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

Focusing question

  • Please log in to reply
9 replies to this topic

#1 steven40

steven40

    Mariner 2

  • -----
  • topic starter
  • Posts: 244
  • Joined: 01 Feb 2012

Posted 24 February 2020 - 12:44 PM

Hi Folks,

 

Had a couple good nights with my Nikon 300mm F4, shooting at F7 with a circular mask. Rosette attached. I focus as carefully as I can using live view.

 

I seem to recall (from Jerry Lodriguss) that best focus is achieved not by seeking the smallest image on the live view screen, but by having a slight color fringe around the star; I can't recall green or magenta.

 

Anyone have any thoughts on this?

 

Steve

Attached Thumbnails

  • Autosave-proc4-crop-shrink-northup-verysmall2-22-20-2237.jpg

  • Starman27 and Jim Waters like this

#2 Jim Waters

Jim Waters

    Soyuz

  • *****
  • Posts: 3,854
  • Joined: 21 Oct 2007
  • Loc: Phoenix, AZ USA

Posted 24 February 2020 - 01:31 PM

Not sure I agree with the above.  On my Canon 5DMkIV I try to get the smallest star at 10x Live View.  Check out the following link.

 

https://www.otelesco...-backyardnikon/



#3 steven40

steven40

    Mariner 2

  • -----
  • topic starter
  • Posts: 244
  • Joined: 01 Feb 2012

Posted 24 February 2020 - 01:59 PM

Hi Jim,

 

I recall that there was some rationale for the approach but I can't recall what it was!

 

Steve



#4 Jim Waters

Jim Waters

    Soyuz

  • *****
  • Posts: 3,854
  • Joined: 21 Oct 2007
  • Loc: Phoenix, AZ USA

Posted 24 February 2020 - 02:25 PM

Maybe for standard quality lenses which have CA. I use high quality lenses when I image.

#5 Jim Waters

Jim Waters

    Soyuz

  • *****
  • Posts: 3,854
  • Joined: 21 Oct 2007
  • Loc: Phoenix, AZ USA

Posted 24 February 2020 - 02:26 PM

BTW - I forgot to comment on your image.  Real nice image Steve.



#6 steven40

steven40

    Mariner 2

  • -----
  • topic starter
  • Posts: 244
  • Joined: 01 Feb 2012

Posted 24 February 2020 - 02:32 PM

Thank you sir!

 

Got this shot of NGC 2264 region with the 300 Nikon last night....

 

Steve

Attached Thumbnails

  • Autosave-2-23-20-300-2264proc-crop-very_small.jpg

  • Jim Waters likes this

#7 Tapio

Tapio

    Vanguard

  • -----
  • Posts: 2,431
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 24 February 2020 - 03:12 PM

Focus looks good. And overall too.



#8 BQ Octantis

BQ Octantis

    Vanguard

  • *****
  • Posts: 2,066
  • Joined: 29 Apr 2017
  • Loc: Red Centre, Oz

Posted 25 February 2020 - 05:32 AM

The Bahtinov mask doesn't lie. But if you must rely on chromatic aberration for focusing, here are the physics:

 

The index of refraction of a real material is wavelength dependent. So the photons going through it are slowed down by different amounts depending on their color (blue more than red), resulting in their beams being bent by different angles (think of white light split into its constituent colors through a prism). This makes the focus point of a lens wavelength dependent. For white light through a lens onto an RGB sensor, it looks like this:

 

Screen Shot 2020-02-25 at 7.24.05 PM.png

 

Since the sensor intersects the light cones on a single plane, the "focus" point is where the center wavelength—green—is brought to a point (technically, minimum spot size, defined by the diffraction limit); the blue light cone has gone past its focus point and expanded, while the red light cone has yet to reach its focus point.

 

For incoming light from a white star, it looks like this on the sensor:

 

halo.jpg

 

Whether or not this is detectable depends on several factors: the brightness of the target relative to its surroundings (high contrast is worse), the focal ratio of the lens (faster focal ratios have more CA), the size of the pixels relative to the size of the CA ring (from circles of confusion—e.g., a full-frame camera's larger pixels will be larger than the CA, whereas an APS-C sensor will show CA from the same lens), and the heroics of exotic glass type and lens combinations implemented by the manufacturer to reduce CA (basically, none, 2-color/achromatic, or 3 color/apochromatic).

 

If you're averse to using a Bahtinov mask, as an alternative to CA focusing I'd recommend stopping down to f/4 (or higher) and using the aperture preview button in 10x Live View on a nearby bright star to focus. That way, you can try to make the smallest point possible vice a magenta halo from a potentially non-white star. Try using some reading glasses (+1 to +2)—in low light, I've found this to help me see the tiny dot more clearly up close on the rear screen of my T3i.

 

BQ


Edited by BQ Octantis, 26 February 2020 - 03:49 AM.


#9 steven40

steven40

    Mariner 2

  • -----
  • topic starter
  • Posts: 244
  • Joined: 01 Feb 2012

Posted 25 February 2020 - 08:57 AM

Thanks BQ,

 

I use a Bahtinov mask for my refractors with great success but don't have one small enough for my 300mm F4 (approx 3 inch aperture); I'll look into that. And yes, at my age reading glasses are a forgone conclusion!

 

Can you please elaborate on this sentence?---

 

"That way, you can try to make the smallest point possible vice a magenta halo from a potentially non-white star."

 

Thanks again,

Steve



#10 BQ Octantis

BQ Octantis

    Vanguard

  • *****
  • Posts: 2,066
  • Joined: 29 Apr 2017
  • Loc: Red Centre, Oz

Posted 25 February 2020 - 04:15 PM

Thanks BQ,

 

I use a Bahtinov mask for my refractors with great success but don't have one small enough for my 300mm F4 (approx 3 inch aperture); I'll look into that. And yes, at my age reading glasses are a forgone conclusion!

 

Can you please elaborate on this sentence?---

 

"That way, you can try to make the smallest point possible vice a magenta halo from a potentially non-white star."

 

Thanks again,

Steve

No worries.

 

What I meant by the sentence was,

 

1. If the star is not white, then the optimum focus will have an imbalance of red to blue, so you'd have to target the right shade of magenta.

2. If you stop down to f/4 (or higher), you make the CA ring at optimum focus imperceptible even on a bright target—so you can just make a point vice a magenta donut.

 

Cheers,

 

BQ




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