Author Topic: Can't create proper stretched image  (Read 196 times)

Offline tomb18

  • Newcomer
  • Posts: 39
    • View Profile
Can't create proper stretched image
« on: 2019 October 11 09:58:45 »
Hi, I have a rather strange issue with an RGB-Ha photo.  All images have been integrated and are still linear.
From the moment I combine the channels to produce an RGB image, the image is perfect.  There is no need at all to do a STF or use histogram transformation.  However, if the image is saved as a jpg or tiff, it looks just like an image that has NOT had the histogram stretched.
So I tried using Histogram transformation.  I used the real time preview and it looks just perfect as in the image itself.  Any attempt to adjust the image and everything looks horrible.  So I tried to just not change anything in the histogram and just applied it to the image.  Fine.  It look identical.  However, saving it as jpg or anything else once again shows it was not stretched at all.
So' i'm kind of lost here.
Thanks, Tom

Offline John_Gill

  • PixInsight Old Hand
  • ****
  • Posts: 320
    • View Profile
Re: Can't create proper stretched image
« Reply #1 on: 2019 October 14 00:11:22 »
Hi,

I have read that you should stretch the images/channels before combining them.
A non-stretched (linear) image will always look "black" in any application. When using HT did you push the apply button (small blue triangle) onto the image.  If yes, then the image should be non-linear and the stars, nebula, etc should be visible.


So, try this : stretch the RGB (If the RGB is individual channels, then stretch each channel spearately) and the Ha and then do a ChannelCombination or as I prefer LRGBCombination. 


space is not black
John


APM 107/700 apo on CGX mount
ZWO Optics - Autoguiding
ZWO1600mm and filters
... when there are no clouds ...

Offline jkmorse

  • PixInsight Padawan
  • ****
  • Posts: 916
  • Two questions, Mitch . .
    • View Profile
    • Jim Morse Astronomy
Re: Can't create proper stretched image
« Reply #2 on: 2019 October 14 07:34:10 »
John,

Another way forward that is easy to get the image to match what you see in the linear form is to open ScreenTransferFunction (STF) and HistogramTransform (HT).  Make sure the image you are interested in is highlighted with a blue band around it and that you have selected it in HT.  Then grab the little triangle at the bottom left on STF and drop it onto the bottom bar on HT.  Then simply apply HT to the image by dragging the little triangle at the bottom left of HT onto the image.

If you are interested I have a workbook that I freely share that should help you get through the starting jitters.  If interested just drop me a note at jkmorse57@gmail.com.

Hope that helps.

Jim 
Really, are clear skies, low wind and no moon that much to ask for? 

New Mexico Skies Observatory
FLI Microline 16803
Planewave CDK17 - Paramount MEII
Planewave IFR90 - Astrodon LRGB & NB filters
SkyX - MaximDL - ACP

http://www.jimmorse-astronomy.com
http://www.astrobin.com/users/JimMors

Offline John_Gill

  • PixInsight Old Hand
  • ****
  • Posts: 320
    • View Profile
Re: Can't create proper stretched image
« Reply #3 on: 2019 October 14 12:16:55 »
Hey Jim,

I usually create a duplicate image and on one I do the STF and HT. On the second image I do two small stretches with ArcsinhStretch and then a little HistogramTransformation.  I normally select the AS and HT as the best. I have played with MaskedStretch but not had the best results.

space is not black
John
APM 107/700 apo on CGX mount
ZWO Optics - Autoguiding
ZWO1600mm and filters
... when there are no clouds ...