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Old 22-11-2015, 11:20 PM
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John K
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Determining optimal exposure levels for a new cooled CMOS ASI 174 mm camera

Hi guys,

I have ordered a cooled ZWO ASI 174mm camera which will cool to 45 degrees below ambient in order to upgrade my 120mm for planetary, but also because I wish to do some mono deep sky imaging. So far, have spent several years doing mono planetary imaging and OSC deep sky, but looks like my two worlds are about to collide!

As I was not prepared to spend several thousand dollars on a mid-range mono CCD camera, I am hoping to see what can be achieved with this new style of CMOS Sony chips. My scope is a 12.5 f/5 scope and I will image from the ASV's dark sky site outside of Melbourne with a portable setup generally shooting one object per night over 4-5 hours with LRGB filters.

I will be using Firecapture to control the camera very much like I do when I do planetary imaging. The software will be able to set the camera temperature, and like with planetary, the user can control the Gain and Exposure length which then determines the Frames per Second / Minute.

The question now becomes, with this type of Camera / CMOS Chip, what is the optimal Gain / Exposure level for this camera for Deep Sky Objects vs optimal Noise / Signal and how do I determine that? For planetary it's the histogram normally and the noise level that I use to determine what my exposure / gain settings are in Firecapture.

Most people that I have seen using this camera in the "un-cooled" version are generally going up to about 60 seconds and stacking several hundred/thousand frames. My thinking is that this camera will be good for "Lucky" style imaging for Deep Sky objects, and based on how I capture and processes my Planetary don't have an issue with this, but need a formula of some sort to determine what may be best for this camera to optimise the Signal / Noise Ratio.

I know that most of you will now say well John it all depends!!.... So, I am attaching some info on things such as Peak Noise (e-); Dynamic Range (dB); Gain (e-/ADU) etc etc with the hope that some of the well informed people on IIS can assist. The data sheets don't however show much in terms of the effect on any of these levels vs temperature and although I have seen some calculators on-line I am struggling to make sense of the best approach.

Otherwise, my approach, will simply be to minimise the amount of noise through low gain settings and then maximise the exposure level to no more that 1-3 minutes per captured frame and be guided by the data I see in Firecapture in terms of the live histogram and noise levels as well as the visual appearance of the subs (which is quite an unscientific approach!)

It may well end up that I simply have to do some noise / gain tests myself on various objects and temperatures??

A long thread to probably a simple question for some! but would appreciate thoughts and comments.

Clear skies and thanks in advance.

John K.
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Old 23-11-2015, 02:25 PM
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Flugel88 (Michael)
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John Congrats on acquiring such an awsome camera i would love to have a play with one of the first cooled ASI.

I have an asi120mm-s the usb 3 version.
It was my first ever Astro camera i purchased to use for guiding but untill ia had the funds for my atik i spent some time playing around this the zwo to see what i could get out of it.

Main issue for me was the heat uncooled for Deepsky is not much fun i had did attempt to modify using a thermo electric cooler but i couldn't come up with a way of preventing dew and ice from forming on the glass window

Playing with the gain and exposure settings i found to be a fun challenge that involved a lot of trial an error but ultimately i was limited in my possible exposure times duo to no guiding.

Here's one of my first ever astro photos.
Im sure you could do a million times better with a cooled camera and bigger chip.

Camera ZWO Asi 120mm-s
ZWO LRGB filters and ZWO filter wheel
Scope lxd75-8SN
Mount EQ-6pro
no guiding

30x20 sec of each colour RGB subs 65 gain
100x10 sec of Lum at 70 gain
stacked in dss using Fit files
processed in photoshop
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Old 24-11-2015, 11:02 AM
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ZeroID (Brent)
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Aha ! it IS the new ZWO DSO camera. I saw the pic and checked the website but no update on the 'Coming soon' status.
Be very interested to hear how it performs.
Can I ask what the price was ? ... so I can start saving ?

