16 replies to this topic

[Michael Covington]

Has anyone done astrophotography with diffractive optics (Canon DO, Nikon PF) lenses?

 

[Jim Waters]

Yes - Canon DO lens.

 

http://www.dianedmil...Skies/i-bq244FV

http://www.dianedmil...tography/Skies/

 

Check out her other astro posts.  IMHO I find it hard to justify the cost of the Canon DO lens.  A well corrected fast 70 or 80mm ...etc triplet with a FR/FF or FF is much less.

[CodeBlind]

Those are some spectacular shots from Diane Miller.

 

I think if money were no object, that 400mm DO II lens is a no brainer (not so for the first gen lens - it is horrendously soft). You get a hand-holdable lens with stellar optics and autofocus for wildlife, and pretty amazing image quality. And aside from dealing with dust motes, you don't need flats because Canon's DPP4 software comes with light falloff correction profiles for this lens. It will save a ton of time, and doesn’t weigh a ton so you can get away with a smaller mount.

 

That said, you don't get a "does everything awesomely" lens for cheap. Between that lens and the 1DXII Ms. Miller is shooting through, you're out almost $12k. But, if you find yourself spending tons of time setting up mounts for moon shots or trying to get decent flats and can spare the cash, it very well may be worth it. That's my $0.02 anyway.

Edited by CodeBlind, 13 November 2017 - 08:35 PM.

[whwang]

I heard that DO lenses produce large halos on bright stars.  Diane Miller's pictures confirm this.

[Michael Covington]

I heard that DO lenses produce large halos on bright stars.  Diane Miller's pictures confirm this.

Indeed they do!  Good catch!

[Rudy Pohl]

I heard that DO lenses produce large halos on bright stars.  Diane Miller's pictures confirm this.

Yes indeed they sure do. My friend began astrophotography with a Star Adventurer and his Nikon 300/4 PF lens and was never able to avoid huge round flare halos around all his bright stars and the longer his exposures the more pronounced they appeared. No amount of tweaking in post processing was able to mitigate them. 

[Alen K]

The brighter stars in her photos also show a curious diffraction artifact: two dark triangular spikes on diametrically opposite sides of the star, cutting into the halo. I have seen these artifacts on other photos and always wondered about them. Perhaps they were also taken with a DO lens.

[whwang]

The brighter stars in her photos also show a curious diffraction artifact: two dark triangular spikes on diametrically opposite sides of the star, cutting into the halo. I have seen these artifacts on other photos and always wondered about them. Perhaps they were also taken with a DO lens.

 

Those have nothing to do with DO.  It's caused by diffraction and the change of the aperture shape at large off-axis angles.

[Alen K]

Those have nothing to do with DO.  It's caused by diffraction and the change of the aperture shape at large off-axis angles.

I knew it was caused by diffraction (hence, you know, me saying "diffraction artifact"). But I didn't know that would happen with off-axis rays. Strange then that some of the stars not too far from the center of her Pleiades photo are showing it. Perhaps the whole cluster was not centered and she cropped to make it so. Still, this particular artifact must be associated with something in the optical design other than the edges of the aperture blades themselves. I have never seen it in my own camera-lens astro images.

Edited by Alen K, 18 August 2018 - 07:28 AM.

[piaras]

Version 1 or current version, causes the Halos? The reviews show that version 2 has a better blacks due the reduction of light scattering. I wonder how it holds up in astronomy usage. It is still out of my price range!

Edited by piaras, 18 August 2018 - 09:43 AM.

[whwang]

Hi Alen,

 

It definitely has something to do with the lens design, particularly how the internal lens elements would occult the entrance pupil.  I only see such on telephoto lenses and astrographs, perhaps because the diffraction effect needs long focal length to magnify.  On the other hand, not every telephoto shows this.  So there is something still unclear to me.  What I am quite certain is that it has nothing to do with the aperture blades.  All examples I have seen are taken with the apertures wide-open.  So the aperture blades are not involved.

[whwang]

Heck, even TAK FSQ has the two dark diffraction spikes:

 

https://www.astrobin.../full/362738/0/

[Samir Kharusi]

There was some discussion about 20 years ago as regards why these triangular double spikes occur when using DSLRs. The wisdom in those days was that they come from the anti-blooming firmware used in DSLRs. They do not occur when using astro CCDs which have vastly more ugly blooming spikes. Of course any very bright star will show spikes of some kind no matter how close to perfectly circular the aperture is. A very, very tiny imperfection will diffract and when the contrast has been vastly increased, then that diffraction spike becomes visible. But these spikes are different from the (presumed) anti-blooming triangles.

[whwang]

They do not have anything to do with sensor.  The orientation of the dark spikes is different in different location, and is axisymmetric around the field center.  Sensors do not do that.

[sharkmelley]

Those have nothing to do with DO.  It's caused by diffraction and the change of the aperture shape at large off-axis angles.

Regarding the "dark wedges" that's exactly right.  It's called aperture vignetting and the effect can be easily simulated and explained using the Maskulator software (or similar) as I did here:

https://www.cloudyni...tars/?p=7388127

 

The direction of the wedges will have rotational symmetry around the image centre and will aligned with the tangential (i.e. not the radial) direction.

 

Mark

Edited by sharkmelley, 23 August 2018 - 05:31 AM.

[whwang]

What’s interesting here is that the aperture vignetting or occulting of entrance pupil happens on nearly all telephoto lenses, but not all of them show obvious dark wedges. I am guessing that the exact shape of the entrance pupil may play a role.

[sharkmelley]

The "almond" shape of the aperture occulting is also the cause of the "cat's eye bokeh" found near the edge of images.  It's often discussed by photographers.  So it would be interesting to know what the bokeh looks like for telephoto lenses that don't exhibit the dark wedges on stars. 

 

Of course, this type of aperture occulting only happens when the telephoto lens is used wide open.  If it is stopped down (which is probably normal for astrophotography) then it is the shape of the blades of the iris that will dominate the diffraction pattern of stars.

 

Mark