Exposure time question

[Haggis]

This may be a dumb question, but I'd like to know anyway...

I recently started my astrophotography hobby since I've finally managed to get all my stuff together. I bought a licencekey of Backyard EOS and like it very much! But is it normal that every time I make an exposure of say, 300 seconds, the timer counts to 300 twice? I thought the exposure itself would take 300 seconds and then maybe a few seconds more to transfer the image to the PC through the USB cable. Is this normal or do I need to change a setting in my Canon 1000D?

If it's normal, what does the camera do when it does the second countdown? The second countdown always takes just as long as the first. A 10 sec. exposure takes 20 seconds on my setup, a 10 minute exposure takes 20 minutes. As a beginner, I'm confused how this works

[laser_jock99]

The camera might be in some kind of 'noise reduction' mode? It could be taking a dark frame of equal length to the original exposure and digitally subtracting noise?

[Capricorn]

Cannot recall where I heard it said but if you take a N second exposure you should then leave the camera to do nothing for the same period.

Seem to have a half recollection of someone saying why, but cannot recall any more then this so cannot supply an more definite idea why this is. Just that it is a vague recollection. So whatever you have may simply be enforcing this.

On that basis if it were me in your position I would not worry overly, but I would however probably drive myself insane trying to find out why.

Actually sure I have heard it on more then one occasion.

[Haggis]

Yes, I have noise reduction enabled. Could that be it? Is it better to leave this setting on even if you take dark frames yourself by putting a lid on the scope? Thanks for the advice!

[Haggis]

I'm not worrying but if I have to wait twice as long as necessary for no reason, then I might as well change whatever is causing it . But if it's normal and not because of the noise reduction setting, then there's nothing I can do about it. Next time, I'll try it with noise reduction off and see if I have to wait 10 sec. for a 10 sec. exposure and not 20 sec.

[pwalsh61]

I got caught out by that with my 1000d last year - every exposure seemed to take ages to write to card - I'd got noise reduction switched on. For every exposure the camera was doing a 'dark' of the same length.

Switch the noise reduction off

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[Haggis]

I will switch it off, thanks for the advice everybody!

[astrofox]

sounds very much like noisereduction algorithym is switched on. Look in the menu and see if you can turn it off. My Nikon behaved just like that

[LukeSkywatcher]

The camera might be in some kind of 'noise reduction' mode? It could be taking a dark frame of equal length to the original exposure and digitally subtracting noise?

That is exactly what the camera is doing. For every exposure you take using the camera, it is also taking a "dark" exposure of the same length of time. So a 60 second exposure takes 120 seconds. Not a bad thing but if you go into the settings you can stop this from happening.

When i bought my 450D, the same thing was happening and it puzzled me. Now i still just let my cam do this for me.

[K3ny0n]

Ill remember that for my 1000d

Rob