Stretching?

[Guest]

Does anyone know of a good description of 'Stretching'. I'm using Registax 6 but I think the histogram is a common feature of many imaging processing programs. I can play around with it and get stuff to happen but I'm not sure what I'm doing exactly. I'm particularly interesting in any guidelines and strategies for using it properly.

In Registax 6 there are three sliders for dark , light and mid-range. I think I know what the light and dark sliders do and it's a bit like what I used to do in the dark room processing photos with dodging and so on . But I'm not sure what the mid-range slider does.

Also I wondered if you can make any deductions about the exposure from the shape of the histogram. For example can you say that a longer or shorter exposure would have been better or perhaps a different ISO.

Cheers

Steve

 

Edited January 6, 2020 by Guest

[Freddie]

What type of image are you processing?

[dobblob]

Registax is usually a program used for stacking video (lucky frame) image capture, of thousands of individual frames commonly of the moon or planets.

It is not intended for use in deep sky image capture and has no use in this so indeed we need to know what type of images and targets you are working from.

If you are taking deep sky shots  of nebulae and galaxies then programs such as DSS (Deep Sky Stacker free!) and StarTools or PI and many others are the way to go.

These are two completely different astrophotography  strategies requiring different techniques and equipment.

If you can at least let us know what your equipment is in your profile or directly it would help.

 

[Guest]

Hi, At the moment I'm taking pictures of the moon - full disc. I found a good you tube video about the method of processing the image. The guy seemed to know what he was doing.  I'm actually quite pleased with the result but I want to understand what I'm doing rather than just following blindly. There are so many switches and options in the software I don't understand the meaning of. Stretching seems to be an important thing so I'm focusing on that right now.

I'm using a SW 200pds on a HEQ5 mount with a canon EOS450D camera. I took 200 frames and stacked them. The sequence I used is like this -

PIPP to crop and sort the frames.

Autosakkert to stack

Registax to stretch and do the wavelet thing

There seems to be quite an overlap in features between the three programs especially Autostakkert and Registax but anyway that's how he did it so I just followed exactly his steps. This is what I ended up with. I thought it wasn't too bad.

I understand what the histogram is but I don't quite understand what's happening when you move those sliders around so any help on that would be appreciated.

Cheers

Steve

 

[geoflewis]

That's a very nice result Steve. Yes, there is overlap between Registax and Autostakkert. I believe that Registax has been around longer and in my (and many others) opinion it's wavelet function is still the best, but Autostakkert does a better job of grading and stacking video frames than Registax, hence many now use the best features of both tools. Autostakkert also has a nice feature that allows you to send the graded, stacked image straight over to Registax already open and ready for wavelet sharpening. This linked process also automatically applies the same wavelets to each subsequent image loaded into Registax from Autostakkert once you've fine tuned the wavelets appropriate for the session, which is really useful when processing a series of planetary image ready for derotation in WinJupos, if you ever want to get into that level of planetary imaging.

[happy-kat]

I found this page useful on Registax, though there's not much on the 'how'

http://www.sunspot51.com/Registax6/Registax6.htm

[michael8554]

Hi Steve

Looking at your moon image:

The "sky" shouldn't be absolutely black, so use the dark control to set this. 

The brightest areas shouldn't be burnt out, so use the light control to set this. 

The brightness of the mid-grey areas of the moon can be adjusted with the mid-range control without influencing the previous two settings, this is where your artistic preferences can be exercised. 

Michael 

[wimvb]

On 06/01/2020 at 21:52, woodblock said:

In Registax 6 there are three sliders for dark , light and mid-range.

In any program that uses "levels" or histogram, the dark slider determines what will be totally black (0%) in the result. The light slider determines totally white (100%) The mid-range slider determines what will be mid-gray (50%). Normally you can't avoid the brightest stars to be white, but you should try to avoid a totally black background. There should at least be some noise visible, as this will give a more natural look. I don't know about other programs, but Pixinsight will show how many pixels and how many % of the image will get clipped when you move the black point and white point sliders. When stretching dso images, the mid-range slider needs to be moved to near the black point in order to get good contrast. In images of the moon, there is already a good amount of contrast, and you may have to use other means of enhancing it.

[Guest]

Thanks Wim,

After playing around with the sliders for a while I think I understand what's going on. It's clear what happens with the two outside sliders. With the middle slider what happens is that the two halves of the histogram divided by the middle slider are stretched independently. So, for example, if you move the middle slider to the right to say the 75% position then that section from 75% to 100%  is stretched on the new histogram from 50% to 100% (stretched by a factor of 2) but the left hand side of the original histogram from 0% to 75% is shrunk on the new histogram to the 0% to 50% range (stretched by a factor of 0.5 ie shrunk)

Looking at the information that's available I often see remarks to the effect that you just fiddle about with the controls until you get something you like. This applies especially to the wavelet feature on Registax. But I don't think that's particularly helpful. 

Steve

 

[wimvb]

38 minutes ago, woodblock said:

Looking at the information that's available I often see remarks to the effect that you just fiddle about with the controls until you get something you like. This applies especially to the wavelet feature on Registax. But I don't think that's particularly helpful.

Most of the information of interest in astro images is in the low part of the intensity scale. So you normally slide the mid-range control to the left. You can either slide it at once, until you get good contrast, or you can set the slider at a certain position, and apply it multiple times. For example, set the mid-range control at 0.25 and apply this a number of times until you get pleasing contrast. Then reset the controls and slide the blackpoint to the right, just to the very foot of the histogram.

You can use either way, but imo you should try both methods on copies of an image, and compare the result. Ie, do one aggressive stretch until it looks ok in a preview on copy A, and a multiple "soft" stretch on copy B. Then compare the images. The multiple stretch method takes a little longer, but gives you better control. Which ever method you prefer, you should always strive for getting a good separation of shades in the middle and lower parts of the image, while avoiding large patches of total blackness and excessively bloated stars. This part of astrophotography is very much an art and up to your liking. But as in any art form, there are guidelines/rules as to what makes a pleasing image.

What we do with sliders on a computer is very similar to what photographers like Ansel Adams and Edward Weston did with b/w film. If you study their methods to achieve contrast and a high dynamic range, you will learn much that you can apply to your astro image processing.