Mirror lock up...is it important?

[MG1]

Just a thought I've been trying to resolve in my cold riddled head...

If I'm exposing for say 5min plus, am I really going to notice and shake from the mirror slap happening in the first fraction of a second?

Does the sensitivity of the exposure reduce with time? Ie is the chip more sensitive at the beginning of an exposure than at the end? I believe this was the case with film...but may be wrong.

Just doesn't seem to make sense....anyway I await some expert feedback.

Michael

[andyhicks]

I am totally new at this but wouldnt the chip get hotter the longer the exposure?

[brianb]

If I'm exposing for say 5min plus, am I really going to notice and shake from the mirror slap happening in the first fraction of a second?

Yes, if there are bright stars in the field. In any case there is almost no penalty for using mirror lock when you have the camera on a tripod or hung off a scope.

Does the sensitivity of the exposure reduce with time? Ie is the chip more sensitive at the beginning of an exposure than at the end? I believe this was the case with film...but may be wrong.

There is no reciprocity failure with digital imaging. With film the cause is that it takes several photons to strike a siver halide grain before it can be developed, and the grain has a bad memory so that e.g. 3 photons arriving within 1 sec will register but 3 photons arriving at 1 min intervals will not because the earlier events will have been "forgotten" by the time the later ones occur. Film is more sensitive to "flashes" than long steady exposure, it isn't more sensitive at the beginning of the exposure than at the end.

wouldnt the chip get hotter the longer the exposure?

Yes, unless the chip is actively cooled. Because noise decreases with temperature (about halving with every 7C drop in temp) this is why dedicated astro CCD cameras are cooled.

[MG1]

Cheers Brian....I'm guessing I can set a custom programme to always engage mirror lock. Although I can't remember if there are issues with custom functions (the programmable knob on the top of canons) and bulb exposure with the EOS utility.

Unfortunately I'm away from home with neither camera computer or scope so will have to wait to have a play.

It's just I keep forgetting it....and when I do remember then forget the second shutter release to start the exposure!

I am a bit slow sometimes!

[jgs001]

I use a custom menu option to set mirror lockup, and always used it with my timer remote. I've started shooting tethered so that I can control the camera and integrate with PHD for controlled dithering with guiding, and turned off mirror lockup. Even pixel peeping I can't see any noticeable effect on the stars from not using mirror lockup. Given the choice is no mirror lockup or amp glow...

I will, at some point, get a proper serial shutter cable so I can go back to using mirror lockup however.

[MG1]

Bit OT,but what's dithering?

[jgs001]

The scope is adjusted a bit between exposures. This puts the target on a slightly different part of the sensor. The benefit is, if you stack with a statistical method (Kappa Sigma or Kappa Sigma Median in DSS) the stacking process removes irregularities, like hot pixels on each frame, as they are in different positions within each frame.

[MG1]

Very clever....will look into it a little more when I advance a bit.

Thanks

[Dave]

I never use mirror lock up for my deep sky imaging.

It would be far too labour intensive and I don't think it notices that much on the faint stuff anyway.

But I always use mirror lock up it for the Moon and planets where possible as the objects are bright enough to register in that short time and will blur the image. In this case I lock up the mirror, wait until the vibrations have died down and then take my short exposure.

I don't think reciprocity failure is a problem on DSLR's as the chip continues to collect photons the longer it is exposed.

Some people leave their camera to cool down between exposures / subs as the camera gets warmer while it is being used, this results in more noise and glow from internal components showing on the image.

Dave