Hi everyone this is Nico Carver from
nebulaphotos.com
hopefully you've already watched part 1
of this series where I showed you how to
capture a bunch of photos of the Orion
constellation now we're going to jump
into processing those photos with two
programs one called deep sky stacker and
the other called gimp if you're
following along with me you can download
these from Google Drive and just follow
along with my example files here or if
you're doing it on your own I would
recommend organizing your files into
these four folders you have them all
ready to go for the next step here which
is we're going to use deep sky stacker
to calibrate the lights with the flats
darks and bias files and then we're
going to register those lights which
means match them based on the star
patterns so that they're all lined up
and then stack them into one complete
image that has all of that data combined
together and it rejects a lot of the
noise both through calibration and just
through the process of stacking the
images together we can reject a lot of
things like hot pixels and and just the
inherent noise of the sky and
uncertainty of light so we're ready to
go here let's go ahead and open up deep
sky stacker which is this right here I'm
using the 64 bit version it does tell me
a newer version is available for
download I'm using 4.1.1 and this says
four point two point three is available
I'm not going to upgrade right now but I
think it works the same way we start
over here on the left hand side with and
we just basically step through these
different steps we start with open
picture files and I'm going to navigate
to that folder of showing you before
Orion 85mm
and I'm gonna start by loading in
my lights I'm gonna press ctrl-a to
select them all and click open okay now
those are all loaded I'm going to go
ahead and check all of those just to see
how many we have here we have 451 lights
I'm now going to go up here back up to
where it says registering and stacking
and choose dark files go into my darks
folder here again click once and then
press ctrl-a to select them all and then
click open okay we loaded in 31 dark
frames I'm gonna click on flat files
load in my flats control a to select all
click open load it in 20 flats I don't
have any dark flats this time I'm going
to use a bias so click on biased files
open up my bias folder press ctrl-a to
select everything and click open and we
load it in 60 bias frames okay so we now
have everything we need here lights
darks flats and bias the next thing I'm
going to do is I'm going to click on
this register checked pictures link
right here we can see here there's an
actions tab and an advanced tab it says
dark flats and bias checked that's good
I'm gonna leave automatic detection of
hot pixels
checked I'm going to go to the advanced
tab and I'm going to compute the number
of detected stars here okay it found 200
stars with a 20% star detection
threshold that seems fine if I wanted to
I could try raising that and then just
try clicking that compute button again
to see what happens okay when I raise
the star detection threshold that number
of stars goes down if I lower that star
detection threshold like let's say to
try 15%
we get a few more stars this is probably
fine I'm gonna leave that at 15% I'm
going to click into recommended settings
and just see what we have here it's
saying that we're using Sigma clipping
combination method and sigma clipping
median for the other calibration frames
that all seems fine basically if this
recommended settings thing found any
problems instead of showing in green it
might show things in red to fix certain
things but this all looks good we can
click on stacking parameters here and
just see what it's doing so it's doing a
median Kappa Sigma clipping you can
hover over these different stacking
modes to find out what they do this all
looks fine I'm gonna click OK and I'm
gonna click OK again and it will start
creating all of our master calibration
frames it starts with the bias then goes
on to creating the master flats and the
master dark and everything else and then
it will go ahead and calibrate
everything and register those check
pictures together once we have all of
that done we can pick the best light
frame to use for the final registration
and stacking and then stack all these
pictures together so I'm gonna let this
go for a while here and check back in
when it's done okay it's now done I'm
gonna go ahead and sort these by score
and you can see the top score here is
1862 so that means that this has been
when deep-sky stacker looked at all the
different lights it found that this one
had the route
and tightest stars in terms of focus so
I'm gonna go ahead and choose that one
as the reference frame I'm going to
right-click on it and choose uses
reference frame we now get this little
asterisk right there
I'm then gonna go over here to stack
checked pictures which is the next thing
down and it looks like I didn't take my
bias frames that is the same ISO as my
darks and flats and lights that's not
good but oh well it's too late now
probably work out fine you can see the
total exposure here is about 15 minutes
because we took 450 frames at 2 seconds
each and ISO 1600 I don't want custom
rectangle what is that
just once standard mode I don't know
what I did there I'm I there's an option
to stack based on a custom rectangle
that I must have somehow messed up and
put that in so you always want to check
all these things make sure that it's
what you actually want I just want a
