Focus & Throughput

Updated on May 10, 2021, 9:44 am

It is very important to get the best possible focus with a fiber-fed instrument like Hydra. A modest error will cause a large fraction of light to miss the fiber. Hydra has the tools necessary to help the observer get a good focus, but the unwary observer can easily fool him/herself into thinking the image is well focussed when it is in fact poor.

Hydra has a camera on the gripper which shows the observer a simultaneous view of the tip of the fiber and the image of the target object. This makes it easy to determine if Hydra is positioning well and is properly focussed, right?

Wrong! Let's see how this camera works. Click here to bring up a drawing of how the image is generated. The gripper camera sees the fibers by means of a "periscope", shown in this drawing. The hydra fibers come in from the side of the field, enclosed in a piece of hypodermic tubing to avoid breakage. A small prism is cemented onto the tip of each fiber. In turn each of these prisms are cemented to magnetic buttons. Hydra moves the buttons with a "gripper" which picks them up and places them at the appropriate locations on a flat steel plate. The plate is then warped into a curve which moves the tips of the fibers to the focal surface.

The periscope permits the gripper's TV camera to see both the target and the fiber simultaneously by means of a pellicle mirror. Light from the target hits the pellicle and is reflected directly onto the camera. The fiber buttons are illuminated from above with LEDs on the base of the gripper. The intensity of these LEDs can be controlled using the Hydra GUI. This light reflects off the back of the pellicle, into a collimating lens and then to a retroreflector, which passes back through the lens, through the pellicle and focusses on the TV camera.

If the light is parallel when it goes into the retroreflector and the distances are the same from the pellicle to the camera and telescope focal surface, the relative positions of the images of the target star and the fiber tip will remain unchanged wherever the images are within the camera field. Thus the observer sees star and fiber and can theoretically bring them into focus and see when light from the object is indeed going straight into the fiber.

This is what one thinks he is seeing but appearances can be deceiving. The gripper camera focusses on a plane, which is that of the unwarped plate. Warping the plate varies the position of the focal surface by up to 3mm over the field. This will increase the diameter of a star's image on the focal surface when the plate is warped by as much as the diameter of the large fibers, but someone looking at the view in the gripper camera will not see any change in the image of the target. The image of the fibers will go slight out of focus.. Additionally, as the plate warps, it pulls the fibers slightly off position. This is compensated in the positioning model which results in situations in which a fiber is well positioned, but appears to be off center or out of focus in the gripper camera.

If the telecsope focus is adjusted to make the image of a target the sharpest in the gripper camera, the telescope will be focussed on the plane of the unwarped plate. It will thus be out of focus everywhere in the field save at the very edge. The image of a fiber as seen through the periscope will only appear to be in perfect focus at the edge of the field.

Thus, though the gripper camera is very useful, it is NOT a reliable gauge of telescope focus. Telescope focus can only be judged with the FOPS fibers. These are bundles of seven fibers 100µ diameter placed on buttons and connected directly to a television camera. They are therefore in the same plane as the object fibers. One focusses by centering on one or more of the FOPS and adjusting the focus to get as much light as possible into the central fiber and minimizing the light in the peripheral fibers.

In summary, the rule is use the Gripper camera to see what is happening and to recover a lost fiber, but check focusing and positioning with the FOPS. Additionally, BE ABSOLUTELY CERTAIN that the plate is in the warped position when focussing with the FOPS. If for some reason you absolutely must focus using the camera on the gripper, do so as near to the edge of the field as possible.

 

5 June 2000
T.Ingerson