Optical aspects of viewing the golden eagle nesting on a cliff at a far distance

The purpose of this article is to explain the requirements and challenges of imaging an eagle nest from a far distance.

Site location

The golden eagle pair is located on a cliff overlooking Whychus creek about 7 miles outside of Sisters, Oregon.  The eagle nest is about 200 feet up from the valley floor and about 30 feet down from the top of a flat shelf at the top of the canyon.  The observation station from which we are down-streaming is located on the opposite side of the canyon where the eagles are located and at about the same elevation as the nest which allows it to be imaged straight across the canyon.

The observation station is an open shelter which protects the optical system from most weather, however within the shelter the optical system is housed in an enclosure to prevent weather from blowing in from the sides.  Additionally the optical/camera system requires a bit of heat around the critical optics to keep the lenses and camera from frost which both can damage the electronics as well as when frosty, the lenses do not provide a clear image.  The optical system is mounted on a concrete pier to minimize shaking from wind.

Optical system used for down-streaming

The optics consist of a 7 inch aperture telescope with a focal length of 2850 millimeters.  The imaged formed by the telescope is directly projected onto the camera’s array.  The camera is high definition and contains its own computer electronics to allow down-streaming and several optical performance adjustments.

Imaging challenges

The golden eagle nest is located approximately 1100 feet from the observation station.  At that distance the telescope must magnify the nest from 100 to 200 power depending on if you want to see the entire nest or zoom in the see feeding of the chicks.  Because of the high magnification the image is greatly affected by variences in the air between the 1100 feet from the camera to the nest.

In the early morning and most evenings the temperatures across that distance is fairly constant so the image is the best at these times.  During the day and especially from about 10:00 am to 5:00 pm thermal variances are greatest across the canyon.  In these conditions the image wavers, goes in and out of focus and generally is not very good.  At times when it is overcast the images during the day are fairly good, but if the sun is out it creates thermals which greatly disturb the image quality.  Interestingly these day time thermals that hurt the optics clarity are very important for the eagles for soaring and allowing them to easily fly the thermals up to the nest as the heat rises from the valley and up the sides of the cliff.

From morning to noon generally the temperature goes up which flexes the long focal length telescope causing the image to be refocused.  The camera can remotely be focused but does not have an automatic focus feature.  It is important to understand that during warm weather the air density changes at various places across the canyon.  This causes the image to waver and go in and out of focus.  During these times it is very difficult to keep the optics focused because it is continually changing from these thermal effects.  Trying to keep the telescope focused at these times is futile.  For this reason we have required a password for focus control.