standard mode where it's framed by the
based on the reference light frame which
we just set
I'll use Kappa Sigma clipping that's
fine
all these other things are fine let's
click OK I'm just gonna click on
recommended settings and just check
through here looks good ok one thing to
keep in mind when stacking a lot of DSLR
frames like this 450 is that it can
temporarily use up a lot of space on
your main hard drive this is just
because it uses a temp folder while it's
doing his work so you can see that this
is going to use 62 gigabytes of my 80
gigabytes free on the C Drive so that's
just something to keep in mind you might
need a fairly large amount of free space
in order to stack a lot of frames at
once like we're doing here but I seems
to have it now so let's go ahead and
click OK and let it do its thing of
stacking these all together
okay it's done stacking doesn't look
like much here but that's because it's
not stretched really here so we're just
seeing the bright blown out parts but we
can see Orion's belt there and things
like that
but we're going to do the stretching in
other programs so all we have to do now
is make sure that it saved the autosave
dot tiff which f I look in my lights
folder here scroll to the bottom there
it is autosave TIFF that's all we need
actually so we're gonna use that in the
next steps and move on to the actual
post processing programs because this
was just what we call pre-processing so
it's calibrating registering and
stacking and then we'll actually do the
fun part right now
all right now that we have our images
calibrated registered and stacked using
deep-sky stacker we're going to move on
to post-processing the image that came
out of deep sky stacker which is this
autosave TIFF in GIMP which stands for
the new image manipulation program it is
a free and open source project it's been
around for a while it's a similar kind
of program to Photoshop but the main
advantage is that it doesn't cost any
money and also being open source you can
contribute to it or fork it and make
your own project
I downloaded the latest version of it at
the time of this recording which was
2.10 0.14 and it's April 4th 2020 so
let's open up the program and let's open
up our autosave TIFF file so I'm just
going to go to file open and I put it on
my desktop here and then I'll click open
again
okay and it doesn't look like much right
away this is because it's not yet
stretched and what we mean by stretch is
we're going to transform the values in
the picture applying sort of a curve
that that takes it from this linear
state to a completely nonlinear state
where we stretch out the histogram
typically this is done if you just shoot
JPEG in the camera with something that's
built into the camera that converts the
raw picture values to a pleasing picture
with a curve so before we apply that
first curve though let's go ahead and
duplicate our layer you can do this by
just going over here to the layers panel
on the right hand side right clicking
and choosing duplicate layer and I'm
also going to rename that layer just by
double clicking into there and selecting
the text I'm going to call this first
stretch okay
now we can go up to the colors menu up
here at the top and choose levels
I want my preview turned on which is
down here in this version of GIMP it's
turned on by default
but I'm not sure if that's true in all
versions so just make sure your previous
turned on and then take this middle
slider here and push it over the image
will become sort of gray that's normal
and just click OK and I'm going to
repeat that process I'm going to go back
to color levels but now what you can see
is we can now see this histogram bump
right here this is most of the
information in the picture and what
we're basically trying to do is broaden
that hump of of information so that we
have more dynamic range in the picture
and we're going to do that by moving
this shadow slider in to the right and
this history the middle slider to the
left
and we're gonna have to probably do this
a few times to get to where we want it
this time I may be just gonna reset my
black level mostly something like that
and if you want to zoom in you can
either use the view menu and there's
some zoom options right there or you can
just use the keyboard shortcuts which
are plus and minus so if I just press
plus which is actually you have to hold
down the shift key and hit the plus
equals button I can zoom in and this can
help see the detail in the picture and
see what we're bringing out so I can see
that I can already sort of see the flame
nebula there and Orion's coming in
nicely and so sometimes it's nice to
zoom in like this while we're doing this
stretching and if I move the black point
slider all the way sort of over right to
the edge of where this histogram bump is
right now I can take a look at what we
can see and I know that we should be
able to bring out the horse head a
little bit better than that so I'm gonna
bring my midpoint slider over again
we're going to keep doing this a few
more times I can sort of see it coming
in now you can see now that the
histogram mountain here is getting
really quite spread out which is what we
we want to see
maybe just a couple more times
look mess something up there cancel that
one
okay I think we go ahead and zoom back
out with the minus key yeah
okay this is I think at a good point
this is gonna be my final stretch for
now again I called this first stretch so
we might do a little bit more stretching
later on but this is this is a good
