Back to forums

What's causing giant blue stars in this image? [Orion 120mm f/5 doublet w/Canon 550D]

Canon EOS 550D / Rebel T2i / Kiss X4 Acquisition Deep Sky M20 NGC7000
5 replies•542 views
Robert Westbrook avatar
Robert Westbrook avatar
This is the image in question.

I've seen plenty of other images on this site from people imaging with the Orion 120mm f/5 doublet that do not have these giant blue stars.

I'm capturing with a Canon 550D (non-modded) and guiding with PHD2 and an Orion SSAG, mount is an iOptron SmartEQ Pro.

This is a raw 240 second frame of M20.

This is a 180 second raw frame of NGC7000.

Are my sub images too long an exposure? Should I keep exposures to a minute or less? Would a dedicated camera rather than a DSLR reduce this effect? Thinking of buying the ZWO ASI183MC.

Thanks!
Engaging
Oskari Nikkinen avatar
Oskari Nikkinen avatar
Which scope exactly is this, you dont mean the achromatic doublet 120mm f/5 one do you?

In any case, achromatic doublet or ED doublet its chromatic aberration from your fast doublet lens and even with an ED doublet you should expect to see plenty of CA at f/5, unless you paid like 5k for it. Others with the same telescope might have taken some steps during processing to mitigate the blue bloat so that its not so easily seen, or used a filter that cuts off the blue/purple wavelengths off.
Helpful
andrea tasselli avatar
andrea tasselli avatar
It is an achromatic refractor and a rather fast one at that. Just the type of refractor you DON'T want to image with an OSC (or ever in fact). You *might* better performance by using a monochrome camera and narrow-band filters so you can control somewhat the amount of spherical and longitudinal aberration, but is a pointless pursuit in my view. Certainly going with a OSC camera with even smaller pixels is NOT going to help. If you want improve upon that level of imaging results you need to get at least a doublet ED or even better an APO.
Concise
James Ross avatar
James Ross avatar
You can bodge things in postprocessing to make the blue bloat look slightly less severe, which I expect the other images you have seen must have done, as I'm not sure there's a way to use that particular scope for imaging in a way that won't regularly suffer from horrendous chromatic aberration. Before upgrading your camera I'd look to upgrading the scope/lens to something better suited to photography. Exposures are also maybe a touch long as the star cores were all blown but with an achromat no matter what you do in terms of exposure you'll keep running into this sort of artefact unfortunately.
Helpful Insightful
Greg Bock avatar
Greg Bock avatar
Hi Robert
as has been discussed by the others before me, the short answer to your question is that the scope is creating the blue haloes. This is a clear and common case of chromatic aberation in this scope design which uses 2 lens elements only in the primary lens.
Other images taken through this scope design will also show blue haloes, but the effects may have been reduced during processing, or they may have used some type of fringe killer filter (google fringe killer to see what i mean here)


These 2-element achromatic scopes aren't designed for astro-photography. To avoid this effect, you need to change the scope.
Your Canon 550D is a great start, and the ASI183MC is a great OSC upgrade, but you will still see the blue haloes if you continue to use this scope.

Scope designs that reduce or eliminate this effect are refractors that employ apochromatic 3 elements in the primary lens. They cost more $$$.
Reflectors don't cause this effect, such as Newtonians, Maksutov-Newtonians, etc.

Do some more research about chromatic aberation and 3 element refractors if you want to stay with a refractor design.
Helpful Engaging
Dale Penkala avatar
Dale Penkala avatar
Agree with everyone here its CA.