As a beginner to astronomy and astrophotography, and with a
limited budget for spending on the hobbies, I am always on the lookout for
‘free’ software. Two fantastic programs are SIRIL and GraXpert. My post editing
workflow uses these two and I do ‘finishing’ edits in Affinity Photo, which I
have had for a few years now.
Like any new programs, the initial learning curve can be
steep and so I found this to be with SIRIL. However, two things came to my
rescue. Firstly, this YouTube channel: Deep Space Astro –
a fantastic channel; I made so many notes and went through his videos so many
times, step by step, whilst actually processing an image in Siril.
I discovered this book via a Facebook post on an
astrophotography group in November which showed a similar flow diagram to one I
was already working on to summarise my own workflow (I am a bit of a visual
learner) which is below:
My diagram above now amended, incorporates some of the
elements of that original flow diagram I saw in the Facebook astrophotography group.
Max's book was fascinating. There was much I knew already from the videos and my internet reserach notes but then there were constant snippets on his pages, where I thought "Oooooh"! And his screen grab images were really helpful.
So below, I now share my SIRIL, GraXpert and Affinity Photo
work flow.
As always, I provide a caveat which is this: Like you, I
am a beginner. I am finding my feet slowly but surely in the new to me fields
of astronomy, astrophotography, and photo editing in AF. If I have made any
mistakes below then please accept my humble apologies and drop me a comment
at the end of the post so that I can correct it immediately.
These are my notes from the videos, internet research etc. I
have had them printed off and they’ve sat alongside my laptop EVERY TIME I do
any post editing of deep space objects since late 2023. They are written in a
shorthand but it should be fairly clear. I have made minor alterations in light
of what I have read in Max’s book. He goes into great detail and includes
really help diagrams and screen grab images. If you get stuck, then DeepSpaceAstro
on YouTube will have a video about it, no doubt. If you want to develop further
– its worth getting Max’s book off Amazon.
Good luck, if you use these notes and please do let me
know how it goes. If you think there should be amendments, then drop me a
comment.
S.I.R.I.L
instructions – my work flow
Firstly, open Siril and familiarise self with home screen.
Select console tab on right hand side.
1.
STEP ONE - STACKING
a. Create a new folder on hard drive –
called Siril
b. Create four new folders within
this Siril folder – name them ‘biases’,
‘flats’, ‘lights’, ‘darks’
c.
Click on HOME (top left hand side) – change directory folder
to this Siril folder created in (a) above
d. Select SCRIPTS on top menu – select
‘OSC processing’ – run this script
e. Watch progress of stacking on the
console tab, right hand side
f.
When it has finished, load up newly created ‘results.fits’ file
from this home Siril folder
g. At this stage, if need to free up
hard drive space, can delete newly created ‘process’ and ‘masters’ folders from
the master Siril folder
h. NOTE:
if not using calibration frames – SCRIPT – select right one; and ensure
you have left flats, darks, biases folders empty
2.
STEP TWO - IMAGE PROCESSING
a. With ‘results.fits’ image open in
screen, on bottom menu select auto-stretch to preview data and review in RGB
b. Now preview separate R, G, B channels
and then return to RGB channel
3.
STEP THREE – SWITCH TO USING GRAXPERT FOR BACKGROUND
EXTRACTION
a. DO THIS BEFORE ANY STRETCHING OF THE
IMAGE in Siril
b. Load up saved ‘results.fits’ image
created in Siril folder
c.
Select advanced tab on RHS and switch on AI ‘Background Extraction’
and ‘AI Denoise’
d. Change bottom panel stretch to 30%
e. Step 1 - Crop to remove any artifacts. Immediately save this image as ‘results.GxpCrp.fits’
f.
Return to 15% stretch at bottom tab
g. Step 2 - Background extraction tab – select AI – smoothing to 0.5% - click
‘calculate background’
h. When done change top tab to ‘gradient
corrected’. (Original on this tab shows pre BG). Now save image as ‘results.GxpCrpBge.fits’
i.
