Apodizing Screen Revisited

Herb Highstone requested that I post this item in the equipment forum and suggested that I am in line for some criticism (oh well, here goes).

Tonight (Jan 26)I had my best view of Saturn yet. Jet black Cassini ring with the outer ring clearly more transparent. The surface had multiple banding with subtle variations of pastel colors. My view was improved by use of an apodizing screen. It is possible that the effect was a reduction of light making the detection of detail easier. Viewing was excellent up to 286X. At 381X there was no improvement in detection of detail. My limiting magnitude was less than +4.5. I have ordered a polarizing filter to evaluate the effect of reduced light which may be the benefit of the screen. We will see, my jury is out, but the screen will see further use. As a footnote, I tried a 4-hole mask as an alternative to reducing light, but it did not have any positive effect on image quality that I could discern.

With what telescope?

Thanks.

Clear skies, Alan

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Judson Mitchell said:
>>
Herb Highstone requested that I post this item in the equipment forum and suggested that I am in line for some criticism (oh well, here goes).

Tonight (Jan 26)I had my best view of Saturn yet. ....
<<

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Hi Judson,

Sorry I can't offer any vigorous criticism, but I did make a short trip to the exhibits at the San Jose Photonics West show this week, and saw a vendor of metallic coatings offering "apodizing filters". So these types of things are commercially available for some applications, although these folks were not using wire screens, it was a metal film on glass. I didn't get a chance to find out the application, but in this case, it did not look like they were for astronomy. Maybe there is a more standard use for these ?

Keep us updated !

Hi!

I know this is a five-year-old thread, but I had to put in my two cents. I have experience with an apodizing mask, know that they can work, and I know why they work. I made one for my 10" f/4.8 Newt. The mask improves contrast on bright planets, allowing more detail to be seen. Also, detail that was seen without the mask is seen more easily with the mask. Apodizing masks improve contrast by reducing the brightness and thickness of the first two diffraction rings and increasing the size of the Airy disk. This is what "apodizing" means. (From the Greek "a" without + "pod" foot: "without a foot.")

An apodizing mask does not work by reducing the aperture, it does not work by reducing the glare (though that is helpful on the Moon), and it does not work by drawing the eye toward the center of the field of view. An apodizing mask works because it changes the diffraction properties of the telescope, thereby increasing contrast, which in turn allows more detail to be seen in the object.

By the way, the idea about charging your eyes with white light before you view planets is a good idea. It works, too! The white light tends to reset your eyes to photopic vision, how your eyes see during daylight. Our eyes can see more detail with more contrast and a much greater color recognition when they are photopically adapted. Mesopic or scotopic (twilight- or dark-adapted) eyes will see less detail, less contrast and less color on planets than photopic eyes. When I'm looking at Jupiter or another planet, I stare at the reflection on a white sheet of paper from a white flashlight every 10 minutes or so to try to maintain my photopic adaptation. It works, and I've explained here why it works. Apodizing masks can also work, if they are constructed and used correctly and with a little patience.

Clear Skies,
Mike