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Old 20-01-2015, 09:51 PM
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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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Old 21-01-2015, 08:15 AM
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Camelopardalis (Dunk)
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Nice one Jeff a good overview of what's involved with these elusive cooled beasts for those of us still looking at colour with our on-screen histogram
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Old 21-01-2015, 08:58 AM
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thanks a lot jeff, i have kept this one for a near future reference, very interesting!
pat
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Old 23-01-2015, 11:42 PM
dylan_odonnell (Dylan)
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Thanks so much for this info. I've yet to get first light on the QHY12 as I needed a few more adapters and now it's cloudy but without this info I would have tripped up for sure!
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Old 24-01-2015, 08:04 AM
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Eden (Brett)
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Hi Jeff,

Glad you are enjoying the move from DSLR to cooled CCD imaging.

I have the QHY12 here, it's a great camera which has performed well for me under a variety of conditions.

The cameras "Fast" mode is intended for use when previewing a target or performing any focusing tasks, not for long-exposure imaging. This is due to the fact that although sensitivity is increased and the time taken to read the image from the camera is reduced, it is done so at the expense of increased noise and disables the anti-blooming gate, which is practically a "must have" for long exposures.

I've tried numerous times to tweak the gain and offset using the so-called scientific method but did not see any measurable improvement. As a result, I have always run the camera at Gain 0 Offset 125 with satisfying results. Perhaps I was missing something in the process, might be worth another try! The other problem with Gain and Offset calculation is that any values you come up with are only good for whatever temperature you obtained them at, requiring you to reevaluate them at different times of the year when you may need to run the camera at different temperatures.

Enjoy your new camera!

Last edited by Eden; 24-01-2015 at 10:51 AM.
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Old 24-01-2015, 10:45 AM
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Thanks Dunk, Pat and Dylan. I hope somebody gets some small benefit from my stumbles ... and eventual enlightenment. Despite being an engineer and computer geek, it still takes me a while to "learn my way" through new equipment and techniques sometimes.

Hi Eden. I too initially tried Gain 0 and Offset 125, but found that I was going nowhere near utilising the full dynamic range of the sensor (i.e. spread of brightness values). I settled on Gain 22 and Offset 120 (my testing with a light box suggested I could go to values of around 24/123, then pegged these back a little). I found from other on IIS and Cloudy Nights forum threads that others have settled on similar settings.

I found that by capturing subs across the full dynamic range, fine details such as "dust" around nebulas became much more evident, even before stretching calibrated/stacked images during processing.

I agree that the "Fast" capture mode has more read noise and is best suited to framing and focus before an imaging run. The mistake I made was thinking I could also use it to experiment with Gain/Offset settings before an imaging run.

So for me, settings of Gain 22 and Offset 120 suited me for most targets through an ED80 at f6 (with 0.8 reducer/flattener), typically with 5 minutes subs. When capturing fainter targets like the Leo Triplet galaxies (through a GSO RC8 scope), I found that getting a good histogram at 5 minute subs involved bumping the settings up to Gain 63 and Offset 140. At lower settings, the captured subs were faint with bunched histograms - requiring longer exposure times and revealing less details during processing.

Ah .... the joys of tinkering.

Last edited by Jeff; 24-01-2015 at 12:29 PM.
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Old 27-01-2015, 06:34 PM
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Jeff, thanks for the info and tips. I haven't made the move to CCD yet but am very interested in learning about it.

Can you post some of your DSLR and CCD images for comparison so I can see how much I may benefit from changing to CCD. I also image with an ED80 so your experience will be quite relevant to me.

Thanks, Kev
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Old 27-01-2015, 10:48 PM
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Hi Kev,

Some images for comparison (3 from 450D at ambient, and 3 from QHY12 at -15C).
Note that I've cranked up the colours and applied Noel Carboni tools heavily on some of these.
ED80 is used with William Optics pflat2 flattener/reducer giving f6.
GSO RC8 is used with a TS flattener leaving the focal ratio close to f8.

Hope this helps.
Attached Thumbnails
Click for full-size image (450D - Carina 2009_04_21.jpg)
161.1 KB106 views
Click for full-size image (450D - Horsehead 2010_01_6&9th (60 x 3min).jpg)
154.1 KB98 views
Click for full-size image (450D - Orion 2009_01_04 layers2.jpg)
178.4 KB114 views
Click for full-size image (QHY12 - M83 RC8 (45x8min).jpg)
100.7 KB123 views
Click for full-size image (QHY12 - Orion ED80 (42x6min).jpg)
164.1 KB121 views
Click for full-size image (QHY12 - Crab RC8 (20 x 5min).jpg)
163.0 KB104 views
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Old 28-01-2015, 03:56 PM
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Dealy (Kev)
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Thanks Jeff,

So the M42 shots - are they of similar exposure time and with the same scope?
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Old 28-01-2015, 06:37 PM
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Hi Kev,

Both M42 shots are through an ED80 with a pflat2 reducer/flattener (giving focal ratio of f6).

450D image was Jan 2009 from a mix of 5min, 3min, and 30sec subs at ISO800 .... how many subs I don't recall, as I only l kept the stacks.

QHY12 image was Jan 2014 from 10x2min subs for the core (Gain 22, Offset 120), and 42x6min subs for the nebulas (Gain 30, Offset 150).

Cheers.

Last edited by Jeff; 29-01-2015 at 07:12 PM.
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Old 28-01-2015, 09:10 PM
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Thanks Jeff,

Great info for future reference

Kev
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Old 10-10-2019, 06:23 PM
Crossy (Paul)
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Qhy12

Jeff,
Wish I had seen this when I first got my QHY12 a while ago. Great advice, I do have one question, is a Gain value of 0 feasible after calibrating. I have ended up Gain 0 and Offset 139. I will adjust whilst actually imaging and see what occurs.
Thanks again for the informative post.
Paul
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Old 10-10-2019, 07:20 PM
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Hi Paul,

Wow … that thread was a few years ago now, but I'm glad that you found some of the information interesting or useful.
I've since moved on to ZWO cameras, but your question is still relevant.

Yes, it's definitely feasible. I sometimes use zero gain to good effect on "high dynamic range" targets with bright and dim areas (M42, Omega Cent, 47 Tuc, etc).

Although my most common method is to take sets of subs at various durations (eg. 30 sec or less for stars, 300 sec for a galaxy or nebulosity), then blend together … to retain colour in the bright stars.

Hope this helps.

I'm others on IIS will have plenty of tips also.

Cheers,
Jeff
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