For those astrophotographers,
Have been trying to do some piggy back astrophotography but the prob is at iso 800, f/8, 70mm lens and an exposure of 90s, i get a washed out pic with stars that are barely shining.
how do you guys minimize the washout?
Thanks
piggy back astro
Hi,
are you engaging in DSLR imaging? Is the "wash out" referring to light pollution superseding signal from stars that you are trying to record? if this is the case, try to shorten your exposures, say a minute or even 30 seconds, and do stacking to minimize background noise. if the wash-out is prominent at the central and reduces radially outward, then you are facing vignetting issues with your system, and will require anti-vignetting corrections which can be done in software techniques.
are you engaging in DSLR imaging? Is the "wash out" referring to light pollution superseding signal from stars that you are trying to record? if this is the case, try to shorten your exposures, say a minute or even 30 seconds, and do stacking to minimize background noise. if the wash-out is prominent at the central and reduces radially outward, then you are facing vignetting issues with your system, and will require anti-vignetting corrections which can be done in software techniques.
you can try stacking from a minimum of 3 sub exposures. Why stack? Basically, as photons gather on each subexposure, the noise gathers as well, but contrary to the photon signals, noise gathers in a random pattern. As you stack successive subexposures, the noise is "stacked" randomly and effectively cancels each noise profile from each subexposure out, and the photon signals are stacked effectively. This results in a smoother background noise profile, and lowered noise too, but signal profile is heightened. How much? square root of the number of stacked exposures. try it!
Thanks for this information! Something I learn again!rcj wrote:you can try stacking from a minimum of 3 sub exposures. Why stack? Basically, as photons gather on each subexposure, the noise gathers as well, but contrary to the photon signals, noise gathers in a random pattern. As you stack successive subexposures, the noise is "stacked" randomly and effectively cancels each noise profile from each subexposure out, and the photon signals are stacked effectively. This results in a smoother background noise profile, and lowered noise too, but signal profile is heightened. How much? square root of the number of stacked exposures. try it!
Wee Nghee the Pooh
One more thing: when stacking, do not use the 'SUM' function if you want to get rid of noise! Use 'Median' or 'Mean/Average' instead.
Cheers!
Cheers!
[80% Steve, 20% Alfred] ------- Probability of Clear Skies = (Age of newest equipment in days) / [(Number of observers) * (Total Aperture of all telescopes present in mm)]
A little more on the type of stacking used.
SUM or if you are referring to ADD functions, can still be used, provided noise profile is not so high. Median is commonly used and good for smoothing out the noise profile, especially when it is good for removing serious spikes or cosmic rays often encountered in CCD imaging. However the trade off are the stars will not be as pronounced and will have slight blurring introduced. I am currently using Addictive stacking in most of my CCD images, and only use Median stacking when the SNR is very good. Or when I will decide to apply a little unsharp masking to "sharpen" up the PSFs of stars.
Use Median when you have multiples of 3 stacks or at least three.
Use Average if you have less than 3 though it is not suggested to stack this little.
Use Add if you are certain all your raw images do not have spike noise or cosmic rays and you are prepared to introduce a little blurring of the stars, and slight suppression in faint nebulosity backgrounds.
If you have more than 9 raw frames for stacking, I will suggest you use SigMa Rejection which is even better, combining both worlds of Add and Median stacking.
But not all software supports Sigma Rejection. MaxIM DL does this.
SUM or if you are referring to ADD functions, can still be used, provided noise profile is not so high. Median is commonly used and good for smoothing out the noise profile, especially when it is good for removing serious spikes or cosmic rays often encountered in CCD imaging. However the trade off are the stars will not be as pronounced and will have slight blurring introduced. I am currently using Addictive stacking in most of my CCD images, and only use Median stacking when the SNR is very good. Or when I will decide to apply a little unsharp masking to "sharpen" up the PSFs of stars.
Use Median when you have multiples of 3 stacks or at least three.
Use Average if you have less than 3 though it is not suggested to stack this little.
Use Add if you are certain all your raw images do not have spike noise or cosmic rays and you are prepared to introduce a little blurring of the stars, and slight suppression in faint nebulosity backgrounds.
If you have more than 9 raw frames for stacking, I will suggest you use SigMa Rejection which is even better, combining both worlds of Add and Median stacking.
But not all software supports Sigma Rejection. MaxIM DL does this.
Oh yes! Image processing is 'addictive'! Heh..rcj wrote:I am currently using Addictive stacking in most of my CCD images,
Agreed! I would strongly recommend MaxIm/DL. Although a bit expensive, you only need this software (and Photoshop also, but compared to the price of Photoshop, this software is a steal!).rcj wrote:But not all software supports Sigma Rejection. MaxIM DL does this.
My Rule of Thumb: If you can see the object clearly in a single frame, then use Median/Average/Sigma Clip. However, if I do not see my object in the single image, then I use 'Add' to bring out the object.
Cheers!
[80% Steve, 20% Alfred] ------- Probability of Clear Skies = (Age of newest equipment in days) / [(Number of observers) * (Total Aperture of all telescopes present in mm)]