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Old 03-12-2008, 11:51 AM
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Omaroo (Chris Malikoff)
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Inter-exposure cool-down period for DSLR?

Hi all

Just wondering for how long most people leave their standard DSLR (i.e. 350/450D) cool down between sequenced takes? If I am running a long sequence of, say 20x360-sec exposures, a fair amount of heat and therefore noise is generated by the sensor within that 360-sec exposure. If I don't let the camera cool between shots I imagine that heat from the prior shot would still be dissipating, and if I started the next exposure too soon after it'd result in a chip that is never given the chance to reach some sort of temperature stabilisation. Amp glow would also be exacerbated I'd assume

I'm considering leaving at least a 1-minute gap between frames. What do others do? Is a minute long enough at average summer night time ambient air temps?

Last edited by Omaroo; 03-12-2008 at 05:07 PM.
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Old 03-12-2008, 12:19 PM
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Octane (Humayun)
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Chris,

I've only ever left 10 seconds between exposures.

Dark frames are your friend.

Regards,
Humayun
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Old 03-12-2008, 12:27 PM
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Hi Humayun

Yep - I take plenty of darks (at least two) every time i change camera timing or ISO settings - as well as a couple for every shift of 10 degrees or so in air temp. I them group shots by these darks and stack them accordingly. I guess I'd still like to know how long chip heat typically tales to dissipate, but 10 secs should see most of it gone I'd presume. It's kinda like a turbocharger, where a 15-second idle at the end of a hard run sees the turbo casing drop several hundred degrees in no time. It then takes ages to actually cool....
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Old 03-12-2008, 03:56 PM
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I routinely leave 30 secs between exposures and have not noticed an increase in noise between frames after a long shoot of lights. I also use a digital thermometer to log dew point as well as ambient temperature so I can match darks for master dark subtraction
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Old 03-12-2008, 04:12 PM
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h0ughy (David)
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LOL doesnt bother me with the 350D (cooled ) but the normal 20D i have might just make sense to leave a bit of time? we need a science experiment to prove such points. how about we do a 5min dark immediately follewed by another 5 min dark within 3 seconds, then do it with the 30 second gap to see if there is a noise difference? gentlemen arm your cameras.....
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Old 03-12-2008, 04:23 PM
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10 seconds...mmmmm!!??

May have to have a rethink.

Thanks for bringing this to our attention Chris

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Old 03-12-2008, 04:26 PM
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With the 5D I don't seem to have any problem, but I do use ICNR with every light though.

Leon
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Old 03-12-2008, 06:21 PM
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A signifcant amount of noise accumulates in each dark frame the longer the camera is left on I've got a collection of darks where you can see this clearly especially as the ambient temperature rises.

No doubt that is why cripser clearer frames are captures in the winter months.

Ideally IMO turning the camera off between frames would be the go but as you know this isn't practical better is someway of cooling the camera which I am working on.

Cheers
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Old 03-12-2008, 06:40 PM
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Quote:
Originally Posted by Omaroo View Post
Yep - I take plenty of darks (at least two) every time i change camera timing or ISO settings - as well as a couple for every shift of 10 degrees or so in air temp.
Keep a close eye on ambient temp changes.
For every 6 deg increase, the dark current doubles.

I keep a very close eye on temp and take darks accordingly.
Also I switch off the preview screen once I begin a run and allow around 15 sec between shots for things to settle.
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Old 03-12-2008, 07:58 PM
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OK guys maybe I am wrong here, and I might just heed to your advice.

Leon
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Old 03-12-2008, 08:13 PM
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mmm, your getting mighty picky there .

Given the investment in a TAK and G11, your seriously due for a cooled astro CCD upgrade. Trust me, its a whole new world of pain, well worth dipping into .
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Old 03-12-2008, 09:52 PM
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Quote:
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mmm, your getting mighty picky there .

Given the investment in a TAK and G11, your seriously due for a cooled astro CCD upgrade. Trust me, its a whole new world of pain, well worth dipping into .
Fred - please call my wife and tell her..... you may do better. Then again - the darling woman has just bought me a nice shiny blue QHY5 Q-Guider so I can give Geoff's (Asterisk) Orion Starshoot back to him, bless her dacron socks.

I'm seriously looking into sending my 350D off to Taiwan to get Yun to apply the filter and cooling upgrades. A dedicated CCD camera like a QHY8 or, dare I say it, an STL is a way off yet.

Maybe not the QHY8.... it isn't quite the moolah of the STL.

