PEST-CDK14 is a home observatory located near the city of Perth, Western Australia.
It’s heart is a very nice Planewave CDK14 (14″ Corrected Dall-Kirkham) telescope. As with previous iterations of PEST, it is protected by a roll-away plywood shelter. The observatory is located on a roof terrace on top of the home we moved into at the end of 2022. The design and selection of equipment are geared towards automated, high quality observations of exoplanets.
The key components;
- A Planewave CDK14 telescope. Optical quality is significantly better than the Meade SCT I had before, with sharp stars across the field of view.
- Mesu-Mount 200 MkII, a computer controlled, friction drive equatorial mount. The motors are equipped with encoders that enable shaft position, and therefore telescope pointing, to be controlled to very high precision. With plate-solving, I can routinely point to within 3″. The friction drives have little to no backlash which enables very good autoguiding performance. The guide errors are usually < 0.5″ RMS, and 0.3″ under good conditions.
- The mount sits on a ‘bended-knee’ tripod, also from Mesu-Optics. This design enables the mount to point anywhere in the sky and avoids the need for a meridian flip as I track a target across the sky.
- QHY268M CMOS camera. Basically a digital camera, except that it is monochrome, 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. Unlike previous generations of CMOS cameras this one does not show any ‘amp-glow’ on long exposure images.
- Computer controlled Esatto 3″ focuser. Enables auto focusing, which I do at the start of an observation run and every hour after. Without re-focusing, the image would drift out of focus as the air cools and the telescope tube contracts.
- 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!
Equipment History
In Sept 2020, my original camera failed. This section is a record of when particular cameras and other equipment were used at PEST.
2010 to 8 Sept 2020: SBIG ST-8XME, ‘Audacious Ant‘. My original camera.
14 Sept 2020 to 26 Feb 2021: SBIG ST-8XME. ‘Bowers Beetle‘. A loan camera from Craig Bowers.
23 Mar 2021 on: QHY183M, ‘CMOS Slug’.
3 Nov 2021 on: A set of Astrodon g’, r’, i’, z’ on loan from Perth Observatory installed in place of the B, V, Rc, Ic filters.
29 Nov 2021: Reinstalled BVRI filters.
18 Jan 2022: Installed Baader g’r’i’zs’ filters.
5 Oct 2025: First light for the PEST-CDK14 observatory. The original PEST is now decommissioned.
Technical Details
The description below is current for Oct 2025 onwards.
Science collaborators who wish to describe the observatory for pre March 2021 observations should refer to this page.
Or this page for the period March 2021 to Sept 2025.
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PEST-CDK14 is a home observatory with a 14” Planewave CDK14 f/7.2 telescope. The camera is a QHY268M with a QHY-CFW3M filter wheel loaded with g’r’i’ and Clear filters. One of the filter slots is occupied by a blocking ‘filter’, used for taking dark frames (the CMOS camera does not have a physical shutter). Guiding is done with a QHYOAG-M off-axis guider with ZWO ASI174MM guide camera. Focusing is computer controlled with a Prima Luce Lab Esatto 3″ focuser.
The observatory is controlled using a laptop running Windows with the following software. Voyager to script observations, control the camera, manage focusing, and guiding through PHD2. ASTAP is used for plate solving and is robust enough to do blind solving. There is ethernet connection to my home network and the internet. Dimension 4 is used to synchronize the PC clock with a set list of atomic clocks. This synchronization is done at observatory startup then every 30 minutes.
The telescope and computer are housed in a roll-off plywood enclosure which is opened and closed manually. The shelter is equipped with a dehumidifier which prevents humidity inside from getting above 70% and an air intake fan that starts in hot conditions (>30°C) to keep the inside temperature at about ambient.
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 32° 0’ S, 115° 52’ E, elevation 20m, 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 field of view is 32 arcmin x 21 arcmin. Image scale for all observations is 0.6 arcsec/pixel. The native image scale is just 0.3 arcsec/pixel but all images are binned 2×2 off-camera.
