Couple of Photoshop ?'s

[nightster]

I am trying to teach myself some of the layering and masking features of photoshop. I've been using a couple of online tutorials and the both show older version's of PS. I'm using the creative cloud version 2016. I'm trying to use a clipping mask to blend my 3 NB channels. First, when do I stretch them?  I assumed I would stretch after combination, but that may be wrong as the examples seem to be stretched already. If so, how do you stretch them so the background is the same level before they are combined? Does that even matter? As far as the clipping mask I can not get the color to apply to the layers. There seems to be a different pop up windows vs the Starizona tutorial's older version of PS. Maybe this is not the best way to combine them either. And as far as PS goes, are there any processes run on the unstreched images?  So much is done in PI before you use the HistgramTrans tool. 

[swag72]

I can do either a stretch before combination or afterwards - It depends on the data.

If the Ha is massively overwhelming then I will stretch the individual data first so that they are all about equal in showing through detail. This will mean a small stretch in Ha and a massive one in the OIII and SII. Then I'll combine. If the OIII and SII are up to the task and quite good and strong then I will merge them before doing anything else.

To merge them easily - Firstly make sure that the three images that you want to combine are aligned and opened. on one of them go to the layers palete and click on channels. Then there's a small menu on the right of that box and if you click on that you can merge the channels. I've got a quick youtube video of it if that helps. It really is the simplest way I have found of merging channels.

Hope that helps.

 

[ollypenrice]

Interesting question about the background levels. In broadband imaging the background sky is very much part of the image and has to be colour balanced. I use the eyepdropper to measure the values in RGB and like to end up with 23/23/23. In NB I wonder if this matters? Many NB images are likely to have faint Ha pretty well everywhere, I guess, whereas this is less likely to be the case in the other filters on a majority of targets.

This is a question for a NB expert. Sara?

Olly

[ChrisLX200]

Easiest way of balancing is to use LinearFit in PI, just choose a background area preview for both channels and the two frames will be balanced for you. Repeat with the third channel. The problem with hard stretches is the bad effect on stars which will be bloated as a result, they must be protected (masked out) during the stretch to avoid producing a mis-match of sizes between channels - that will result in coloured halos (often blue if the OIII has been strongly stretched). It may still end up being necessary to reduce star sizes in individual channels to more closely balance the relative sizes - either that or remove them completely and replace with RGB stars from separate exposures.

ChrisH