I have recently noted that you cannot use exposure time below 1s with Ekos and Atik CCD driver.
Apparently you can do it directly with Atik CCD on INDI, not in Ekos. That's why I would point out at Ekos rather than Atik driver.
Jasem, could you take a look at this?

What exactly do you mean by "cannot use"? Are there failures? Anyway, I'll test with our two Atik cameras today.

The exact result is that you cannot set exposure value below 1s in Ekos CCD.

No issue here with Atik 314L and 383L+, can select exposures down to 0.001s. I'm using latest KStars and INDI 1.2.0 on Fedora 23 here. What is your setup?

I can also set exposures below 1sec with Ekos and my Atik 383L+. I think I've even used 0.001 for Bias frames

Hi there. The problem persists. Exposures below 1s are just not available in Ekos CCD tab (and ONLY here - not on the Ekos Focusing tab or on Atik Titan CCD driver's tab in INDI client - these work as usual). See attached screenshot - note gray button at exposure field. It is disabled, I cannot click it so the exposure is set below 1s. If I set exposure value manually by entering anything below 1s (ex. 0,001) the value is reset to 1s while capturing.

My setup is as follows.
INDI server: Raspberry Pi 2, Ubuntu 15.10, armv7l arch with 4.1.15-v7+ kernel, libindi 1.2.0+r1013.250
INDI client: PC, Ubuntu 15.10, x86_64 arch with 4.2.0-34-generic kernel, libindi 1.2.0+r1013.251, kstars-bleeding 5:15.12+r5640.251

The problem exists in Ekos CCD tab ONLY.

Hi, probably some locale related issue ("," instead of "."). Peter

Bingo! Thank you Peter. This solves the issue.

So the solution is to keep locale configuration consistent between you INDI server and PC running Kstars & Ekos.
You can check your locale setting on linux system by running "locale" command.
If your locale (especially LC_NUMERIC) differs between systems i.e. decimal point is not consistently "." or "," you get into trouble.
Just set your default locale to the same on both systems and you're done e.g. in Ubuntu you just change locale in /etc/default/locale to permanently set it for your entire system.