<p>This web page -- <a href="https://melinasworldblog.wordpress.com/2020/04/13/here-is-why-the-hubble-space-telescope-only-looked-a-few-times-at-venus-and-why-it-looked-at-the-moon-instead/" rel="noreferrer">&quot;Here is why the Hubble Space Telescope only looked a few times at Venus (and why it looked at the Moon instead)&quot;</a> -- seems like a pretty good answer to your main question (note:  &quot;MAST&quot; = <a href="https://archive.stsci.edu" rel="noreferrer">Mukulski Archive for Space Telescopes</a>):</p>
<blockquote>
<p>There are only a few times the Hubble Space Telescope did look to
Venus according to MAST. Once in 1995 with the Wide-Field and
Planetary Camera 2 (WFPC2) and the High Resolution Spectrograph (HRS),
the second time was STIS [an optical spectrograph] in 2013.</p>
<p>So the two WFPC2 F218W and F255W [near-UV] images that we have of Venus with the
Hubble Space Telescope are maybe the only sharp images of Venus by
Hubble.</p>
</blockquote>
<p>And of course the reason for this paucity is not too mysterious:</p>
<blockquote>
<p>The first rule for (non-solar) Space Telescopes: Do not look at the
sun. This rule is the reason why the Hubble Space Telescope did look
at Venus only a few times. Venus is one of the inner planets and the
Hubble Space Telescope is in an orbit around the Earth. You will find
Venus always on the evening and morning sky because Venus is seen
close to the sun from our position. Looking close to the sun with a
Space Telescope is risky. If anything goes wrong during such an
observation it would be the end of the telescope.</p>
</blockquote>
<p>So, apparently no near-infrared images with <em>HST</em>.</p>