starting point here the first thing that
we can see is that we've now brought out
the Orion Nebula quite well we can see
both the core and the outer edge of it
here this right above that is called the
running man Nebula and I just do just
like it clicked on that accidently and
then up here we have the horse head I
try to zoom in on that a bit right in
there and the flame nebula the next
thing I want to do with the image is try
to crop it a bit and then after we've
cropped it I'm gonna get rid of some of
this gradient all of the interesting
stuff in this image is right here in
there Ryan constellation and out here
it's not as interesting if we if we zoom
in on on the corners we can see there's
actually a lot of misalignment and and
weird stars and things out here too so
we might want to just crop that out of
the picture so I'm gonna choose my crop
tool here it's the second row all the
way over to the right and I'm just going
to pull out a crop box something like
that with a Ryan roughly centered we're
keeping in this bright star Rigel which
is of interest yeah I think that looks
good so I'll just press Enter or return
to accept the crop and now we have less
weird stuff to deal with and this this
gradient looks a little bit more mad
- and that's the next thing we're gonna
deal with is is trying to remove that
gradient a bit so I'm going to actually
duplicate this first stretch layer twice
I'm going to call this one now actually
I'll just duplicate it once for now and
we'll call this BG Knute for background
neutralization but right above that
background neutralization layer I'm
going to create a new layer just a new
blank layer and in GIMP the way to do
this is look down here at the bottom of
the layers panel and all the way over on
the left is this little icon with the
plus sign go ahead and click that and we
just want a new visible layer that's
good
so click OK and nothing in the image
changed because this right now this this
layer is transparent you can tell that
because we have the checkerboard pattern
over here in the thumbnail but we're
going to fill this transparent layer
with a gradient to create the gradient
and that what the gradient is gonna be
is it's gonna model the background sky
so disregarding the stars and the nebula
which is what we want to keep in the
image we're gonna try to to model this
light pollution effect that's why it's
it's brighter down here at the bottom is
because light pollution is almost always
worse the closer you get to the horizon
line where cities and human civilization
is and it's it's better up here the
further you get away from the city
lights so we're gonna try to model that
with a gradient to do so I'm gonna pick
my color picker which my color picker
tool sorry so if I look over here in the
tool set in the second row second from
the left is my color picker I'm gonna
set I'm gonna set the foreground color
first so for the foreground color I'm
gonna pick something up here at the top
and then I'm going to set my background
color down here oh I know what I did
wrong
sorry let's try that again so I'm gonna
first click on my background
neutralization layer over here so that's
what we're going to be sampling on and
then with my color picker I'm gonna
first click up here and you want to make
me be sure that you're not clicking on a
star so if you accidentally click on a
star you're gonna get a weird result see
what it did to my foreground color there
it made it this sort of pinkish color
what I want to be clicking on is the
background sky and so you see there
again I think I clicked partly on a star
and that's why we get that bright pink
there so just click around until you get
this sort of sky looking value then
we're gonna do the same thing for the
background color so over here in the
color picker options I'm going to choose
set background color over here on the
left and then I'm gonna go down here to
this corner so I think is the brightest
corner and I'm going to choose a sky
value down there
let's click around a little bit till I
see one looks nice and bright that looks
pretty good okay and then we're going to
draw out a gradient using this
foreground and background color so let's
choose our gradient tool here which is
this one you can also get to it by just
pressing G for gradient if you are new
to GIMP and this is like you know you
just start it up just like me then your
gradient will be and it's the your
gradient tool Weena's default state and
in its default state it just picks the
foreground and background color to
create the gradient here if yours
doesn't look doesn't do that for some
reason then just click on the
Swach here the gradient Squatch and
choose custom and then what it should do
is do a foreground to background RGB
gradient but you want it to basically
look like this we want a linear shape we
want foreground to background and we
want it to be a custom gradient and then
we're just going to draw a line from the
foreground to the background in the
image and what I mean by that is from
the top of the image to the bottom
because that's how we picked those two
colors so I'm going to just do it from
where I picked that color which was up
here in the upper left of the image down
to the lower right and when I look at
that my gut reaction is that's not quite
right but let's just go ahead and turn
oh I should have me undo one thing I
should have mentioned here is we wanted
to be doing this actually on the blank
layer right here let me just actually
call this gradient so I remember okay so
