Guider jumpingOcculting dome Deflated airbags"Intergalactic battle"Internal reflections"Squiggle" meteoritesasteroids satellites Earthquakes

The 2.1 m Telescope OAN/SPM - BC, Mexico

Problem:

Double images of objects are usually caused by motion of the guider. Trailed images are caused by bad guiding and guiding parameters can be adjusted to correct for this. Two distinct images may be created if the guider suddenly guides on a new position (i.e. the guide star shifts within the guide box and the guider guides with it there rather than shifting it back to the original location in the box, or the entire guide probe slips a fraction and pulls the telescope with it). The latter is what likely occurred here as this happened every time the telescope was positioned at an HA of about 3h (large inclination of the telescope).

Fix:

Make sure the guiding parameters are adjusted for optimal guiding, and that the guide star is correctly positioned in the guide box before starting. If the guider probe itself is loose and thus slips, have a support technician fix this.

C.1. Guider jumping: stars appear as double images.

C. 2. Occulting dome: a distorted stellar image plus a bright partial doughnut-like feature around the star.

Problem: It is not quite clear why this image looks the way it does. The stellar appearance is distorted and there is a partialdoughnut to the lower right of the star. It turned out that the edge of the dome was located very close to where the telescope was pointing. Very likely, the cause of the problem is a reflection off some part of the dome.

Fix: After the dome was moved out of the way, the next image was fine. By the way, the bright feature on the left side of theCCD (see panel) is a so-called "warm edge" of the CCD. This feature is visible only on very short exposures and goes away when processing the image.

Dropout Occulting

Problem: On this run, the dropout shutter was stuck shut and at the end of the night, as the target field set below the level ofthe dropout, the telescope field of view became occulted by the dropout. In this bias corrected, flattened image, the filter vignetting (already present) is more obvious and the donuts havenot flattened out due to different illuminations in the flats vs. this image.

Fix: Make sure the telescope is not occulted by the dome - make sure the dome is following the telescope or remember to move the dome yourself, and stop observing once the object is too lowto be viewed through the dome opening.

C. 3. Deflated Airbags - loss of support of primary mirror: stars appear very non-circular, but rather triangular in shape (due to the 3 hard support points of the primary mirror).

OAN-SPM 2.1m telescope, San Pedro Martir.

Position of the 18 force actuators (air bags) and three hard points in the mirror cell.

Problem: The airbags supporting the primary mirror were deflated when this image was taken. The reason for the triangular shapes of the stars in this focus image was that the mirror is supported at 3 "hard" support points. This, of course, made it impossible to obtain a good focus.

- loss of support of primary mirror: stars appear very non-circular, but rather triangular in shape (due to the 3 hard support points of the primary mirror).

Fix: DO NOT MOVE THE TELESCOPE! The problem here was that the PC which ran the program responsible for the air pressure in the airbags was hung up. When it was rebooted, the airbags were re-inflated, and the problem was taken care of. A word of caution, however: moving the telescope with deflated airbags can actually cause damage to the telescope itself.

Problem: The airbags supporting the primary mirror were deflated when this image was taken. The reason for the triangular shapes of the stars in this focus image was that the mirror is supported at 3 "hard" support points. This, of course, made it impossible to obtain a good focus. The teardrop like images are the triangular shapes of the stars with the focus setting substantially different from true focus.

Fix: The problem in this case was that the PC which ran the program responsible for the air pressure in the airbags was hungup. When it was rebooted, the airbags were re-inflated, and the problem was taken care of. A word of caution, however: moving the telescope with deflated airbags can actually cause damage to thetelescope itself.

Another Example

C. 4. Intergalactic battle: infrared, stellar image with bright loop features all around it.

Problem:

This was just a 2 sec exposure of a bright star taken in the IR H-band. These loops showed up with every bright star regardless of its position on the chip. A good part of the light was lost in this loop structure surrounding the star.

Fix:

This star was surely saturated. Fainter stars did not exhibit this sort of loop structure. The loops are a consequence of light scattering in the multitude of optical elements in this setup.

