I am reading a paper here

enter image description here

What is off band filter for? If we have already known the positions of the sources we are interested in, then off band filter is useless, correct? So an off band filter is to discovery sources?

An off band filter is usually self-designed? There is no unified standard(central wavelength, band width, etc. )?

For the photometric calibration, when is continuum needed? Since there are standard stars, why is continuum needed?

Thank you. My questions are long, so I put them here.

We assume the continuum is flat, right? The off-band filter is for 1) photometric calibration, 2) discovering new sources?

How to do the photometry then? When do we use continuum and when do we use a standard star?


1 Answer 1


The on- vs. off-band filter is to let you easily measure the strength of an absorption or emission line in the object's spectrum. "Off-band" doesn't refer to a shift in position, but rather in the wavelength that the filter lets through. It is "off" the wavelength of a particular spectral line.

This illustrates the idea:

enter image description here

(from Michael Richmond's page here, used under CC-BY-NC-SA 2.0 license).

In this case, the light through the filter shown in red would be affected by the H$\alpha$ absorption line in the star's spectrum, while the light through the filter shown in green would not. So the ratio of the two would give you a very good measurement of how strong the line is, without worrying about how bright the star is overall, i.e. what the overall continuum level is.

In the paper you're reading, they would be looking for emission lines from planetary nebulae, but the idea is the same - the on-band filter measures line+continuum, and the off-band filter only measures continuum, so it lets you correct for the continuum emission and measure only the line strength.

The design of the filters would depend on which lines you're looking for. In this case they are targeting the [O III] line which is normally at a wavelength of 5007 Angstroms, but because they are looking at distant objects, the emission is red-shifted by the expansion of the universe, so they use a custom filter whose wavelength is shifted to match the red-shifted wavelength of that emission line for this particular group of galaxies.

  • $\begingroup$ We assume the continuum is flat, right? The off-band filter is for 1) photometric calibration, 2) discovering new sources? How to do the photometry then? When do we use continuum and when do we use a standard star? $\endgroup$ Jul 14, 2020 at 14:57
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    $\begingroup$ Regarding continuum slope, you do need to assume that the continuum is relatively flat if you are using a filter like the one shown. Sometimes off-band filters will be designed so that they transmit light on both sides of the spectral line; in that case, the continuum can be sloped and the two sides average out. $\endgroup$ Jul 14, 2020 at 17:29
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    $\begingroup$ The off-band filter is neither for calibration nor (necessarily) for discovering new sources. It is for giving you something to compare the on-band filter to. Look at the black line above, the spectrum of the star. The on-band filter measures one point on that line, and the off-band filter measures a second point. Comparing those two points, you can tell if the line is in absorption, emission, or absent, based on whether the in-band flux is less than, more than, or equal to the off-band flux. Calibration is done as for other photometry, by observing sources with known properties. $\endgroup$ Jul 14, 2020 at 17:41
  • $\begingroup$ What is the typical way of doing this? If we care line flux(-continuum), we need off-band filter(or a broader filter, which is rough) and a standard star. If we need line flux+continuum, then a standard star is enough? $\endgroup$ Jul 15, 2020 at 9:31
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    $\begingroup$ @questionhang If we don’t care about separating out the line flux, then a single filter is ok - but as for any photometry, we would still need to observe a standard star if we want to convert relative brightness values to an absolute scale. If we want to measure line flux separately from continuum flux, then we need two filters, as you say, with standards observed for each. But I’m not sure if I have understood your question. Are you asking about how to calibrate photometry in general? $\endgroup$ Jul 15, 2020 at 12:29

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