Step 3 - Denoising tab – default strength is 0.5 – can experiment with 0.8
or 0.9; click ‘denoise’ – zoom in to see changes (hold down control and use
mouse wheel). When done save image as ‘results.GxpCrpBgeDno.fits’
j.
Change top tab to denoise gradient
k. (NOTE: can do Bottom tab – saturation
(adds more colour to image) now or leave to Siril)
l.
Return to SIRIL and reopen this saved image ‘results.GxpCrpBgeDno.fits’
and continue SIRIL work flow
NOTE: How to do background extraction
in Siril
If
working on Background extract in SIRIL:
1.
With newly created image file ‘results.fits’ – open either green,
red or blue channel tab but NOT the RGB one.
2.
Image processing top menu – click ‘Background extraction’
3.
On new pop up dialogue box - Change grid tolerance to 3 if there are missing orange squares
on part of image background OR left click in that area to add a square. Keep
only the BG squares – take squares off nebula and around really bright
stars. To remove a star - right click on square to remove. Click dither. Keep
on subtraction. Now click ‘compute background’ button – and then click apply
4.
NOTES:
(polynominal tab – select 1 to 4 depending on how bad a gradient is – worse the
gradient the higher the number to
select). (RBF tab – recommended to use – smoothing = how aggressively applied –
0.5 is default setting)
If
working on denoising in Siril
1.
Select ‘Auto-stretch’ mode on bottom tab
2.
Top tab – image processing – ‘noise reduction’
3.
In new pop up dialogue box - Check ‘salt and pepper’ box. Colour denoising – off.
Modulation 0.5
4.
Default setting is ‘no secondary denoising stage’ – modulation 1 = full
noise reduction – zoom in 100% on image and hit apply – experiment and do undo
and redo to see changes
5.
However, Anscombe VST OR data adaptive is better method - do trial and error between them –
use undo/do buttons. Too much smoothing or smearing at edges of nebulae on new
image – undo - adjust modulation slider – redo
6.
Save image under new file name – this means can always go back a step and
eload previous image if something goes wrong
4.
STEP FOUR - COLOUR CALIBRATION IN SIRIL
a. Make sure have image open - ‘results.GxpCrpBgeDno.fits’
b. Set lower tab to ‘autostretch’
c.
Top tab – image processing – select colour calibration – select ‘Photometric
calibration’
d. In new dialogue box - search for DSO by
entering its catalogue number e.g M101 or M42. Alter focal length inputs or use
metadata within photo box to set the focal length details if used ASIair. Tick ‘flip
image’ if need be. Select ‘auto’ under ‘catalogue limit’. Then click apply
e. NOTE: BG reference – choose one of the
colour channels – draw square on image in area representing best background
area – Tick use current selection on dialogue box – BG neutralisation – do
white reference on bright white star – apply
f.
Go to RGB and use backwards and forwards arrows on top menu to toggle
between before and after changes. When
happy save this refined image as ‘results.GxpCrpBgeDnoPcc.fits’
5.
STEP FIVE - DECONVOLUTION IN SIRIL
a. Bottom tab – select ‘Linear
b. Top tab - Image processing – ‘deconvolution’
c.
In
newly popped up dialogue box – Default is ‘blind deconvolution’ to generate PSF image – click
apply
d. NOTE: for experienced people - Other
option to try – ‘PSF using stars’
e. OR: see below –
f.
Deconvolution MUST USE LINEAR NON STRETCHED IMAGE so ensure in linear mode on bottom tab
i.
Simple option approach a - hamburger icon top right – image info
– dynamic PSF – blind DC box ticked(essentially as steps above)
ii.
Approach b – image processing – ‘star processing’ – ‘full resynthesis’
– click the cog wheel. Drag the pop-up dialogue box to RHS
iii.
Back on the image - create green ‘selection box’ over centre
of image – right
hand mouse click - crop;
iv.
