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Old 20-01-2015, 09:51 PM
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Jeff
Starry Eyed

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Join Date: Dec 2007
Location: Wonga Park
Posts: 692
Experiences Migrating from Canon DSLR to QHY12

Like many others who have enjoyed imaging deep sky objects using Canon DSLR cameras, I have made the transition to using a dedicated/cooled astrophotography CCD camera. After 5 years enjoying imaging with an IR modified Canon 450D, I wanted a cooled CCD camera to go deeper - capturing more dust in nebulas and peering into distant galaxies without a hammertone finish caused by sensor thermal noise (especially during those warm summer nights).

I spent many moons contemplating an upgrade – reading reviews, looking at posted DSO images, comparing brands, analysing specs, vacillating about costs, getting in my partner’s good books in prep for another outlandish astro purchase. How we love this strangely addictive hobby with its mix of technical problem solving, awe inspiring scientific learning, and enjoying the beauties of nature!

In the end, the gods took pity on me when my 450D finally bit the dust after 5 years of wonderful service … victim to my homemade attempts at apply TEC cooling to the sensor. My wife very kindly agreed to the purchase of a cooled CCD camera, so I contacted Theo and purchased a QHY12 One Shot Colour CCD camera (http://www.gamaelectronics.com.au/QHY12.html) within the next few days before she could change her mind.

I guess choosing a camera can be quite subjective like choosing suburbs, houses and cars. Some people have passionate opinions and lifelong brand loyalties or dislikes, while others are more focussed on recent product history and price/performance trade-offs. The key criteria for me were needing to be one-shot-colour (due to limited spare time), cooling, chip dimensions, pixel size, price point, and product maturity/support.

Early Challenges – Some Learning Required

As with all previous significant purchases of astro equipment, there was a subdued excitement of progressive familiarisation and testing. The camera and accessories were well packaged in an aluminium case, and the camera was wonderfully lightweight compared to the DSLR. The fan was a little noisier than expected, but the two stage TEC cooling was amazingly fast and effective.

However, during the first couple of weeks owning the QHY12, I found that I had some learning to do. I had hoped to be in long-sub / low-noise imaging nirvana on day one or two, based purely on image capture/processing experience with the 450D. But I had a few knowledge gaps which I needed to bridge before producing decent images. The constant learning is one of the things to love about this hobby.

The key things I needed to learn were as follows:
1. Subs must be converted to colour
2. Gain/Offset experimentation is required

Balanced conversion of OSC CCD subs to colour

When using a DSLR such as the 450D, the bundled EOS Utility software captures and displays images (CR2 files) in instantly gratifying colour with pleasing balance and saturation. However, most astro image acquisition programs I have used capture images from OSC CCD cameras in 16-bit greyscale FIT format which needs to be explicitly converted to RGB before colour can be seen.

Some software such as EZYCap (bundled with the QHY12 for first light testing) or Nebulosity can perform this conversion “on the fly” as each image is read from the camera. With other software such as MaxIm DL or CCDSoft, the conversion needs to be performed separately later … giving an opportunity to calibrate images (applying bias/dark/flat frames) to raw data before colour conversion. Experimentation is required to achieve nice colour saturation and balance.

I found it easiest to test during daylight using various software packages and settings. With the QHY12, the quickest and easiest way to achieve balanced colours is to use Nebulosity 3 with the native QHY12 driver (rather than ASCOM), and set the Acquisition Mode in preferences to “Recon (RGB/Square): Quality”. This stores images to disk as nicely colour balanced 16-bit RGB FITS (rather than 16-bit greyscale FITS). Craig Stark has done an excellent job balancing offsets and gains for the QHY12 in Nebulosity, so whites come out white (like the old OMO ads), and colours look good without needing to tweak saturation.

I know that staying in greyscale FIT format until bias/darks/flats have been applied is theoretically best. However, being a “one-shot-colour instant gratification” person, I prefer to capture my bias/dark/flat frames in the same RGB FIT format, then calibrate/stack in Nebulosity, DSS, or some other program.

It is also possible to achieve similar results using MaxIm DL or CCDSoft, but additional steps are required along with experimentation to determine optimal conversion gains for red, green and blue. I found it difficult converting to decent daytime colour balance and saturation when capturing via Maxim DL 5.23.

Gain/Offset experimentation is required

This one really had me stumped!

With a Canon 450D I always imaged at ISO800 and just varied the sub duration to avoid over exposure of the images.

Unlike Canon DSLRs, cooled CCD cameras such as the QHY12 allow user adjustment of camera Gain and Offset to allow the full dynamic range of the CCD sensor to be utilised for a given scope/target/exposure combination without clipping the left or right ends of the histogram. Increasing offset translates the histogram to the right, whereas increasing gain stretches the histogram out.

With the QHY12, the PDF manual includes brief instructions to initially calibrate these values using a combination of Bias shots and saturated shots (starting with default values of Gain=0, and Offset=125). However what the manual fails to mention is that initial calibration settings need to be done separately for “fast” and “normal” readout speeds.

As a result, I spent the next few nights testing the camera on 47-Tuc and the Tarantula Nebula, with some perplexing results. I would take test shots and adjust gain/offset at “fast” readout speed, then switch to “normal” readout speed for my imaging run only to find that images were barely perceptible (lumpy and dim).

After a few days of scratching my head and suspecting there was a camera sensitivity problem when using normal readout speed, I contacted Theo who kindly invited me to his house and was very generous with his time teaching me how to quickly analyse and optimise gain/offset settings.

In summary, I needed a higher gain setting.

I eventually settled on leaving settings at gain=22 and offset=120, then experimenting with sub durations to get a good histogram spread for a given optical train and imaging target. Nice and simple!

Below is a link to a very good guide which I subsequently found explaining How to Scientifically Determine CCD Gain and Offset Settings:
https://www.astrofactors.com/images/scientifically%20determining%20ccd% 20gain%20and%20offset.pdf

My Lessons Learned

1. Acquisition software typically loads OSC CCD images as 16-bit greyscale FITS which then need to be converted to colour. The easiest option and best colour balance for QHY12 cameras is using Nebulosity with the native QHY12 driver.

2. The histogram is king. For a given optical train and target, it is necessary to adjust gain and offset settings (with sub durations) to get a nicely spread histogram without clipping/truncation at either end. Required settings can vary significantly not just for different binning modes, but also for different CCD readout speeds.

After sorting out the above I have had a ball with the QHY12, and hope some of this is helpful to others considering migrating from Canon DSLRs to a cooled CCD camera.

Last edited by Jeff; 23-01-2015 at 11:57 AM. Reason: typos
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