CC BY-SA 4.0
2021-02-26T14:40:45.263Z
Thanks for your post! Okay, yes this seems to be the answer to my question. I bumbled through MAST far enough to get "1 rows displayed, but 2 are available" which seems to confirm what the blogpost states. We'll have to stay tuned to this to see what re-testing of the (pesumably) backup versions of the cameras on Earth reveals about the filters; maybe they have some extra wiggles below their nominal cutoffs.
uhoh
CC BY-SA 4.0
2021-02-26T15:20:16.497Z
I get the impression (from reading around a bit) that WISPR doesn't have any filters, so it may be a question of whether the sensor is more sensitive at the edge of the near-IR than they assumed.
Peter Erwin
CC BY-SA 4.0
2021-02-26T15:29:49.293Z
With spectral ranges of 490–740 and 475–725 for the two cameras (see question) I can't see how this can be achieved without a filter *somewhere!* **update:** the paper I cite in the question says "The bandpass for each telescope is selected using a combination of long/short wavelength cutoff filters deposited on internal lens surfaces similar to SECCHI/HI."
uhoh
CC BY-SA 4.0
2021-02-26T13:49:08.383Z
2021-02-27T09:39:44.073Z
2021-02-27T09:39:44.073Z
Peter Erwin
<p>Phys.org's <a href="https://phys.org/news/2021-02-parker-solar-probe-stunning-view.html" rel="noreferrer">Parker Solar Probe offers stunning view of Venus</a> includes the image below taken by the Parker Solar Probe during it's most recent gravitational assist flyby of Venus as it continues to rid itself of energy to get ever closer to the Sun.</p>
<p>Apparently the image is surprising because it shows surface features of the planet, which was not expected to happen.</p>
<p>The article says:</p>
<blockquote>
<p>&quot;<strong>WISPR effectively captured the thermal emission of the Venusian surface,&quot;</strong> said Brian Wood, an astrophysicist and WISPR team member from the U.S. Naval Research Laboratory in Washington, D.C. <strong>&quot;It's very similar to images acquired by the Akatsuki spacecraft at near-infrared wavelengths.&quot;</strong></p>
<p><strong>This surprising observation sent the WISPR team back to the lab to measure the instrument's sensitivity to infrared light.</strong> If WISPR can indeed pick up near-infrared wavelengths of light, the unforeseen capability would provide new opportunities to study dust around the Sun and in the inner solar system. If it can't pick up extra infrared wavelengths, then these images—showing signatures of features on Venus' surface—may have revealed a previously unknown &quot;window&quot; through the Venusian atmosphere.</p>
<p>&quot;Either way,&quot; Vourlidas said, &quot;some exciting science opportunities await us.&quot;</p>
</blockquote>
<p>Wikipedia's <a href="https://en.wikipedia.org/wiki/Akatsuki_(spacecraft)" rel="noreferrer">Akatsuki_(spacecraft)</a> says that it had:</p>
<ul>
<li>Lightning and Airglow Camera (LAC, 552-777 nm)</li>
<li>Ultraviolet imager (UVI, 283–365 nm)</li>
<li>Longwave infrared camera (LIR, 10 μm)</li>
<li>Infrared 1 μm camera (IR1, 0.90–1.01 μm) is imaging on the night side heat radiation emitted from Venus's surface and help researchers to spot active volcanoes, if they exist. While on the day side, it sensed the solar near-infrared radiation (0.90 μm) reflected by the middle clouds. Unavailable for observation after December 2016 due to an electronic failure. (references)</li>
<li>infrared 2 μm camera (IR2,, 1.74–2.32 μm) studied the night side lower clouds' opacity to the thermal emission from the surface and deeper atmosphere. It also sensed on the day side the CO2 band at 2.02 μm, which can be used to infer the altitude of the top of the clouds. Finally, the 1.65-μm filter was used during the cruise phase to study the zodiacal light. Unavailable for observation after December 2016 due to an electronic failure.</li>
<li>Ultra-Stable Oscillator (USO) for performing radio occultation experiments.</li>
</ul>
<p>Wikipedia <a href="https://en.wikipedia.org/wiki/Venus" rel="noreferrer">gives</a> the average surface temperature of Venus as 737 K which is 464 °C. I've plotted a <a href="https://en.wikipedia.org/wiki/Planck%27s_law" rel="noreferrer">Planck distribution</a> for that temperature below.</p>
<p>Venus is hot enough where thermal radiation can be found in &quot;near IR&quot; and not just &quot;thermal IR&quot; which explains the mixing of the terms in the article.</p>
<p><a href="https://link.springer.com/article/10.1007/s11214-014-0114-y" rel="noreferrer">The Wide-Field Imager for Solar Probe Plus (WISPR)</a> (paywalled but also found <a href="https://orbi.uliege.be/bitstream/2268/200751/1/The%20Wide-Field%20Imager%20for%20Solar%20Probe%20Plus%20(WISPR).pdf" rel="noreferrer">here</a> and <a href="https://www.researchgate.net/publication/273351399_The_Wide-Field_Imager_for_Solar_Probe_Plus_WISPR" rel="noreferrer">here</a>) gives the specifications for the two cameras comprising WISPR:</p>
<pre><code>Table 4 WISPR Optical Design

