<p>They must be referring to two different filters:</p>

<ul>
<li>the (Bessel) I filter which has a central wavelength of $\lambda \simeq 800\,\mathrm{nm}$  and a width of $\Delta\lambda\simeq 150\,\mathrm{nm}$, and</li>
<li>the (Sloan) z' filter, which has $\lambda,\Delta\lambda \simeq 970,255\,\mathrm{nm}$.</li>
</ul>

<p>Hence their transmission is in the near infrared ("NIR"), and they're both above 700 Å.</p>

<p>The exact transmission curves vary a little depending on the producer, so if you want to calculate exact magnitudes, for instance, you need to find the transmission curves for the telescope in question. From the <a href="http://www.ati.ulg.ac.be/TRAPPIST/Trappist_main/Equipment.html" rel="nofollow">TRAPPIST equipment website</a>, it seems they use filters from <a href="http://www.astrodon.com" rel="nofollow">Astrodon</a>. Astrodon don't seem to provide filter curves for these exact filter, but if you're okay with a good approximation, you can find the curves here:
<a href="http://svo2.cab.inta-csic.es/theory/fps/index.php?id=NOT/ALFOSC.Bes_I&amp;&amp;mode=browse&amp;gname=NOT&amp;gname2=ALFOSC#filter" rel="nofollow">Bessel I</a> and
<a href="http://www.aip.de/en/research/facilities/stella/instruments/data/sloanugriz-filter-curves" rel="nofollow">Sloan z'</a>.</p>

CC BY-SA 3.0
2016-06-01T09:53:08.523Z
2016-06-01T09:53:08.523Z
pela
<p>On the TRAPPIST telescope, the filter wheels are described to have a filter with the following description:</p>

<blockquote>
  <p>NIR Luminance I+z' (>700nm)</p>
</blockquote>

<p>After a bit of googling, I wasn't able to find what the specifics of this filter are (its FWHM and mean wavelength).</p>

<p>How would I go about finding these qualities of this filter?</p>



<light><observational-astronomy><terminology>
What does the filter name I+z' mean
Joeytje50
# What does the filter name I+z' mean

(Asked by: username_0 on 2016-05-31)

On the TRAPPIST telescope, the filter wheels are described to have a filter with the following description:

> NIR Luminance I+z' (>700nm)

After a bit of googling, I wasn't able to find what the specifics of this filter are (its FWHM and mean wavelength).

How would I go about finding these qualities of this filter?

**Tags:** light, observational-astronomy, terminology


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## 1 answers

### Answer by username_1 on 2016-06-01. Score: 3 | Accepted answer

They must be referring to two different filters:

* the (Bessel) I filter which has a central wavelength of $\lambda \simeq 800\,\mathrm{nm}$ and a width of $\Delta\lambda\simeq 150\,\mathrm{nm}$, and
* the (Sloan) z' filter, which has $\lambda,\Delta\lambda \simeq 970,255\,\mathrm{nm}$.

Hence their transmission is in the near infrared ("NIR"), and they're both above 700 Å.

The exact transmission curves vary a little depending on the producer, so if you want to calculate exact magnitudes, for instance, you need to find the transmission curves for the telescope in question. From the [TRAPPIST equipment website](http://www.ati.ulg.ac.be/TRAPPIST/Trappist_main/Equipment.html), it seems they use filters from [Astrodon](http://www.astrodon.com). Astrodon don't seem to provide filter curves for these exact filter, but if you're okay with a good approximation, you can find the curves here:
[Bessel I](http://svo2.cab.inta-csic.es/theory/fps/index.php?id=NOT/ALFOSC.Bes_I&&mode=browse&gname=NOT&gname2=ALFOSC#filter) and
[Sloan z'](http://www.aip.de/en/research/facilities/stella/instruments/data/sloanugriz-filter-curves).


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