Image processing – star processing – cog wheel – dynamic psf – click ‘detect
stars icon’ – aiming for white star that appears in top RHC to be unclipped
in PSF preview box i.e. star is fully within the box and has some blackness all
around it
v.
On resulting chart – click column a to sort from dimmest to brightest stars – want to keep
only those 0.1 to 0.7 amplitude – fill in boxes at bottom to get this
range; profile type to ‘moffat’.
vi.
Click ‘detect stars’ icon
vii.
Reduce PSF box down out of sight in RHC of screen
viii.
Top tab - Image processing – deconvolution – select PSF from stars – generate
PSF; then click download arrow alongside generate PSF button to save this
image. Save as ‘results.GxpCrpBgeDnoPccDecon.fits’
ix.
Bring back dynamic PSF box into view from bottom RHC – now doing test
settings on the cropped image and when finally done will click undo right back
to where image was uncropped – meanwhile on cropped image select ‘Richard lucy
algorithm’ and ‘non blind deconvolution’ – start with default settings – keep
other algorithm to gradient descent – click apply
x.
Zoom in and check for star fringing, ringing etc – (control hold down and
use mouse wheel); toggle before and after arrows in top menu to judge change
xi.
Click undo arrow and increase gradient step size a little and repeat zoom
in to see effect
xii.
Repeat this pattern until happy with changes – note the settings used
xiii.
Click undo now until returning to uncropped image – in Deconvolution box
put in settings and click apply on full image. Save any alterations as ‘results.GxpCrpBgeDnoPccDecon.fits’
6. 6. STAR SEPARATION/STRETCH/RECOMBINATION WORKFLOW USING USING
STARNETT++ in SIRIL
NOTE: if you haven’t downloaded STARNET++ add on into your
Siril program – do so now.
a. Top tab - Imaging processing – ‘star processing’
– ‘starnet++ removal’
b. On new pop-up dialogue box – tick ‘pre-stretch
linear image’ and ‘generate star mask’ –
make sure still in linear mode on bottom bar – then click execute
c.
Stretching the new Starless NEBULOSITY image which is the one that should be on screen now (NOTE
YOU CAN DO THIS IN AFFINITY PHOTO – SEE NOTES AT END OF THIS SECTION)
i.
start in linear mode on bottom tab
ii.
on top tab – image processing – ‘histogram
transformation’ (MTF)
d. In new popup dialogue box - Click
on cog symbol – if happy with changes to image then click ‘apply’. If not happy click ‘reset’ and then
following below
i.
Set ‘zoom amount’ in box (top tab LHC) to 1. Adjust middle triangle on
base of histogram leftwards as far as
can go. Now increase the zoom number value at top to create more space at base
of histogram – slide the middle triangle leftwards again until gaining pleasing
image; TIP – keep this first stretch slightly dark to
avoid blowing out any cores – if happy with image – click ‘apply’ and image as ‘results.GxpCrpBgeDnoPccDeconStarless.fits’
e. Alternative from AFTER step(b) above
i.
stretch line sliders below the histogram graph area until
‘nebulosity’ appears on black image; use -/+ button to do black point
adjustments OR
ii.
middle triangle at base of histogram to LHS to get
histogram off LHS. Then alter shadow slider in to base of histogram – keep
figures on right at 0 to 0.1. Choose colours menu and do appropriate for
each colour as prefer (can draw box onto part of nebula – hone in on different
parts of it – then go to symmetry point – hit eyedropper on LHS – then move
stretch slider)
iii.
SAVE FILE TWICE–
firstly, as results.GxpCrpBgeDnoPccDeconStarless.fits’. Then resave
again but this time as results.GxpCrpBgeDnoPccDeconStarless.tiff’ (as tiff 16 bit unsigned).
iv.
