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Grainy Ha?

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#1 Deesk06

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Posted 01 June 2020 - 06:58 PM

Hello,

 

I have a quick question, how can I reduce the grain in this image? It seems like it's Ha that is grainy and almost "speck" like. This is one of those things that kind of annoys me. Is this something I am going to have to live with? I see some people have good data that looks smooth. if you look around the edges of the image you will see how much grain there is. Also, please do not mind if I cooked the image, I am experimenting with a lot of different edits. Trying to find my way in this hobby. 

 

Edit: I should add that I had so much gradient in this image. I have two bright lights shining into my backyard. Both have totally affected the image. I can tell that because of the glow after stacking, its was insane. Need to try and block them somehow. So this is pretty decently cropped.

 

Data Acquisition 

 

212 x 60" Light

45 x 60" Dark

45 Flat 

80 Bias

 

Unguided (No dithering, maybe this is why its grainy?)

 

North American Nebula NGC7000

Edited by Deesk06, 01 June 2020 - 07:09 PM.


#2 kathyastro

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Posted 01 June 2020 - 07:11 PM

60 seconds is pretty short for Ha.  I shoot 600 second subs for mine.  Granted newer cameras can shoot shorter, but I would guess that you are not getting above the noise threshhold.  Try increasing the exposure time per sub.  And definitely dither.


Edited by kathyastro, 01 June 2020 - 07:12 PM.

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#3 Deesk06

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Posted 01 June 2020 - 07:19 PM

60 seconds is pretty short for Ha.  I shoot 600 second subs for mine.  Granted newer cameras can shoot shorter, but I would guess that you are not getting above the noise threshhold.  Try increasing the exposure time per sub.  And definitely dither.

Thanks, I will try longer subs. I had to go short because I didn't want to blow the image out. The moon was lighting up the sky. I will see how far I can push it 



#4 Madratter

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Posted 01 June 2020 - 07:24 PM

The Moon has little affect on Ha unless it is very close in the sky, which this wasn't or the Ha is very wide band, which this probably isn't.


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#5 scadvice

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Posted 01 June 2020 - 07:30 PM

60 seconds is pretty short for Ha.  I shoot 600 second subs for mine.  Granted newer cameras can shoot shorter, but I would guess that you are not getting above the noise threshhold.  Try increasing the exposure time per sub.  And definitely dither.

Another little piece of information for my notebook. I recall hearing that tidbit a time back but you forget this stuff sometimes. Thanks Kathy.


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#6 wrnchhead

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Posted 01 June 2020 - 07:37 PM

Kathy is one is those ones that slides in and fixes my problems with apparent ease, lol. Just got my first Ha filter and so glad to know this!

Edited by wrnchhead, 01 June 2020 - 07:38 PM.

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#7 kathyastro

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Posted 01 June 2020 - 07:50 PM

Thanks, I will try longer subs. I had to go short because I didn't want to blow the image out. The moon was lighting up the sky. I will see how far I can push it 

When the Moon is bright, that's when I shoot Ha, because the Moon isn't reflecting a lot of Ha.  Narrowband is often the only DSO work you can do on a full Moon.



#8 ks__observer

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Posted 01 June 2020 - 08:31 PM

Not sure i see the "graininess."  

IMO, exposure time is not going to change things much.

The swamp factor formula for determining percent best:

(noise real camera) / (ideal noiseless camera) = sqrt(X) / sqrt (X+1), X = swamp factor

For X = 10, you get 95% best.

For X = 1, you get 71% of best. 

I don't think a few extra hits of read noise from shorter exposures should have a drastic impact on your picture.

Total integration time is most important factor.

Also, experiment with denoise tools.



#9 SilverLitz

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Posted 02 June 2020 - 07:42 AM

It might not be grain, but a VERY dense star field.

 

I would expect trying to get Ha, which requires significant longer exposure times, will be very difficult unguided, unless you have a high end mount, e.g. AP, Paramount,...

 

When I shoot Ha with a mono camera and at a fast f/4, I use 240s.  If your scope is slower, you will need even longer exposures, e.g. a f/7 scope will take ~3x longer than f/4.


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#10 Deesk06

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Posted 02 June 2020 - 08:01 AM

It might not be grain, but a VERY dense star field.

 

I would expect trying to get Ha, which requires significant longer exposure times, will be very difficult unguided, unless you have a high end mount, e.g. AP, Paramount,...