Last edited by Omaroo; 03-12-2008 at 10:06 PM.
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Old 04-12-2008, 12:38 AM
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But you want the STL more Donnntcha!!

Hell... I'd trade my QHY8 for an ST4k let alone a STL11k
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Old 04-12-2008, 07:25 AM
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But you want the STL more Donnntcha!!

Hell... I'd trade my QHY8 for an ST4k let alone a STL11k
hehe... I very nearly bought Monte's STL11k a couple of weeks ago Alex - except he caught me too early unfortunately. I'm going to have to wait for our dollar to shore up a bit against the greenback until that becomes a reality now. AU$15-16k is a somewhat ridiculous a price at this time.
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Old 04-12-2008, 09:41 AM
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Before I built my fridge I worked out what was happening to the sensor temperature of my 5DH. When you turn the camera on it uses about four watts at idle and the sensor temperature goes up by about 2 deg C from ambient. When doing a long exposure the camera uses about eight watts and the sensor temperature then goes up by about 2 deg again. This takes some minutes to reach equilibrium.

You can measure the temperature of the sensor with one of those IR remote thermometers when the mirror is up.

If you have ever taken a sequence of exposure with ICNR from idle have a look at the first image and you will see 'holes' caused by the first 'dark' being taken at about 2C above the first 'light'. These holes are then much dimmer in subsequent exposures since the camera has reached equilibrium and 'darks' are at the same temperature as the 'lights'.

Turning the camera off between exposures is a waste of time as it takes many minutes for the camera to cool.

The only way to overcome these ambient temperature variation problems is to put the camera in a constant temperature environment. The fridge I built is temperature controlled within 0.1 C. This temperature can be set to any temperature 28 degrees C below ambient. So if the night starts off at 18C I set -10.0 C and in summer say -5.0 C or even higher on a very hot night.

I always take some test exposures for focus and start collecting data as soon as possible to stop the first exposure being 2 C cooler than the rest.

Bert
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Old 04-12-2008, 11:56 AM
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Thanks Bert - much appreciated.

So - from that are you saying, I gather that it may not be the absolute temerature of a CCD or CMOS array that determines the point at which thermal noise will start to become prevalent, but how far from its "equilibrium" it is at any point? I wouldn't have thought that a measured 2-degree rise in temerature of an array sounds like it should be enough to trigger thermal noise (but I'm not knowledgable in this area so might be sprouting rubbish), so is your method of measurement accurate at pixel fence level - or are you looking at the temperature of the sensor and its casing through a glass filter over an average area? I can imagine that the photodiodes might each be a good deal warmer than that average - triggering their noise response.

If you leave the cool-down period between frames very short - without a giving heat a chance to dissipate, does chip heat accumulate to a point that it is indeed very warm? What is, in fact, "very warm"? At what point does noise start to show? Is it the product of a temperature differential between the array and its environment, or is it due to an absolute temperature - above which you get noise and below you dont?

Last edited by Omaroo; 05-12-2008 at 04:24 PM.
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Old 05-12-2008, 04:19 PM
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Chris I have got a ST1100 and I leave 60 sec between exposures. It is also worth while jogging the imager between exposures as well. You can get a burnin effect on the brighter objects within the frame. Also helps reduce the effects of dust.
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Old 05-12-2008, 05:54 PM
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Chris thermal noise varies with temperature. I only mentioned equilibrium as it is the only way to get your darks to match your lights as far as temperature of your sensor is concerned.

Bert
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Old 05-12-2008, 06:05 PM
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If you dont cool the cam, just use ICNR, so Darks will track the image temp. Imaging sessions will take twice as long, but I saw an M42 pic Mike Salway did this way, and it looked very low noise.
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Old 06-12-2008, 09:53 AM
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Quote:
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If you dont cool the cam, just use ICNR, so Darks will track the image temp. Imaging sessions will take twice as long, but I saw an M42 pic Mike Salway did this way, and it looked very low noise.
I did a series of 35 2min exposures of M45 last night between 9:00 and 10:30 pm attached is the first exposure and the last. (temp varied from around 18-15C) INCR was off but it may be the way to go on warmer nights, I might give that a try tonight to see how much difference it makes.

I don't know if two minutes is too long for this object but even after applying lights and darks I can't get a good image with about 1 hrs worth of exposure.

(the first image was compressed to 8 in PS7 whereas the second I had to set as 4 to get under the size for posting both where saved at 800 x 532)

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