration when moving, and if you’re using a GoTo system, it won’t slew or point your telescope correctly.

Make a note of the weight of each object that the mount holds up. All mounts have a specific load capacity, and if your set up (the optical tube assembly, the tube rings, camera, and any additional accessories) weighs slightly more than the max load, then a well balanced telescope will save the day!

When I am imaging, it’s a different balance setting versus when I’m visual observing, and I don’t like to switch back and forth between the two modes as the eyepieces will affect the balance differently versus the camera.

Make a Note of Where Your Focal Point is!

There is a “magic line” where the camera is perfectly focused through the eyepiece barrel. I myself marked it down with a sharpee pen just in case I need to find it, as the focal points with the eyepieces will be different.

With DSLR’s, you can see bright stars in the live view mode, and thus you can use it to get the star(s) to appear as sharp and small as possible. If your focus is good, then even a few dimmer background stars will start showing up in the live view, which you can use to help orient your frame in the image.

In the case with using a GoTo system, When I perform the alignment procedures, I get the focus set and then I never take the camera off until I’m done imaging. As long as I did it correctly, I can simply flip between different objects with ease, snap the pictures I want, and then move onto the next without having to keep taking the camera off and check through the eyepieces.

Make Sure the Autoguider is Calibrated

Assuming you have one. If you don’t, then this section doesn’t apply.

While it is time consuming, each time you set up your mount, you definitely want to re-calibrate the autoguider to the sky you’re working with. Your polar alignment may not be the same as before, and the seeing conditions also affect how much your guider needs to correct.

You also may need to re-calibrate it if you switch between two objects on opposite sides of the meridian, as the gears move slightly differently with an object that is rising versus an object that is setting.

The only way to know for sure if the guider is working is through test exposures, trial and error.

Time is Valuable!

Whether it’s wanting to image as many objects as possible, or getting as many sub-exposures for a long composite, time that is spent not imaging is a waste. There will be only so many hours you can work with.

You have to account for how long your camera batteries can last, how long your laptop battery can last, and most importantly how long your telescope battery can last. Colder nights can result in shorter battery life. There’s also the duration of how long the night lasts before the Sun comes back, or how long the Moon is below the horizon.

DSLR’s have a noise reduction feature that activates with every long exposure, and it processes for as long as the image was exposed. Imagine taking a five minute sub and then waiting another five minutes before you actually see if the image is any good or not. Because I know I can fix the noise later through image processing, I have the noise reduction feature turned off.

The Telescope Must Remain Undisturbed!

It’s dark, and it’s not always easy to see the tripod at night. But bumping into it will surely mess with the alignment, and if it happens during an exposure, the image is ruined! Having a low red light pointed down, or putting glow in the dark tape on the tripod can help you see it when you have to move around.

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An example of a 60 second exposure of m20 ruined by wind shaking the telescope.

You must also be wary of wind, even light wind conditions can shake a large telescope tube.

Imaging is best done away from people who don’t “get it.” But if you’re at a popular location, while you can try to be polite about it, ultimately there is nothing you can do about people shining lights in your direction.

Be Patient!

I time my exposures with a stopwatch; the autoguider software and stopwatch window can both fit on my laptop screen. Watching the time, and waiting until you close the shutter can be a dull experience. If it’s an even longer sub, you can get nervous while you wait, hoping that the equipment worked correctly the entire time, or that it didn’t get disturbed by the wind or distant lights from a car.

For me, I mainly listen to music to help pass the time, and sometimes consult with star charts to plan on the next object I want to image. If I had another telescope at my disposal for visual observing, I’d spend time looking through that while waiting.

There’s So Much Within This World!

I don’t claim to have all the answers for prime focus astrophotography, but I can still provide insight to those getting into it.

I thought the pictures I took when I started doing prime focus were amazing, but then as I got better, I made the older shots of the same objects pale in comparison.

The photographer in me is never satisfied, and shots that may seem perfect to most will always be flawed to me, even in minuscule ways.

It’s okay to admit when you’re not sure, and trust me, when you think you know a lot, there are plenty of online communities dedicated to astrophotography that make you feel like you’re just getting started again.

TL;DR – Keep the telescope well balanced, autoguider calibrated, be wary of the weather, and make good use of your time!

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