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dbk21au04.as setting question, gain vs. exposure

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#1 ourobouros2k2

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Posted 20 November 2012 - 08:02 PM

Greetings all..

I am currently imaging with a Nexstar 8SE OTA on an Orion Sirius mount. I have had a few imaging runs with the above camera, and have noticed some excessive noise after processing. I don't know if this is the result of more gain or what. I know that if I increase the gain and keep exposure around 1/30 of a second I can keep my framerate at about 30fps, but of course the framerate drops when I increase exposure and dial down the gain. Which is preferrable, more exposure or more gain? Is the increased gain the reason for the noise in my images?

much thanks,
Andy

#2 edsplace

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Posted 20 November 2012 - 09:29 PM

Yes, and yes. You want to keep the frame rate down, and the gain down. Excessive gain will add noise to the image, to long of an exposure will during times of poorer seeing reduce sharpness.

With gain you can overcome noise by using more frames in the stack to reduce the noise. If the planet is moving around the capture area at all noise patters will also be eliminated by a large number of frames, if the planet does move much or stays on the same exact stop, noise patters on the chip will be noticed in your final image. I use a high gain setting and faster frame rate when the seeing is average or below, I also will move the planet around the chip a bit while imaging.

Longer exposure times will allow blurring in the image during times a bad seeing, but when the seeing is very good increasing the exposure time will not has as much an adverse effect on the final image. A shorter exposure time even when the seeing is good will probably produce a clearer final image. I will use longer exposure times when the seeing is above average and reduce the gain. An image with lower gain will require fewer frames to get a smooth image.

So as I say yes you want low gain, and you also want a short exposures and low gain. It's a balancing act. What I have done is found the highest gain that I find "workable" in post processing and then use that to figure out what exposure to use that evening. What I want is the highest number of frames per second and shortest exposures with a gain low enough to produce a smooth image.

I am sure others on this site have different strategies and they may work well, but that is mine.

#3 ourobouros2k2

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Posted 20 November 2012 - 09:45 PM

I have noticed to my dismay that as seeing varies from session to session, I have to readjust my approach. I understand that it is a balancing act, one I must practice more with. I appreciate the response and information. I will try some more experiments this weekend provided mother nature cooperates. I had it down pretty well I thought for solar imaging, but I am finding out that planetary imaging is a whole other fickle beast.

again much thanks for the info, can't wait to apply it.

#4 ourobouros2k2

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Posted 20 November 2012 - 09:49 PM

Also, I tend to keep my avi runs around 500-1000 frames, depending on framerate. I remember reading somewhere that for jupiter, I should keep my runs to about 90 seconds due to fast rotation. Is this too short, or how much do you think I can get by with timewise before rotation-based feature blurring occurs?

thanks in advance,

#5 ToxMan

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Posted 20 November 2012 - 10:06 PM

Are you using the histogram during live view and capture? It takes out a lot of guess work because it helps maintain consistency without all the other variables turning it into a guessing game.

With IC Capture, the button is on the upper left of the toolbar. If you adjust gain until 50%, or "127" along the intensity axis (0 - 255) you have a reference point for comparing all future videos and images.

#6 edsplace

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Posted 20 November 2012 - 11:11 PM

I run 1000 frames min per AVI, if I can get to 60 FPS I run 2000 frames. I use the DMK21 so I end up with 3 min from begining to end and can work woth that. When I used a DBK a few times I captured 3000 frames and it worked well. I stacked the best 1000. As for the histogram I fill mine to 75%, I find it leaves enough room for sharpening while gathering a bit more dit depth in the image.

#7 ourobouros2k2

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Posted 20 November 2012 - 11:54 PM

Here is a cap I took on one of my last imaging runs. I am thinking the histograms are too low? When you say fill to x% I am assuming you mean the red, green, and blue portions? What is the fluctuating yellow portion on the far side of the histogram?

I can't wait till this weekend to try these tips and thank you all for the advice.

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#8 edsplace

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Posted 21 November 2012 - 12:11 AM

Yes, and those exposures look good to me.. Thy are between 50 and 75%

#9 edsplace

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Posted 21 November 2012 - 12:17 AM

I find that I can run the gain up to 750-800 with little effect on the final image.

#10 ourobouros2k2

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Posted 21 November 2012 - 12:32 AM

Thanks again for the help, much appreciated :)

#11 zAmbonii

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Posted 21 November 2012 - 06:14 AM

Don't be afraid to go longer with your captures. I do ~3 minute captures, just mainly to get as many frames I can to stack. I usually have a total of around 3000 frames and stack about 1000 of them. More frames would be important when seeing is not so good (so you can get more good frames). If seeing is really good you could shorten your capture time and stack a higher percentage of the frames you get.

Depending on image scale, Jupiter can rotate quite a bit in those 3 minutes, but the stacking programs do a really good job of avoiding smearing. You might want to experiment some to find out what works best for you.

The below image is from a 3 minute capture, and 1000 out of 3400 frames were stacked. (Used an 8SE OTA + 3x barlow)

Posted Image

#12 ourobouros2k2

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Posted 21 November 2012 - 10:56 AM

Beautiful shot, I really gotta get a 3x barlow!






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