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Will taking more Light frames clean-up my stacked image?

Astrophotography
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#1 CrystalizedCarbonInTheSky

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Posted 12 May 2021 - 12:55 AM

I tried to take light frames of Sombrero Galaxy using a 80mm Refractor and a 2X barlow with a DSLR at 4mins ISO3200 and the noise is really bad. I got about 21 usable frames and stacking them did not produce decent results. Will taking more light frames clear up the image? Maybe there is something else wrong with my process?

 

Here is a sample light frame:

Capture.jpg

 

Here is a sample of the unprocessed stacked image:

Capture.jpg

 

I did take dark frames as well but the problem is I am using a DSLR and there's no way for me to know the sensor temp. I also have 50+ bias frames. I don't have flats or dark flats. My camera NR option is OFF.

 

What I think I can improve on is exposure time to lower the ISO but that's not something I think I can do because I am only using a motorized CG-4. A problem I think I have is that the temps of my dark frames don't match my light frames. I was shooting through the night and the temps change. I took the darks past midnight, so the sensor is probably cooler.

 

Maybe there's some arcane photoshop skill I need to learn?

 

Any thoughts on what my options are?

 



#2 nyx

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Posted 12 May 2021 - 01:12 AM

I have no experience on astrophotography using a barlow, but I doubt this will give you that results you want.

 

Shooting "small" galaxies like the Sombrero galaxy with a 80mm refractor? Not the right scope in my opinion.

 

No experience with the CG-4, though it looks like it might not be up to the task for such long exposures.

 

As far as noise is concerned, dithering is - in my opinion - more important than darks. I'd shoot flats and bias and skip darks on a DSLR. But you really, really need to dither do get read of all the inherent color noise.

 

Either way, I believe you're using the "wrong tools" for the job: small refractor, cheap mount.

 

Edit: are you guiding?


Edited by nyx, 12 May 2021 - 01:15 AM.

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#3 james7ca

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Posted 12 May 2021 - 01:13 AM

Yes, more light frames will help with the quality and your signal to noise. Signal to noise varies by the square root of the number of light frames. That means, however, that you need four times as many light frames to get a 2X improvement in the signal to noise.

 

I generally find that a few hundred frames is my limit on patience, going beyond that to see significant improvements might take a thousand frames which on a DSLR isn't that practical. Of course, with darker skies you can expose each frame for a longer period of time (guiding and other factors permitting) which means that darker skies are another way to increase your signal to noise (since longer exposures can deliver more signal with only somewhat more "noise" --- read noise stays fairly constant, but dark current and amp glow will increase with exposure time).

 

So, you either need more light frames or darker skies. Plus, that barlow is decreasing your signal per unit area (or pixel) on extended objects by a factor of 4X.


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#4 CrystalizedCarbonInTheSky

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Posted 12 May 2021 - 01:39 AM

Yes, more light frames will help with the quality and your signal to noise. Signal to noise varies by the square root of the number of light frames. That means, however, that you need four times as many light frames to get a 2X improvement in the signal to noise.

 

I generally find that a few hundred frames is my limit on patience, going beyond that to see significant improvements might take a thousand frames which on a DSLR isn't that practical. Of course, with darker skies you can expose each frame for a longer period of time (guiding and other factors permitting) which means that darker skies are another way to increase your signal to noise (since longer exposures can deliver more signal with only somewhat more "noise" --- read noise stays fairly constant, but dark current and amp glow will increase with exposure time).

 

So, you either need more light frames or darker skies. Plus, that barlow is decreasing your signal per unit area (or pixel) on extended objects by a factor of 4X.

Thanks for the input Nyx. I'm really a bit limited right now. I hear from other posts that the CG-4 with an autoguider should get you around 2 mins of exposure so I was really happy I was able to get 4 mins. Once I get a bigger scope and better mount, I'll try again.

 

Yes, more light frames will help with the quality and your signal to noise. Signal to noise varies by the square root of the number of light frames. That means, however, that you need four times as many light frames to get a 2X improvement in the signal to noise.

 

I generally find that a few hundred frames is my limit on patience, going beyond that to see significant improvements might take a thousand frames which on a DSLR isn't that practical. Of course, with darker skies you can expose each frame for a longer period of time (guiding and other factors permitting) which means that darker skies are another way to increase your signal to noise (since longer exposures can deliver more signal with only somewhat more "noise" --- read noise stays fairly constant, but dark current and amp glow will increase with exposure time).

 

So, you either need more light frames or darker skies. Plus, that barlow is decreasing your signal per unit area (or pixel) on extended objects by a factor of 4X.

Thanks James. I've also been starting to read about this. I guess it's best to lower the noise per frame first before trying to deal with it by taking more frames. But I am curious, is there an example somewhere that shows the improvement between 1 frame and a stack of 100? I think the DSS website only shows an example that is up to 32 frames.



#5 rj144

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Posted 12 May 2021 - 02:49 AM

Was that 4 total minutes or 4 minute per sub?  Either way, more will help.

 

You also don't want to shoot at ISO 3200, probably, either.


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#6 CrystalizedCarbonInTheSky

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Posted 12 May 2021 - 04:43 AM

Was that 4 total minutes or 4 minute per sub?  Either way, more will help.

 

You also don't want to shoot at ISO 3200, probably, either.

It was 4 minutes per sub. I had 21 4-minute subs. Yep. Pretty much wasted last night to learn that.



