Emily LakdawallaSep 16, 2009

How does Hubble compare?

Apologies in advance for the number of acronyms in this post. There's no way to get around them.

Last week when the new, post-servicing-mission-4 capabilities of Hubble were unveiled, I kept asking myself: How do Hubble's new capabilities compare to what it could do before the servicing mission, and how do they compare to what we can do from the ground? I fired off some questions to Heidi Hammel, and did some research on my own within the Hubble instrument description documents.

There's four main Hubble instruments I was interested in comparing with each other and to Earth:

  • The Wide Field and Planetary Camera 2 (WFPC2), an ultraviolet- to very near-infrared imager, Hubble's workhorse from 1993 to 2009;
  • the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS), a near-infrared imager, installed in 1997, which has had periodic problems with its cooling system and was most recently offline from November 2008 to August 2009;
  • the Advanced Camera for Surveys (ACS), an ultraviolet- to very near-infrared imager, in use since 2002 but with a hiccup in 2007, mostly repaired in May 2009; and
  • the newly-installed Wide Field Camera 3 (WFC3), which replaced WFPC2 in May 2009 and was also intended to largely supplant NICMOS.

Here's the executive summary: what WFC3 brings to the party is a killer combination of large field of view and pretty high resolution. It's an improvement on WFPC2 in almost every way. It improves marginally on the resolution of the largest-field-of-view optical images that Hubble was capable of before (ACS Wide Field camera), and it really beats the pants off of the field of view of NICMOS, though at lower resolution than NICMOS can manage. (So it's a good thing that they're now reporting progress on getting NICMOS' cooling system working again.) And although Earth-based instruments at the Keck, VLT, and Gemini observatories can beat Hubble resolution using adaptive optics, that's only at near-infrared wavelengths; shorter than 800 nanometers (and especially in the ultraviolet) there's nothing on Earth that can get sharper images than Hubble.

I spent a couple of hours this morning noodling around the Space Telescope Science Institute website reading instrument descriptions. I came up with the following diagram that compares the capabilities of the different instruments. The top row (above the dashed line) compares fields of view -- how much of the sky each one takes in. (For comparison, the Moon at 30 arcminutes or 1800 arcseconds across would be about twice the width of the whole diagram.) The bottom row (below the dashed line) compares pixel sizes. Ideally you'd like to maximize field of view and minimize pixel size.

Some Hubble instrument fields of view and pixel sizes

Emily Lakdawalla, based on Space Telescope Science Institute documentation

Some Hubble instrument fields of view and pixel sizes
Top row: the fields of view for five Hubble instruments are compared. Bottom row: single pixel sizes for four of the instruments. STIS: Space Telescope Imaging Spectrograph. NICMOS: Near-Infrared Camera and Multi-Object Spectrometer, which has three independent cameras NIC1, NIC2, and NIC3. ACS: Advanced Camera for Surveys, which has three channels: Wide Field, Solar Blind, and High Resolution. WFPC2: Wide Field and Planetary Camera 2, which has two channels: Wide Field and Planetary Camera. WFC3: Wide Field Camera 3, which has two channels: Ultraviolet-Visible and Infrared.

Here's a table summarizing the same geometric information, plus info on detector size and wavelength sensitivity.

(arcsec /
range (nm)
WFC3 UVIS162 x 1620.042@ 2051 x 4096
(35-pixel gap in between)
WFC3 IR123 x 1360.131014 x 1014850-1700
WFPC2 PC34 x 340.046800 x 800115-1100
WFPC2 WF2, WF3, WF4150 x 150 (L shaped)0.1Three 800 x 800115-1100
ACS WFC202 x 2020.05Two 2048 x 4096350-1100
ACS SBC34.6 x 30.10.034 x 0.0301024 x 1024115-1700
ACS HRC29 x 260.028 x 0.0251024 x 1024170-1100
NICMOS NIC111 x 110.043256 x 256800-1800
NICMOS NIC219.2 x 19.20.075256 x 256800-2450
NICMOS NIC351.2 x 51.20.2256 x 256800-2300
  • NIRC2 on the Keck II telescope on Mauna Kea, Hawaii;
  • NaCo on the Very Large Telescope (VLT) at Cerro Paranal, Chile; and
  • the NIRI imager with ALTAIR adaptive optics system at Gemini North Observatory, also on Mauna Kea.

Why not just pick a few interesting objects out there in space and actually look at some comparable images? Nancy Atkinson over at Universe Today already did that for some Hubble targets, though she didn't specify which cameras the before and after images came from. I searched for some things to compare between the newly released Hubble images and the Earth-based ones that Heidi suggested I check, but I couldn't find anything comparable. I believe that the reason for that is the newest Hubble releases were selected in order to showcase the broad field of view of the new Wide Field Camera 3, and none of those Earth-based imagers holds a candle to WFC3 (or ACS's Wide Field channel, for that matter) in field of view.

In the end I feel like this was not a very productive use of my day! I guess what I've learned is that there isn't an easy answer to the question of how all these facilities compare. Which one you'd like to use depends upon a complicated game, trading between resolution, fields of view, wavelength sensitivity, the kinds of filters available (something I didn't even get into here), and how hard it is to get a proposal approved for time on the instrument of your choice.

With that, I'd better quit, and move on to something else! Hopefully tomorrow I'll have more to show from a day's work.

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