Astrophotography – deep sky

Image processing lore

There is an article in January’s “Sky at Night” magazine under “Skills” and “Image Processing” entitled “Improve your deep-sky images with luminosity layers”. While not entirely BS, it seems to me that it is a complicated way of doing something relatively simple. There is nothing new about the point that most of the resolution information in a colour image is in the luminance part of the image. After all, this is how colour TV signals have been transmitted for ever. With colour TV, the image is decomposed into a luminance, or black and white signal and a chrominance signal that contains the colour information. Luminance is then transmitted on a high-bandwidth channel, and the chrominance on a low-bandwidth sub-carrier. They are combined again inside your TV receiver.

The author of the article used Photoshop, but you can do essentially what he did (and to me, much more simply) with GIMP. How to do it is in an article from 3 years ago on our own web-site at

AND, of course, as we keep reminding everyone, GIMP is FREE!


Hi All,

Got a new toy for Christmas and just waited for a clear night. Only one night so far, so have only managed a quick image of the B30 nebula in Orion.

The new toy is for processing, it’s the Straton star removal program.

Having taken only 6x5minute images in each of Red, Green, Blue and Ha, the data was stacked in Deep Sky Stacker. Next, each one was put through Fits Liberator. These master images were then all aligned in Maxim DL.

The RGB images were put through Photo Shop curves and combined then tweeked in the colour adjustments.

The Ha image was put through the new toy and had all the stars removed from it in 35 minutes (automatically of course!).

This was then put back in PS and combined in “Levels” with the RGB image. After a bit more fiddling in curves I got this result.

It sounds a long and complicated process……….IT IS!!

Anyway, for a first attempt I’m quite pleased with the result.



Playing with the Pleiades (M45)

Last night, from the window-sill, I went comet hunting (ultimately unsuccessfully!)

I set up the PD camera with the (£25) 50mm f/1.4 lens (no telescope) on the window-sill, and since I haven’t used that for a while, went looking for a target in order to set it all up. Spotted the Pleiades, not ideally located in the window (right in the corner), but given Ken’s recent splendid image, thought I would give the box Brownie (!) version a go.

Here is the result, after a total exposure of around 200 seconds and a bit of GIMP-ery.


ts been a while now since i posted my preliminarily preview of the Esprit 120 mostly due to a combination of work/family/bad weather and Xmas parties. On Friday evening the forecast was clear until around 7ish so i was able to get a few shots of M45. I chose M45 as its a great star cluster which would be good to determine star shapes in the corner of my sensor.

I was able to get 26×2.5min before the clouds started to roll in. For this test the position of M45 wasn’t great so there is a light pollution gradient etc, which isn’t important to determining star shapes, optical errors.

Anyway looking at my RAW images id say the scope covers about 98% of the sensor. For the remaining 2% a very small amount of vignetting is visible with a strong histogram stretch and an ever so slight amount of coma on stars at the extremities of the image. If your using a 36mm sensor like me then a very slight crop is necessary, although most people would do this anyway. If your using a standard APS-C Dslr then your stars are pinpoints to the edge! Very impressive in my book. Zooming into a bright star i can see no signs of star bloating.

I don’t know how Skywatcher are able to achieve such high quality for such a low price (in comparison to high end competitors) but im delighted!

The image ive posted is about 75% crop, for your perusal if anyone wants a full uncropped image please let me know.