NOTE: advanced action - could at this point – open tiff file in affinity photo an do some
levels and curves adjustments. Save as 16 bits Tiff file and then bring back to
Siril. Open it and immediately save it as fits file! This is one which will
then carry on with in Siril.
7.
STEP SEVEN - GHS STRETCHING AND COLOUR SATURATION IN SIRIL
The aim of a
GHS stretch, as I understand it, is to stretch the pixel values out across the
whole range of brightness. We do a series of stretches to different areas of
image to bring out detail and improve contrast where interesting features
lie. Starting with an overall stretch, successive
stretches are applied to different areas to maximise the potential of image.
Principles of GHS: ‘S’ curves (to
flatten either end of pixel distribution and boost mid-range pixels.) Done by
a) defining symmetry point (sp) = centre of curve and b) local intensity – the
range of the S curve. Slider adjusts strength of stretch. AIM: to bring out
most detail and contrast in image
a. Bottom tab – make sure selected linear
mode
b. Top tab - Image processing – ‘GHS’
c.
In pop up window
i.
Symmetry point to 0.5 or click middle of line on histogram; zoom level is
‘1’
ii.
Method one: if focusing on nebulae or galaxies – focus on outer rim
details:
1. GHS rim detail – left mouse click on image to draw
box in area of interest e.g. outer rim detail
2. Click on dropper symbol to set the SP
value
3. Stretch factor slider moved to get
level of detail required
4. Local stretch factor slider moved to
modify extent of stretch factor slider
5. Go back and forth between two until
pleasing result (could try Local stretch slider full across to rhs and then
Stretch factor slider until nebula appears – stop before background gets too
grey); aiming to form an S curve on histogram – click ‘apply’ when happy
6. Then change to a ‘linear black point
stretch’ in top menu of pop up box –
slide BK point slider to close gap base of LHS of histogram curve
7. NOTE: Stop highlights blowing out when adjusting for midtones by
pulling back the highlight protection point - on logarithmic scale the red line on
histogram will be very steep where detail is but then the incline should be far
less steep for rest of the histogram
iii.
Method two – for GHS odust detail – draw box on area wanted to impact
1. Set the SP point as above with box
2. Adjust strength and local intensity
values as above to get effect pleasing
3. Move shadows and highlight protection
sliders to stop highlights blowing out
iv.
On both methods above -
1. Change back to GHS at top - logarithmic scale – check box on left –
aiming to get straight line from peak to bottom lowest point (humps = areas
where lower contrast needed; valleys = where more contrast needed) – click to
left of very top of histogram curve to set symmetry point – now stretch slider
a little; then local stretch slider to add or remove contrast and to try and
smooth the line.
2. Repeat for bottom of curve rhs –
click to set symmetry point an repeat step above – use local stretch curve to
add/remove contrast
v.
Method three – GHS colour stretching
1. Aim to improve colours – turning
colour buttons on and off
2. Boost blue – a tiny amount – click off other
colour channels – draw square around area of DSO that represents brightness
area where we want to boost blue – click on dropper to set SP. Play with strength and local intensity sliders
to get effect as outlined above.
3. Use shadow and highlight protection
point sliders to squeeze the affected area away from edges of histogram by
keeping them close to the SP
4. May need to move SP to get blue boost
in right place - then click apply
5. Add touch of red – use same process outlined above
LESS IS BETTER
6. Improving saturation – top right menu to colour saturation
stretch. Boosts all colours. Check zoom at 1.
vi.
Remember to save your image after GHS: ‘results.GxpCrpBgeDnoPccDeconStarlessGhs.fits’
8.
If not doing colour saturation above as part of GHS work then - COLOUR SATURATION
a. Top tab - Image processing – ‘colour
saturation’ – hue on global.
b. Amount > increases colour
saturation – try between 0.25 and 0.5.
c.
Click background factor to allow sky background to be protected from
colour saturation
d. Experiment and when happy - apply
e. Click save: ‘results.GxpCrpBgeDnoPccDeconStarlessGhsCos.fits’
9.
