Imaging Mars with the Hubble Space Telescope
photo credit: Max Mutchler/Space Telescope Science Institute
Photo Information from Max Mutchler: What you see above are individual Hubble WFC3 images of Mars in chronological order. I cycled through 4 different filters (F275W, F410M, F502N, F673N) and hedged on exposure times and other parameters - such as post-flashing - since we never know exactly what bright features, like clouds or polar caps, that we might get. You can see one frame with significant saturation and bleeding, but most frames had little or none. I tried to strike the right balance between going deep without saturating. As you can see, some guesswork was involved in adjusting the camera settings.
Astronomers who take gorgeous images of the planets have a very specific system for their imaging runs, customized for their own telescopes, observing sites, and imaging equipment. Telescopes are set up on precisely balanced and oriented mounts, a webcam or dedicated CCD dropped in, autoguider locked on, data and power cables plugged in and set, with software like Photoshop and Registax ready on their computer for a post-processing enhancement often lasting much longer than the initial imaging run.
Now, imagine that your telescope is the Hubble Space Telescope and your next target is Mars. How do you actually go about taking that spectacular image and scrub it up for public viewing? We were able to talk with someone who actually did just that: Max Mutchler, Research and Instrument Scientist at the Space Telescope Science Institute in Baltimore, Maryland. Max was kind enough to answer questions from amateur astronomers about how he used the Hubble Space Telescope to take the latest amazing image of Mars. Our questions and his answers are below.

Hubble has been around for over a quarter century. In that time astronomy and astroimaging on Earth has changed quite a lot -- webcams and dedicated CCDs have revolutionized planetary imaging on Earth. Does Hubble still use the same cameras when imaging Mars as it did in the 90's, or have they been upgraded?

Max: I used a 4th-generation HST camera (Wide Field Camera 3 or WFC3) to make this Mars image. WFC3 was installed by astronauts during our final Space Shuttle servicing mission in 2009. Many other Hubble cameras preceded WFC3 -- in chronological order we have had WF/PC, FOC, WFPC2, NICMOS, ACS, WFC3 -- and most have imaged Mars.

Most amateur astronomers use software like Registax and Photoshop for their images of Mars and other planets. What software does scientists imaging with Hubble use to process the images? Is it custom software, off the shelf, or a mix?

Max: Starting with the raw images (in FITS format), I use AstroDrizzle to resample the images and decide which pixels to include or exclude. AstroDrizzle is really designed to combine multiple images but we can’t do that with planets because they rotate several pixels between frames so we’d be blurring the images. A collaborator Mike Wolff then deprojected the images which allows us to align and combine any subset of the 25 images (cycling through 4 different filters) that we obtained during our one Hubble orbit. Greg Bacon wrapped those deprojected and aligned “maps” around a sphere to reconstruct Mars without any rotational blurring. I use SAOimage ds9 to display FITS images, but then when we hand them off to Lisa Frattare and Zolt Levay they use FITS Liberator to ingest them into PhotoShop where they create color composite images. 
How was the date picked for imaging Mars? Why wasn't Mars imaged by the Hubble Space Telescope (HST) at the exact date of Mars' closest approach, or at opposition?

Max: Our News team often makes the call in such a situation: we wanted our image to be out just before and during opposition, and it took many days of work to produce the image you see. Our image would have come out “late” -- after all the stories about opposition have already been published. This way, our Hubble image appears in many of those articles instead of some older or lower-quality image.

How far in advance was the Mars imaging scheduled for the Hubble Space Telescope?

Max: We schedule most Hubble observations about 2-3 weeks in advance, and this Mars program was no different. But we starting talking about this observations months ago, and I had to submit a Phase I proposal and then a Phase II observing program in advance.
How long does it take for a picture taken by Hubble to be processed and released to the public by the Hubble team?

Max: It depends because every observation is different and there is no one “Hubble team”. For most observations there is no plan or at least no rush to create a press release image. But for timely events like the Mars opposition, and a program conducted by the Hubble Heritage Team, we can produce a press release image much faster than most other teams -- often within days or a week or two depending on our goal.
Do HST folks use stacking to process their images? Do they process one great exposure? Or do they actually choose the best picture out of a series of exposures?
Max:.For images of stars, nebulae, and galaxies we always combine multiple images to reject cosmic rays and bad pixels, or to make a big mosaic. For moving Solar System targets we do not do any simple image combinations (as described above), but we may be picking the best from a series of images. For this Mars image I specified 25 different exposures and not all of them were great (see the image at the top of the page)! 

Amateur astronomers have to rush to finish their imaging before sunrise. Often they focus on just one or two objects before packing up. Does Hubble has a similar "sunrise" in orbit where it needs to stop taking pictures due to interferences from the Sun, Earth, or Moon, or other factors, or can the telescope simply image anything at any time, all the time?

Max: Not exactly, but we do have similar scheduling constraints. For most objects the Earth itself gets in the way for half of Hubble’s orbit. Then scheduling is dictated by the target’s location, avoiding the Sun and Moon. There is also something called the South Atlantic Anomaly (SAA) where the Earth’s magnetic fields dip down and we get more cosmic rays in our images, so we schedule around it.

Is Mars the closest object to Earth that Hubble can safely and effectively observe?

Max: No. I recently contributed to an observation of one of the nearest objects to ever be observed by Hubble: Comet 252P/LINEAR  with Jian-Yang Li. Technically, we “observe” the Earth itself (cloudtops) sometimes to make flat fields, but that doesn’t count in my mind.
Hubble image of Comet 252p LINEAR
How does Mars' atmosphere affect the quality of images of its surface? If the interference is problematic, what does the team do to help get around the interferences?
Max: Sometimes Mars can be nearly covered by clouds or dust storms and surface features may be obscured. Since we are scheduled weeks in advance, there is not much we can do -- we just had to accept whatever we got on May 12. In this case, I would say we got an interesting amount of cloud features -- the bright clouds over Syrtis Major are very interesting -- but they didn’t obscure surface features too much. Just about perfect, I’d say!

Image of Mars taken on May 12, 2016 by the Hubble Space Telescope
Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), J. Bell (ASU), and M. Wolff (Space Science Institute)

The stunning final result!
The NSN would again like to thank Max Mutchler for his time and for his excellent answers. You can find out more about his work by checking out his page on the STScI's site. If you would like to know more about about Hubble's Mars portrait, you can check out the official release on NASA's site. You can also see a detailed release here.
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