Observation by Peter Anderson: Omega Centauri Globular Cluster

Uploaded by

Peter Anderson


Peter Anderson


2020 Mar 22 - 00:00


2020 Mar 25 - 01:27


Omega Centauri (NGC5139)

Planetarium overlay



Field centre

RA: 13h26m
Dec: -47°31'
Position angle: -87°49'

Field size

0°55' × 0°42'

  • Skywatcher 150mm F8 ED refr.

3200 ISO 30 secs


Brisbane, Australia.

Target name

Omega Centauri Globular Cluster


Omega Centauri Globular Cluster

About this image

Compare with the previous achromat image and note that the foreground flared O and B stars have lost their flares.  Unfortunately because I used the same 1.25" adaptor I used for the barlow shots, (my fault) the field edges suffer from vignetting. 

The whole series of stellar object images (5 images X 2 - one for each scope) further demonstrate the differences between a standard and an ED refractor of the same specifications for stellar images. It is to be noted that there is still some residual flaring and colour in the brighter stars with the ED noticable in some images and particularly on the Rigel comparison test exposures. These would probably be effectively removed in a triplet system.  We are all familiar with the magnitude of stars which increment in brightness by 2.512 times per magnitude. Virtually the same applies for the cost of optical tubes from standard refractor, to ED, to triplet, namely a magnitude jump each time! It becomes a question of what is cost effective for the purpose.

I hope that the images I supply will be useful to members.

PLEASE NOTE - For unknown reasons some images in the sets have been inverted. After considerable frustration I have managed to adjust these last two.

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Grant Privett
Grant Privett, 2020 Mar 25 - 15:12 UTC

Presumably the difference isnt as obvious when using narrow band filters like H-alpha. So for nebulosity rather than stars the cheaper option may be as good - or am I misunderstanding?

Peter Anderson
Peter Anderson, 2020 Mar 26 - 02:14 UTC

My main field of observations are lunar, and to a lesser extent asteroidal occultations. I also follow current phenomena but have not delved deeply into filters, nor deep sky imaging. I am a bit of an instrument tragic as my profile on this website will attest, owning a variety of instruments. So, though not strictly a dilettante, I am not in a position to comment upon the finer points.

What did amaze me with the previous achromat was the extent of chromatic aberration and the blue violet excess, though at F8 this is what theory predicted. Why make a 150mm telescope no longer than F8? The simple answer is commercial reality in the sheer size and mounting and the prospect of insufficient sales. So the visual and photographic images, whether solar system or stellar suffered. The difference in price between the achromat, ED, and triplet is about one magnitude jump (2.5 times) with each step.  The ED is still reasonably affordable and I was astounded by the difference it made as demonstrated in the images I posted. (I will post more on lunar/planetary as they are obtained.)

I suppose my purpose was to show the obvious differences and improvements. Much more subtle, and possibly caused by the lenses and coatings etc. is the subtle shift in colour balance if you compare the images. 

Thank you for your suggestions, but this was merely an exercise to compare. I have prepared a powerpoint presentation where I align the images in following slides and it is fun to toggle back and forth. I am happy to share this.

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