ALTERING CONTRAST AND DETAIL in SIRIL
a. Top menu tab - Image processing – ‘contrast
limited adaptive histogram equalisation’ (CLAHE)
b. Use it lightly – small adjustments to avoid
introducing artefacts
c.
Use preview box to see before and after
d. Save as ‘results.GxpCrpBgeDnoPccDeconStarlessGhsCosCon.fits’
10.
REMOVAL GREEN HUES
a. Top menu tab - Image processing – ‘sub-chromatic
green noise’
b. After using default settings – save image
as ‘results.GxpCrpBgeDnoPccDeconStarlessGhsCosConGno.fits’ If unhappy with file name this long – try
saving as ‘resultsfinalstarless1.fits’
11.
Note: you can do stretching etc in affinity photo rather than
here in Siril. When you open your image in AP – you can do
your curves and level adjustments, shadows/highlights etc to sort contrast.
Critical is the saving bit at end – save the image in AP – export it as a tiff RETURN
TO SIRIL
Open this Tiff in
SIRIL; click save down arrow and rename file as starless etc but FITs at
end. Click 32 bit and now work on this fits image.
NOTE: you can do similar for working on the star image below –
if AP if desire
12.
STRETCHING ON STAR IMAGE IN STARNET++
a. Open the unstretched stars image from
your master Siril folder – it will be labelled as resultsstarmask.fits
or something similar
b. Top tab - Image processing – ‘histogram
transformation’ – making sure display option on bottom tab is LINEAR
c.
On pop-up box - zoom factor is 1 and all graph colour buttons below
histogram are set to on
d. At base of histogram - Middle ‘mid
tones’ triangle slide to left until stars begin to appear on image
e. Adjust zoom number at top - > it,
triangle will move to right – move it back left again
f.
Keep going – stars will get brighter and more numerous – keep going until
value in midtones box is around 0.002 and zoom value is around 60 – 70 If
happy with amount of stars visible Click
apply if unhappy – reset the histogram
and start again – experiment trial and error
g. When happy save as ‘resultsstarmaskstr.fits’
or something similar
13.
STARNET STAR RECOMPOSITION PROCESS
a. Top menu tab - Image processing – star
processing – star recomposition
b. Load both images back up in relevant
histograms. MAKE SURE RIGHT IMAGE GOES WITH RIGHT HISTOGRAM: your stretched stars image goes in ‘star
stretch parameters’. Your starless nebulosity image loads in to ‘background
stretch parameters’.
c.
Make tint slider adjustments under each histogram. When happy with each click
apply under each histogram and check progress in console tab on RHS.
d. Save this recombined image as something like ‘M42final.fits’ or
whatever target you have been imaging
Other
things you can do in Siril
14.
REMOVING GRADIENTS IN SIRIL
a. Image processing – BG extraction –
Polynominal – simple gradients. The higher the number chosen, the more complex
the gradient
b. Best to use RBF method – choose small
smoothing changes – default setting is 0.5
c.
The larger the number of amples per line – the longer taken to process
d. Grid tolerance – adjust so that
samples not on stars or DSO’s that do not belong to the background sky
e. Add dither
f.
Generate creates grid – clear – clears it
g. Correction – subtracts background sky
15.
TO REDUCE BANDING IN IMAGE
a. Auto-stretch
b. Image processing – banding reduction
– apply (use default settings)
c.
To prevent it affecting brightness of image – tick protect from
highlights box; select vertical banding if it is also present
16.
STAR REDUCTION IN SIZE
a. Image processing – star reduction MTF
– it will close image down; bottom right hand script menu – green light will
show processing – when red – means finished. Save image
b. To compare this new image with original
– open original file used before doing star reduction process; open this final
one working on
c.
Use little triangle button next to open – toggle between new and original
image to see differences
d. Want smaller – re-run process again Save file as reduced stars_final.fit
Using
affinity photo for final polishing edits
17.