Michael : I came up with a way to keep it dry and ice free on the 120mc if you are interested in further experimentation. Temps down to -20 or so, 50 Watt TEC, sealed dry chamber for the sensor. Just wasn't happy with getting .bmp files from the camera.
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Old 24-11-2015, 01:44 PM
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That's an interesting beast John - should be a really good match for your scope.

the specs are quite different from those published for conventional 16 bit CCD cameras, presumably to allow for high speed 12 bits operation. Looks to me like your strategy is most sensible: get the best dynamic range with DSOs, by working down in the 0-100 gain range. Because the ADU values will not be padded out to 16 bits, the signal will be stuff all (maybe a few ADU), but the read noise will also be minimal at around 1ADU, so you will still get a good image. Even though you cannot get fractions of an ADU in a sub, you can after stacking with a floating point operator, so quantisation noise should not be an issue if you have enough subs.

I think that the equation here will still apply for determining sub lengths http://www.iceinspace.com.au/forum/s...d.php?t=117010
eg, at a gain setting of 100, the calculation suggests that you aim for sky ADU values of ~90 + whatever the bias is. When stacking subs with such low ADU values, you could consider using additive stacking rather than average, simply to make the data easier for the display algorithms. I don't understand the implications of the published specs sufficiently well to offer an optimal sub time, but as a general observation, a fairly low noise camera on a reasonably fast scope should require maybe 5-10 minutes per sub. Be very interesting to see what you find.

Last edited by Shiraz; 24-11-2015 at 03:19 PM.
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Old 24-11-2015, 02:05 PM
glend (Glen)
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Quote:
Originally Posted by ZeroID View Post
Aha ! it IS the new ZWO DSO camera. I saw the pic and checked the website but no update on the 'Coming soon' status.
Be very interested to hear how it performs.
Can I ask what the price was ? ... so I can start saving ?

Michael : I came up with a way to keep it dry and ice free on the 120mc if you are interested in further experimentation. Temps down to -20 or so, 50 Watt TEC, sealed dry chamber for the sensor. Just wasn't happy with getting .bmp files from the camera.
Brent the price is $300 USD on top of the standard camera price, so about $899 USD based on the ZWO website price. They are available now according to Sam at ZWO.
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Old 24-11-2015, 02:38 PM
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John K
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Quote:
Originally Posted by Shiraz View Post


I think that the equation here will still apply for determining sub lengths http://www.iceinspace.com.au/forum/s...d.php?t=117010
eg, at a gain setting of 100 for example, the calculation suggests that you aim for sky ADU values of ~90 + whatever the bias is. When stacking subs with such low ADU values, you could consider using additive stacking rather than average, simply to make the data easier for the display algorithms. I don't understand the implications of the published specs sufficiently well to suggest an optimal sub time, but as a general observation, a fairly low noise camera on a reasonably fast scope should require maybe 5-10 minutes per sub.
Thanks Ray - good info - I still don't fully understand the ADU value concept and how I measure this so more reading for me to do on that front.

Quote:
Originally Posted by ZeroID View Post
Aha ! it IS the new ZWO DSO camera. I saw the pic and checked the website but no update on the 'Coming soon' status.
Be very interested to hear how it performs.
Can I ask what the price was ? ... so I can start saving ?

Michael : I came up with a way to keep it dry and ice free on the 120mc if you are interested in further experimentation. Temps down to -20 or so, 50 Watt TEC, sealed dry chamber for the sensor. Just wasn't happy with getting .bmp files from the camera.
Brent - the price in fact will normally be $899.00USD but in fact for Beta Testers and as an intro price it was cheaper for me by about $100USD - not sure if this will be offered to everyone but even at $899USD still good value in my mind if it delivers with Deep Sky on top of Planetary.

Also interested in your sealed chamber solution as I have seen the horror stories with several cameras like QHY. Sam has assured me that due to the small sensor/window and built in decadent it will be ok. My back up plan is to also try something else (see image attached below!) but will not jump any guns including a dew heater seal on the front if I start getting any issues.

Quote:
Originally Posted by Flugel88 View Post
Playing with the gain and exposure settings i found to be a fun challenge that involved a lot of trial an error but ultimately i was limited in my possible exposure times duo to no guiding.

Here's one of my first ever astro photos.
Im sure you could do a million times better with a cooled camera and bigger chip.
Hi Michael - thanks for the info and sharing the image. You might also be interested in the attached image from a QHY user who simply pumps warm air into the nose piece through a hole using a small hose and a hole he drilled in the nose piece.

