<p><a href="https://www.spacetelescope.org/images/potw1917a/" rel="nofollow noreferrer">Spacetelescope.org</a>
indicates under "Colours &amp; filters" that the blue channel of the image is from a 658 nm filter (red)
and the red channel is from an 814 nm filter (near infrared).
The cyan and orange rows of the table also list these filters; presumably the green channel is a blend of the two.</p>

<p>The <a href="http://www.stsci.edu/hst/acs/documents/handbooks/current/c05_imaging2.html" rel="nofollow noreferrer">ACS instrument handbook</a> says filter F658N is narrow, passing only NII or slightly redshifted H&alpha; emission (Table 5.1, Figure 5.4).
Filter F814W is wide, passing wavelengths between 710 nm and 960 nm (Table 5.2, Figure 5.1).</p>

<p>NGC 2903 is one of the brighter galaxies not in the Messier catalog.
Here is a wider angle, more natural RGB image by <a href="http://bf-astro.com/ngc2903/ngc2903.htm" rel="nofollow noreferrer">Bob Franke</a>, rotated for easier comparison.
The reddish emission nebulae here appear blue in the HST image.</p>

<p><img src="https://i.sstatic.net/JHDtw.jpg" alt="NGC 2903 by Bob Franke"></p>

CC BY-SA 4.0
2019-05-04T06:12:28.567Z
wow that was fast! I don't fully understand how the spacetelescope.org's Colours & filters table works. There are four "categories" with the word "Optical" appearing four times in for font colors (blue, cyan, orange, yellow). The first two both map to 658 nm and say "NII", the second two to 814 nm and say "I". Any idea why there are four entries like that? For example, [this one](https://www.spacetelescope.org/images/heic1803a/) has seven and they are all unique.
uhoh
CC BY-SA 4.0
2019-05-04T06:42:32.937Z
Maybe they think it's better to have multiple rows per filter than to have multiple filters per row. I guess the NGC 3344 image put three filters in the B channel, one in G, two in R, and one in both B and G.
Mike G
CC BY-SA 4.0
2019-05-04T06:02:37.570Z
2019-05-04T15:10:13.800Z
2019-05-04T15:10:13.800Z
Mike G
<p>C|Net's <a href="https://www.cnet.com/news/hubble-spots-drop-dead-gorgeous-spiral-galaxy-tucked-into-leo/" rel="nofollow noreferrer">Hubble spots drop-dead gorgeous spiral galaxy tucked into Leo</a> links to NASA's <a href="https://www.nasa.gov/image-feature/goddard/2019/hubble-spots-stunning-spiral-galaxy" rel="nofollow noreferrer">Hubble Spots Stunning Spiral Galaxy</a> which shows the image below.</p>

<p>The caption on the NASA page doesn't mention the color coding, nor does it mention a reference to a technical page for the image's construction or history.</p>

<p><strong>Question:</strong> Is this close to a "straight RGB" image of NGC 2903, or were filters used at certain wavelengths, possibly including IR or UV, in order to highlight certain regions and emissions, then assigned false colors. If the latter, is it a standard color coding?</p>

<p>The NASA page says: </p>

<blockquote>
  <p>NGC 2903 is located about 30 million light-years away in the constellation of Leo (the Lion), <strong>and was studied as part of a Hubble survey of the central regions of roughly 145 nearby disk galaxies.</strong> This study aimed to help astronomers better understand the relationship between the black holes that lurk at the cores of galaxies like these, and the rugby-ball-shaped bulge of stars, gas and dust at the galaxy’s center — such as that seen in this image. Text credit: ESA (European Space Agency)</p>
</blockquote>

<p>Click to view full size!</p>

<p><a href="https://i.sstatic.net/MRzzb.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MRzzb.jpg" alt="New NGC 2903 Hubble image, wow!"></a></p>

<photography><hubble-telescope><image-processing>
False-color wavelength assignments in this "drop-dead gorgeous" image of NGC 2903?
uhoh
# False-color wavelength assignments in this "drop-dead gorgeous" image of NGC 2903?

(Asked by: username_0 on 2019-05-04)

C|Net's [Hubble spots drop-dead gorgeous spiral galaxy tucked into Leo](https://www.cnet.com/news/hubble-spots-drop-dead-gorgeous-spiral-galaxy-tucked-into-leo/) links to NASA's [Hubble Spots Stunning Spiral Galaxy](https://www.nasa.gov/image-feature/goddard/2019/hubble-spots-stunning-spiral-galaxy) which shows the image below.

The caption on the NASA page doesn't mention the color coding, nor does it mention a reference to a technical page for the image's construction or history.

**Question:** Is this close to a "straight RGB" image of NGC 2903, or were filters used at certain wavelengths, possibly including IR or UV, in order to highlight certain regions and emissions, then assigned false colors. If the latter, is it a standard color coding?

The NASA page says:

> NGC 2903 is located about 30 million light-years away in the constellation of Leo (the Lion), **and was studied as part of a Hubble survey of the central regions of roughly 145 nearby disk galaxies.** This study aimed to help astronomers better understand the relationship between the black holes that lurk at the cores of galaxies like these, and the rugby-ball-shaped bulge of stars, gas and dust at the galaxy’s center — such as that seen in this image. Text credit: ESA (European Space Agency)

Click to view full size!

[![New NGC 2903 Hubble image, wow!](https://i.sstatic.net/MRzzb.jpg)](https://i.sstatic.net/MRzzb.jpg)

**Tags:** photography, hubble-telescope, image-processing


---

## 1 answers

### Answer by username_1 on 2019-05-04. Score: 1 | Accepted answer

[Spacetelescope.org](https://www.spacetelescope.org/images/potw1917a/)
indicates under "Colours & filters" that the blue channel of the image is from a 658 nm filter (red)
and the red channel is from an 814 nm filter (near infrared).
The cyan and orange rows of the table also list these filters; presumably the green channel is a blend of the two.

The [ACS instrument handbook](http://www.stsci.edu/hst/acs/documents/handbooks/current/c05_imaging2.html) says filter F658N is narrow, passing only NII or slightly redshifted Hα emission (Table 5.1, Figure 5.4).
Filter F814W is wide, passing wavelengths between 710 nm and 960 nm (Table 5.2, Figure 5.1).

NGC 2903 is one of the brighter galaxies not in the Messier catalog.
Here is a wider angle, more natural RGB image by [Bob Franke](http://bf-astro.com/ngc2903/ngc2903.htm), rotated for easier comparison.
The reddish emission nebulae here appear blue in the HST image.

![NGC 2903 by Bob Franke](https://i.sstatic.net/JHDtw.jpg)

> **Comments:**
> **username_0**: wow that was fast! I don't fully understand how the spacetelescope.org's Colours & filters table works. There are four "categories" with the word "Optical" appearing four times in for font colors (blue, cyan, orange, yellow). The first two both map to 658 nm and say "NII", the second two to 814 nm and say "I". Any idea why there are four entries like that? For example, [this one](https://www.spacetelescope.org/images/heic1803a/) has seven and they are all unique.  
> **username_1**: Maybe they think it's better to have multiple rows per filter than to have multiple filters per row. I guess the NGC 3344 image put three filters in the B channel, one in G, two in R, and one in both B and G.  

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