GO TO AFFINITY PHOTO
a. curve and level adjustments for
colour, contrast, brightness) and do for R,G, B individual colours as well.
TINY changes.
b. Shadows/highlights.
c.
Sharpening and denoise.
d. Colour adjustments and saturation
e. Other level adjustments as see fit.
f.
SAVE AS TIFF; SAVE PNG AND/OR JPEG AS WELL; SAVE AS AFFINITY PHOTO WORK
FILE
18.
IF NOT DOING STARTNET++ REDUCTION in Siril
a. go back to section 2 image processing – this
will now follow on from point 11:
b. do a histogram stretch – select
linear mode on bottom menu – image processing – histogram transformation –
auto-stretch cog wheel – apply. Repeat step and make fine adjustments with
sliders.
c.
Colour saturation – image processing – colour saturation – experiment
with different levels etc
19.
TIPS AND TRICKS IN SIRIL
|
Ctrl
|
+O
|
open an image
|
Ctrl
|
+Z
|
Undo previous
command
|
|
Ctrl
|
+S
|
Save loaded
image
|
Shift+Ctrl
|
+Z
|
Redo previous
command
|
|
Shift+Ctrl
|
+S
|
Save loaded
image with new file name
|
Ctrl
|
+I
|
Negative
transformation
|
|
Ctrl
|
+W
|
Close image
|
Ctrl
|
+east arrow
|
Rotate 90d
clockwise
|
|
Ctrl
|
++
|
Zoom in
|
Ctrl
|
+west arrow
|
Rotate 90d
anticlockwise
|
|
Ctrl
|
+-
|
Zoom out
|
Ctrl
|
+vertical arrow
|
Horizontal
mirror
|
|
Ctrl
|
+0
|
Fit image to
window
|
Ctrl
|
+down arrow
|
Vertical mirror
|
|
Ctrl
|
+1
|
Zoom image to
100%
|
Ctrl
|
+
|
|
|
|
|
|
|
|
|
|
Ctrl
|
+P
|
Open
preferences
|
Shift+Ctrl
|
+A
|
Open plate
solving dialogue
|
|
F10
|
|
Open
application menu
|
Shift+Ctrl
|
+P
|
Open
photometric colour calibration
|
|
Ctrl
|
+F6
|
Open dynamic
PSF window
|
Ctrl
|
+/
|
Search an
object in the image
|
|
Ctrl
|
+D
|
Change working
directory
|
Shift+Ctrl
|
+C
|
Copy snapshot
to clipboard
|
|
Ctrl
|
+F
|
Full screen
on/off
|
F5
|
|
switch to plot
page
|
|
Ctrl
|
+T
|
Hide/show
toolbar
|
F6
|
|
Switch to the
stacking page
|
|
F1
|
|
Switch to
conversion page
|
F7
|
|
Switch to the
output logs page
|
|
F2
|
|
Switch to
sequence page
|
Ctrl
|
+Q
|
Quit the
application
|
|
F3
|
|
Switch to
pre-processing page
|
|
|
|
|
F4
|
|
Switch to
registration page
|
|
|
|
I hope these notes are of help – they are my workflow –
written as a beginner and as I gain further experience, I am sure I will refine
them. I made the mistake of trying to get to grips with astronomy and
telescopes, astrophotography, and then basic photo editing in affinity photo –
all at the same time. Stupid mistake but there we go – heck of a learning
journey, I guess!
As always, if you are more experienced at all this and have
any useful tips regarding workflow order, please share them in the comment box
below so that we beginners can see them and learn from them. Appreciated.
Clear skies. Stay safe, have fun out there.
Steve
PS – I can not place enough emphasis on this – two fantastic
resources that beginners should know about:
Youtube channel: DeepSpaceAstro
Book: Astrophotography Image Processing with Siril, GraXpert
and GIMP by Max Dobres FRAS
ISBN 9798338039373
Pub: 2024
