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Old 10-04-2016, 07:29 PM
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NorthernLight (Max)
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Coma in a refractor?

Hi Guys,

I have been playing around with my newly Baader modified 450D on a ED80 with field flattener plus a EOS clip in CLS filter. What I note is that star halos seem to be stretched towards the edge of the frame as I've had it with my Newton. It annoys me that I can't figure what the culprit is and hence I seek your advice on this matter.
Image attached, look out for the big orange stars towards the edge of field.

Many thanks for looking into this.

Cheers,
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Old 10-04-2016, 07:55 PM
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janoskiss (Steve H)
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Refractors have coma for sure (camera lenses are a uuell knouun example of that). But the ED80 should be pretty damn good and not exhibit any coma.

Not sure about Baader mod (you had the Baader filter removed??) but possibly you're imaging beyond the visual spectrum after the mod. So it's reasonable to expect that the infra-red is pushing a scope designed for visible light.

I cannot see anything uurong uuith your image by the uuay. Looks like a stunning photo to me.

(sorry uu stands for double-u; my keyboard is busted.)
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Old 10-04-2016, 08:23 PM
raymo
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With my screen at 400% I can't see any aberrations.
raymo
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Old 10-04-2016, 08:44 PM
glend (Glen)
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Assuming you have had a full spectrum mod done, where both the LP1 and LP2 filters are removed, you should be running a UV/IR Cut filter in the light path to cut off the lower end of the spectrum (below Ha). The image looks like there is no Cut filter in front. Based on my own full spectrum results on this target that's what I think.
I had a look at the spectrum profile of the CLS filter and it does not cut off the spectrum below Ha allowing 97% passage below 650nm.

Last edited by glend; 10-04-2016 at 08:58 PM.
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Old 11-04-2016, 05:18 PM
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NorthernLight (Max)
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Hi there,



and thanks for your responses. Just to clarify, the camera has had the IR filter replaced with a filter from Baader (BCF). That filter allows passing of more Ha light.

The issue I am seeing is that in particular orange stars seem to have their halos shifted to towards the lower right image corner.

I was reading up on perpendicularity issues but noted that stars with blue halos are not equally affected.

That's why I was wondering whether the ED80 might produce coma in that spectrum or whether perhaps it's an issue with the filter assembly in front of the sensor or maybe a contribution of the CLS clip in filter and my camera not sitting square on the bayonet mount.

Maybe someone has experienced the same and found an answer.



I attached another image - same thing.


PS: last night I found out that stars appearing as straight streaks in the image are a result of the camera lenses image stabilisation.
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Old 13-04-2016, 02:02 PM
raymo
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Well, it is chromatic aberration. If you look at the large/bright, blue/white
stars at a screen mag of 300% or more, you will see that they have an
offset blue halo on the opposite side from the halos on the orange ones.
APOs, ED or fluorite, do not entirely remove CA, just varying amounts
of almost.
raymo
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Old 13-04-2016, 04:48 PM
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Thanks Raymo. Good to know. I had a suspicion when Insaw that other thread here with the guy also using an ED80.
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