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Author Topic: Here comes CMOS  (Read 917 times)

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Offline chris.bailey

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Here comes CMOS
« on: 09:10:30, 05 November, 2016 »
I have seen a few reviews of CMOS sensors being used for deep sky imaging and the results have been pretty impressive so I dipped my toe in the water with a couple of cameras bought primarily for Solar and Lunar Imaging. The key to using these devices for deep sky work is to keep the exposures short and stack LOTS of frames. CMOS sensors have quite a small well depth so are easily saturated but they are very sensitive, have a very high Quantum Efficiency (QE ~ 78%) and have a lower read noise than CCD sensors.

First up comes the little Altair GPCAM ARO130M, a 1/3" sensor with 1280 x 960 3.75 microns which has a frame rate of up to 30fps in video mode.

M27 - 150 x 15 seconds
NGC891 - 200 x 10 seconds
Bubble - 200 x 15 seconds

Given that this camera costs less than £200 and the the total integration times on these is fairly short, I am pretty impressed.
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Offline chris.bailey

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Re: Here comes CMOS
« Reply #1 on: 09:21:38, 05 November, 2016 »
Moving on to the Altair Hypercam174. This has a 1/1.2" chip with 1920 x 1200 5.86micron pixels. In video mode this USB3 camera has a frame rate of up to 130 fps at full resolution and I have seen nearly 250fps with a smaller area of interest.

M15 - 140 x 20 seconds (45mins total)
M33 - 350 x 30 seconds (3 hrs total)
M31 - 300 x 30 seconds (2.5hrs total)

When you get large numbers of frames (300 plus) this camera really starts to sing. The M33 and M3 have no applied noise reduction and really don't need it. They do however have a large number of dark frames applied (200) and as the camera has no set point cooling, these are best taken just before or just after the light frames so as to be a reasonable match temperature wise.

My initial frames with this camera showed a few odd artefacts. This may be a form of residual image retention as I found a small delay introduced between frames reduced it and taking a zero length exposure between each light frame cleared them still further. I also didn't do flat frames with any of these but really should have done as there are a couple of nasty dust bunnies.

(posted full size - apologies for those with small monitors)
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Offline chris.bailey

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Re: Here comes CMOS
« Reply #2 on: 09:31:55, 05 November, 2016 »
I have to say the results with the 174M really surprised me on this range of quite challenging targets. The M31 in particular shows very good control right into the core though longer sub frames did start to burn it out. It really is a viable deep sky camera whilst also being capable of being used for Solar and Lunar imaging.

In summary

Pros -

* VERY sensitive with high QE (more of those photons get turned into usable signal) and low read noise (I measured the 174 at about 3e!)

* Work best with short exposures so no need for guiding, though there is a need to re-centre the target now and again. The M33 frames were taken with lost of passing cloud and I just let it run and deleted those that were clouded out!

* Multi purpose - Deep sky, Solar and Lunar in one camera.

Cons -

* LOTS of files to deal with (hundreds) so a need to be organised and processing is quite lengthy - 350 frames takes quite some time to stack!

* There is some residual banding that is not fully removed with dark frames. PI has a banding reduction script which works very well.

* Small well depth needs care with exposures. Star cores will burn at much beyond 30 seconds.

* Amp glow - largely dealt with with darks. Amp glow is part and parcel with CMOS sensors and does not change even with deep cooling.

* USB 3.0 - can be quite cable sensitive and I have yet to find an active USB 3.0 cable that works.

To my mind the pros largely outweigh the cons. This would be very suitable for a small mobile setup (a star adventurer and 80mm refractor) as the 174 weights a fraction of a DSLR. My next step is to put one on a filter wheel and try some RGB and Ha. Just need some more clear skies!
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Offline ChrisLX200

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Re: Here comes CMOS
« Reply #3 on: 10:55:41, 05 November, 2016 »
I guarantee you that this 15m USB3 active cable works https://www.amazon.co.uk/gp/product/B01BLPSVZS/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1
and the amazing thing is, it even works without the extra 5v feed at the far end. I have an ASI178MM on the end of it and get totally reliable transfer at full frame rate.

Yes, CMOS may well be the way forward, and if they can sort the amp glow it offers the potential for a cheap(er) option in DSO imaging. All those files though! I generally save all my raw data but that's not going to be possible with the GB's per session using one of these!

ChrisH

Edit: Oops, wrong link - back in a minute!

Well, they don't list the 15M USB3 cable any more, just the 7.5M https://www.amazon.co.uk/MutecPower-female-cable-extention-chipset/dp/B01BLPSUTA/ref=sr_1_1?s=computers&ie=UTF8&qid=1478343950&sr=8-1&keywords=MutecPower+15m+USB+3

I clicked on my order for the 15M USB3 and it took me to the first link above - the USB2 cable - that's all wrong. If I wanted to quickly re-order the cable I;d bought without checking then I would have ended up with the wrong cable!
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Offline chris.bailey

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Re: Here comes CMOS
« Reply #4 on: 12:18:54, 05 November, 2016 »
Cheers for the link Chris. 7.5m is more than adequate so ordered!

Yes the huge number of files is a consideration. Calibrating 300 odd files takes quite some time, as does aligning and then stacking. I did use the PI Batch Processing script for some of these so you can just leave it to get on with it for an hour or so.

I was quite concerned when I saw the degree of amp glow (stretched 30s master dark below) but as long as you take a fair few darks they do seem to correct it pretty well. Darks are a MUST have. Bias is pretty much zero so not worth taking.

Stretching is quite interesting. As all the data is contained in a very narrow band, the stretch needed is immense but when you stack 300 frames the noise floor is tiny even in comparison to the data.

I don't think I am quite yet ready to ditch the CCD's for faint object hunting but I have proved to myself that COS deep sky imaging is a viable lower cost alternative.

Chris
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Offline Einari

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Re: Here comes CMOS
« Reply #5 on: 12:25:23, 05 November, 2016 »
Thanks for this Chris. Examples like M33 and M31 really looks promising.
I have seen really outstanding results taken with these cmos cameras and those were taken even with shorter exposures and more of them, thousands even.
Also must say that QHY has made some impressive tricks to deal with amp glow.

Btw, have made some experiments with fast frame rates and slow motion with my ASI174MC Cool.
In my tests I have gone up to 1000 fps and more but here's an example of slower, about 600 fps if remember correct:
https://vimeo.com/179216265
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