 

When I shoot Ha with a mono camera and at a fast f/4, I use 240s.  If your scope is slower, you will need even longer exposures, e.g. a f/7 scope will take ~3x longer than f/4.

This seems likely. Kathy said so as well. I wa susing a lens at f/4. Could use longer exposure I guess. 



#11 ks__observer

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Posted 02 June 2020 - 08:11 AM

In PixInsight, and I presume other programs, you can reduce stars in the star field with Morphological Transformation. 



#12 sharkmelley

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Posted 02 June 2020 - 08:39 AM

You are using a modified Canon 600D with an f/4 lens in a bright suburban sky with a fair amount of moon and no Ha filter?

 

If so then 60sec exposures are absolutely fine.  The noise level in your stacked image will be driven by the brightness of the sky - i.e. the combination of light pollution and moon.  Your result is actually pretty good.  To improve on this you either need to shoot a lot more subs or travel out to a dark-sky site.  Dithering will also help.

 

Mark 


Edited by sharkmelley, 02 June 2020 - 08:43 AM.

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#13 RJF-Astro

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Posted 02 June 2020 - 08:39 AM

I do not read that the OP is using a Ha-filter. Is this correct? Do you mean the Ha-part of the nebula or is this a colorized stack made with a narrowband filter? Star color appears to be RGB, so I am guessing no Ha-filter.

 

Agreed that it is probably the dense starfield around NGC7000. Looks nice enough to me for 3-4 hours RGB.



#14 SilverLitz

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Posted 02 June 2020 - 02:00 PM

I was assuming he was using an Ha filter, given the title of the thread.  If he was not, my relative exposures would be 50% of that, e.g. 120s for RGB at f/4.  But, my reference is an ASI183mm-Pro with EFW at unity gain and Bortle 5+ skies.



#15 Ryou

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Posted 02 June 2020 - 02:49 PM

I agree with what the others have said and say that you're probably just seeing a dense star field. It is very surprising how many stars are actually out there compared to what the eye can see (even at a dark site), and of course you have to remember that things aren't really a flat plane. So there could be any number of stars in front of or behind (as seen from earth) that are there. Especially with the thinner/whispier parts of a DSO the ones "behind" can easily shine through.

 

That all said, keep in mind that internet (and especially forum) compression/resizing of images really gets rid of noise well. What we really need to see is either the full resolution image or a crop (that is 1:1 when uploaded) of the part of the image you are most concerned with.

 

Also, and I don't think anyone has said it yet, however not guiding or dithering your images is fine and is not going to magically increase the noise. Guiding is more about keeping the stars round at long exposures and doesn't do anything for noise except let you dither. My understanding of dithering is it is mainly helps lower the walking noise (fixed pattern noise with slight drifting between frames due to seeing/not perfect PA) in images. It probably will help the noise overall somewhat also due to the outlier rejection methods of stacking, however again I think this mostly is the fixed pattern that turns into walking and if you don't see that or your sensor doesn't suffer from it... Well I mean if you're guiding why not dither, however if you don't need to guide and still get round stars it may not be the worse thing in the world.



#16 Deesk06

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Posted 02 June 2020 - 09:38 PM

I do not read that the OP is using a Ha-filter. Is this correct? Do you mean the Ha-part of the nebula or is this a colorized stack made with a narrowband filter? Star color appears to be RGB, so I am guessing no Ha-filter.

Agreed that it is probably the dense starfield around NGC7000. Looks nice enough to me for 3-4 hours RGB.


Yes, no Ha filter. Sorry for throwing everyone off, I shouldn't have said that. I was trying to say the Ha part of the nebula, thinking that maybe my camera didn't pick it up well so any red on the out part of the image is grainy. Again, sorry for throwing everyone off and sorry if it is misleading. I guess maybe I should have just said the color red!

#17 Deesk06

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Posted 02 June 2020 - 09:41 PM

You are using a modified Canon 600D with an f/4 lens in a bright suburban sky with a fair amount of moon and no Ha filter?

If so then 60sec exposures are absolutely fine. The noise level in your stacked image will be driven by the brightness of the sky - i.e. the combination of light pollution and moon. Your result is actually pretty good. To improve on this you either need to shoot a lot more subs or travel out to a dark-sky site. Dithering will also help.

Mark


Yes correct! No Ha filter, my mistake for throwing everyone off. I shouldn't have said that. I will look to try and get more data on my next run out. I ended up removing all of the stars and I can really see the "grain" from noise and maybe a low red channel signal


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