Note that earthquakes can sometimes produce images similar to this one.

C. 5. Internal Reflections: multiple, well-defined, offset rings around bright stars.

Problem: This exposure of Betelgeuse shows a series of bright rings offset from the stellar image. Most likely, these are a consequence of internal reflections inside the optical system. Part of the light hitting the CCD surface is reflected back toward the dewar window and the filter, then reflected back toward the CCD surface where it is perceived as out of focus (thus the doughnuts). There seem to be multiple internal reflections in this example here. In general, the farther away from the CCD a reflecting surface is, the larger of an image it will produce. Depending on this particular setup, the smaller doughnut might be a reflection off the dewar window and the bigger ones from the filter. The fact that the centers of the doughnuts are offset from the star indicates that either the reflecting surface is slightly tilted, or that the CCD itself is tilted. If the latter is the case there should be a focus gradient across the chip in the direction of the tilt (e.g., stars in the middle of the image are in focus, on one edge the focus is below true focus and above true focus on the other edge). Although this might at first glance appear as a collimation problem, the fact that the "hole" in the doughnuts is exactly in the center indicates that the collimation is not a problem.

Fix: There is no easy fix to this problem. Since the fraction of the light which is reflected essentially is 1 -the quantum efficiency of the CCD, there is not much one can do, given a certain CCD. This problem is presumably hardly noticeable at all with fainter stars, though, since only a small fraction of the light is reflected by the CCD surface.

C. 6. Squiggle: unusual appearance of an otherwise familiar satellite or meteorite trail through the image - satellite trail with wavy pattern perpendicular to the direction of motion.

Problem:

This squiggle appeared in a single 400sec image and runs about halfway through the image. Straight lines running through images are usually caused by meteors, asteroids, or satellites. In this case the motion perpendicular to the direction of the satellite's motion is very likely due to a high frequency oscillation of the telescope itself. What exactly caused this oscillation is not clear, but a very likely explanation would be wind hitting the telescope.

Fix:

This might be an indicator that the declination axis shaft-to-telescope joint is loose. This kind of problem is not easy to solve, and should be fixed by the telescope staff.

As a quick fix, you might try to protect the telescope from exposure to strong wind. Suggestions would be to close the drop-out shutter in the dome and/or close other ventilation louvers, windows, or doors in the dome so as to not allow any kind of draft going through the dome. Keep in mind, though, that less ventilation in the dome can frequently cause the seeing to degrade.

MeteorProblem:

The streak running through the image is either a meteor or satellite. The monotonic change in brightness over the extent ofthe image leads me to believe this is a meteor. Satellites frequently exhibit periodic changes in brightness due to their rotation as they cross the field of view.

Fix:

Take enough images of your fields so that upon combining, streaks like these will be removed.

Problem:

This object did not move very much at all during the course of a 600sec exposure. In a single image it appeared as a slightly elliptical star. Over the course of 4 images, however, its motion across the sky becomes noticeable. This is a summed image of 4 600-second images showing the asteroid trail. Other smaller streaks visible in the image are just cosmic ray hits.

Fix:

If the asteroid actually passes over a target object, take another exposure.

Asteroid

Problem:

Satellites often display this kind of periodic brightening due to their rotation as they go through the field of view. Meteors often exhibit a constant or monotonically changing brightness during their passage through the small field of view of the CCD. However, as one can see, there seems to be something branching off the main trail which seems to imply that this is a meteor after all.

Fix:

Not possible.

Satellite or Meteor

C. 7. Earthquake: stars appear with trails due to motion of telescope from earthquakes.

Problem:

An earthquake occurred during the taking of this image. As a result, the guider lost the guide star, resulting in the elongation of the stars in the image. The "jellyfish" like appearance of the bright stars is due to the actual motion of the telescope during the earthquake (only the bright stars produce enough signal to leave a trail, the equivalent trails of the fainter stars do not show up in the image).

Fix:

None possible.