Kevin_A

OK, here is a detailed, and more accurate measurement.

The measurement set up looks like this:

The Rokinon 135mm/T2.2 is at the far end, and we are looking at a Canon EOS flange. A ruler is placed at the focal plane of the lens (44mm from the lens flange).

At the near end, there is a ASI294MM camera with a Fujinon low distortion 16mm lens (HF-16HA). with a ring light around it (to illuminate the ruler).

The camera is mounted on ARCA dovetail plates (ring light not shown) to allow us to center the camera onto the optical axis of the lens.

This is what a centered aperture looks like from the Rokinon lens. Notice the red crosshair lines with the vertical line coinciding with the "60mm" marking on the ruler. The Rokinon lens has the lens hood mounted (not an issue, but I will mention it), and the T-stop set to 4.0 (close to f/4.0 on a non-cine lens).

The ASI294MM camera is now moved sideways in this next image, with the crosshair placed at 60mm + 22mm. The 22mm offset at the focal plane corresponds to a 44mm diagonal of a full framed camera.

Notice that the shape of the bright aperture remains the same 9-bladed iris, undistorted. There is no aperture vignetting, even at the corner of a full frame camera.

However, star images will show quite pronounced radial spikes (9 spikes + 9 more spikes flipped, for a total of 18 spikes).

The lens is next set to its full opening (T/2.2), and a circular aperture mask is placed in front of the lens. The mask is made up of a 77mm-55mm step-down ring, followed by a 55mm-37mm step-down, to create a composite 77mm-37mm step-down ring. A 37mm Tiffen UV filter, with its UV glass removed is place on the step-down ring, creating a clean circular aperture of 33.7mm, equivalent to an f/4.0 aperture of a 135mm lens.

Notice the aperture is completely round, instead of looking like a regular 9-sided polygon. This should produce stars that are void of radial diffraction spikes, at least for a star near the optical axis.

Next, we move the camera so that it is centered at the "70mm" mark on the ruler. A star that is 10mm away from the optical axis, will be seeing this following aperture:

Notice that the aperture is still completely circular, but simply displaced. The star will suffer from other lens aberrations, but it is no aperture vignetting.

We now place the crosshair at 14mm -- this corresponds to a star that is 14mm away from the optical axis on the focal plane, i.e., a star that is at a corner of a APS-C frame that has 28mm diagonal.

Notice that there is some slight non-circular irregularity on the left side of the circular disc.

A star at the corner of an APS-C frame will suffer from a slight (very slight) amount of aperture vignetting. This will create two notches in the PSF around a bright star, as discussed here:

https://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/

We now move to the corner of a full frame camera with 44mm diagonals (i.e., 22mm away from the optical axis):

There is now severe aperture vignetting.

What this implies is that a 33.7mm circular mask that is placed just in from of the Rokinon lens will probably be usable for APS-C cameras, but bright stars towards the edges of a full frame camera will show the diffraction notches.

Interestingly, a larger mask will show less aperture vignetting! This is an aperture that is made from a Tiffen 55mm filter (with its glass removed), from the same 22mm away:

The Tiffen 52mm filter with the glass removed has a clear aperture of 49.7mm. Producing a f/2.8 aperture for the 135mm focal length.

The above appears to imply that for APS-C camera can at most be masked to an f/4.0 on this lens (and there may be a slight, perhaps not really visible except on the brightest stars, aperture vignetting at the far corners). But when masked to look like an f/2.7, the stars are void of aperture vignetting all the way to the corners of a full frame camera.

The next image is from a 46mm UV filter with the glass removed, producing an f/3.3. Offset of 22mm (corner of full frame camera), so it may be possible to mask the lens down to a smaller f/3.3, and not suffer aperture vignettng even on a full frame camera.

Chen

Kevin_A Here is a comparison of 2 hours at HGC vs 3 hours with reducer at Gain 198.

No contest, IMHO. But you also spent more time, and the system already collected more photons per pixel with the reducer.

You might be able to get rid of the CA by using something like one of the Astronomik L-1, L-2, or more extreme, the L-3. One of those should eventually be enough to rid of the purple fringe. IMHO, the L-3 cuts too much.

Check out the blue channel response of the ASI2600MC -- it has only dropped to 50% of its own max by 400nm. The L-2, for example, will knock out everything below 380nm from entering the camera, and "pretending" to be "blue" when it is more like UV already.

Because of the camera's spectral response, you really need at least a UV-IR cut filter, even when you are in Bortle 1. A lot of scopes bloat the stars in UV, which then looks like blue because the blue channel of the camera is sensitive to it. Don't trust the UV-IR cut window of the 2600MC.

