Question on Darks

[bloodhound31]

If the temp in the obs was 12 degrees last week when I imaged, and it is 12 degrees this week while I image, can I use the same darks to stack with tonights subs or do I have to take darks again?

Baz.

[JohnG]

No reason why not, so long as they are the same length/ISO as your lights.

I have various Master Darks made up to suit various exposure lengths, I use a standard ISO 800 so it easy to accomodate my lights.

I also use a made up Master Flat and Bias as well, you can probably use these for about 6 months before making a new set.

Cheers

[bloodhound31]

Thanks John, Very timely reply mate, you just saved me a lot of work and a couple of hour sleep!

God bless ya li'l cotton socks!!!!:thumbsup:

Baz.

[mostschaedel]

Sorry to disturb your planned sleep, but my experience with darks of DSLR`s is different.
If your outside temperature today is the same as yesterday, that does not
mean, the camera inside temperature is the same!!
This changes during imaging the whole night and from day to day.
Depending on the heat developed in the camera from the electronis and the
usage of the screen.
In my experience the darks during the night vary and using an old master dark will work somehow but not if you want to press everything out of the data you got on the light image.

The difference in the darks will always be less the lower the temperature is.
The higher the temperature the bigger the difference in the thermal noise
from one dark to another.

ONLY temperature regulated CCD`s can use a master dark from a special temperature for longer! DSLR`s are NOT temperature regulated therefore
you dont have control over the darks.

Usually i am giving 25% of the exposure time to the darks and 75% to the lights, means 45 minutes light frames 15 minutes afterwards dark frames every hour and so on.

You can test what i wrote very simply.
Make a dark today at a fixed room temperature do the same tomorrow with the same temperature,
subtract the images in photoshop, push the histogramm of the result in photoshop and you will see a lot of difference.

Gerald

[gbeal]

While not overly qualified with DSLR darks, I am leaning with Gerald.
With my cooled CCD I can usually get away with a master dark.
My gut feeling with the DSLR though is that you should take them at the same time, and preferably interspaced with your lights. Take a few lights, shoot a few darks, take some more lights etc. That way the sensor temp is near or perhaps closer to that which exposes the darks.
Anyway Baz, try Gerald's experiment, can't hurt. Post the results.

[JohnG]

Well, sorry, that is not my experience.

I have found that so long as you take your Darks, say 3-5 at the beginning of your run, 5-6 or so during the middle of your run and the rest at the end, combine them to make a Master Dark, they are good for any Lights taken around the same temp, I normally make Master Darks every 3-6 months, depending on the season.

Please feel free to correct me but a Master Dark is the combined average of the 20 or so Darks taken during an imaging run done as above, therefore you are covering a multitude of temps, starting from when the DSLR chip is cold to when it heats up over the period of the imaging run.

Every persons setup is different, every person mileage is different, this works fine for me when Lights are taken around a similar temp. You need to test your own setup and find out what works for you.

Just my opinion from a lot of experience.

Cheers

[higginsdj]

In reality, the dark current on a CCD will vary considerably with temperature variations as small as 5 degrees C ambient. However, I believe all the current DSLR's are using CMOS chips and as far as I am aware they behave somewhat differently to a CCD chip.

You'll need to look at the dark current values for your chips/cameras and make your own value judgments based on the quality and type of images you wish to produce. (ie you'll need to characterise your cameras by taking test images under various conditions and usage, measure those images and then compare them)

Cheers

[bloodhound31]

Well, I used my old darks for last nights shooting of the Eagle nebula. I got 42 minutes worth and am not terribly happy with it.

Have a look here (http://www.asignobservatory.com/Astrophotography.aspx) at the nebula thumb and look at the first one, M16 Eagle nebula, to see what I'm talking about.

I think I will have a go at taking a few lights, then darks, then more lights etc.

It seems like a real pain in the butt to do, but this IS a game of patience and perfection, so I'm up for it.

This just means I will probably have to resist being greedy for multiple targets in one night, and just dedicate myself to getting ONE GOOD ONE in a night.

I just get so excited....:P

Baz.

[mostschaedel]

Hi!
Think thats the right approach. :thumbsup:
Most of us start with imaging 3 different
objects in one night. You cant resist...
Later we end up with one object in 3 nights to get
the real deep sky objects and to be able to push the images to the limits.