alacant Posted March 21, 2018 Share Posted March 21, 2018 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. Link to comment Share on other sites More sharing options...
vlaiv Posted March 21, 2018 Share Posted March 21, 2018 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? Link to comment Share on other sites More sharing options...
spillage Posted March 21, 2018 Share Posted March 21, 2018 Are you sure that you are not too far away from the sensor? Link to comment Share on other sites More sharing options...
Demonperformer Posted March 22, 2018 Share Posted March 22, 2018 These diagrams of what happens when the imaging distance is too close/too far have been posted on SGL previously, but are possibly worth repeating. First one is too far, second one is too close. Link to comment Share on other sites More sharing options...
alacant Posted March 22, 2018 Author Share Posted March 22, 2018 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: Link to comment Share on other sites More sharing options...
Gerry Casa Christiana Posted March 22, 2018 Share Posted March 22, 2018 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: Hi Alacant. What coma corrector are we talking about here. Didn't know focal length affected distancing! Gerry Link to comment Share on other sites More sharing options...
alacant Posted March 22, 2018 Author Share Posted March 22, 2018 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 Link to comment Share on other sites More sharing options...
alacant Posted March 28, 2018 Author Share Posted March 28, 2018 Hi. Getting somewhere. Here we are with the cc back at 55mm, a further 2mm from the sensor. Better, but still not quite there; it looks like the gpu spacing graph is correct:) Link to comment Share on other sites More sharing options...
Gerry Casa Christiana Posted March 28, 2018 Share Posted March 28, 2018 6 hours ago, alacant said: Hi. Getting somewhere. Here we are with the cc back at 55mm, a further 2mm from the sensor. Better, but still not quite there; it looks like the gpu spacing graph is correct:) Your at 1200mm right? So 54.7 for you? I would be happy with stars like that Gerry Link to comment Share on other sites More sharing options...
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