Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Should I be getting a ghost image when out of focus?

  • Please log in to reply
6 replies to this topic

#1 Ittaku

Ittaku

    Ranger 4

  • -----
  • topic starter
  • Posts: 354
  • Joined: 09 Aug 2020
  • Loc: Melbourne, Australia

Posted 21 September 2020 - 11:05 PM

I'm currently using a ZWO ASI224MC with a CPC1100 Edge HD, mostly for planetary imaging. At the moment I'm taking most images with the camera directly mounted on the back with a 25mm extension and 2x barlow, and focusing with a Bahtinov mask. One thing that strikes me is I always see a duller "ghost image" of what's on screen usually off diagonally from the main image, and as I move it into focus, the ghost image eventually merges with the real image and more or less disappears. I've been wondering if there's something wrong with my setup or if there's a reflection somewhere the way I'm setting it up, or something else untowards. The final images are decent, but I'm not sure if there's something wrong for that image to be there, or if it's a normal part of focusing? Thanks.



#2 bjulihn

bjulihn

    Mariner 2

  • -----
  • Posts: 226
  • Joined: 19 Jan 2019
  • Loc: Langley, BC. Canada

Posted 22 September 2020 - 12:03 AM

I haven't used an Edge Scope and I am no expert on them. However, a ghost diagonally off to one side suggests collimation issues or tilt in the imaging train. But tilt would likely show up in differential star shapes in different parts of your image.

 

Have you done anything with collimation? Have you tested star shapes by intentionally defocusing until you see a star in the center of the field of view as a "doughnut". Is the "doughnut hole" centered or off-centered. If it's off center, you need to collimate. Okay, someone more experienced can jump in any time. Anyone . . . Anyone . . .

Brad



#3 Ittaku

Ittaku

    Ranger 4

  • -----
  • topic starter
  • Posts: 354
  • Joined: 09 Aug 2020
  • Loc: Melbourne, Australia

Posted 22 September 2020 - 12:07 AM

I haven't used an Edge Scope and I am no expert on them. However, a ghost diagonally off to one side suggests collimation issues or tilt in the imaging train. But tilt would likely show up in differential star shapes in different parts of your image.

 

Have you done anything with collimation? Have you tested star shapes by intentionally defocusing until you see a star in the center of the field of view as a "doughnut". Is the "doughnut hole" centered or off-centered. If it's off center, you need to collimate.

I've spent the last two weeks trying to fine tune the collimation to the best of my ability. Each time I think I've nailed it, I reassess and tweak it a littler further. I do believe I'm very close.



#4 Der_Pit

Der_Pit

    Surveyor 1

  • *****
  • Posts: 1,604
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 22 September 2020 - 08:01 AM

I would guess the direction of that ghost varies, depending on where in the FOV the star is?  I assume it's some backreflection between the sensor and the camera window.  Also assuming that you use quite bright stars for focusing I think it won't create issues with darker / extended objects when in in focus.

If, it would show up as halos around/close to stars.


  • Ittaku likes this

#5 Ittaku

Ittaku

    Ranger 4

  • -----
  • topic starter
  • Posts: 354
  • Joined: 09 Aug 2020
  • Loc: Melbourne, Australia

Posted 22 September 2020 - 08:43 AM

Thanks, that makes sense. Yes it's only very bright things, and relative to where the centre of the field of view is. However I'm usually imaging planets, so to me it happens all the time, but I don't get halos in my final images.


Edited by Ittaku, 22 September 2020 - 08:46 AM.


#6 Ittaku

Ittaku

    Ranger 4

  • -----
  • topic starter
  • Posts: 354
  • Joined: 09 Aug 2020
  • Loc: Melbourne, Australia

Posted 22 September 2020 - 10:52 PM

I'm wondering if it's related to the "fairy wings" and flaring I'm getting my imaging, as per my other thread:

 

https://www.cloudyni...-stacked-image/



#7 Ittaku

Ittaku

    Ranger 4

  • -----
  • topic starter
  • Posts: 354
  • Joined: 09 Aug 2020
  • Loc: Melbourne, Australia

Posted 13 October 2020 - 10:15 PM

Follow up in case anyone comes back to this thread with a similar problem. I replaced my 2x barlow with a 2x powermate and the ghost image went away. It appears it was all internal reflection on the cheaper glass.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics