Last month, I began describing a typical night of remote astrophotography, covering opening the dome, cooling the camera and pointing the telescope in the right direction. This month, I will continue by discussing focusing the telescope and the process of guiding during long exposures.
One of the biggest challenges with astrophotography is getting the correct focus. One might think that if you set the focus at infinity you will never have to change it. Unfortunately, the telescope tube expands and contracts with changing temperature, which slightly changes the ideal focus point. And filters, which select which part of the spectrum reaches the camera and come in a variety of different thicknesses and types of glass, also change the focus slightly.
If you remember the days before autofocus cameras, you are familiar with having to adjust the focus until you get the sharpest image. But stars are very hard to see well when looking through the eyepiece of a single lens reflex camera. A great focusing aid for this situation is called a Bahtinov mask. This mask is just a sheet of thick plastic or cardboard that has dozens of slits arranged in a very specific pattern. When you put the mask over the front of your telescope, the light getting through the slits diffracts into a different pattern when inside or outside of focus. Make sure to remove the mask when you are ready to take your picture.
Autofocus cameras do the same thing we do manually, but they typically do it better and faster. Since daytime exposures last a tiny fraction of a second, focusing can happen so quickly that you often don’t notice. However, with nighttime exposures of at least a second or two, special software is required, and the process can easily take several minutes. And most nights, focusing will need to be repeated as the temperature drops.
The last big challenge is how to keep your telescope pointed at your target for exposures lasting several minutes. As the Earth rotates, the stars move across your field of view. The telescope drive motor follows the Earth’s rotation, but additional small corrections are always needed. This analogy is not completely correct, but when you use the cruise control on your car to manage the overall motion, you always need to use the steering wheel for the small corrections to stay in your lane. To stay on-target, a second camera is used. While a very long exposure is being made using the primary camera, the guide camera takes quick shots of a single star every second to see if the telescope has remained on target. If not, the appropriate corrective information is sent to the telescope mount to put it back on target.
If all goes well, the only thing you should have to worry about is an airplane or satellite going through your image. This happens more often than you might expect, which is why many astronomers are concerned about the new Starlink constellation of satellites. A satellite streak across your frame is only one reason why multiple exposures are usually needed to get a good single image through a single filter. Processing the resulting exposures is a task for another day, and a discussion of it is a task for another column.
This monthIt is hard to miss Venus as the evening star in the western sky at sunset. During the second week in February, you can also see Mercury in the western sky at dusk, but it will be much closer to the horizon than Venus. The other naked-eye planets, Jupiter, Saturn and Mars will be visible in the morning before sunrise. On the morning of Feb. 18, the moon will occult, or pass in front of, Mars.
If you haven’t seen or heard about the star Betelgeuse recently, it is getting dimmer. The red star is the one to the left and above the three belt stars in Orion. Historically, it has been almost as bright as Rigel, the blue star to the right and below the belt stars, and one of the 10 brightest stars in the sky.
Betelgeuse has always been a variable star, but not by this much. Over the last few months, it has noticeably dimmed by a full magnitude, or to much less than half of its previous brightness. The social media speculation is that it is an imminent supernova. While that might be true, imminent on an astronomical scale means sometime in the next 100,000 years. So don’t hold your breath.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at firstname.lastname@example.org.