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Help with processing stars

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#1 Becomart

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Posted 20 February 2020 - 01:07 PM

Hi folks,

 

wondering if a few good people would look at my image here https://www.astrobin...o4otqb/?nc=user

and give me a bit of feedback on my stars. I really struggle to process stars for some reason and they never quite look right....I know this is a slightly different case being M45 but the medium size stars look quite mottled (again) and I’m finding the colours hard to establish. I used the WBBP script and have done the process manually as well and find my stars take on a nasty yellow colour at the start of processing. When I look through many other experienced imagers work, the stars look fantastic. How’d the pros do it? I’m using all the usual masking masking techniques. 



#2 Madratter

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Posted 20 February 2020 - 01:23 PM

First of all, you are unduly critical of what you have. The stars may not be excellent, but they aren't bad.

 

Second, for now dump all those fancy masking techniques and concentrate on getting good stars without them.

 

Third, my opinion about what makes a good star. Tight but without a hard outline with good color.

 

Basic ways to achieve that:

1) Don't overexpose

2) Be very careful not to bring up your black point too high. It creates hard edges

3) You can use a moderate amount of deconvolution or you can use a stretching technique such as masked stretch for your L.

4) Be very light handed on the use of noise reduction. Too much noise reduction can absolutely destroy any chance of good stars.

5) Use photometricColorCalibration

6) Saturate while data is still linear.

7) Be very careful with any sharpening, and sharpening should be very close to your last step if done at all. This is another area that just kills good stars.

 

Not all the above apply to you. Have in mind what you are trying to achieve. Some things are a tradeoff, and depending on what you are trying to achieve you might need to sacrifice your stars somewhat to achieve something else.


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#3 DrGomer

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Posted 20 February 2020 - 01:31 PM

To add to what Madratter said, assuming your stars aren't saturated (if they are, change exposure, or use HDR to recover the tops), you can also de-emphasize them with various processing tricks.  if you have pixinsight, masked stretch is very handy. Further reduction can be done with morphological transformation, and de-emphesis with starmask+curves.

 



#4 Becomart

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Posted 20 February 2020 - 03:17 PM

First of all, you are unduly critical of what you have. The stars may not be excellent, but they aren't bad.

 

Second, for now dump all those fancy masking techniques and concentrate on getting good stars without them.

 

Third, my opinion about what makes a good star. Tight but without a hard outline with good color.

 

Basic ways to achieve that:

1) Don't overexpose

2) Be very careful not to bring up your black point too high. It creates hard edges

3) You can use a moderate amount of deconvolution or you can use a stretching technique such as masked stretch for your L.

4) Be very light handed on the use of noise reduction. Too much noise reduction can absolutely destroy any chance of good stars.

5) Use photometricColorCalibration

6) Saturate while data is still linear.

7) Be very careful with any sharpening, and sharpening should be very close to your last step if done at all. This is another area that just kills good stars.

 

Not all the above apply to you. Have in mind what you are trying to achieve. Some things are a tradeoff, and depending on what you are trying to achieve you might need to sacrifice your stars somewhat to achieve something else.

Thanks Madratter. I think we’re all hyper critical of our own images though I’d really like to progress from competent to good. A lot of your ideas ring true, particularly overexposed subs...I kinda knew this when I was imaging as the estimated calculations were telling me to stop at 1min but I stubbornly continued at 2 mins. I’ve not tried a masked stretch yet so will look into that as a next step. I used the auto colour script which was quite useful and my noise reduction was better than usual but I didn’t sharpen at the best point. Thanks for your help...really useful. 



#5 Becomart

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Posted 21 February 2020 - 09:43 AM

UPDATE

 

For anyone else on a similar journey - I’ve been doing a bit of experimenting on stretching images but protecting medium size stars (by medium, I mean in M45 - in most images, these would be large stars) from bloating and smudging an image. I have tried 3 different methods:

1. David Ault star mask creation by combining 3 different sets of variable size star masks. This process worked well but it was very difficult to use the combined  mask and avoid dark rings upon stretching. I tried doing a stretch with the mask applied in a completely linear state then removing the mask and continuing to stretch. I also tried stretching to a certain point then applying the mask and doing final stretches. The results were not pleasing but a useful process. 
 

2. I cloned my original luminance image in a linear state, auto stretched to a permanent state, applied the starnet+ process to create a starless image and then subtracted this from the stretched version to give a star mask. This star mask was much better than method 1 capturing all the stars and then having options to saturate or not. However, much like method 1, it didn’t work well for stretching purposes again introducing black rings if pushed too far and you can’t stretch more than a very small amount without introducing problems. 
 

3. Masked stretch. This saw an improvement in small and medium size star sizes but also added very bright saturated cores to the largest stars in M45. These proved challenging to remove even with the GAME script mask and didn’t provide anything more aesthetically pleasing than my original image. 
 

I am therefore left to conclude the following - getting the exposure length correct is vital to having good quality stars in an image! Interestingly, even prior to stretching the channels, I could see over 20 well exposed stars in the image and this has made the data very difficult to work with. Lesson learnt for next time and I may have got away with it more on a different target. 




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