How cold do you go?

[DMouse]

Hi, I am curious just how cold people like to get their CCDs. Here in Canada it gets cold in the winter, so cooling to -40C below ambient could mean I could get as low as say -60C, or lower. But practically what is the lowest I should go, and what is the lowest people generally use?

Currently I use -25C as my set point. Would -40C make a big difference. So what is the coldest you've gone and what do you normally use?

Clear Skies

Paul

[ollypenrice]

I'd say just try it and see. I default to -20 because I can almost always get down to that and it simplifies the darks. In winter I could go a lot lower but have never bothered. Maybe I should take my own advice and try it!!!

Olly

[opticalpath]

I have standardized on -20C as I can reach this on all but exceptionally warm summer nights.

It's actually a hard question to answer because it depends ...! By cooling the sensor you are reducing thermal noise, but that is just one of several sources of noise that are present. The objective of cooling should be to reduce thermal noise until it's an acceptably low proportion of the total noise accumulated in the image from all sources. If you cool beyond that point, it's a case of diminishing returns and you increase the likelihood of residual bulk image (ghosting). If you're imaging LRGB under good to average dark skies, the sky background noise is likely to be the major contributor of noise and very deep cooling is unlikely to give much real benefit if any. If you're imaging in exceptionally good dark conditions where sky background noise is very low, and/or you're using NB filters, it's likely that other noise sources, including thermal, will be more significant, so somewhat deeper cooling may be beneficial.

There are a couple of excellent papers that cover just this subject in some detail - worth a read:

http://www.starrywonders.com/coolinghidden.html

http://www.hiddenloft.com/notes/CCDtemp.pdf

Adrian

[Gina]

Having recently bought an Atik 314L+ mono CCD camera, I was wondering that myself. I've been using -15C for no particular reason (maybe because that's what I can get with my cooled DSLR). I thought that if I got any noise showing I might go down to -20C.

[carastro]

I spoke to Steve Chambers at Atik when I first got my Atik314L, and he suggested -10 degrees. So this is what I have been using.

Carole

[Gina]

Well, I'm not seeing any noise at -15C so -10C could well be enough Anyway, you got it "from the horses mouth" so that's fine

[James]

-15 seems to be working for me. I'll have to see if it all works when the outside temperature is 25+ C but can anyone remember such warm weather?

James

[ian_bird]

Have to say - I am a bit confused by regulated cooling. The set point is x degrees below ambient - but ambient varies (a lot). So how can it be said to be regulated?

If I take a bunch of darks indoors at say 25 degrees C with my set point at - 20 C - then the CCD should be at five degrees C. But if I am imaging outside at say minus 2 degrees C with the same set point - then my CCD is at minus 22 degrees C.

So do I need to do my darks at the same temperature as when I take my lights? Ditto for Bias?

I wanted to build up a library of darks (refreshed every few months). Do I also have to note how cold it was when I took them? And then try to match that with how cold (or warm) it was when I took the lights?

Any clarification warmly welcomed!

Cheers

Ian

[ollypenrice]

Have to say - I am a bit confused by regulated cooling. The set point is x degrees below ambient - but ambient varies (a lot). So how can it be said to be regulated?

If I take a bunch of darks indoors at say 25 degrees C with my set point at - 20 C - then the CCD should be at five degrees C. But if I am imaging outside at say minus 2 degrees C with the same set point - then my CCD is at minus 22 degrees C.

So do I need to do my darks at the same temperature as when I take my lights? Ditto for Bias?

I wanted to build up a library of darks (refreshed every few months). Do I also have to note how cold it was when I took them? And then try to match that with how cold (or warm) it was when I took the lights?

Any clarification warmly welcomed!

Cheers

Ian

Ian, the 'regulated' bit means simply that you can control and fix the temperature. The usual term is 'set point cooling' so you can make a darks library at minus x, not x below ambient, which could be anything - as you suggest.

Users of the sublime Sony chips have nothing much to worry about and the difference between -10 and -15 will not matter. Kodak (Motorhead!!) chip users like me need to be far more careful and set point cooling is a boon.

Tonight, after a chat with Yves, we are running his SXVH36 at -30 because it is sub zero ambient here. We are also trying 30 min luminance subs because we are imaging the notoriously noisy M78. (19 hours and we are still bawling to be heard above the noise!) It will be interesting to see what the extra cooling and longer subs can deliver.

285 chip users? Forget it and go and laugh at the rest of us...

Olly

[Gina]

The "set point" is the actual temperature. They quote the ability of the camera to be cooled a certain number of degrees below ambient as the limitation of the cooling system. ie. if I set mine to -15C with the limit defined as 25C below ambient then I wouldn't be able to get the camera down to that temperature it the ambient were more than 10C.

[Tom OD]

Here is a scren shot of a -10C verses a -20C 300s dark. The lower the temp the less column defects I get and the smoother the noise pattern. The -10C almost has a gradient on it too.

[adamsp123]

Another one using -20C - simplifies the darks and in our climate I reckon it will never struggle to get to that a night.

[opticalpath]

That's an interesting difference, Tom, especially regarding the gradient effect you noticed.

The point I was trying to make earlier though is to assess the contribution to overall noise that the thermal (dark) noise is making. If sky background is already making up 95% of the noise in your signal, it's unlikely that cooling another 5 or 10 degrees will show significant improvement in overall S/N. The trick is to find, for your specific camera sensor (they vary hugely in thermal noise, as we know), for your specific sky background conditions, and type of imaging (broad or narrow band) ...... what's the temp. at which the thermal noise becomes a very small part of the total noise. And stop there, rather than continue cooling lower and lower - which will certainly reduce thermal noise further but may not be noticeable beside the other much larger noise sources that aren't going away.

Another thing to consider when you're comparing dark frames is that it's not strictly the amplitude of the dark signal that's critical (because we remove that when we subtract a dark frame), but rather the *noise* in the dark image ..... which is what remains after the dark has been subtracted and is what affects overall S/N.

Adrian

[ian_bird]

Gina, Olly

Thanks for the explanations. I thought I must be missing the point somehow! Not the first time - won't be the last time!

Cheers

Ian

[kirkster501]

Interested in how these cameras work please ....? Do they have a small freezer or soemthing like that built in? How is it powered - via the USB?

[Gina]

They use a Peltier Thermo Electric Cooler which is a semiconductor device that produces a temperature difference between it's two surfaces. For more info see Google - lots of info on there. Incidentally, they will also work the other way round, generating electricity if a temperature difference is applied. The cold side is placed at the back of the image sensor to cool it and the hot side is cooled with a fan like the cooler on a computer processor. They generally run off 12v DC and take around an amp or so of current. More when cooling down and less current when maintaining the set point temperature. Cheaper cameras have just two settings - full on or off. The dearer ones allow you to set the temperature and it will cool down and then maintain that temperature.

[Uranium235]

-10 or -15 with the 314L+

I dont bother with darks for that camera because its already super quiet, and youre more likely to add noise by using darks. The few hot pixels you get can be easily removed with software.