July 29th: The Evryscope Sky Survey System


Podcaster: Cosmic

Apogee PodcastTitle: The Apogee Podcast –  The Evryscope Sky Survey System

Links: astroandmusic.blogspot.com

Original Paper: http://arxiv.org/abs/1407.0026

Description:  In this Apogee podcast, Cosmic discusses purpose of sky survey telescopes, and details the work of one such system under development.

Bio: Cosmic is a self- and crowd-funded independent research astronomer and space musician.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2014, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.


Hello! This is Cosmic, and welcome to the Apogee podcast! This podcast is still relatively new and subject to evolution. For now, the format of this podcast will be myself describing, discussing, and critiquing an article of my choice from an astronomical journal. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.

If you have any articles to suggest for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT yahoo DOT com.

The music you hear in the background are my own compositions, which can be downloaded from my soundcloud channel ‘cosmiclettuce’. I hope you enjoy listening to space music as much as I do. The apogee for this podcast took place at 03:28 UTC on 28 July 2014. The lunar distance at that time was 406568 km, which is 637 km further away than last apodee on the 30 June, and 46 km closer than the next apogee on 24 August.

How do we know how far away the moon is?

  • observed enough to calculate an orbit
  • laser range measurements (light travel time)
  • parallax

This month, the moon doesn’t appear near any bright background object (like a planet) at any opportune time, so we’ll have to wait for another lunation or two to once again try to measure the distance to the moon using parallax.

On with the paper!

This apogee’s article is entitled “The Evryscope: the first full-sky gigapixel-scale telescope” by Law, Fors, Wulfken, Ratzloff, and Kavanaugh coming out of the department of physics and astronomy at the univ of north carolina at chapel hill.  The term ‘Evryscope’ means “wide seeing” in Greek, as opposed to ‘telescope’ which means “far seeing”.

The goal of every sky survey is to image a large part of the sky as deep as possible (see the faintest objects possible with good signal to noise), as often as possible.  I guess the ultimate system would be one, for example, that images the entire sky to a visual magnitude of 22 several times every second.

But why is this so important?  Ultimately, I think it has to do with our innate and intense curiosity about things.  We like to know what’s going on and the ultimate sky survey system I mentioned before would most definitely allow us to be aware of many things going on.  We also like to study and measure things, and there are LOTS of things out there in the universe and they are always changing.

So what kinds of things can a sky survey allow us to watch?  Well, that depends on the cadence of the survey — how often does a particular patch of sky get imaged?  On a one year cadence, you can track the brightness of long-period variable stars.  On a month and week cadence, you can detect exoplanets that orbit their stars in months or years.  On a daily basis, you can detect the motions of asteroids, comets, planets, and moons.  You can also get a good look at seasonal changes in the sky (like sky brightness versus time of year) You can track the brightness of short-period variable stars.  On an hourly cadence, you can start detecting transiting exoplanets and supernovae as they go off.  On a one minute cadence, you can starf looking at gamma-ray bursts. A cadence of one second would allow us to watch meteors burn up as they enter our atmosphere.  Generally, cadences shorter than about 30 seconds mainly just increase the temportal resolution of the data in the next-longer cadence – a very nice thing in order to monitor both long and short term activity.  These are just a few examples of what can be studied.  Most of these are what astronomers call “transient events” — things that happen from time to time in an unpredictable way (you just need be at the right place and the right time to see them!).

The Evryscope that these guys are developing takes an image of the sky once every two minutes to a limiting visual magnitude of 16.5 with millimagnitude (a few thousandths of a magnitude) precision, with a possibilty of getting as faint as 19th magnitude if images are co-added — essentially creating a long-exposure image.

They aren’t going to do this with a single telescope.  I don’t know if that’s even technically feasable.  Instead, they are going to construct an array of 23, 7cm telescopes arranged on a “mushroom” affixed to a german equitorial mount (see figure 1 in the article).  Each telescope will be pointing to a different part of the sky with their edges overlapping (see figure 2).  Each telescope has a field of view of 25.4 by 18 degrees.  The pixel scale of this system is 13 arcsec per pixel.  To put that into perspective, that would mean that the full moon would be about 138 pixels wide.  The full field of view of the Evryscope will be 9060 square degrees.

The group at UNC Chapel Hill has calculated that the Evryscope will produce a 660 MPix image every two minutes, or 5MB per second.  The 20TB data storage units can hold about three months worth of data.  The group intends to produce two data products.  First and foremost, photometrically and astrometrically calibrated images.  Second, millimagnitude precision photometric light curves for every star in the field of view.  There are about 500 million stars that are brighter than magnitude 16.5 and there are 9.3 billion stars visible to magitude 19.  Wow that’s a lot of lightcurves.  The data will be available “to interested astronomers on request”.  Hmmm, I would rather that this be available via a website that allows anyone to search for and retrieve whatever data they want anytime.  Perhaps they don’t have the resources or know-how to be able to make this data available in the web.  That’s a very tough thing to do, but there are many websites that have very nice user-interfaces and database back-ends to allow people to access data so I’m not quire sure why this group isn’t able to do the same thing.

For the past two years, a single telescope system has been operating and has produced very satifying results.

A prototype version with several (the article doesn’t specify exactly how many) 7cm telescopes will be deployed in early 2015.  This system will allow the group to validate the mushroom design concept, test the overal system performance, and begin doing some science with an exoplanet search using a transient monitoring program.

This sounds like a very exciting sky survey project and I look forward to seeing the results of the prototype system and ultimately having access to what will be a very incredible ever-expanding data set.

So until next time, Peace.

End of podcast:

365 Days of Astronomy

The 365 Days of Astronomy Podcast is produced by Astrosphere New Media. Audio post-production by Richard Drumm. Bandwidth donated by libsyn.com and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. In the new year the 365 Days of Astronomy project will be something different than before….Until then…goodbye

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