Astrophotography – deep sky

The dehalo method for deep sky objects using GIMP.

As previously discussed in

Here is the method in words:


We need to do 2 things.

  1. We need to create an image which will provide the in-fill for the dark haloes.
  2. We need to create a layer mask that just has the haloes, but is otherwise opaque. We then use this mask and the in-fill image to fill-in the haloes.

To do 1 above we are going to use a “median blur”, and it is probably worth-while just explaining what this is.

Normally, when we filter, smooth or blur data we use an averaging technique. A straightforward “moving average” is just that – an average of the data in a “window”. A “Gaussian blur” is similar, but this is a “weighted” moving average.

For example, let us consider a very simple example where the image data represented as numbers is a smooth progression, say (1,2,3,4,5). The average of the numbers in this window is (1+2+3+4+5)/5 =3.

A Gaussian weighting scheme might be (1,4,5,4,1), and this weighted moving average is        (1X1+ 4X2 +5X3+4X4+1X5)/15 = 3 again. Note that this time the divisor is the sum of the weighting coefficients.

Now suppose we have a very “bright” point replacing the “4” point with “100”. The data is now (1,2,3,100,5). The average is now 22.2 and the weighted moving average is 28.6. Obviously the effect of the “100” point is reduced, but it is still there in the blurred data.

A “median” is the point where there are as many points above the point as below. So the median of (1,2,3,4,5) is 3, and the median of (1,2,3,100,5) is also 3. In other words the anomalous “100” point has completely gone from the “blurred” data.

So let us now apply this to an image with haloes in it.


  1. Open image file
  2. Click on “Windows”, then on “Dockable dialogs”, then “Layers”. This opens a “Layers” window.
  3. Right click on the layer that is there and then on “Duplicate layer”. We now have 2 copies of the layer
  4. Highlight the top layer, then click on “Filters” then “Blur” then “Median Blur”
  5. Expand the “Radius” until the haloes disappear. As explained above, because this is a median blur, the bright stars have little effect.
  6. Drag the lower layer up so it is on top. We are now going to create the layer mask.
  7. Right click on this, and click on “Add layer mask”.
  8. Select “Greyscale copy of layer” from the resultant window.
  9. Right click the top layer, click on “Show Layer Mask”, and, operating on the mask, click on “Filters”, then “Enhance”, then “High pass”.
  10. We now have a high pass filtered version of the mask.
  11. Click on “Colours”, then “Curves”. Manipulate the “Curves” to get a dominantly white screen with black where you want to in-fill (ie the haloes).
  12. We can now expand the haloes on the mask. Click on “Filters” then “Blur” then “Median Blur”.
  13. Reduce the “Radius” to 1, and the “Percentile” to around 20.
  14. Right click the top layer and uncheck the “Show Layer Mask” and you should see the result.
  15. If you don’t like it, Click on “Edit” and “Undo” and manipulate the parameters on the mask and try again.
  16. Once you have something you like, highlight the top layer, click on “Layer” on the top toolbar and then “Merge Down”.
  17. Export the result with a name and format of your choice.

All this might seem a bit involved, but it’s a bit like trying to explain how to ride a bike in words. Like riding a bike, once you get the hang of it, it is dead easy!

And here it is with some screenshots:

A dehalo method 3



Another attempt at using the Straton software.

The target is a large Planetary Nebula in Cancer. However, as always, it is quite dim. Only used the Ha so far but it does take a good narrow band. This was using the Atik 450 before it was sent back to the maker.

Total time on this is 1hour 30mins over 3 nights.

As before, having stacked in DSS and then Fits Liberator, the image was put through Straton to remove the stars. The remaining image was sent to PS and Stretched as much as I dared. Then the stars were put back and the result is above. When I get the camera back I’ll be adding more Ha and the O3 &S2 (That will probably be next year!!)


Window-sill Trapezium dehaloed

As I have previously posted, I have been trawling through my archive looking for likely candidates for the “dehalo” processing i came up with to get rid of those irritating dark haloes around bright objects the PD camera sometimes produces. (I think it is probably an artifact of the PD’s compression/noise reduction algorithms). Again, I’ve already posted some of these. M42 seems particularly to benefit from this:

Digital Astronomy – things to do when it is cloudy!


However, I found this one too, that I was particularly taken by. Here is the original and the dehaloed version.

It is quite simple and quick to do in GIMP and I’ll post the method if anyone is interested, but I suspect it is just my kit and methodology that creates the issue. “Proper” imagers don’t seem to get the problem!


Spider and the Fly

Hi all,

Taken with big camera Atik 4021. Spider on right (sh2-234) and Fly on left (sh2-237) in the constellation of Auriga in one session last week.

This is a total of 3 hours made up of Ha, O3 and S2 each 1 hour in 5 minute subs using the Takahashi Epsilon at f2.8.

The images I took last time with the Atik 450 only just covered the bright part of the spider and the fly.  It is now back with Atik as it has now given up the ghost completely. So I’m back with the big camera for a while.

Finally getting close with the focus which is now working with the electronic focuser after I found a screw to tighten !







Christmas tree

Images taken on the 26th Feb 31×2.5min D2 filter and 28×2.5min NB1 filter. Ive realized i  really need to start adjusting focus during the night as temperature differences and seeing conditions have shown me in this image. Looking at my RAW images a huge difference can be seen from the beginning of the evening. Also the christmas tree is reasonably low on the horizon for me with a lot of light pollution so im pleased to get this result.

Cone and California Nebulae (Manflu=Processing Opportunity)

A touch of lurgy at the moment has given me an opportunity to catch up on some imaging processing. The cone was the trickier of the pair- it’s much less bright and because there’s lots of gas around it, finding some background to calibrate on was not easy. Also, in my efforts to strip light pollution (both moon and LED) from the RGB subs I’ve removed any Oiii signal which I know is present somewhere in this! California was easier as it has higher surface brightness.


14/2/19: Ha – 14x 10 min subs (yes- my wife is very understanding)

21/2/19: RGB – 40x 2.5mins subs


14/2/19: Ha – 14x 10 mins subs

9/1/19: RGB – 12x 5 mins subs


DSOs 27-28/02/2019

It was such a nice night, at least for starters last night that I was tempted away from the window-sill!

First, M46 in Puppis, an open cluster that also seems to contain a planetary nebula, NGC2438. It isn’t in the cluster, in fact, it is just a line of sight effect. Since I couldn’t get the whole of M46 in field, to set the scene, the first image below is from the window-sill telescope from last year, followed by last night’s images:

Here is an enlarged and further processed version of the planetary nebula.

Then on to two little known open clusters in Gemini, NGC2129 and IC2157

Then some quite tricky galaxies in Coma Berenices, NGC4274 and NGC4559 (C36).

Finally, another pair of tricky galaxies in Virgo, the “Siamese Twins” NGC4567 and NGC4568