                             Spectral    Entrance                        RMS Spot 
                   FOV      Range (nm)  Pupil (mm)   F#    # of lenses   Size (µm)
                ---------   ----------  ----------  -----  -----------   ---------
Inner Telescope 40◦ × 40◦    490–740      7.31      3.83     5-element       19
Outer Telescope 58◦ × 58◦    475–725      8.08      4.04     6-element       20
</code></pre>
<p>With nominal cutoffs at 725 and 740 nm one might not expect to pick up much radiant light from Venus' surface compared to reflected sunlight from the clouds in these spectral ranges, <strong>thus the excitement!</strong></p>
<p>The NASA JPL page <a href="https://www.jpl.nasa.gov/images/venus-cloud-tops-viewed-by-hubble" rel="noreferrer">Venus Cloud Tops Viewed by Hubble</a> has an image of Venus taken by Hubble in ultraviolet.</p>
<p><strong>Question:</strong> Has Hubble photographed Venus in near IR? If so how does it compare to the new Parker Solar Probe image?</p>
<p>Wikipedia's <a href="https://en.wikipedia.org/wiki/Hubble_Space_Telescope" rel="noreferrer">Hubble Space Telescope</a> gives the spectral range of the WFC3 as 0.2–1.7 μm (<a href="https://esahubble.org/about/general/instruments/wfpc2/" rel="noreferrer">WFC2</a> was 120 to 1000 nm) so If there is a filter that coincides with whatever spectral window of Venus' atmosphere is involved in this, it's possible that the effect may have been spotted earlier.</p>
<p><a href="https://i.sstatic.net/RwS28.jpg" rel="noreferrer"><img src="https://i.sstatic.net/RwS28.jpg" alt="enter image description here" /></a></p>
<hr />
<p>The figure's caption says:</p>
<blockquote>
<p>When flying past Venus in July 2020, Parker Solar Probe's WISPR instrument, short for Wide-field Imager for Parker Solar Probe, detected a bright rim around the edge of the planet that may be nightglow -- light emitted by oxygen atoms high in the atmosphere that recombine into molecules in the nightside. <strong>The prominent dark feature in the center of the image is Aphrodite Terra, the largest highland region on the Venusian surface.</strong> Bright streaks in WISPR, such as the ones seen here, are typically caused by a combination of charged particles -- called cosmic rays -- sunlight reflected by grains of space dust, and particles of material expelled from the spacecraft's structures after impact with those dust grains. The number of streaks varies along the orbit or when the spacecraft is traveling at different speeds, and scientists are still in discussion about the specific origins of the streaks here. The dark spot appearing on the lower portion of Venus is an artifact from the WISPR instrument.</p>
<p>Credit: NASA/Johns Hopkins APL/Naval Research Laboratory/Guillermo Stenborg and Brendan Gallagher</p>
</blockquote>
<p><a href="https://i.sstatic.net/wpGQm.jpg" rel="noreferrer"><img src="https://i.sstatic.net/wpGQm.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2021-02-26T12:15:36.167Z
I confess to being a bit dubious about the "we're seeing emission from the surface" claim. As far as I can tell, the Akatsuki imager doesn't see emission from the surface in any of its near-IR filters; it's all coming from the clouds, dozens of km up. (And I'd kind of expect any thermal emission to be overwhelmed by the reflected solar radiation, anyway.)
Peter Erwin
CC BY-SA 4.0
2021-02-26T12:16:09.740Z
A recent paper on Akatsuki imaging of Venus: https://www.researchgate.net/publication/323149701_Venus_looks_different_from_day_to_night_across_wavelengths_morphology_from_Akatsuki_multispectral_images
Peter Erwin
CC BY-SA 4.0
2021-02-26T12:18:21.387Z
@PeterErwin every planet has a night side, no? From the quoted Wikiblurb: "Infrared 1 μm camera (IR1, 0.90–1.01 μm) *is imaging on the night side heat radiation emitted from Venus's surface"* Wouldn't any night side disk images have to come from thermal radiation from something at least?
uhoh
CC BY-SA 4.0
2021-02-26T12:33:39.130Z
I assume this is a dayside image. (WISPR is supposed to be sensitive to $\sim 480-750$ nm, so why would you point it at the nightside of Venus?)
Peter Erwin
CC BY-SA 4.0
2021-02-26T12:39:09.913Z
@PeterErwin Phys.org article says "WISPR is designed to take images of the solar corona and inner heliosphere in visible light, as well as images of the solar wind and its structures as they approach and fly by the spacecraft. At Venus, the camera detected a bright rim around the edge of the planet that may be nightglow—light emitted by oxygen atoms high in the atmosphere that recombine into molecules in the nightside." It's a flexible instrument, intended to image "diffuse glowing stuff in space" rather than planetary disks. At Venus I think you would only point it at the night side.
uhoh
CC BY-SA 4.0
2021-02-26T12:43:57.377Z
You're right in that at 1.01 microns, the IR1 channel of Akatsuki can see some thermal emission from surface (https://www.stp.isas.jaxa.jp/venus/docs/vco.pdf). But that's nightside-only, and 1.01 microns is, I suspect, beyond the plausible sensitivity of the WISPR sensor anyway.
Peter Erwin
CC BY-SA 4.0
2021-02-26T12:57:43.293Z
OK, I was wrong; the article does mention it's a "nightside" view of Venus,
Peter Erwin
CC BY-SA 4.0
2021-02-27T11:56:56.510Z
While Q1 seems to be answered by one question below, Q2 is cryptic. It seems that you intend seeing a surface by virtue of its emission only, but reflection of the sun rays will let you image that point nevertheless. To see what you you likely mean is possible just in the nigh sight. That is all. To make it easier, to spot a green field you right need sun light. The principe is the same in the ir, with the difference that the field can be seen in the dark. So in my opinion the question should be how we can be sure that the features are surfaces based and not coming from the clouds.
Alchimista
CC BY-SA 4.0
2021-02-27T18:54:23.300Z
