Astrophotography – deep sky

M51 – Box Brownie version!

Seeing  all the recent splendid images of M51 made me regret that I didn’t acquire some images of M51 when I had the chance last week!

So,I had to satisfy myself with re-processing images I acquired a couple of years ago using processing methods learned in the intervening time

This one therefore is a stack of 9X20-second images from 2015 and 25X20 second images from 2016 – 680 seconds in total.

The box-brownie snapshot version!


Decided since its galaxy season to have a crack at M51. Actually i was planning this only as a test before my new scope arrives in July but didn’t expect such a result. There is a bit of field curvature in stars around the galaxy (pointing outwards) but from this experience i expect to remove that in future subs by better centering the object before and after a meridian flip. 93×1.5min. I will also increase exposure next time although i was concerned by mirror flop etc etc which didn’t show its face.



This is my go at Messier 51 from Thursday night, using the 200p f5 Newtonian that I picked up earlier this month. My first effort at deep sky with the scope had a few issues with collimation and focus, but a bit of time spent tuning it up seems to have yielded a much better result. This is one of my favourite targets, both for observing and images. To get a grandstand view of a galactic collision with an 8 inch dob in my suburban garden is one of those special moments that makes me love this hobby. In images, I love the stream of stars flung out into space from the encounter- this is the first time, after 3 attempts, that I’ve caught a bit of it.

This image is 35x 4 minute subs with 20 each of flats, darks and bias. Camera is a modded Canon 600d, stacking is in DSS and processing is in Pixinsight.

A Virgo galaxy selection 28-29/03/2019

As Rob alluded to the other day – so much choice, so little time!

Anyway here is a selection of images from last night.

Firstly a nice pair NGC 4762 and NGC 4754.


Then on to NGC 4222, NGC 4216 and NGC 4206, a nice triplet of edge-on galaxies. This one is a mosaic of 3 images, in fact.

A nice dust lane is visible in NGC 4216.



NGC 4216 has a 15.4 magnitude companion – PGC 39247

Then on to NGC 4608. This is a barred spiral, but I could only get the bar.

Finally, a pair of ellipticals, NGC 4461 and NGC 4458

All these images were acquired with a senseup of 1024 and are a stack of 20 of these – around 400 sec exposure in total.

Stacked in Registax 5 and processed in GIMP.

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!!)