General advice on image processing please..

[SkyExplorer]

Hi,

Wondering if anyone could help on the following points....

1. If I have different exposure length images, should I combine these in DSS or should I create a HDR image later in Photoshop?

2. I've just started to use Noise Ninja on an image of M33, I masked off M33, and used NN on the background, but I'm then left with noise in M33. How do I get rid of this?

3. After using Curves and Levels to stretch the image, I set the light and dark values. If I can't set the dark levels to 30 for RGB without going into the "hill" on the histogram does that mean I've stretched the image too much?

Thanks

Steve

[SteveP]

Steve

1. To understand more about what you are doing, why are you using subs with different exposure lengths?

3. I assume you mean 'black' and 'white' points. If so when setting the black point in Levels after a curves adjustment, it's a good practice not to move the black point so far that the data is clipped

HTH

Steve

[SkyExplorer]

Hi,

I had a lot of 105sec exposures of M33 that I took unguided. Over Christmas I got a QHY5t and my first target was M33 guided with 300s exposures. So I thought I'd combine the two. The longer exposures can cause the centre of the galaxy to be overexposed and so combining the two can help this.

With the Black and White point, what I am reading from your answer is that is that if I have to clip the data to get the black point correct, then I have over stretched the image....?

[SteveP]

If you want to use the shorter exposures to avoid burning out the core then I suspect you're going to need some form of masking in PS so that you can blend the short with the long exposures

You may or may not have overstretched the image but you should be able to check by inspecting it. To set the black point just move the marker to the left of the start of the histogram. Don't be getting too concerned with the precise value at this stage of processing

Steve

[SkyExplorer]

What stage of processing should I start to get worried about setting the black point?

So if I can't reach say R30,G30,B40 without clipping a lot of data then I should not worry about it and leave the values at say around the 80-100 mark?

[SteveP]

Everytime you apply Curves or Levels in PS, you will be stretching the histogram. What this does is to re-calibrate the entire range of 'brightness' values. So even if you set the black point at say 30, then after some more Curves/Levels adjustments, that value will have been 'stretched' to another value. It's good practice to set the black point after every Curves but don't get concerned about its precise value.

The most important point is not to adjust the black point too far to the right that some data in the histogram is lost. Once lost, it cannot be recreated. If you are wanting to use the R/G/B histograms to establish a particular colour of the background sky then I suggest you do that towards the end of your processing when you do your final tweaks.

[SkyExplorer]

Thanks for the information Steve, it's very useful.

Can anyone answer my second question regarding Noise Ninja and removing noise for galaxies?

Thanks

[MartinB]

1) To combine exposures of different exposure lengths you need your software to perform a "normalisation" routine. The software analyses a selection of each image comparing grey scale values and then applies an appropriate weighting against a reference frame. I don't use DSS but image it can do this.

2) If you are getting noise in parts of your target despite applying a mask you are best to create a greyscale mask using your image. Duplicate your image. Apply the noise reduction to give optimal results for the dark areas. Now create a "new version" window. Unfortunately I don't have PS in front of memory this can be found under the Windows menu item at the bottom of the first sub menu (hope that makes sense!) Arrange your workspace so that you can see both clearly but make sure you keep your working image highlighted. Copy your background image. With the duplicate layer highlighted create a mask by clicking on the mask icon at the bottom of the layers palette. Select the mask by Alt clicking the mask which will now appear alongside the duplicate image in layers. This will copy a greyscale version of your image and you will see the effect of this in the new version image. You can tweak the mask by applying a gaussian blur and adjusting levels to suit. The new version lets you play around on the fly - very powerful!

3) RGB images typically have a colour bias. To get the dark sky areas right you have to use levels to adjust each channel seperately. If you want to do very precise adjustments you are better to use curves and the dropper tool. In practice levels is quicker and works fine. Just move the black point in each channel so that they are a consistent distance from the start of the histogram. This should give you a neutral background. How close you go to the histogram depends on what processing routines you are using to increase colour saturation and the final adjustment comes down to personal preference. There aren't any magic numbers.

Sorry, have done this in a bit of a rush. If it is gobbledygook please let me know!

[SkyExplorer]

Thanks Martin,

I'll have a go tonight and let you know if I get it....

[ollypenrice]

I very much doubt that your 300sec exposures are really overexposed. I used 600 seconds at f7 and 300 secs at f5 without difficulty on a sensitive CCD. But you might need to make two stretches, one for the core and one for the fainter spirals. Then you can blend them in layers. I also doubt that your short exposures are really going to add much other than noise, if you are not careful. They might be useful for the core but on anything faint I doubt them being much help.

Just have a quick look to see if, on a partial stretch, you haven't got good core data in there. Of course I might be up a gum tree here because I image from a very dark site, in which case my apologies!!

Cheers,

Olly

http://ollypenrice.smugmug.com/Other/Galaxies/M33-PIX-FINAL/1016601171_mSYWy-X2.jpg

[themos]

DSS stacking method:

Entropy Weighted Average (High Dynamic Range)

This method is based on the work of German, Jenkin and Lesperance (see Entropy-Based image merging - 2005) and is used to stack the picture while keeping for each pixel the best dynamic.

It is particularly useful when stacking pictures taken with different exposure times and ISO speeds, and it creates an averaged picture with the best possible dynamic. To put it simply it avoids burning galaxies and nebula centers.

Note: this method is very CPU and memory intensive.