Keep the info coming IIS's!!
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Last edited by John K; 24-11-2015 at 02:51 PM.
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Old 03-12-2015, 07:12 PM
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So just a bit more info on this now whilst my camera is being shipped.

The creator of Firecapture, Torsten Edelmannhas, has been good enough to now release a version which allows up to 50 darks and flats to be taken and averaged and then subtracted on the fly whilst the camera is capturing images. In addition to being great for deep sky imaging, this will also be great for things such as capturing images of Uranus, Neptune and Methane Jupiter images.

One of the questions I have is, what would be the optimal number of dark frames to subtract from each deep sky image captured on the fly?

I don't envisage going past more than 30 seconds exposure for the faintest objects in order to try and do Lucky type imaging.

Lots of testing to do hopefully next week at LMDSS in Vic when the camera arrives.

John K.
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Old 04-12-2015, 07:36 AM
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ZeroID (Brent)
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Here's the thread on the sealed chamber ZWO 120 I did earlier.
http://www.iceinspace.com.au/forum/s...ghlight=zeroid

Any questions just post 'em up. I can draw you a diagram of the guts of it if required.
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Old 04-12-2015, 07:37 AM
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ZeroID (Brent)
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Quote:
Originally Posted by John K View Post
Thanks Ray - good info - I still don't fully understand the ADU value concept and how I measure this so more reading for me to do on that front.



Brent - the price in fact will normally be $899.00USD but in fact for Beta Testers and as an intro price it was cheaper for me by about $100USD - not sure if this will be offered to everyone but even at $899USD still good value in my mind if it delivers with Deep Sky on top of Planetary.

Also interested in your sealed chamber solution as I have seen the horror stories with several cameras like QHY. Sam has assured me that due to the small sensor/window and built in decadent it will be ok. My back up plan is to also try something else (see image attached below!) but will not jump any guns including a dew heater seal on the front if I start getting any issues.



Hi Michael - thanks for the info and sharing the image. You might also be interested in the attached image from a QHY user who simply pumps warm air into the nose piece through a hole using a small hose and a hole he drilled in the nose piece.

Keep the info coming IIS's!!
Hey !! That was MY idea and it worked well..

Having just recently splashed out on the 1200D DSLR I suspect my purchasing power is somewhat diminished and probably frowned upon by the Finance Minister. But patience, .... patience will be rewarded.
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Old 04-12-2015, 09:12 AM
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John, my thoughts are... it depends

Seriously though, it will depend...what I would be looking for is consistency of the dark frames. How effective the cooler is at regulating the temperature of the sensor will very much impact what's in the dark frames. If there is too much variance, a larger set of dark frames won't necessarily be better than a smaller set, but introduces its own challenge as to which set you apply

Btw, my impression (and it is just that) of lucky imaging was that it used very similar techniques to planetary imaging...thousands of very short (sub-1 second) subs with some capturing good detail in the fleeting seeing.
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Old 04-12-2015, 10:53 AM
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John
Calibration with CMOS cameras may be a bit different. Instead of a single A/D, you have one for each column, which means that you can get fixed pattern noise due to slightly different bias on each A/D. My limited understanding is that at least one manufacturer (Aptina) fixes this by doing a chip calibration at power up and this equalises the A/Ds to remove fixed pattern bias noise. An unwanted outcome of this process can be that each time the chip powers up, you end up with a slightly different bias set, since the process never converges to exactly the same solution. Thus, you will need a new bias calibration for each time you switch the camera on. I don't know if Sony does the same, but they must do something to fix the problem - bear it in mind when trying out calibration processes, since variable bias will need careful handling. Maybe ZWO can provide some advice on calibration - might head off an ulcer.

Because the chips are quite low noise and so small that vignetting will not be an issue, it might be a reasonable approach to dispense with calibration altogether (probably need bias subtraction though) and rely on dither to randomise any residual fixed pattern noise so that it integrates out. If doing lucky imaging, you will have enough subs for this to work well.

Torsten's on-the-fly calibration sound promising - if you use it, suggest that you employ as many darks and flats as the software will allow (more is always better)

Last edited by Shiraz; 04-12-2015 at 04:23 PM.
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