Darks and temperature, question

[Shawn]

Ok heres the scenario...

Thought I would play around, I understand the correlation between exposure time and thermal noise, not knowing and being ignorant of the consequences of missmatched darks, and its cloudy, I did an experiment, as I said its cloudy so I cant use the darks anyway, Its more an experiment into where half a charge of battery dissapears to on a 350D,

Scenario.

Room held at 23.5 C, not using the scope so it doesnt matter.
Took a sequence of darks, with the 350 , 30 seconds each , spaced at 30 seconds, mirror settle turned off.

Stuffed a simple thermocouple up the tripod hole on the camera.

Camera temperature rose from 23.5 to 24.9 degrees, the exposure sequence has just finished and is as I type dropping in tenths very quickly... Now I ask, what is the procedure then for getting acurate darks, without regulation, and are these darks any use, does the cooldown period need to be a lot longer...

Your advice please , confused and runnig blind,

shawn

[Shawn]

I should mention , I want to archive masters for use at any later time, and it was 10 exposures all up...

Cheers

Shawn

[Shawn]

Update, It too 12 minutes for the camera to back to room temperature...

[Shawn]

At least predictable , 2nd sequence from 22.7 to 24.1...

Room stable at 22.7

hmmmm

Dropping the room to 10 , see what happens...

S

[Shawn]

Same again at 10 degrees, predictable. over a degree in heat change. and that is from a simple couple stuffed up the tripod hole.

Questions are

What difference do these temperature variations make to the darks, ?

And how do you combat it if they do..

As I mentioned its cloudy, as usual, so I cannot experiment. and its bloody cold in here , so Im off for a while...get my woolies...

[JohnG (John)]

Hi

Your darks should be taken in exactly the same time, delay sequence as your lights, preferable at the same temperature and at the same time as well.

EG: 10 x 180 second, lights with 30 shutter delay followed by 6 x 180 second darks with 30 second shutter delay.

With my 350D I use ImagesPlus to make a Master Dark for subtraction.

Cheers

JohnG

[Shawn]

The thought did occur to me that batteries being used also generate heat, however it is still tranfered into the camera body, Im surprised at the results of this cloudy nite experiment...

All the best guys...


S

[Shawn]

Thanks, but it took 12 minutes for the camera to cool down to ambient, surely you dont have to wait 12 minutes between you lights and darks...Im getting really confused here..

[Shawn]

What Im asking , is, that in a window of really good seeing, when you dont really want to waste time on darks. what variance is acceptible for use if ou have archived darks in the scenario Ive mentioned...

Any advice is really appreciated....

Shawn

...

[JohnG (John)]

OK, now I understand what you are trying to achieve.

If you have a reasonable window to take your lights, go ahead and take them, you can always expose your darks later, eg, when you are packing up, shutting down. Just remember that when you expose the darks, they need to be at the same length, ISO and whatever delays you set, it is preferable that they be taken at the same temperature, by that I mean, they can be taken at the end of your imaging run. The aim of the darks is to map the electronic noise that has been built up during an exposure and when combined to make a Master Dark, you then subtract it from shot.

I have a set of Master Darks I have made covering various exposure times and have used them on shots of the same length, this is not ideal and I usually try to take a set of darks whenever I shoot.

Hope this is what you are after.

There is a good article on this in the archives somewhere, can't seem to find it at the moment.

Cheers

JG

[Shawn]

PS

It took the 350 only 6 minutes to come down to ambient of 10C, measured from camera chassis, again..

Is this variation going to influence the efficiency of the darks, or am I getting too carried away...

S

[JohnG (John)]

Here it is:

http://www.iceinspace.com.au/index.p...63,211,0,0,1,0

Cheers

JG

[Shawn]

Hey Thanks JG,,

Im beng too fussy, I guess, I have no idea at this stage what temperature will do to the effectiveness of my darks, what you are saying is that I do not need be so fussy, yes, ?...

cheers shawn

..

[JohnG (John)]

That article I just posted can explain it better than I can, me, I just take my darks as a matter of course, some I have used for a while, others I take when I am imaging.

As I said, just make sure they are the same as your lights and you will have no trouble, and no, no need to be so fussy you will be mapping any thermal noise anyway.

Most of all, enjoy taking your shots.

Cheers

JG

[Shawn]

thanks for the link to another post , It helped a bit, but Im still confused by what is acceptible in applying a dark at say 20C to an image at 23C, although I assume that if the start piont is the same ,ie 21.7C same exposure lenght and cool down then the temperature increase inside the camera will be the same and the darks will be adequate,

but what is the margin of acceptibility...?

I guess its trial and error, huh ...

[JohnG (John)]

Personally I wouldn't worry if that was the difference, if you take 5 or 6 darks it will even itself out over the length of the exposure, your darks will have the mapped thermal noise anyway, this is what you want.

Cheers

JG

[Shawn]

Thanks again JG

S