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Stretching Tutorial

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#1 damarks913

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Posted 20 September 2021 - 11:17 AM

OK, so I thought I was doing a passable job on stretching, but in hindsight I was likely clipping more data than a urologist running a vasectomy special.

 

It also seems like there are 2000 different ways to go about the stretching process.

 

So what tutorials, articles or go-to's would you recommend to better learn how to accomplish this critical step in post-processing. In fairly accessible terms BTW. Some I looked at assumed I knew more than I did.

 

Learning to crawl here, then I have to go back through a year of stacked images from the DSLR that likely could be a whole bunch better.


Edited by damarks913, 20 September 2021 - 11:22 AM.

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#2 KungFood

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Posted 20 September 2021 - 11:19 AM

What software are you processing in? The theory on stretching applies to any, but the method can be specific to program.



#3 M_Johnson

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Posted 20 September 2021 - 11:33 AM

OK, so I thought I was doing a passable job on stretching, but in hindsight I was likely clipping more data than a urologist running a vasectomy special.

 

 

Can't help without knowing the processing program but THAT is funny!! Thanks for the laugh.


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#4 stargzr66207

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Posted 20 September 2021 - 11:35 AM

If you’re using PhotoShop, the Curves tool in the Image Adjustment menu is the way to go. Remember that the left side is the faint zone, so move this section of the line up first. Remember 3 things: 1) make a number of small adjustments, not one big one. 2) remember to have a flat line from your last adjustment point to the right side of the curve. 3) always go back to levels and reset the black point, being careful to NEVER move the black point slider into the “hill” of the histogram: this will clip faint data!
Ron Abbott

#5 damarks913

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Posted 20 September 2021 - 11:38 AM

What software are you processing in? The theory on stretching applies to any, but the method can be specific to program.

Photoshop



#6 bobharmony

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Posted 20 September 2021 - 11:50 AM

When I started out doing AP, I used PhotoShop for post-processing.  There were a few tutorials by Doug German that explained things in laymans terms which I found very helpful.  He used to have his own website, but when I searched today I found the material on youtube at

 

https://www.youtube....1E1162212A88E25

 

It appears he has added more advanced topics as well over time.  Hope this helps.  I have since moved on to dedicated processing software, but this was a good starting point.  His videos on using DSS for stacking were also helpful at the time.

 

Bob


Edited by bobharmony, 20 September 2021 - 11:51 AM.

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#7 jonnybravo0311

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Posted 20 September 2021 - 11:53 AM

I nearly spit my coffee on the screen reading the "vasectomy special" comment lol.gif


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#8 Ibuprofen200mg

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Posted 20 September 2021 - 12:29 PM

Siril is free and has an auto-stretch feature that is pretty good. I typically manually stretch my own images in GIMP however for more control. The general process regardless of program is an iterative exercise in pulling the peak of the histogram toward the middle by adjusting the midtones followed by resetting the black point (without clipping the data, i.e. erasing the left side of the peak). You would repeat this process until you are satisfied with the outcome. When you first start stretching, the histogram peak will be all the way to the left and be very 'thin', by the end it should be nice and 'fat' and peaked around the the left 1/4 or 1/3 of the scale. 


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#9 sharkmelley

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Posted 20 September 2021 - 12:47 PM

I'll assume that you are calibrating and stacking raw files so the final stack is linear.

 

Make sure your stack uses 32bit data because Photoshop opens 32bit files with a linear profile (i.e. gamma = 1.0).  In one fell swoop this solves a big part of the stretching problem because you are already viewing the data correctly i.e. the brightness displayed on your screen is directly proportional to the pixel values in the image.  You can then do the majority of your background subtraction while the data are still linear, before changing the mode to 16bit for the rest of the post processing.

 

A common mistake is that linear data files are often (wrongly) displayed with the default sRGB non-linear profile.  They are then displayed incorrectly and look far too contrasty.  An enormous amount of (unnecessary) stretching is then required before they begin to look reasonable.

 

Mark


Edited by sharkmelley, 20 September 2021 - 12:53 PM.

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#10 KungFood

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Posted 20 September 2021 - 12:53 PM

Trevor Jones (Astrobackyard) has a series of youtube tutorials, and writeups on his website addressing post processing in Photoshop. Worth a look.


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#11 damarks913

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Posted 21 September 2021 - 03:22 PM

thanks everyone...last night took a shot at the Veils with the OSC and the L-Extreme. The Skyguider was dragging, even at 30 seconds, so the stars look like a tribute to the NFL, but again, the possibilities when I finally figure all of this out in 8-10 years.

 

Tonight, adventures in guiding...

 

Veils

 

 



#12 jonnybravo0311

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Posted 21 September 2021 - 03:53 PM

That image is quite nice. What is your imaging setup? You mention the SGP and OSC. What is the OSC? What is the scope/lens?

 

If you're using a small refractor, do you have a field flattener on it? The issue I see in the image certainly looks more like a spacing issue than anything else. For reference:

 

post-288137-0-08304100-1615744113.png


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#13 damarks913

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Posted 21 September 2021 - 04:05 PM

That image is quite nice. What is your imaging setup? You mention the SGP and OSC. What is the OSC? What is the scope/lens?

 

If you're using a small refractor, do you have a field flattener on it? The issue I see in the image certainly looks more like a spacing issue than anything else. For reference:

 

post-288137-0-08304100-1615744113.png

Thanks, looks to me like the latter, and I will try to back it off a bit next night I'm out

 

Set Up is Zenithstar 61, yes with FLAT61R .8 Reducer/Flattener...camera is QHY168C


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