#7 michael8554

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Posted 12 May 2021 - 06:17 AM

I tried stretching your stack, and IMO you need a lot more data.

 

Crop the stack result instead of Barlowing.

 

Forget the Darks, Load your Bias into DSS as Darks.


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#8 Hypoxic

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Posted 12 May 2021 - 07:50 AM

+1 on Dithering.


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#9 idclimber

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Posted 12 May 2021 - 08:00 AM

To answer your question, yes more data will help reduce the noise. The problem as eluded to in a previous answer is this is not a linear relationship, but a logarithmic one. Very quickly the math will have you needing weeks worth of data to get the next step in noise reduction. 

 

You will get more with the little data you do have if you actually do proper calibration. This means lights, darks, flats and bias frames. Yes, temperature with a DSLR is a pain, but you either figure out how to manage it with your lights and darks or live with the increased noise. The only other solution is to use a cooled astro camera. 

 

Increasing ISO has a cost. Not just noise but also in reducing the dynamic range. This means you lose the subtle variation in tones in the final image. 

 

Obviously most of this can all be "fixed" by replacing/upgrading gear. Alternatively I would suggest undoing most of what you are attempting on this target. Remove the Barlow. Change the ISO to what is recommended for your DSLR (probably 400 to 800) and learn to take calibration frames. Either change your target or live with the image scale you have. In a couple months you can start imaging M31, and the large field of view will be perfect. 


Edited by idclimber, 12 May 2021 - 08:01 AM.

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#10 CrystalizedCarbonInTheSky

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Posted 12 May 2021 - 08:36 AM

To answer your question, yes more data will help reduce the noise. The problem as eluded to in a previous answer is this is not a linear relationship, but a logarithmic one. Very quickly the math will have you needing weeks worth of data to get the next step in noise reduction. 

 

You will get more with the little data you do have if you actually do proper calibration. This means lights, darks, flats and bias frames. Yes, temperature with a DSLR is a pain, but you either figure out how to manage it with your lights and darks or live with the increased noise. The only other solution is to use a cooled astro camera. 

 

Increasing ISO has a cost. Not just noise but also in reducing the dynamic range. This means you lose the subtle variation in tones in the final image. 

 

Obviously most of this can all be "fixed" by replacing/upgrading gear. Alternatively I would suggest undoing most of what you are attempting on this target. Remove the Barlow. Change the ISO to what is recommended for your DSLR (probably 400 to 800) and learn to take calibration frames. Either change your target or live with the image scale you have. In a couple months you can start imaging M31, and the large field of view will be perfect. 

Thanks idclimber. These are good points. I already did a non-barlowed 4-min ISO800 exposures of this target. Which turned out OK for me (It's my profile pic). I'm just trying the limits of my gear while patiently waiting for the backordered stuff I bought. grin.gif



#11 Midnight Dan

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Posted 12 May 2021 - 09:47 AM

I have to agree with Nyx.  A barlowed 80mm scope will not give you the results you want.

 

First, an 80mm scope has limited resolution.  Adding a barlow does not change that.  Even if you were to get noise-free images, they would be soft and not contain much detail.

 

You didn't say what scope you're using, but if it's an f/6 or f/7 scope, barlowing it reduces it to f/12 or f/14.  That makes it extremely slow and would require many, many hours of integration to get a usable level of signal to noise in the stacked image.

 

Galaxies require lots of aperture and long focal lengths. That scope won't do it.  

 

If you want to use an 80mm scope on galaxies, I'd recommend doing so without the barlow.  The galaxy will be small, but the needed integration time will be reduced by a factor of 4.  That puts you within the capabilities of your gear.

 

-Dan


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#12 Hypoxic

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Posted 12 May 2021 - 10:42 AM

Here is M81 and M 82 with a TS SD-APO 72mm f/6 @ just under 3 hours total integration from a Bortle 5/6 zone (just outside of Paris).

This project is ongoing for me but I'm posting this image to aid in your decision making.

 

Image lightly cropped to remove only the two night integration irregularities at the edges.

Attached Thumbnails

  • Bodes Nebula.jpg

Edited by Hypoxic, 12 May 2021 - 10:46 AM.

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#13 Peregrinatum

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Posted 12 May 2021 - 11:25 AM

You need to determine if the 4 min exp is swamping read noise sufficiently (lots of threads and calculators on how to do this).

Can the gain be adjusted to decrease read noise?

+2 on dithering, it will help reduce fixed pattern noise from the sensor, with 21 subs you should dither every time

Yes, increasing total integration will reduce noise...you only have 84 min on the stack, unless you have decently dark skies thats not nearly enough... in my Bortle 7 skies I need at least 10 hours to make a decent integration


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#14 JCDAstro

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Posted 12 May 2021 - 09:16 PM

I just posted this the other day, but having a larger scope with much higher quality glass really helped me with noise (these are stacked, stretched and some effort in noise reduction has been made, but they are processed using largely the same workflow and settings where possible).

 

2f8PCPj.jpg

 

I literally had to stack >10 hours of data with the starter 90mm to notice any appreciable differences.  Upgrading my optics was completely worth it and made me enjoy the hobby much more.  This isn't a totally fair comparison, the EON is much bigger (with better glass) than the 90mm doublet, but based on Hypoxic's post above, just having better glass in front of your camera should help alot in my opinion.


Edited by JCDAstro, 12 May 2021 - 09:53 PM.

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