From: Milton Aupperle <milton@outcastsoft.com>

Date: February 22, 2010 8:34:32 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] 60fps vs 30fps imaging

Hi Jim;

On 22-Feb-10, at 7:05 PM, jimchung2338 wrote:

Last night I was doing some DSO imaging and I decided to pack it in early because transparency was falling.  Since the C8 was on the mount, I swung it to Mars and had a rare visual peek with a 4mm ep.  The seeing was quite stable and I could see a lot of surface detail.  Not wanting to waste an opportunity, I imaged Mars with my DMK21F04 colour ccd at both 60 & 30 fps.  At 60fps I had to turn the gains to about 90% so I expected a much noisier stack result than with the 30fps stack.  Both stacks exhibited very similarly shaped sharpness distribution histograms and the cutoff was chosen at 80% for both.

Both the 60 & 30fps stacked image were sharpened the same amount.

It appears that the level of noise reduction as a result of stacking many more images in the 60fps image enabled it to withstand a level of sharpening high enough to cause the 30fps image to breakdown and show artefacts.

http://tech.groups.yahoo.com/group/Astro_IIDC/files/Planetary/DMKMars.jpg

Well you are using about 2.5 times as many frames in the 60 fps case as you are in the 30 fps case too.

In the past I've always shot at lower frames rates thinking that I was ahead by having individual frames with lower noise.  This was a tradeoff I was willing to make even though a high frame rate would enable better freezing of the seeing.

Frame rate is part of it, but it's really CCD exposure time that freezes the image. You could probably shoot at 30 fps with a 16 ms exposures  at those gain levels and get the same result. But at 30 fps recording to disk, you do get fewer frames captured.

I've shot lunar images as lows as 1 ms (1/1000th of a second) with the Grasshopper and still had really bad turbulence between frames.

It appears that stacking tolerates quite a bit of noise and a higher frame rate becomes the more desireable attribute.

Stacking is primarily to reduce noise and amplify signal.

Basically it's all trade offs depending on your seeing. If your seeing is average and you have "normal" type curve, you can stack quite a few frames to reduce noise and get reasonably sharp stacks. However if you have my typical seeing, where I get a few good frames in a sea of blurry masses, well then you only get sharp stacks if you can use those few sharper frames. However since your using fewer frames, you have to be very careful with your gains because your not suppressing noise with enough frames.

So it's all a tradeoff.

PS:

In Astro IIDC, you can remove the red fringe on the upper right side of Mars by offsetting the Red channels by -1 horizontal and -1 vertical. I did that pretty easily in Astro IIDC using the "Color Channel Offsets:" of the Image Processing.

TTYL..

Milton Aupperle