How deep-sky astrophotography works at the Old Fort Lewis Observatory

Saturday, Jan 11, 2020 4:03 AM

Greetings, Stargazers.

I must admit that I spend more time looking at the stars and other celestial objects with the aid of a camera than I do with my naked eye.

I have written a couple of columns about cameras and filters, and even a column on low-cost astrophotography techniques, but never one talking about the steps needed for deep-sky imaging. This is the process of taking pictures of things that are typically too dim to be seen through the eyepiece of even a large telescope. Although the photographic basics are the same no matter what the target is, it is interesting how some of the details differ from daylight photography. This month, I will discuss some of the steps used at the Old Fort Lewis Observatory.

All the following tasks can be done manually, but using a computer interface makes things more reliable. To complete a night’s session, the computer needs to be able to (with user input): open the dome, cool the camera, point to a target, focus the telescope, guide the scope during long exposures, and finally turn things off and close things up.

Opening the dome is much like opening your garage door – you just press a button. Or make your computer press the button. In this case, there are four buttons – separate open and close buttons each for both the north and south sides of the clamshell dome. Simple relays and an Arduino micro-controller are used for this button-pushing activity. To verify the dome has actually opened as expected, night-vision monitor cameras are set up both inside and outside the dome. More importantly, these monitor cameras are used to make sure the dome completely closes after the observing session is finished. Since night-vision cameras “see” by shining lots of infrared light on their targets, they must be turned off once the astrophotography session starts.

Cameras made to photograph the night sky differ in two main respects from “standard” daytime cameras. The first is that they are significantly cooled in order to reduce electronic noise that can overwhelm the starlight you are trying to record. I typically cool the camera to -30 Celsius (-22 Fahrenheit). For this reason, the first step in an evening of photography is to turn on the camera and let the temperature drop and stabilize.

The second difference is that these cameras are essentially black-and-white cameras that record the entire visible (and infrared) spectrum unless you use a filter. A filter wheel between the camera and the telescope is used to select which part of the spectrum will make it to the camera sensor. To make a color photograph, you need to set up a series of pictures using at least three different filters, for example, red, green and blue.

Fortunately, pointing the telescope in the right direction is one of the easier tasks of the evening. However, this is only once the scope is synced with the computer software. Aligning things in the first place can be quite a challenge, especially because different mounts have different ways of doing things. Unfortunately, the older telescope mount at the Old Fort requires a manual re-sync whenever the power goes off, so we usually just leave it parked, but with its internal computer still on.

Next month, I can discuss the rest of the steps – focusing and guiding for long exposures.

This month

It is hard to miss Venus as the evening star in the western sky at sunset. By the end of the month, Venus will be joined by Mercury in the evening sky. Also by the end of the month, the other naked-eye planets – Jupiter, Saturn and Mars – will be visible in the morning sky just before dawn.

Friday night was the first full moon of the year. Although it was not visible from Durango, this was also the first of four penumbral lunar eclipses this year. A penumbral eclipse is when the moon passes through part of the shadow of the Earth but still receives a bit of direct sunlight.

Rather than seeing part of the moon blocked by the full shadow of the Earth, the full moon will simply appear a bit dimmer than usual for the few hours of the eclipse. We should be able to see the penumbral eclipses in July and November.

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 hakes_c@fortlewis.edu.