make sure you have the gradient blank
layer selected and when I just did that
I noticed that it didn't look quite
right there was a little bit too much
blue over here well actually this is a
fairly up-and-down gradient so I'm
actually thinking that blue isn't quite
right let's try actually switching out
our color samples and picking one we're
in the middle here yeah it's a little
bit darker and a little bit more in the
middle here
so I'm at background neutralization
layer you're gonna reset these just to
see if I can get a little bit better
result okay then I'm gonna go back to my
gradient tool make sure I have my
gradient blink layer selected over here
and let's try this again just going from
the top to the bottom this time rather
than from corner to corner okay I like
this a lot better this is looking right
and notice if I move this sample up
that's going to reposition where it's
sort of changing from dark to light so I
think maybe ending it right about there
looks about right and I'm just gonna
press ENTER to accept that let's go
ahead and try turning the visibility of
that gradient layer off and on to see
how it looks I think that looks pretty
good next one we're going to do is we
want to subtract this gradient from the
layer below the way to do that is we're
going to change its imaging or its layer
mode so right here in the layers panel
we have mode and right now it's set to
normal meaning that whatever is on top
here in the layers panel is what's
showing but we're going to change it to
subtract which is down here not quite at
the bottom but
there we go okay so you can see this
made a really dramatic change to the
image it looks immediately a lot more
dramatic because the sky background is
now dark because we subtracted out that
light pollution through this gradient
removal process if I turn it off and on
there's before and there's after before
after and really this is probably gonna
be the most important thing I show you
in this GIMP tutorial that really just
what we've done so far the stretching
and the the gradient removal everything
from here on out is really more optional
I'd say if we zoom in now we can see
that everything already looks pretty
nice I think the star colors look pretty
good
see the Horsehead and the flame and that
ryan running man here down here got the
bright star Rigel if I'm looking here at
the at the very bottom of the image you
can see that we still have a bit of
residual brightening and it's sort of
greenish looking maybe greenish yellow
looking same thing over here on the
left-hand side if you just decided you
know there's nothing really that
important to me in the image on those on
the side and the bottom maybe I would
just do some further cropping so if I
think you know the I still think I could
crop in a little bit further this this
side and the bottom here don't really
matter I can just grab my crop tool
again and crop those areas out
I'm gonna undo that just to show you
another option if you if you like the
framing you know this wider framing
better is we could just apply another
background gradient removal so to do
this let's just first merge this
gradient subtraction down into the
background new Claire so to do that just
with the gradient layer selected go up
to layer and choose merge down what that
means is it's going to combine it with
the layer below it just like that so now
we've we made that that decision
permanent we could undo of course but
we've basically combined those two
layers let's now go ahead and duplicate
the background neutralization layer and
call this I don't know
BG noot - and I'm just going to repeat
the process so I'd make I'd make a new
layer click on BG new to pick my color
samples and this time I'm gonna try to
get rid of this brightening over here in
this corner yes I'm going to pick my
color picker tool I'm just gonna go from
out here for my foreground
two down into this corner for my
background those look pretty close maybe
I'll be four my foreground I'll go
somewhere out here
huh wonder if that will pink means
something different than I think maybe
it just means this is black I don't know
okay let's try that
so grab our gradient tool go up here to
the blank layer and let's just draw a
gradient down into this corner yeah that
looks cool so it's basically saying out
here don't really do anything down here
let's subtract some of this greenish
crap okay I'm gonna press ENTER to
accept that gradient and then I'm gonna
change this to subtract mode and it did
it did it it did its job but it went a
little bit too far so what I mean by
that is the opposite of green is magenta
and now this whole corner looks to me a
little bit magenta blueish rather than a
more neutral black so what I'm gonna do
is I'm gonna play around with the
opacity of that layer which I should
name here just call this corner
until this is looking more neutral okay
I think that's pretty good so I'll go
ahead and just merge that layer down
into BG new - so layer merge down and
then we can turn that layer off and on
and see what it did before fairly
extreme green cast over there after
still a little bit of a color cast I'd
say it's a little bit more of like a
blueish magenta now but I think it's a
lot less distracting than that green
cast and if we kept fiddling with it we
could probably completely eliminate it
since this is just a quick tutorial I'm
not gonna I'm not going to keep going
with that but if you if you really you
know have some time you could just keep
fiddling with the technique I showed you