@Alchimista I wrote that before we came to the conclusion that this is the night and not the day side. I'll just delete it; it's not necessary to explain how light works to me.
uhoh
CC BY-SA 4.0
2021-02-28T09:08:16.403Z
@uhoh sorry but I just commented to your question. It looked like that something irradiated it cannot be seen, at least in the first version of the question you were posing that to "see" a surface, that can't be "illuminated". That is obviously no true and I have commented on that. I have no idea what you know of light, and it looked like you did not know or momentarily skipped that point.
Alchimista
CC BY-SA 4.0
2021-02-28T09:33:29.170Z
@Alchimista There are several simultaneous issues involving both a possible unexpected spectral transmission window in the filters of at least one of the cameras, and also a  possible unexpected spectral transmission window in the atmosphere. There are workable scenarios for this unexpected image in both day and night imaging, but we've since verified that it is a nighttime image as the exposure was intended to look at other weak optical signals from the atmosphere and surrounding ionosphere. The dark region has been specifically identified as a geographic feature so it's not "clouds".
uhoh
CC BY-SA 4.0
2021-02-28T09:36:16.163Z
@Alchimista this is mostly unrelated, but it's sort-of the opposite; the narrow and dark solar Fraunhofer (absorption) lines allow spacecraft with narrow spectral transmission windows to image faint fluorescence from chlorophyll molecules in leaves on Earth during the day  https://space.stackexchange.com/a/33823/12102
uhoh
CC BY-SA 4.0
2021-02-28T10:11:05.687Z
@uhoh interesting. That is about the point. Actually all this makes me think on how we distinguish surface and atmosphere in far object. I mean a pic of Earth is easy to interpret because we know how clouds look. But for Venus, what exactly let us discern the two? It is about assumptions and combinations of various knowledge?
Alchimista
CC BY-SA 4.0
2021-02-28T10:55:03.113Z
@Alchimista maybe my answer will not be so exciting, but Venus has been mapped in great detail by radar, so planetary scientists that study Venus (Venusologists?, Venonomers?) will already have an idea what clouds *usually* look like and be primed to recognize surface features when presented them. Those associated with the spacecraft mission will have ready access to the path of the flyby and the longitudes visible at the moment the photo was taken, so they could make a direct comparison. I'm totally baffled by Jupiter and Saturn that look like they have real structures but are just swirly gas
uhoh
CC BY-SA 4.0
2021-02-28T14:13:18.967Z
@uhoh sure radar is there. Also one can take pics at different times. :) my curiosity was a bit naive
Alchimista
CC BY-SA 4.0
2021-03-01T00:00:21.857Z
@Alchimista I think that there's always been quite a public "spotlight" on Mars, and so all the work that's been done on Venus is a lot less widely popularized and appreciated.
uhoh
CC BY-SA 4.0
2021-04-05T09:30:53.747Z
@uhoh  Interesting ! But it it's really the surface of Aphrodite Terra, with an emission from roughly 1000-2000 nm, why should there be a previously unknown "window" ?
Cornelis
CC BY-SA 4.0
2021-04-05T11:10:57.157Z
@uhoh If those are the contours of Aphrodite Terra, then north of it should be another highland. There isn't ! Couldn't those bright features be nightglow as well ?
Cornelis
CC BY-SA 4.0
2021-04-05T23:02:17.823Z
@Cornelis when I posted this question the image was a surprise. My understanding is that they weren't sure if there was an unexpected spectral window in *the atmosphere*, or in the *camera's filters*. Silicon's long-wave cut-off is not sharp. It's roughly 1100 nm but temperature and doping and to some extent thickness can have an effect. This [figure](https://i.sstatic.net/60fUM.png) from the top of page six in Hamamatsu's [Si photodiodes](https://www.hamamatsu.com/resources/pdf/ssd/e02_handbook_si_photodiode.pdf) suggests a little bit of sensitivity out to 1200 nm, but no conditions given
uhoh
CC BY-SA 4.0
2021-04-05T23:06:30.573Z
@Cornelis in [this comment](https://astronomy.stackexchange.com/questions/41691/?noredirect=1#comment85502_41696) I mention that the filters were applied as coatings directly to the lens surfaces. It's possible that if the coatings were incorrect thickness or nonuniform they may not be completely blocking the longer wavelengths as expected. We have to wait and see, but interference filter design is tricky, and it's really hard to completely block a wide range of wavelengths using interference only. Even a tiny manufacturing error can lead to incomplete cancellation and therefore some "leakage"
uhoh
CC BY-SA 4.0
2021-04-05T23:08:20.977Z
@Cornelis as far as the landforms are concerned I'm clueless. If you have some insight about that, what we're looking at in the image, maybe you can add a supplemental helpful answer comparing what the image caption says in the question to a good map of Venus, so we can see what they're talking about.
uhoh
CC BY-SA 4.0
2021-04-11T13:55:39.797Z
Thank you for all the informative comments, that figure could explain it.  I've read in this article, https://earthweb.ess.washington.edu/space/ESS495/venus_arney.pdf that oxygen airglow is at 1.27 $\mu$m and that a 1.0 $\mu$m window has 96% of the surface thermal emission, so I think they should be more surprised to see that airglow.
Cornelis
CC BY-SA 4.0
2021-04-11T19:57:44.427Z
@Cornelis Oh that's amazing! What a cool presentation, thanks! They (very roughly) imaged the surface of Venus from Earth by spectrally looking through the clouds.
uhoh
<photography><venus><hubble-telescope><instruments><parker-solar-probe>
Has Hubble photographed Venus in near IR? If so how does it compare to the new and exciting Parker Solar Probe image?
uhoh
# Has Hubble photographed Venus in near IR? If so how does it compare to the new and exciting Parker Solar Probe image?

