Bias duration?

[UlteriorModem]

I know the basic rule for Bias duration is "As short as possible".

But I just looked up the specs for the ASI071mc and its minimum shutter speed is 64us. That is insanely fast.

I just went with .1 seconds.

Any thoughts on this?

[JayTee]

That is fast. I use 1/4000th for my DSLR and that equals 250µ. 64µ is roughly 1/16,000th second - how does it go so fast?

JT

PS, is my math right on this?

[UlteriorModem]

It uses a 'rolling shutter'.

Here check out this thread.

viewtopic.php?f=28&t=6918&p=59543&hilit ... ter#p59543

[JayTee]

Yeah, I watched that video from the original posting. I guess I did not equate the rolling shutter being able to "catch" such a quick glimpse.

JT

[bobharmony]

64 microseconds should be fine. With my DSLR I can get 1/8000 which is equivalent to 125 microseconds.

Bob

[UlteriorModem]

I dont think SGP will let me set .000064 seconds

[bobharmony]

I dont think SGP will let me set .000064 seconds

Sounds like a bug in SGP or the camera driver If the camera supports it, the software should recognize it. Maybe you should look for 1/15625 seconds!

Bob

[UlteriorModem]

To be honest I have no idea how many decimal places SGP will let you take it out to.

[STEVE333]

Hi tom - The reason for taking such short exposures for Bias frames is to ensure that there is no Dark Current contribution to the signal. Since you have a cooled camera the Dark Current will be extremely low, so, even 0.1 sec should be fine.

Cheers,
Steve

[Stuart]

With SGP I don't set a duration. I just use the drop down menu and hit "bias" and leave the duration blank.

[UlteriorModem]

All selecting 'Bias' does is append the file name. It does not determine a duration. A duration 'should' be set.

I have no idea what duration you will get if you leave it at 0. BTW SGP goes out to three decimal places just FYI.

[Stuart]

I'll see if I can find one and post it.

Edit: I just checked. I dumped all my individual bias frames and only have masters. Next time I take bias frames I'll screenshot how I take it and show the result. It works fine.

Here is from the SGP manual. I set mine at zero

[UlteriorModem]

Find one what? A bias? Does it have the duration info in the fits header?

[Stuart]

Hey Tom, yes. That's what I was looking for, but I don't have any stored online or on this PC. I may have an individual bias frame on another PC--I'll check when I get home. I'm pretty sure it says duration "zero". If you're interested, here is a bias master but it's a master and doesn't have duration on the fits header.

[UlteriorModem]

I just asked over at the SGP forums and was told that many cameras are not capable of a zero duration and will most likely throw an error. I guess your camera does not.

I will set a duration even if it is .001 sec ::D

Might try one at 0.000 just to see what happens. But it will be a couple of days and I am not dragging out the camera for just that

[Stuart]

Hey Tom,
CMOS may be different than CCD. I'm ignorant with CMOS cameras. I'll find one and share with you later.
Stu

[Stuart]

Hey Tom,
Here is a bias frame