MagAO pupils and Fourier optics

Today we are going to explore the MagAO pupils and their corresponding transforms in the image plane, courtesy of Fourier optics.

So let’s have a look at the pupil.  Here is a photo of the ASM, taken with a digital camera.  This was from before Clio was mounted, so that we just stood on the Nasmyth platform and put the digital camera where Clio is now.  The light source is the sky, and the light path is primary + secondary + tertiary.

ASM image, from before Clio was mounted. You can see bird poop on the tertiary, a splotch at about 11:00 in this image, and the lollipop-shaped "slot" is at 9:00.

The main features of the pupil are the outer diameter of the mirror, the inner diameter of the secondary obscuration, the support spiders holding up the secondary, and the slot.  Here, then, is the pupil mask:

Pupil mask

Since we know what the pupil looks like, we can create simulated images of the focal plane by taking the Fourier transform modulus squared:

What the PSF will look like in the image plane (log scale)

If we really stretch the color table, you can see the diffraction off the spiders, but it is not a big effect.  Also, I couldn’t find the diffraction off the slot, so it is negligible:

Stretching the color table to saturate the inner part of the PSF allows you to see the diffraction spikes off the spiders, but they are very faint

Now, Clio is an infrared camera, going out to 5 um, and so it has its own pupil mask, a cold stop.  So let’s look at the pupil through Clio, by taking a pupil image (which we did after Clio was mounted).  Here is an image of the pupil plane through the whole system, taken with Clio by putting in a powered lens to the focal plane to make a pupil image:

Clio pupil image, 3.4 um

It’s pretty cool because you can see the 2 spiders holding up the secondary obscuration on the cold stop, but you can also see the 4 telescope spiders and the ASM slot!  Here’s just the Clio cold stop pupil mask:

Pupil mask - Clio cold stop

And here is its Fourier-transform-modulus-squared: The simulated PSF:

Simulated PSF for Clio cold stop (log scale). Diffraction off the spiders is a little bit visible here, since they are slightly wider than the telescope spiders.

And here is the zoomed-out, saturated version so that you can better see the diffraction spikes:

Clio cold stop PSF -- scaled to bring out the diffraction spikes.


Top: ASM slot + telescope spiders pupil image and mask. Bottom: Clio cold stop image + pupil mask.

Top: ASM slot + telescope spiders PSF. Bottom: Clio cold stop PSF.

The two simulated PSFs look very similar, and diffraction off the spiders and slot has a very minimal effect compared to the Airy rings.

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