(Asked by: username_0 on 2021-02-26)

Phys.org's [Parker Solar Probe offers stunning view of Venus](https://phys.org/news/2021-02-parker-solar-probe-stunning-view.html) includes the image below taken by the Parker Solar Probe during it's most recent gravitational assist flyby of Venus as it continues to rid itself of energy to get ever closer to the Sun.

Apparently the image is surprising because it shows surface features of the planet, which was not expected to happen.

The article says:

> "**WISPR effectively captured the thermal emission of the Venusian surface,"** said Brian Wood, an astrophysicist and WISPR team member from the U.S. Naval Research Laboratory in Washington, D.C. **"It's very similar to images acquired by the Akatsuki spacecraft at near-infrared wavelengths."**
>
> **This surprising observation sent the WISPR team back to the lab to measure the instrument's sensitivity to infrared light.** If WISPR can indeed pick up near-infrared wavelengths of light, the unforeseen capability would provide new opportunities to study dust around the Sun and in the inner solar system. If it can't pick up extra infrared wavelengths, then these images—showing signatures of features on Venus' surface—may have revealed a previously unknown "window" through the Venusian atmosphere.
>
> "Either way," Vourlidas said, "some exciting science opportunities await us."

Wikipedia's [Akatsuki\_(spacecraft)](https://en.wikipedia.org/wiki/Akatsuki_(spacecraft)) says that it had:

* Lightning and Airglow Camera (LAC, 552-777 nm)
* Ultraviolet imager (UVI, 283–365 nm)
* Longwave infrared camera (LIR, 10 μm)
* Infrared 1 μm camera (IR1, 0.90–1.01 μm) is imaging on the night side heat radiation emitted from Venus's surface and help researchers to spot active volcanoes, if they exist. While on the day side, it sensed the solar near-infrared radiation (0.90 μm) reflected by the middle clouds. Unavailable for observation after December 2016 due to an electronic failure. (references)
* infrared 2 μm camera (IR2,, 1.74–2.32 μm) studied the night side lower clouds' opacity to the thermal emission from the surface and deeper atmosphere. It also sensed on the day side the CO2 band at 2.02 μm, which can be used to infer the altitude of the top of the clouds. Finally, the 1.65-μm filter was used during the cruise phase to study the zodiacal light. Unavailable for observation after December 2016 due to an electronic failure.
* Ultra-Stable Oscillator (USO) for performing radio occultation experiments.

Wikipedia [gives](https://en.wikipedia.org/wiki/Venus) the average surface temperature of Venus as 737 K which is 464 °C. I've plotted a [Planck distribution](https://en.wikipedia.org/wiki/Planck%27s_law) for that temperature below.