Exoplanet transit observations are usually done with the r’ filter for maximum precision or with alternating g’ and i’ to check for chromaticity. For images in focus the star FWHM achieved is between 2 to 4 arcsec. Intentional de-focusing is not usually done because the small image scale ensures the star image is spread over many pixels anyway, and most observations also aim to resolve close neighbouring stars in order to make sure they are not the source of the transit signal.
A normal night’s observation starts with the observatory being rolled back, laptop connected and telescope switched on. Voyager is then started. Prior to the this I will have prepared the Voyager script (a ‘Dragscript‘) for the night. I usually re-use a standard script which just needs to be modified with target information, and calibration frames requested. Connection to all equipment is made and camera cooling started. The script waits for the sun to set to specified altitude, then if required, starts an automatic sky flat sequence. My current practice is to take 200 raw flats per filter, with exposure times down to 0.5s.
Once this is finished the script waits for darkness, then slews the scope to a point near the zenith, does a blind plate solve and synch. I do this so that subsequent slews will be more accurate. If the target has not risen yet, dark frames may be taken at this point. At the start of target observations a focus run is done by slewing to a nearby bright star and focusing, before slewing back to the target. Focusing is then re-done every hour. Voyager automates all these, including selecting focus stars.
The full observation sequence takes place unattended, although Voyager has a nice feature which enables messages to be sent as the script runs. I use Telegram as the messaging facility, although you can also use SMS. Through the night, if all is well I will get a message every hour. Any errors that arise will be notified as well.
As the sky brightens just before dawn, or if the target has set, Voyager will stop data acquisition, then wait for further light before starting to take sky flats, if needed. On completion of the night’s sequence, Voyager parks the scope, copies data to my home NAS, and warms up the camera. All I have to do when I wake up is to turn off the telescope, disconnect the laptop and close the observatory.
Camera Technical Data and Linearity
The QHY268M camera has excellent linearity and low noise, in most respects better than the QHY183M it replaced. As I did for the previous camera, I ran some very detailed tests to characterise it, which are documented here.
Binning
Unlike the QHY183M, the camera analog digital convertor (ADC) is native 16-bit. The plate scale on sensor is only 0.3 arcsec/pix so stars are ‘oversampled’ and I can bin without detriment to photometry. My practice is to bin 2×2 to reduce processing and storage requirements.
Binning is done by averaging each group of 2×2 pixels with my custom written FITSbin utility. I don’t bin on-camera because the camera driver bins by adding ADUs, which would result in loss of information if the sum exceeds 65,535. See a much deeper discussion of binning strategies here.
Mode, Gain and Offset settings
The camera has a number of Mode options. I selected Mode#5 ‘High Gain 2CMS’, Gain = 0 and Offset = 20. The rationale for these choices are described fully in the Characterisation document but is a balance between read noise, well depth to achieve high photometric precision.
The Offset setting was selected in order not to zero-clip low signals but not so high as to eat into dynamic range at the top of the ADU range. The bias level (i.e. ADU level without any light) is about 250 minimum.
Measured gain and read noise
The gain of my QHY268M camera with the selected settings and binned 2×2 is 3.2 e-/ADU and read noise is 7.0e-. I used the method described by Michael Newberry of Mirametrics in this technical note.
Linearity
Linearity is excellent up to about 64,500 ADU, just short of the 16-bit numerical limit of 65,535 ADU. It is likely that the linear region extends beyond 64,500 ADU but to be conservative I mark all pixels above this as invalid i.e., not used in photometry.
Well depth
The number of electrons captured per pixel at saturation is the well depth. Saturation is assumed at 64,500 ADU and unbinned gain = 0.8 e-/ADU, so well depth = 64,500 x 0.8 = 51,700 e-. Since I bin 2×2 the effective full well per binned pixel is an impressive 103,400 e-.
Random Telegraph Noise (RTS) behaviour
I had concluded before that although RTS is present in the QHY183M it would not have a significant impact on photometry. Testing of the QHY268M confirms the same conclusion. Much more detail can be found in the Characterisation document.