In fact, I plan on using the L-1 as the UV-IR cut "prefilter" which I described earlier.

Chen

Kevin_A Vignetting and a weird centre spot.

I used a pixel peeper and it looks like the bright ring around the center is not really brighter than the center (or barely brighter). All measured 125 or so on 8 bit scale, and falling as it gets farther fom the center. The human eye is playing tricks :-).

This is intensity vignetting, though, and has nothing to do with the diffraction pattern (aperture vignetting; i.e., non-symmetrical illumination). Different animals. Given just this image, you cound get, or not get, diffraction spikes. Only a star test will tell us if it is clean of diffractions.

BTW, re that Horsehead taken with the IDAS NBZ. IDAS has reissued an NBZ II, and I have one on the way. Should work well with fast lenses. The NBZ II is narrower, 10nm H-alpha, 8 nm OIII, instead of the original with 12nm at both wavelengths.

Chen

Kevin_A I will test it out but compared to the prior results the vignette is much softer and not as abrupt.

Yeah, you cannot tell from taking flat frames, soft or not. Diffraction spikes appear when the aperture itself is not perfectly circular (happens to both the iris blades and with the aperture vignetting).

If you want to see what diffraction looks like, you can take a Fourier Transform of the images that I measured (the bright central part), and you should be able to see the diffraction spikes . Same as Bahtinov Mask -- take a Fourier Transform of the Bahtinov mask, and you see those 6 spikes. The Fourier Transform of a perfect circle is a J1 Bessel function, and that is the familiar "Airy pattern." You need real math for this. (Out of focus diffraction is actually even more interesting -- you need to multiply the Fourier kernel (i.e., exp(-ift))by a quadratic term in x, y (where x is radial distance and y is the distance along the optical axis. Pretty deep math, but I found a Purdue class lecture that talks about this, if you are interested to discover the secrets of diffraction patterns (Maskulator does not implement out of focus diffraction pattern, by the way).

Chen

Kevin_A Let me know how yours turns out and I will post when the skies clear!

Not a chance my skies will clear before yours :-). (Yeah, it is raining at the moment.)

I don't take the covers off my mount when there is a chance of rain, :-).

By the way, I have an NBZ-II coming my way next week. I also have a 365nm LED flashlight that I can subjectively test to see how good various UV-IR blocks are. When the NBZ-II appears, I will parade my dozen or so filters that claim to block UV.

Not a great way to measure (the center wavelength of that LED can be 10nm off). I don't have a good white light source for my spectrometer, unfortunately; and most cheap (less than $20,000) spectrometers cannot measure the blocking aspect to the dynamic range that just measuring the max ADU of a frame with a flashlight in it, anyway).

But at least I will be able to tell what blocking filters actually blocks UV, and which are bogus. A quick test in the past has shown the ZWO UV-IR block to be placebo - caveat emptor; but I should be able to compare it with other filters next week. I had kept it around just for such an occasion.

UV will excite the blue channel of a sensor, and will look blue in an image, even though it is way past violet. Without a controlled experiment, it is not easy for an unsuspecting customer to buy a UV-IR cut that is placebo (well, not placebo in terms of losing money). 365 nm should be past the point where a decent UV cut filter should work, so any significant energy that is passed to the camera sensor at 365 nm will indicate a moderately poor UV-cut filter (thus giving blue fringes around stars from an OTA that whose spot diagram bloats in the blue side of the spectrum).

This collection of spot diagrams is from a Cloudy Night posting. Notice the blue bloat from an Askar even at the optical axis (0.00mm). The spot diagrams may not be scaled properly, but you can see the blue bloat clearly on the Askar.

We'll see how my simple tests will turn out. I bought the flashlight a couple of years ago to use for bonding the UV-sensitive epoxy (like the kind used by dentists today). Have to find my UV blocking goggles too :-).

BTW, ZWO swears up and down that their color ASI2600MC uses an UV-IR cut window (see the last few posts in the following foum thread - some very recent). I happen to also have a pristine (never opened) ZWO D60 IR-CUT filter for just such an occasion. We'll see if the 60mm glass window is as poor as ZWO's 2" mounted filter. If it is poor, it is time we expose some of the marketing nonsense. The curve is from a Agena page -- notice that they claim a brickwall at 400 nm, we'll see if even a cheap Optolong UV-IR cut beats it.

https://bbs.zwoastro.com/d/13239-asi2600mc-protective-window/5

They also do have an AR version of the 60mm, and I might have installed it in my ASI2600MC. So, using an external D60 glass is the only sure way to measure the window.

Chen