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strange coma


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Hi everyone. I'm trying to diagnose the tails on the edge of frame stars. The stars seem quite well defined, but have tails. I think it's coma as it's pointing toward centre frame but not like the coma I'm used to. My next move is to move the cc away from the sensor, but any other diagnoses most gratefully received. Cheers and TIA.

f5 Newtonian with gpu cc @ 53mm to 700d.

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I've seen such thing previously (sort of inverse coma, with regular coma tail is pointing away from center, this one is pointing towards center) with focal reducers that are placed too far away from imaging plane for their specs.

How do you calculate your distance? Flange distance for EOS is 44mm and standard EOS-T2 ring is usually 11mm optical path - that would mean you at least have 55mm distance. You must be using very low profile T2-EOS adapter to get to 53mm?

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7 hours ago, spillage said:

Are you sure that you are not too far away from the sensor?

Hi. Almost certainly, see the diagrams in Demomperormer's post. If I'm too far away, the coma is orthogonal to the centre. Mine is toward the centre...

8 hours ago, vlaiv said:

How do you calculate your distance?

44mm Canon, 9mm OAG =53mm.

Oh, and it's only on bright stars...

--- --- ---

**Just found the gpu blurb. It seems it's focal length dependent rather than f ratio. I'm at 1250mm:

aplanatic1-gpu.jpg.f7ec9c22e8070b6a78e5c8c725497844.jpg

 

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3 hours ago, alacant said:

Hi. Almost certainly, see the diagrams in Demomperormer's post. If I'm too far away, the coma is orthogonal to the centre. Mine is toward the centre...

44mm Canon, 9mm OAG =53mm.

Oh, and it's only on bright stars...

--- --- ---

**Just found the gpu blurb. It seems it's focal length dependent rather than f ratio. I'm at 1250mm:

aplanatic1-gpu.jpg.f7ec9c22e8070b6a78e5c8c725497844.jpg

 

Hi Alacant. What coma corrector are we talking about here. Didn't know focal length affected distancing! 

Gerry

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1 hour ago, Gerry Casa Christiana said:

coma corrector are we talking about here.

Hi. It's the gpu coma corrector, sometimes called -marketed as- sw aplanatic.

1 hour ago, Gerry Casa Christiana said:

Didn't know focal length affected distancing! 

Neither did I. Another case of RTFM on my part;)

Cheers

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