Problems with the measurement of Flatfield frames (Flats)

With flats are to be eliminated mainly dirt effects from the recorded spectra.

  1. Such dirt effects are dust particles in the slit. They cause dark horizontal stripes in the photographs, because the dust in the slit blocks a part of the light.
  2. Dust particles near the camera's focus (dust on the thin glass plate on the CCD camera chip), produce shadows, which usually take the form of dust rings in the photographs. They are always found at the same place.
  3. The CCD pixels have different sensitivity to photons of different wavelengths (quantum efficiency, QE). This causes wave like differences in brightness in the direction of the dispersion when the wavelength is shown at low dispersion sufficiently large.

Some informations and a flat are shown already on the page data reduction. Other basic knowledge see Wikipedia.

In practice, the production of good flats is not trivial. For light source only line free lamps come into question. They are, for example light bulbs (halogen lamps). The light should take the same path as the starlight, so that all influences (dust, vignettings) are recorded properly. The flat therefore must be taken using the complete set how it is used for recording the spectra (telescope, spectrograph, slit width, grid position ...). It would be ideal if the light used had a flat wave front like the starlight (lamp very far away) However this is not really feasible. An approximation to the ideal beam path is a featureless white surface (paper, cardboard, wall), evenly illuminated by a lamp, and placed far away as possible from the telescope.

In the case that the shadows of dust particles in the flat are not identical with those in the star image then during the correction by the flat artificial structures will be introduced into the spectrum (artifacts that significantly alter the "true" spectrum). It is therefore dangerous to use flats, when the light path is not identical to that of starlight. The check is not easy.

Below I show the results of an experiment (C14 telescope, spectrograph Lhires III,I1200/mm grating , 40 µm slit width).

Method 1:

Using a 60W light bulb (with frosted glass front). The lamp shines directly into the telescope, spacing about 40 cm from the front Schmidt plate of the C14. Exposure time 30s.

See the next photo. Just without the sheet.

Method 2:

Using a 60W light bulb with diffuser surface. The lamp shines on a white sheet in front of the telescope.The white cloth serves as a second diffuser. Exposure time 60s. .

Method 3:

Using a 60W light bulb with diffuser surface. The lamp shines on a white board which is about 80 cm before the telescope. Exposure time 180s.

Some darkcorrected flats was averaged. Below are plotted two screen shots. They show the 3 flats (method 1 left, center and right methods 2 and 3). A horizontal or vertical row / column profile is selected in the middle flat, so that the huge dust ring is cut in the photo (profile = white line) . The profiles show the rippled brightness differences in the dispersion direction, which is seen in all 3 flats directly by eye.

The left flat shows some sharply depicted dust particles. The same shadows are in the other two flats available at exactly the same place, but represented as fuzzy rings. Obviously, the method 1 (the lamp shines directly into the telescope) is fundamentally different from the "diffuse" methods 2 and 3. Because the dust particles are sharply imaged in method 1, the cone of light that falls locally on the CCD, has a much smaller angle of divergence as do the methods 2 and 3. To study in more detail the relationships I have dark corrected the 3 flats with the help of SMS. The hotpixels are removed and then the flats are normalized (then the median of the pixel is 1). The results are summarized in the following table. With a mouse click on the pictures you obtain the corresponding fits files for your own experiments.

Normierte Flats  

Method 1:

Lamp lights directly into the telescope. Sharp dust grains and a white horizontal line near the top (origin unknown).

Method 2:

Obliquely illuminated TShirt. Dust grains out of focus. In the dust ring asymmetric light distribution. Two-dimensional periodic structures (brightness variations).

Method 3:

Obliquely illuminated white cardboard. Results like method 2, different light distribution in the dust rings.

The column 920 of all three normalized flats is shown for comparison. While the first method (green line) shows a rather abstruse intensity distribution over the column (and all other columns also not shown here), the columns of the two diffusely illuminated flats are in the right half virtually identical. The intensity curves show long waves.


Here, the comparison of a row has been carried out (N ° 693). On the right is the effect of large dust grains to be seen. Again, the two methods 2 and 3 are almost identical. But the dust grain is depicted something different. This can be seen directly in the pictures: The dust grains have the same scale shown, but the light distribution within the shadow ring of a big dust particle differs significantly. Using such flats differences may result causing artifacts in the continuum and line profiles.

That is the difference between the flats obtained by method 2 and 3. A mouse click on the image allows you to load the corresponding fit file. Interestingly, the large rings of dust grains all have the same asymmetry in the intensity of light showing in the same direction. Probably the aperture of the telescope has been asymmetrically illuminated at least one of the two methods. That demonstrates the great danger of importing artifacts when the light goes different optical paths in the flats and the object frames.


Preliminary Conclusions:

The stellar light comes on as a parallel beam running approximately parallel to the optical axis of the set. The diffuser can be obliquely illuminated. But only the light comes through the telescope to the CCD, which lies within the view angle of the telescope and falls through the slit. The diffuser is virtually a bright sky and creates the sky background along the slit. On the CCD front glass plate the dust grains are obliquely illuminated by the light cone of the whole view angle and their shadow is therefore different from that in the light cone of the star.



Method 4:

Flatfield foil

As a fourth method, I've tried in January / February 2009 a white electroluminescent film as it is to buy from Gerd Neumann. The luminescent film has the advantages of uniform illumination and vertical alignment relativ to the optical axis of the telescope (symmetry). It is quickly installed, allowing to get the flats without shifting the telescope. The observatory remains obscure. In my tests it has proven itself, so I bought myself now own. After praises now the results. On left side a normalized masterflat, taken with this film, 360 s exposure time. The traces of dust are visible.

A comparison of the row 500 of method 3 and flatfield foil method 4 is shown. The flats were not token at exactly the same grating position, so the long waves can not be directly compared. Its amplitude is of similar magnitude.

Whether the waves are coupled to the wavelength scale was determined in the following experiment.

With the same telescope position two foil flats were recorded at 5800 Angstrom, with a slightly shifted grating. Rows N° 600 from the two flats are plotted over. The associated calibration spectra (Neon) are plotted with. Obviously, the flats have a spectral shift of about 90 Pix. = 31 Angström. A similar shift is seen even the intensity waves of the two rows.

The reproducibility of the grating position setting is about 10 Angström. From this result, so I must see that every newly adjusted grid position, a new flat must be made.

That's a time consuming procedure.

The comparison of the rows N° 500 clearly shows that the dust shadow at 1300 Pix. is not affected by the wavelength displacement of the long waves

It remains to examine what causes the waves.The following options are clear:

  1. Characteristic of the used grating (blaze function).
  2. Characteristic of the KAF-1603 ME 1603ME CCD chip in the Sigma camera.
The waves are seen in varying intensities in the bulb and the foil flats to. Even in spectra of hot stars, the waves are clearly to see if no flat correction was made. The spectrum of the bulb does not seems to be the cause for the intensity waves.

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