Venus is hot enough where thermal radiation can be found in "near IR" and not just "thermal IR" which explains the mixing of the terms in the article.

[The Wide-Field Imager for Solar Probe Plus (WISPR)](https://link.springer.com/article/10.1007/s11214-014-0114-y) (paywalled but also found [here](https://orbi.uliege.be/bitstream/2268/200751/1/The%20Wide-Field%20Imager%20for%20Solar%20Probe%20Plus%20(WISPR).pdf) and [here](https://www.researchgate.net/publication/273351399_The_Wide-Field_Imager_for_Solar_Probe_Plus_WISPR)) gives the specifications for the two cameras comprising WISPR:

```
Table 4 WISPR Optical Design

                             Spectral    Entrance                        RMS Spot 
                   FOV      Range (nm)  Pupil (mm)   F#    # of lenses   Size (µm)
                ---------   ----------  ----------  -----  -----------   ---------
Inner Telescope 40◦ × 40◦    490–740      7.31      3.83     5-element       19
Outer Telescope 58◦ × 58◦    475–725      8.08      4.04     6-element       20

```

With nominal cutoffs at 725 and 740 nm one might not expect to pick up much radiant light from Venus' surface compared to reflected sunlight from the clouds in these spectral ranges, **thus the excitement!**

The NASA JPL page [Venus Cloud Tops Viewed by Hubble](https://www.jpl.nasa.gov/images/venus-cloud-tops-viewed-by-hubble) has an image of Venus taken by Hubble in ultraviolet.

**Question:** Has Hubble photographed Venus in near IR? If so how does it compare to the new Parker Solar Probe image?

Wikipedia's [Hubble Space Telescope](https://en.wikipedia.org/wiki/Hubble_Space_Telescope) gives the spectral range of the WFC3 as 0.2–1.7 μm ([WFC2](https://esahubble.org/about/general/instruments/wfpc2/) was 120 to 1000 nm) so If there is a filter that coincides with whatever spectral window of Venus' atmosphere is involved in this, it's possible that the effect may have been spotted earlier.

[![enter image description here](https://i.sstatic.net/RwS28.jpg)](https://i.sstatic.net/RwS28.jpg)

---

The figure's caption says:

> When flying past Venus in July 2020, Parker Solar Probe's WISPR instrument, short for Wide-field Imager for Parker Solar Probe, detected a bright rim around the edge of the planet that may be nightglow -- light emitted by oxygen atoms high in the atmosphere that recombine into molecules in the nightside. **The prominent dark feature in the center of the image is Aphrodite Terra, the largest highland region on the Venusian surface.** Bright streaks in WISPR, such as the ones seen here, are typically caused by a combination of charged particles -- called cosmic rays -- sunlight reflected by grains of space dust, and particles of material expelled from the spacecraft's structures after impact with those dust grains. The number of streaks varies along the orbit or when the spacecraft is traveling at different speeds, and scientists are still in discussion about the specific origins of the streaks here. The dark spot appearing on the lower portion of Venus is an artifact from the WISPR instrument.
>
> Credit: NASA/Johns Hopkins APL/Naval Research Laboratory/Guillermo Stenborg and Brendan Gallagher

[![enter image description here](https://i.sstatic.net/wpGQm.jpg)](https://i.sstatic.net/wpGQm.jpg)

