| Telescope | MDM 1.3m |
| Date | 11/25/97 |
| Instrument | Echelle CCD |
| Filter | V |
| Exposure Time |
15 sec |
Problem:Dust on the dewar window and filters will show up in all images as out of focus rings. Filter dust rings are the large rings and have a tendency to move throughout the observing run (and night!) as the telescope and filter wheel are moved. In this image a large ring can be seen about halfway down on the left side. The smaller dust rings arise from dewar window dust and are fairly stationary throughout the run. |
Fix:Since the dust rings will show up in the flats, they will just flatten out during normal processing. Watch out for the filter dust rings however. These might require both evening and dawn flats if the dust has moved throughout the night. If you see any of these, you may want to simply clean the filters so as to not have to deal with keeping a close watch of them. It may be difficult to entirely get rid of dustgrains on the images when the difference in the way the light scatters off the grain between your flats and your images is sufficiently large. This could occur when, for example, a bright star falls very close to the dustgrain on the image, or when twilight flats are taken very early in the evening and the sky is still very bright. If you encounter this problem, you would have to take flats under the same conditions as the data images themselves, i.e., take dark sky flats. Furthermore, you may dither your exposures slightly, so that any bright light source on the image is sufficiently far away from the dustgrains. |
Another example of an image with dust rings (includes a readout error).