"Subs" and "darks"

[lateblumer]

In an older post about widefield imaging with a DSLR and a barn door tracker, this caption was written: "7 subs w/ 5 darks, 120sec @ f3.8, ISO 800" I understand the second part of the caption explaining the exposure time, f-stop and speed, but what is meant by "darks" and "subs." And how do you create them? Thanks in advance to anyone who can offer a bit of knowledge to a newbie!

[nmoushon]

Subs is another word for a singe image. So 7 subs is 7 images at 120sec. Darks are images taken with the lens cap on. Thus the name darks. This is a picture of all the hot pixels that build up on the camera sensor with long exposure photography. Doing this maps them so that the computer software can subtract them from you lights and gives you a better final image.

[lateblumer]

Thanks! Now concerning the computer software, I use Photoshop CS6 on a daily basis in my design business, so I am very familiar with all its' features. Can I use it to stack my light images? And if I turn the "darks" into negative images and stack them would that accomplish what the photo stacking software programs do? Or how else might I subtract the hot pixels?

[lensman57]

Thanks! Now concerning the computer software, I use Photoshop CS6 on a daily basis in my design business, so I am very familiar with all its' features. Can I use it to stack my light images? And if I turn the "darks" into negative images and stack them would that accomplish what the photo stacking software programs do? Or how else might I subtract the hot pixels?

Hi,

You can use PS but it is a long drawn out process, download a free copy of the lastest Beta version of the Deep Sky Stacker and it automatically does the calibration and stacking of the frames ( Subs ) for you. You can then use PS to stretch the stacked file or use an Astro software to do this and finish off in PS.

A.G

[JKB]

Yep as stated above!

One thing though that is often forgotten: Make sure the viewfinder on the camera is covered as well as lens when taking the dark frames. All too easy for stray light to enter the camera and spoil them.

Cheers

Jamie

[AndyUK]

Another thing that's important with taking darks is that they should be the same duration as the "subs" (or lights) and preferably taken at the same time. This then helps to ensure that the subs and darks are taken at the same ambient temperature. When I first started with a DSLR, I used to take 20 darks, 10 before I started and 10 after - It wasn't such an ordeal when unguided and using up to 120s exposures, but if/when you start guiding and start using 600s exposures that becomes more tricky (over 3 hours to take the darks alone!).

Fortunately you can re-use darks (as long as you record what duration and temperature they were taken at) so over time you can build up a library of different exposures at different temperature ranges. I cheated a little bit and put the DSLR in the fridge for a day with the bodycap on and then set the remote timer to take 20 of a range of different exposures. Bear in mind though that whilst the fridge temp may be c. 4C, over time and as you increase exposures, the DSLR chip heats up. The EXIF data records the actual chip temp, and I discovered that the chip temp leveled out at about +12C from ambient... but that would also happen if you were out under the stars anyway