Ø FringingØ Dust rings Ø CondensationØ Frost / IceØ Cosmic raysØ Reflections ü Long reflection ü Large blob ü Comet - like ü Scattered Light ü Bright Streak ü Persistent reflection streak (did not disappear when telescope was moved) ü Romulan Warships

The 1.5 m TelescopeOAN/SPM - BC, Mexico

Interference fringing in the spectral image. (FALSE COLORS)

Problem: Interference fringing is obvious in this image. This effect occurs due to multiple reflections internal to the CCD substrate or between the supporting substrate and the silicon. It is usually most visible in the I band, due to the abundant night sky emission lines in this wavelength range. If the fringing is very bad, it can even cause larger scale gradients in the image (on the order of half the CCD diameter), i.e., after the basic bias and flatfield correction, there is residual large scale structure in the image. This can only be corrected by using fringe flats.

Fix: This effect cannot be avoided. It can, however, be corrected for later in the calibration with either the help of so-called fringe flats one has to take during the observation run, or by using some specific computer algorithms (which work best in non-crowded fields and with no extended objects).

~4000 Å ~5500 Å ~7000 Å

B.1. Fringing

B.2. Dust rings: Dust on the dewar window and filters will show up in all images as out of focus rings.

SCORPIO multi-mode focal reducer

CCD camera.

Optical window

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. Inthis 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 dust grains 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 dust grain 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 dust grains.

Another example of an image with dust rings (includes a readout error).

Problem: The saturated strip on the bottom of the image is due to a read out error. These usually happen in the electronicsof the instrument, but they can occur due to a software error as well.

Another feature visible on this image are the doughnuts due to out-of-focus dust grains on the dewar window which are a very common phenomenon.

Fix: The best fix is to retake the image. If the problem reoccurs, the advice of the technical personnel responsible for the health of the telescope to the astronomer would (very likely) be not to touch anything and to call them for assistance.

The dust doughnuts will go away when the image is processed with a flat field image. Larger doughnuts due to dust on the filters can potentially be a bigger problem as they may change position throughout the night due to the motion of the filter wheel. See link below to the other example of an image with dust rings for a more detailed explanation.

B.3. Condensation on Dewar Window: bright filamentary structure or dark doughnuts towards the center of the image.

Problem:

Large dewar windows (which are especially cold) can have condensation forming at the center of the window whenever the humidity starts to rise. Different telltale symptoms can show up. One giveaway is more pronounced (darker) dust rings in the center. Other possibilities are the sudden appearance of filamentary structure or bright 'explosions' in the center of the image due to light from bright stars being dispersed by the dew. In some cases, the condensation on the dewar window may freeze, causing frost on the dewar window.

Fix:

Fix depends on the observatory but may include dewar window heating or blowing dry nitrogen into the setup.

Condensation on Dewar Window

Problem: Condensation has formed around the dust at the center of the dewar window when the humidity began rising. Humidity levels are still safe for observing, but too high for this particular setup.

Fix: Fix depends on the observatory (possibilities include blowing dry nitrogen into the setup or heating the dewar window enough to keep the dewar window dry).

Problem: This was just a 2 sec exposure of a bright star to be used for checking the pointing of the telescope. High humidity over 3 days had allowed for water to condense at the center of the dewar window. The light of this star is being refracted by the water droplets on the dewar window.

Fix: Observatory dependent.

Condensation on Dewar Window

Problem:

This bright filamentary structure is caused by condensation forming on the dewar window at high relative humidity (> 40%). Starlight is refracted by the water on the dewar window.

Fix:

Fix is observatory dependent

Problem: This strange smoke-like feature appeared in only one image taken immediately after the dewar was refilled. It was gone when the next image was taken just 1 minute later. The "smoke" is likely due to a very low level condensation buildup or icing on the dewar window caused simply by filling the dewar. This kind of condensation clears up quickly.

Fix: No fix seemed to be necessary since the problem disappeared by itself.

B.4. Frost/Ice on Dewar Window (outside)

Filamentary structure similar to water condensation symptoms, or dark "fingers" on the outside of the field; dust doughnuts can appear especially dark.

Problem: In extreme cases, the condensation on the dewar window can be frozen, causing features like the ones above to appear (filamentary structure in the field of view -- somewhat similar to fringing - or dark "fingers" on the side of the field).

Fix: Fix depends on the observatory but may include dewar window heating.

Frost/Ice on Dewar Window (inside): For some cameras (particularly electronically cooled ones), a bright ring can be seen which increases in radius as the condensation evaporates (if frost is on the inside of the camera window).

