The Observatory

In comparison to most professional telescopes, the PEST is tiny.  And it’s not located on a mountain top away from light pollution.

It’s an off-the-shelf 12″ scope sitting in my suburban backyard, protected by a roll-away plywood shelter.  But it has some nice features, all geared towards automated, high quality observations of exoplanets.

The key components;

  1. A 12″ Schmidt-Cassegrain telescope on a computer controlled, motorised fork mount.  Enables computer controlled pointing and tracking of a target as it moves across the sky.  The fork mount avoids the need to re-position the telescope as the target crosses the meridian (its highest point in the sky).  Some mount designs would require a ‘meridian flip’.
  2. Charge Coupled Device (CCD) camera.  Basically a digital camera, except that it is monochrome (yes, old fashioned black and white pictures!), and its sensor is cooled while in operation.  The cooling is to reduce image noise – graininess or speckles in long exposure photographs.  I usually run at -5 °C.
  3. Computer controlled focuser.  Enables auto focusing and maintaining focus as the temperature drops through the night.  Without it, as the telescope tube contracts, focus quality would deteriorate.
  4. A computer to run everything.  Observations are programmed at the start of the observing session and will run unattended through the night.   No need for me to stay up!
The PEST Observatory: Image Credit: TG Tan
View of the back of the scope showing the focuser and camera. Image Credit: TG Tan
View of the telescope with the shelter rolled forward. Image Credit: TG Tan

 


Technical Details

PEST is a backyard observatory with a 12” Meade LX200 SCT f/10 and focal reducer yielding f/5.  The camera is an SBIG ST-8XME with a filter wheel loaded with B, V, Rc, Ic and Clear filters.  Focusing is computer controlled with an Optec TCF-Si focuser.

The observatory is controlled using a PC running Windows XP with the following software.  CCD Commander to script observations, FocusMax for focuser control and CCDSoft for control of the CCD camera.  PinPoint is used for plate solving.  Internet connection is available.  AtomTime Pro 3.1 is used to synchronise the PC clock with the atomic clock in Boulder, CO.  This synchronization is done at observatory startup then every 3 hours.

The telescope and computer are housed in a roll-off plywood enclosure which is opened and closed manually.

This setup enables completely automated (other than roll-off/on) observations throughout the night, including the acquisition of sky flat frames at dusk and/or dawn, and if necessary other calibration frames.

Location

PEST is located at 31° 58’ S, 115° 47’ E, elevation 24m, in a suburb of the city of Perth, Western Australia.  Distance from the city centre is about 8km, and the site suffers from the resulting skyglow.

 


Observations

The image scale obtained is 1.2 arcsec/pixel and a full frame image covers 31 arcmin x 21 arcmin.

Exoplanet transit observations are usually done with the Rc filter and individual image integration times of 120s.  For images in focus the usual star FWHM achieved is about 2.5 to 3.5 pixels.  For target stars dimmer than about Vmag = 11.5,  images are taken with the telescope in focus.  For brighter stars, the telescope is de-focused slightly to FWHM of about 6 pixels in order to keep within the CCD’s limit of linearity.  This is done by focusing through the Clear filter for Rc images.  CCDSoft allows the use of ‘filter offsets’ that moves the focuser slightly when a filter is changed.  This enables fine and consistent control of the amount of de-focus.

A normal night’s observation starts with the observatory being rolled back, power and computer switched on and CCD Commander, CCDSoft, and FocusMax started.  Connection to the camera is made and camera cooling started.  The telescope is then started.  I enter the target(s) for the night into a CCD Commander script.  The script waits for the sun to be at a specified angle below the horizon then starts an automatic sky flat sequence (if necessary).  Once this is finished there is a further wait for darkness, then a focusing run is done.  This involves a slew to a ~mag 5 star, plate solve to centre the star as well as to synch to the right sky coordinates.   FocusMax then does focusing.  If there is no change of filter through the night this is the only focusing I will do.  The Optec focuser has temperature compensation that will compensate for contraction of the optical tube through the session.

The rest of the observation sequence takes place unattended.  As the sky brightens just before dawn (specified in terms of sun altitude), CCD Commander will stop data acquisition, then wait for further light before starting to take sky flats.  When the sky is too bright for any further flats, CCD Commander parks the scope.  All data, with the exception of the flats are written straight to my home office computer through my (wired) home network.  Flats are saved onto the observatory hard disk because the slight additional write time through the network would mean I get fewer flats in the limited time between when the sky is too bright and too dark.  All I have to do when I wake up is to turn off the telescope and computer and close the observatory.

 


CCD Technical Data and Linearity

The gain of my ST-8XME CCD camera is measured to be 2.71 e-/ADU with a read noise of 19.9e-.  I used the method described by Michael Newberry of Mirametrics in a technical note.  I have included on this site a full calibration sheet for my CCD.

CCD Gain Measurement

The note does not mention it (and in fact I have not found anything in the literature that describes this way of assessing non-linearity) but if you extend the same graph used for gain measurement to higher ADUs, there is a point at which it takes a sharp departure from the straight line.  I have found that this provides a much more sensitive assessment of where the CCD’s linearity ends.

For my CCD, non-linearity starts at about 40,000 ADU, as indicated in the chart of ADU vs Variance (CCD response measured at a range of light levels) below.  This is important in the kind of high precision photometry needed for exoplanet observations.  This deviation from linearity is quite subtle but if a star image from the CCD has pixels exposed to above 40,000 ADU, its photometry does show higher scatter.

Onset of non-linearity

The camera is run un-binned at a temperature of -5 °C.  Readout time for a full frame is approx. 7s.