to really neutralize the background more
next thing I'm going to do is I want to
just select the bright parts of the
image - basically the stars in the
nebula to increase the saturation on
them a little bit so I'm going to go
ahead and duplicate this layer again and
I'll call this new one saturation and
I'm going to choose select by color
there's also a tool so select by color
tool same thing it's the fifth tool over
here's the tools options right here I'm
gonna choose to feather the edges but
just by say one pixel and where it says
select by composite I'm gonna choose
select by lightness because we just we
don't really
care about picking a particular color we
just want everything that is bright in
the picture and then I'll just click on
something bright in the picture I'm just
gonna click on a Ryan Nebula here and
you can see that then it made initial
selection of many of the stars and the
bright parts of the nebula but you can
see it's not yet picking up on some of
the dimmer nebula so I'm going to raise
the threshold and click again and now
it's picking up on a lot more let me go
ahead and zoom in check this out yeah
and I think that's doing a pretty good
job you can see it's picking up on the
stars not picking up on the background
so much see how it's doing on these
nebulae pretty good okay let's zoom back
out now with all of this stuff selected
I'm gonna go back up to my colors menu
and I'm just gonna choose the saturation
command
and I'll increase the scale here
it's a little hard to see because of the
selection preview it's probably some way
to turn that off I'm not sure you zoom
in a little bit here okay let's just try
turning the preview off and on hmm okay
let's try increasing the scale a lot
more
because I can see it starting to work
alright let's apply it
okay here's where we can turn off self
show selection let me go ahead and turn
that off a second and I'm going to turn
my saturation layer off and on to see
what it did there's with it off there's
what's it on okay that's pretty subtle
let me try zooming in a bit I can
definitely see a difference but it's
it's pretty subtle what to view the
selection again to see if it's still
active okay it is so I'm going to turn
the selection off and let's go back to
colors this time I'm gonna try
hue/saturation and let's just see if
this one is a little easier to see the
difference yeah I think that one is
working a little better so let's try
turn the preview off and on that we can
go even a little higher
another thing we could play around with
here is the lightness let's see if we
make the light parts lighter yeah I like
that actually makes it pop a little bit
more it's basically like increasing
contrast because we're not increasing
contrast to the sky background but we
are increasing the brightness of the
stars and nebula
okay so I've just increased saturation
and lightness on our selection I'm going
to click OK and you know what I'm pretty
much happy with this I'm not going to do
much more with it at the moment because
I think it looks pretty good there which
probably is of you know other things we
could do but I like how this looks so
let's go ahead and save it if you if you
think you're gonna come back to it in
GIMP you just want to choose file save
and save it in the xcf format call it
Orion here okay but if you're saving it
as anything else then you'll go to file
export and by default
exports in PNG which is a good choice
it's um it's doesn't lose much
information that way there's a bunch of
little things you can choose but you
don't need to just click export
okay there's the Ryan PNG on my desktop
the one downside to the PNG format is
because it is fairly lossless meaning
it's not losing much information the
file sizes are quite big so this is 43
megabytes so if we wanted to save it
instead in JPEG we could do file export
as and just change the ending to jpg and
then it's set to recognize the file type
by extension so then we click export and
it gives us a different set of options
here instead we have this little quality
thing that's going to be how much it
compresses the image so I'm just going
to do 85 and export it and because JPEG
is a lossy format that got it down to
4.3 megabytes but I think it'll still be
of perfectly acceptable quality let's go
ahead and make that fullscreen okay and
there's our final result if I was gonna
critique it a little bit some of the
stars are a little bit magenta so we
could work on the color balance there
there's still a little bit of a sky
gradient especially over here on this
side and down here in this corner but
other than that I'm pretty pleased with
this considering it is just an untracked
simple shot of the rhein constellation
we pulled out the bright belt stars
Rigel Orion and Running Man the
Horsehead Nebula the flame nebula and
there's actually a little bit of another
one up here this little guy up here is
m78
just a nice little reflection nebula up
there you can you can clearly see the
horse head in there and then this one is
it's called the flame there they're
surrounding this bright star called
Olney talked
well I hope that this tutorial showed
you that even with free software and
minimal gear you can get a nice pleasing
deep sky Astro photo and if you like
this kind of video please subscribe and
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that I have a patreon which you can
support me and help me keep making these
videos until next time this is Nico
Carver from nebulaphotos.com
Clear skies
