Duration between subs (DSO dSLR imaging)

lacad01 (Adam) · #1 12-02-2010, 10:45 AM

Got a dumb question but wondering what difference it makes as to duration between subs when imaging DSO's with a dSLR? So for example if I was to image an object 20 subs @ 2min / sub, what's the interval between subs, do you wait a certain duration in seconds or just keep snapping with little to no interval between exposures? Apart from the additional time it adds to a session are there any implications for having a long duration between shots?

Jeff · #2 12-02-2010, 11:58 AM

Hi Adam,

I use a Canon 450D, usually at ISO800 with subs of 3 to 5 minutes (depending on target brightness and focal length). To help manage thermal noise, I typically use 1min spacing between subs to allow the DSLR sensor to cool down a bit. If using short subs (eg. 2 to 3min) on a coolish night (below about 15C), I usually reduce the gap to about 30 seconds. Seems to suit the 450D.

Cheers,
Jeff

lacad01 (Adam) · #3 12-02-2010, 12:02 PM

Thanks Jeff, that's helpful
cheers

Octane (Humayun) · #4 12-02-2010, 05:25 PM

Adam,

I think it's a good idea to have a gap between sub-exposures, but, in the past I've only ever had a gap of 5 seconds in between exposures.

Now, I pay a bit more attention to gaps between exposures -- using Nebulosity for capturing (RAW straight to card), I find the time it takes for the dithering between frames is sufficient enough a gap between exposures.

H

lacad01 (Adam) · #5 12-02-2010, 07:04 PM

Thanks H, I'll check Nebulosity out once I start getting serious about imaging and when these darn clouds start to clear

Dennis · #6 12-02-2010, 07:28 PM

Hi Adam

Just a reminder – if you have In Camera Noise Reduction (ICNR) turned on, then don’t forget to factor this in for the interval timer.

So, if you are shooting say, 2 min subs, then add 2 mins for the ICNR and then say, 20 secs for the write/download which would make the interval between each shot 4 min 20 secs.

Cheers

Dennis

lacad01 (Adam) · #7 12-02-2010, 09:54 PM

No worries, thanks for that Dennis. Haven't mucked around with ICNR yet. The odd wide fields that I've taken I've just shot separate darks and used DSS

avandonk · #8 13-02-2010, 02:51 PM

It is my experience that any gap between exposures barely let the sensor cool down. It is marginal at best.

It takes about forty minutes for a sensor to fully heat up with continuous exposures. The time constant is in the order of many minutes for heating up and cooling down.

It is a waste of time!

Bert

citivolus (Ric) · #9 14-02-2010, 04:43 PM

I've seen a graph of this thermal noise somewhere, and as mentioned above, unless you are actively cooling the sensor, you'd need to let it sit for 5-10 minutes (with the camera off) to see an appreciable difference, especially when you consider that the electronics located around the entire inside of the camera are warming up too.

I use long enough to get the image onto the memory card.

dugnsuz (Doug) · #10 15-02-2010, 06:29 PM

Quote:

Originally Posted by avandonk

It is my experience that any gap between exposures barely let the sensor cool down. It is marginal at best.

It takes about forty minutes for a sensor to fully heat up with continuous exposures. The time constant is in the order of many minutes for heating up and cooling down.

It is a waste of time!

Bert

I agree with Bert on this one - cooldown time would need to be too long in an imaging session to be of any significance.
I just allow a 20 second gap so that I can check the sub quickly for errors before the next one fires off. Time is precious when you're taking 15min subs with the additional 15mins ICNR thinking time!
This is my "Machine-Gun" sub theory

Keep firing them off!!!
Doug

[1ponders] (Paul) · #11 15-02-2010, 06:38 PM

I use 30 sec simply to have enough time to check the subs if I want to.

lacad01 (Adam) · #12 16-02-2010, 08:58 AM

Thanks for the feedback & info...once I get some clear skies and free time I'll get amongst it

avandonk · #13 16-02-2010, 11:43 AM

Quote:

Originally Posted by dugnsuz

I agree with Bert on this one - cooldown time would need to be too long in an imaging session to be of any significance.
I just allow a 20 second gap so that I can check the sub quickly for errors before the next one fires off. Time is precious when you're taking 15min subs with the additional 15mins ICNR thinking time!
This is my "Machine-Gun" sub theory

Keep firing them off!!!
Doug

This is called the 'American Method' shoot first and ask questions later. My boss at CSIRO when talking about xray data collection always said 'when in doubt just collect data'. Both these descriptions mean the same thing. My philosophy is to only dither the mount between exposures!

Bert