**Tags:** photography, venus, hubble-telescope, instruments, parker-solar-probe

> **Comments:**
> **username_1**: I confess to being a bit dubious about the "we're seeing emission from the surface" claim. As far as I can tell, the Akatsuki imager doesn't see emission from the surface in any of its near-IR filters; it's all coming from the clouds, dozens of km up. (And I'd kind of expect any thermal emission to be overwhelmed by the reflected solar radiation, anyway.)  
> **username_1**: A recent paper on Akatsuki imaging of Venus: https://www.researchgate.net/publication/323149701_Venus_looks_different_from_day_to_night_across_wavelengths_morphology_from_Akatsuki_multispectral_images  
> **username_0**: @username_1 every planet has a night side, no? From the quoted Wikiblurb: "Infrared 1 μm camera (IR1, 0.90–1.01 μm) *is imaging on the night side heat radiation emitted from Venus's surface"* Wouldn't any night side disk images have to come from thermal radiation from something at least?  
> **username_1**: I assume this is a dayside image. (WISPR is supposed to be sensitive to $\sim 480-750$ nm, so why would you point it at the nightside of Venus?)  
> **username_0**: @username_1 Phys.org article says "WISPR is designed to take images of the solar corona and inner heliosphere in visible light, as well as images of the solar wind and its structures as they approach and fly by the spacecraft. At Venus, the camera detected a bright rim around the edge of the planet that may be nightglow—light emitted by oxygen atoms high in the atmosphere that recombine into molecules in the nightside." It's a flexible instrument, intended to image "diffuse glowing stuff in space" rather than planetary disks. At Venus I think you would only point it at the night side.  
> **username_1**: You're right in that at 1.01 microns, the IR1 channel of Akatsuki can see some thermal emission from surface (https://www.stp.isas.jaxa.jp/venus/docs/vco.pdf). But that's nightside-only, and 1.01 microns is, I suspect, beyond the plausible sensitivity of the WISPR sensor anyway.  
> **username_1**: OK, I was wrong; the article does mention it's a "nightside" view of Venus,  
> **username_2**: While Q1 seems to be answered by one question below, Q2 is cryptic. It seems that you intend seeing a surface by virtue of its emission only, but reflection of the sun rays will let you image that point nevertheless. To see what you you likely mean is possible just in the nigh sight. That is all. To make it easier, to spot a green field you right need sun light. The principe is the same in the ir, with the difference that the field can be seen in the dark. So in my opinion the question should be how we can be sure that the features are surfaces based and not coming from the clouds.  
> **username_0**: @username_2 I wrote that before we came to the conclusion that this is the night and not the day side. I'll just delete it; it's not necessary to explain how light works to me.  
> **username_2**: @username_0 sorry but I just commented to your question. It looked like that something irradiated it cannot be seen, at least in the first version of the question you were posing that to "see" a surface, that can't be "illuminated". That is obviously no true and I have commented on that. I have no idea what you know of light, and it looked like you did not know or momentarily skipped that point.  
> **username_0**: @username_2 There are several simultaneous issues involving both a possible unexpected spectral transmission window in the filters of at least one of the cameras, and also a  possible unexpected spectral transmission window in the atmosphere. There are workable scenarios for this unexpected image in both day and night imaging, but we've since verified that it is a nighttime image as the exposure was intended to look at other weak optical signals from the atmosphere and surrounding ionosphere. The dark region has been specifically identified as a geographic feature so it's not "clouds".  
> **username_0**: @username_2 this is mostly unrelated, but it's sort-of the opposite; the narrow and dark solar Fraunhofer (absorption) lines allow spacecraft with narrow spectral transmission windows to image faint fluorescence from chlorophyll molecules in leaves on Earth during the day  https://space.stackexchange.com/a/33823/12102  
> **username_2**: @username_0 interesting. That is about the point. Actually all this makes me think on how we distinguish surface and atmosphere in far object. I mean a pic of Earth is easy to interpret because we know how clouds look. But for Venus, what exactly let us discern the two? It is about assumptions and combinations of various knowledge?  
> **username_0**: @username_2 maybe my answer will not be so exciting, but Venus has been mapped in great detail by radar, so planetary scientists that study Venus (Venusologists?, Venonomers?) will already have an idea what clouds *usually* look like and be primed to recognize surface features when presented them. Those associated with the spacecraft mission will have ready access to the path of the flyby and the longitudes visible at the moment the photo was taken, so they could make a direct comparison. I'm totally baffled by Jupiter and Saturn that look like they have real structures but are just swirly gas  
> **username_2**: @username_0 sure radar is there. Also one can take pics at different times. :) my curiosity was a bit naive  
> **username_0**: @username_2 I think that there's always been quite a public "spotlight" on Mars, and so all the work that's been done on Venus is a lot less widely popularized and appreciated.  
> **username_3**: @username_0  Interesting ! But it it's really the surface of Aphrodite Terra, with an emission from roughly 1000-2000 nm, why should there be a previously unknown "window" ?  
> **username_3**: @username_0 If those are the contours of Aphrodite Terra, then north of it should be another highland. There isn't ! Couldn't those bright features be nightglow as well ?  
> **username_0**: @username_3 when I posted this question the image was a surprise. My understanding is that they weren't sure if there was an unexpected spectral window in *the atmosphere*, or in the *camera's filters*. Silicon's long-wave cut-off is not sharp. It's roughly 1100 nm but temperature and doping and to some extent thickness can have an effect. This [figure](https://i.sstatic.net/60fUM.png) from the top of page six in Hamamatsu's [Si photodiodes](https://www.hamamatsu.com/resources/pdf/ssd/e02_handbook_si_photodiode.pdf) suggests a little bit of sensitivity out to 1200 nm, but no conditions given  
> **username_0**: @username_3 in [this comment](https://astronomy.stackexchange.com/questions/41691/?noredirect=1#comment85502_41696) I mention that the filters were applied as coatings directly to the lens surfaces. It's possible that if the coatings were incorrect thickness or nonuniform they may not be completely blocking the longer wavelengths as expected. We have to wait and see, but interference filter design is tricky, and it's really hard to completely block a wide range of wavelengths using interference only. Even a tiny manufacturing error can lead to incomplete cancellation and therefore some "leakage"  
> **username_0**: @username_3 as far as the landforms are concerned I'm clueless. If you have some insight about that, what we're looking at in the image, maybe you can add a supplemental helpful answer comparing what the image caption says in the question to a good map of Venus, so we can see what they're talking about.  
> **username_3**: Thank you for all the informative comments, that figure could explain it.  I've read in this article, https://earthweb.ess.washington.edu/space/ESS495/venus_arney.pdf that oxygen airglow is at 1.27 $\mu$m and that a 1.0 $\mu$m window has 96% of the surface thermal emission, so I think they should be more surprised to see that airglow.  
> **username_0**: @username_3 Oh that's amazing! What a cool presentation, thanks! They (very roughly) imaged the surface of Venus from Earth by spectrally looking through the clouds.  