Problem: This camera uses Peltier cooling to achieve the necessary temperatures (of around 15-20 K colder than ambient temperature), which is a technique based on electrical heatsinks(rather than liquid nitrogen cooling which is widely used at a number of observatories). As a result of this technique, there is a fan attached to the camera to blow out the residual heat from the system.

When the cooling starts, residual humidity inside the camera may condense and freeze on the inside of the camera. The whole CCD field starts to look "dirty" as a result of ice crystals forming rapidly. After a while, an ellipsoidal ring forms in the center of the field, the inside of which is free of ice (see left image which is a flat field image). As the humidity is decreasing, one can see this ring starting to grow on timescales of about 1 minute (see larger ring on right image). On the edge of the frame more dust-like ice crystals can form (left image), but the center clears out. It takes about 10-30 minutes for the whole CCD to become clean (depending on the temperature difference between the inside of the camera and the ambient temperature) .

N.B.: the groove-like features (arc-shaped) across the whole field are most likely caused by the corrector lens of this Schmidt telescope.

Fix: Several fixes are possible, though they are somewhat particular to this setup. One may either (a) wait until the entire CCD is ice-free, (b) start cooling the CCD earlier so that it is ice-free when observing starts, or (c) cool more slowly to avoid condensation altogether (often a temperature 5 C above the recommended value is good enough for observing).

B.5. Cosmic RaysProblem: In this image, a large number of cosmic rays can be seen (several in this small portion of the image are circled in green). These omnipresent streaks and dots in the image are caused by cosmic rays (high energy particles) striking the CCD. The ones that look like streaks are easy to identify and disregard. The pointlike ones are slightly less obvious, but can be distinguished from stars by their radial profile which is pointlike rather than gaussian (the shape of the stellar profile).

Fix: Reducing your exposure time will reduce the number of cosmic rays in your image but there is nothing you can do to entirely avoid cosmic rays. However, if you take multiple exposures of the samefield, median combining them together will remove the cosmic rays from the final image.

Problem: This profile of a cosmic ray shows that it is pointlike. The plot looks like a delta function (remaining at constant sky brightness until suddenly rising steeply) rather than a more gaussian profile a star would exhibit.

Fix: Take multiple images and median combine them. This will remove the cosmic rays from the final combined image.

B.6. ReflectionProblem: Star light reflecting off of some part of the telescope setup causes reflections and scattered light to show up in images. These could be from external sources (off the dome or part of the telescope structure itself), or internal such as off of the filter holders. Often these will appear in specific fields with bright stars nearby. In this particular image, a star just out of the field of view was probably reflecting off the filter holder.

Fix: For the most part, nothing can be done while observing to avoid occasional reflections of nearby bright objects. Scattered light from the dome may be corrected for and reflections off of other parts of the telescope setup may be corrected for after investigation into the problem. For scattered light showing up in many images, look for light sources in the dome). To see other examples of common and uncommon reflections, check out the following links:

Problem:

Reflection likely of star just outside the field of view, reflecting off of the metal filter holder.

Fix:

Move telescope slightly.

Similar long reflection

Large blob

Problem:

Another example of bright starlight reflecting off of some part of the setup.

Fix:

Since it is in the corner, just ignore.

Comet-like

Problem: This comet looking flare is the most common type of reflection I have experienced. Ones near the corners are even more comet-shaped. Note that this image has 2 reflections - one probably scattered light from the first.

Fix: Try moving the telescope enough to get rid of the problem. This may be difficult depending on the observing target.

Reflection - Bright Streak

Problem: Reflections can be caused by several things (Moon, bright stars, internal light sources, etc.) and can occur in several places (dome or internally). This particular one seems to be caused by a bright source (Moon or star) close to, but outside of, the field of view, and seems to happen internally (since it reoccurred in every image of this field, but in none of the other ones, and since the positioning precision of the dome is probably not good enough toprovide that kind of repeatability of the problem).

Fix: There is no general fix for reflections, due to their various causes and origins. This particular one could not be avoided other than moving away from the field of view, which would move the observing target out of the center of the image or even out of the image altogether.

Scattered LightProblem:

Another example of reflections: scattered light leaves a long streak here.

Fix:

No fix possible other than moving the telescope slightly, and/orlocating the offending reflecting surface and rendering it non-reflective.

"Romulan Warships"

Problem:

These features are stray reflections. They are due to a part of the prime focus cage that was not properly baffled. They appear when a bright star off the edge of the field reflects off a metal plate onto the camera. The wavy parts are due to the machining grooves in the metal plate.

Fix:

A new baffle was inserted into the prime focus cage.