Combining Images

[Simon Hicks]

Hi PixInSight,
                        I'm brand new to this forum, so apologies if this question has already been covered somewhere else. I'm trying to find a way to combine images, i.e. so I can use the well developed low intensity parts of a very long exposure and the unsaturated high intensity parts of a short exposure and be able to control the transition between them. Probably quite standard stuff for you guys  

I've just been getting to grips with masks in PixInSight. I've never got the hang of them until you put them into the ACDNR module.....which is an absolute joy to use and makes the masking bit extremely easy and intuitive....brilliant work!  Please think about adding this feature to more of the modules....basically anything that would benefit from masking.

Back to combining images....I've done this by creating a mask image from the extracted luminance of one of the images and then using PixelMath to do the following;

(Lum(imagemask)*image1)+((1-Lum(imagemask))*image2)

This seems to do the trick, but I was wondering if there was a better / easier / more interactive way to do it. What would be really handy would be a module that did this and had the Luminance Mask sliders and preview features that are in the ACDNR module.

Cheers....and great software!

Simon

[Carlos Milovic]

Hi Simon

We have plans for a module like the one you mentioned. A tentative name is HDRStack... and it's goal is to combine images with different dynamic range, specially when saturated information is present. It has to wait until next year, because we are working right now writing the documentation and polishing other details before the commercial release.

Now, about the masks... well, you may use the standard masking capabilities of PixInsight. I recommend you to read PixInsight LE's documentation about this topic, while we finish the documentation for the standard release.
With the mask system, youĺl just have to write the following expression on PixelMath: "image2" and apply it to image1, which has imagemask selected as an active mask.

[vicent_peris]

Hi Simon,

read my small tutorial on HDR stacking in this forum. Also, for dynamic range control, I suggest to try the HDR Wavelet Transform, it's very easy to use.


Vicent.

[Simon Hicks]

Hi Carlos and Vicent,
                                      Many thanks for your responses. I've tried the HDR wavelet transform and its amazingly powerful...a really great tool. I've found it can very easily be too agressive (I've never used more than one iteration!), but with a bit of patience it can really be used to powerful effect....I like this one.

But it's still not going to help me capture the dimmest nebulosity and the brightest core on the same image...so that's why I'm interested in the combination method.

Vicent...I will carefully read your tutorial.

Carlos: Wow...using masks is obviously even easier than I thought...it wasn't obvious to me that it was that simple, so I'm keen to try out your suggestion tonight (thick foggy skies in Glasgow, so nothing else to do!). And I'm really looking forward to seeing your HDRStack module when it comes out.

Many thanks again.

Cheers
             Simon

[Juan Conejero]

Hi Simon,

Welcome to PixInsight Forum!

You've addressed an important topic. HDR image combination is one of our priority tasks.

As Carlos has said in a previous post, we'll have a general HDR combination process in the set of standard PixInsight modules during the next year. Vicent's method is the best and most rigorous way to combine HDR astronomical images. It's indeed laborious, but extremely efficient. His latest processing of comet Holmes is a good demonstration.