Arp's wish has been realised?

[Nereid]

In Seeing Red, Arp wrote:

what was needed was a wide field optical survey of the dark sky from above the earth's atmosphere (a space Schmidt). That would have revealed the crucial relationships of different kinds of objects to each other.

While I have not (yet) been able to find any document (on the internet) which describes this proposed space Schmidt in Arp's own words (does anyone have a reference?), I have found several documents on proposals for such a telescope.

For example, Space Schmidt telescope:

The complete survey takes in 3627 fields, each 4.87 deg in diameter, arranged in a hexagonal pattern superimposed on the celestial equatorial coordinate system. The declination bands are spaced every 3 deg, 20 min. The optical instrument is a folded all-reflecting Schmidt system with an aperture of 0.74 m, a focal length of 2.0 m (f/2.7), a circlar field with a diameter of 4.87 deg, and a limiting image diameter of less than 2 arcsec over the entire field. The detector is an electrographic camera having a photocathode diameter of 170 mm. In discussing the telescope structure, it is pointed out that the optical support system is to be of graphite-epoxy construction. The focal tolerance (the most critical optical tolerance) is to be + or - 12 microns. Regarding contamination control, it is expected that with appropriate design it will be possible to operate in sunlight for observations in a restricted portion of the sky, at least more than 90 deg from the sun, depending on the geometry and reflectivity of the platform or spacecraft configuration.

While SDSS (Sloan Digital Sky Survey) was (is) not based in space, it would certainly seem to compare favourably with the above!

It is a 2.5 m telescope (vs a 0.74 m one), has a field of 1.5 degrees (vs 4.87), has imaged 11663 sq. deg. (vs planned ~17600), and an image resolution of 1.4" (vs <2"). While the wavebands (or filters) of the planned space Schmidt are not specified (in the above), and would have likely included some in the UV not accessible on the ground, SDSS' five bands cover the optical from ~330nm to ~1 micron. The sensitivity, and so the 'limiting magnitude', is also not specified; SDSS' is ~22 (21.3 in the i band, 20.5 in the z).

All the SDSS observations are available online, for free. The site also offers several powerful tools for searching and data mining, tools that astronomers in the 1980s and 90s could only have dreamed of.

It gets better.

GALEX is somewhat like a space Schmidt in that it has an all-sky survey objective (which it has achieved 96% of), through two UV wavebands, and a limiting magnitude comparable to that of POSS (~20.5); however, its image resolution is not so good (~5"). All data is publicly available, though the access doesn't have quite the ease of use and power as that provided by SDSS.

In my next post I'll briefly describe what else is already available, what is coming soon, and what is planned; I'll also include a graphic (well, a link to one) which summarises the depth ('limiting magnitude'), wavelength range, and sky coverage of the many surveys which together add up to far more than Arp refers to (in the quote above).

My thanks to Aristarchus for drawing my attention to this.

[Nereid]

The Southern Sky Survey (SSS) is currently underway, and when completed - in a year or two - will complement SDSS by covering most of the sky not observable from the location of the SDSS telescope, and by achieving similar angular resolution, observing with the same filter set, and achieving the same 'limiting magnitude' (it will actually go somewhat deeper). However, it will not do spectroscopy (I can't say for sure, but it would seem that Arp's 'space Schmidt' proposal did not include spectroscopy either).

The first phase of the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) is already taking data. If it lives up to its design specs (and almost all large astronomical projects in the last few decades have, even if there were some bumps along the way), it would do all that Arp could have wanted, and more. It will go much deeper than SDSS (24 mag), and will do imaging and photometry on much more of the sky (SDSS has imaged about a quarter of the sky; Pan-STARRS will image about three-quarters of it). Its angular resolution should also be better, because the observing site has considerably better seeing, consistently, than the SDSS one. However, it may not have the same waveband coverage as either SDSS or SSS.