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## 1 answers

### Answer by username_1 on 2021-02-26. Score: 10 | Accepted answer

This web page -- ["Here is why the Hubble Space Telescope only looked a few times at Venus (and why it looked at the Moon instead)"](https://melinasworldblog.wordpress.com/2020/04/13/here-is-why-the-hubble-space-telescope-only-looked-a-few-times-at-venus-and-why-it-looked-at-the-moon-instead/) -- seems like a pretty good answer to your main question (note: "MAST" = [Mukulski Archive for Space Telescopes](https://archive.stsci.edu)):

> There are only a few times the Hubble Space Telescope did look to
> Venus according to MAST. Once in 1995 with the Wide-Field and
> Planetary Camera 2 (WFPC2) and the High Resolution Spectrograph (HRS),
> the second time was STIS [an optical spectrograph] in 2013.
>
> So the two WFPC2 F218W and F255W [near-UV] images that we have of Venus with the
> Hubble Space Telescope are maybe the only sharp images of Venus by
> Hubble.

And of course the reason for this paucity is not too mysterious:

> The first rule for (non-solar) Space Telescopes: Do not look at the
> sun. This rule is the reason why the Hubble Space Telescope did look
> at Venus only a few times. Venus is one of the inner planets and the
> Hubble Space Telescope is in an orbit around the Earth. You will find
> Venus always on the evening and morning sky because Venus is seen
> close to the sun from our position. Looking close to the sun with a
> Space Telescope is risky. If anything goes wrong during such an
> observation it would be the end of the telescope.

So, apparently no near-infrared images with *HST*.

> **Comments:**
> **username_0**: Thanks for your post! Okay, yes this seems to be the answer to my question. I bumbled through MAST far enough to get "1 rows displayed, but 2 are available" which seems to confirm what the blogpost states. We'll have to stay tuned to this to see what re-testing of the (pesumably) backup versions of the cameras on Earth reveals about the filters; maybe they have some extra wiggles below their nominal cutoffs.  
> **username_1**: I get the impression (from reading around a bit) that WISPR doesn't have any filters, so it may be a question of whether the sensor is more sensitive at the edge of the near-IR than they assumed.  
> **username_0**: With spectral ranges of 490–740 and 475–725 for the two cameras (see question) I can't see how this can be achieved without a filter *somewhere!* **update:** the paper I cite in the question says "The bandpass for each telescope is selected using a combination of long/short wavelength cutoff filters deposited on internal lens surfaces similar to SECCHI/HI."  

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