Why take multiple exposures?

[luminova11]

I have no experience of astrophorography other than some dslr shots of the Moon but do have a lifetime of experience in terestrial photography. Something has had me puzzled for a while; I can understand taking a number of exposures at different lengths and stacking them, similar idea to HDR imaging, but why spend hours taking multiple exposures of the same length. Surely once you have one image of an acceptable quality you could just stack multiple layers of the same image. Wouldn't a stack of 30 second subs of a dso just be a stack of 30 second subs whether it came from one image or many. I don't have any plans to get into AP anytime soon, I am a visual dob user, I am obviously missing something here but I must admit it leaves me more than a little perplexed.

[laser_jock99]

Mainly to even out random camera noise and thus improve the signal-to-noise ratio.

[knobby]

Great question!

[lensman57]

I have no experience of astrophorography other than some dslr shots of the Moon but do have a lifetime of experience in terestrial photography. Something has had me puzzled for a while; I can understand taking a number of exposures at different lengths and stacking them, similar idea to HDR imaging, but why spend hours taking multiple exposures of the same length. Surely once you have one image of an acceptable quality you could just stack multiple layers of the same image. Wouldn't a stack of 30 second subs of a dso just be a stack of 30 second subs whether it came from one image or many. I don't have any plans to get into AP anytime soon, I am a visual dob user, I am obviously missing something here but I must admit it leaves me more than a little perplexed.

Hi,

I did and I am still doing some " terrestrial " photography for over 25 years now, some time even as fine art and landscape photographer and made a few quid out of it. I can tell you that there very little similarity between Terrestrial and AP and IMHO there is none but some common nomenclature. In AP we have a great obsctacle ahead of us and that is mainly the weak signal that is recieved by the sensor. To combat this very long exposures are required to make sure that enough photons from the faint objects hit the sensor so thecan be converted into current for further processing. In doing so we create as big a problem for ourselves in the shape of the random noise which if not removed will swamp most the signal that you have painfully collected. Stacking multiple exposures will remove most of the random noise therefore increasing  the SNR as the signal is constant and the noise is being reduced. There are also different types of noise to deal with such as the read noise, the sensor thermal noise, sky back ground noise and more but stacking multiple exposures will help reduce the random noise. There is another technique to reduce the noise in the captured images ( SUBS we call them ) in particular with DSLR imaging and that is called  dithering where the position of the camera-mount is moved in relation to the target by the order of a few pixels between subs and this is done by the guiding software and then the stacking of multiple subs with the help of sophisticted algorithms will further reduce the noise and the nasties and will help to increase SNR. Only if we had perfect sensors and true dark noiseless skies. I hope that this helps a bit and welcome to the crazy world of AP.

Regards,

A.G

[luminova11]

Thanks for a great explanation, I thought it must be something to do with noise reduction but couldn't quite get my head around why. I assume dark exposures are also to aid with noise reduction, hot pixels ans software calibration, but what about flats. I know, so many questions from a non imager, but it was this curiosity that led me to asking for a telescope for my 8th birthday many years ago.

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[lensman57]

Thanks for a great explanation, I thought it must be something to do with noise reduction but couldn't quite get my head around why. I assume dark exposures are also to aid with noise reduction, hot pixels ans software calibration, but what about flats. I know, so many questions from a non imager, but it was this curiosity that led me to asking for a telescope for my 8th birthday many years ago.

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Hi,

There are mainly 3 calibration frames that need to be applied to the subs, Bias ( sometimes called the offset ) which is the steady state noise of the sensor as it is switched on and a suitable Bias frame is one that is taken at the fastest shutter speed of the camera with the lens cap on ofcourse. Dark ( this is not necessary for the modern cooled Sony CCDs )  which is to help reduce the dark current noise of the sensor at the same exposure of the subs, if your  subs were taken @ 600s then the darks must be taken @ 600s and more importantly at the same sensor temperature, this is where a DSLR is at a great disadvantage to a cooled CCD imager and finally the Flats, these are with the scope _camera _ Lens at exactly the same positon as the subs and the same orientation and at exactly the same focus point using a uniform flat illumination light source, a flat panel for example. The purpose of these is to remove imperfections in the optical path of the imaging rigg and they will also help in producing a flat back ground so faint detail can be extracted during the processing stage of the data, post stacking. For a DSLR all calibration frames are necessary. A minimum of 50 Bias, 35 Darks and 50 Flats will greatly help to sort the light frames out. For a  modern cooled CCD -20 C the Darks are not necessary and the Bias will do the job just as well . While using a CCD I normally take 101 Bias, 51 Flats along with as many Light subs that I can get away with.

Regards,

A.G

[luminova11]

Thank you for the explanation. That has sated my curiosity; for now. I can barely afford one avenue of photography being in the process of setting up my own landscape and location portraiture business, so as much as it calls to me I will enjoy the efforts of other astrophotographers instead. I love visual observation anyway, but there is the new camera mount from SkyWatcher

http://www.teleskop-austria.at/shop/index.php?lng=de&m=2&kod=mont-adv-ent-01

.......

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[dph1nm]

Only if we had perfect sensors and true dark noiseless skies

You would still need to stack, as the signal from the objects themselves is also intrinsically noisy!

NigelM