The Large Scale Synoptic Telescope (LSST) is in the development and construction phase; it is like Pan-STARRS on steroids! It too will be at one of the world's best observing sites, will go even deeper than Pan-STARRS, will cover the sky multiple times, and will observe in many wavebands (Iikely more than SDSS did). As it is in the southern hemisphere, it will complement Pan-STARRS. Like Pan-STARRS, it will not do spectroscopy.

The Wide-fields Infrared Survey Explorer (WISE) is a space-based mission that will release most of the data it has (or will have) acquired early next year. It operates in the mid-infrared (four wavebands) and has a sensitivity comparable to POSS; it has completed its all-sky survey (but the results have not yet been released). It's a bit like Arp's space Schmidt, but covering a much wider swath of the electromagnetic spectrum.

(I'll cover one more infrared survey - the UKIDSS - in a later post; I'll also briefly summarise surveys done, or planned, in the gamma-ray, x-ray, far-IR, microwave, and radio regions).

Here is a figure which summarises some astronomical surveys in the optical region, as well as UV, IR, and microwave (sub-mm actually); to get an idea of sky coverage, SDSS had about 20% at the time this figure was made, and GALEX about 40% (the JWST, ULIRG, and Brown Dwarf lines can be ignored):

(source)
Caption:

This figure shows the 5 sigma point source sensitivities of WISE and previous or planned all-sky surveys. The planned wavelength range for the JWST is indicated. The dot size shows the planned sky coverage. GALEX is a small Explorer (SMEX) which was launched by NASA in 2003, SDSS is the groundbased Sloan Digital Sky Survey, DPOSS is the groundbased Digital Palomar Observatory Sky Survey, ASTRO-F is the Japanese satellite, renamed Akari after launch on 22 Feb 2006, and Planck is the European CMB mission to be launched in 2009 which also includes a good sub-mm survey capability.

The net: the sky has been (or soon will be) imaged, consistently, to considerable depth (sensitivity), from hard UV (~100nm) to the edge of the microwave (~1mm), and all data is (or soon will be) available.

This, surely, is more than Arp asked for, and may be more than he had dreamed of, back in 1990!

[Harry Costas]

G'day Nereid

Thank you for that info, you always give your best.

One of Arp's wishes was for science to be applied and not the MOB.

Arp has given to some extent more to cosmology than any other scientist.

[Nereid]

Arp has given to some extent more to cosmology than any other scientist.

To what extent, in your opinion, Harry?

Edit: Where are my manners?!?!

Thank you for that info, you always give your best.

Harry, thank you for your kind words.

[Harry Costas]

G'day


Smile

I expected that question from you.

Maybe I just followed his work more so than any others. Just following his works is just incredible, easy to do.

I should not limit the work of others, there are some quite workers out there on the front line focusing on the science issues and not the Gold Star Collection.

[Nereid]

UKIDSS, or the UKIRT Infrared Deep Sky Survey; UKIRT refers to the UK Infrared Telescope (UKIRT) in Hawaii ... it's that telescope that the survey instrument, WFCAM, is on.

The survey plans to cover ~7,200 square degrees of the (northern) sky in its main three surveys. In terms of sensitivity, UKIDSS is comparable to SDSS; its angular resolution comparable too (i.e. about 0.5").

In terms of public access, the latest data release is DR5, unless you're in "the UKIDSS community", in which case it's DR8.

The main survey, the Large Area Survey, will be done in four wavebands; the other two large-area surveys in three.

Unlike SDSS, there is no spectroscopy.

[Nereid]

Some other surveys.

NVSS (NRAO VLA Sky Survey) - a rather sensitive, northern sky radio survey at 1.4 GHz, with an angular resolution of ~45".

SUMSS (Sydney University Molonglo Sky Survey) a southern counterpart, but in a different frequency (843 MHz).

HIPASS (HI Parkes All Sky Survey) is more like NVSS, in that it is at 1.4 GHz (21 cm), but unlike in that it also has good 'velocity resolution'.