Masked stretch without success..

Daniel Renner

I have never got masked stretch to work, i have watched Adams videos (both fundamentals and horizon) and i can get a good image. The background always look SUPER ugly even when i try to adjust the settings. 

I know that Adam often uses very good data from bortle 1 skies but everyone cant afford that ;) 

Here is a failed masked stretch: I also have some very strange vingetting type?

DAVID JENKINS

Can you provide us with more details on how you generated the mask that doesn't appear to be protecting the sky?

BlockHead

I don't think masked stretch in particular is the issue. It is determining the brightness of the background- which is high- but if you used any stretch (AutoSTF/HistogramTransformation, Curves) you would see the same basic result.

It appears as if there are multiple things at play. The nature of the noise indicates there were originally speckles in the calibrated data that perhaps were smeared out during image registration and/or other noise reduction of small scale stuff.

In addition, there is indeed a small flat error here too. The flats were not perfectly characterizing the image.

All of this is to say the first thing, I think, to do is pick a threshold- basically a level below which you simply "let be" . This is what I call the threshold between possible "useful" information..and everything else.

As you figured out in your other post, you might be able to fiddle with this not-useful regimes and make it so even at pixel-peeping views it looks reasonable. But I would not think with this kind of data:

1. You cannot *usefully* show this level of faintness of the objects. You simply do not have enough information. Darker skies and longer exposures (or total integration) are really the only big helpful things. (Bigger telescope doesn't hurt either..lol).

2. The speckled nature of the initially calibrated data probably needs to be examined more closely. Are the dark frames really doing their job well... or adding unnecessary extra noisy stuff?

I don't need to protect the sky with the data I used ( you should work with the SAME data I demonstrate with with respect to the WorkFlow examples) because the useful information is significantly brighter than then noise. You can certainly change the output background level of Masked stretch (without using a mask)- the background you see in this attached image will be exactly the same...but darker- and the galaxies not quite as bright. This is OK in the sense that the faint stuff of the galaxies that becomes harder to see is the not-useful information I am talking about. 

I hope something in the above is helpful.

-the Blockhead

Daniel Renner

thank you Adam for the detailed answer! It worked better for me if i put the clipping factor to 0. =)