Masked Jupiter

by Matt Tarlach

With all the recent hullabaloo on Shallow-Sky about various forms of aperture masks, I finally got myself down to Tap Plastics to pick up some materials and make some masks. I tried making an apodizer according to Steve Waldee's instructions but with fiberglass screen instead of metal, and it didn't work. The fiberglass strands (at least in the material I used) are too thick; the outermost portion of the screen was almost completely opaque. The fiberglass screen was also too flexible and difficult to fix in the right shape. I will try again with some metal screen, as the article specifies.

I did succeed in making a simple aperture mask, and the results were very positive. I had previously tried a 4" off-axis mask on my 12.5" f6 Dob-Newt, and was never pleased with the results. Brightness and resolution were too much reduced; under poor seeing the image was steadier with the mask but it was a case of "the baby with the bath water." This time I made an on-axis mask with an aperture of 8.75". With my 2.14" diagonal, this kept central obstruction around 25%, above which several authors agree that planetary contrast begins to fall off sharply.

I tried the 8.75" mask last night under poor seeing conditions (2 out of 5) and the effect on Jupiter was noticeably positive The view was steadier and easier to look at for an extended period, and despite the larger relative obstruction, contrast on Jovian features and the shadow of Europa seemed improved compared to the unmasked view. I tried some star tests and noticed that the smaller exit pupil (at the same x) reduced the astigmatism introduced by my own eye, which certainly contributed to the increased contrast on fine detail. The view at 315x was also slightly dimmed and more comfortable than the same mag in the unobstructed scope.

I don't think the gain in my case can be attributed to masking a turned edge. My optics (primary by Steve Swayze) star test very well at full aperture, not perfect but by comparison to the images in Suiter's book the system has a smooth figure with about 1/8 wave spherical abberation on the wavefront. Under ideal conditions the same scope has provided awesome full-aperture views of Jupiter and Saturn, at powers up to 630x. I also double checked collimation both before and during my testing with the mask.

Rather than masking defects in the telescope, I believe the main benefits of the mask were partial suppression of the distracting effects of seeing, and an effective shrinking of the exit pupil, which improved the performance of the observers's own eyeball. In the past I've achieved a smaller exit pupil by increasing power, but pushing a dob at 425x, under seeing conditions too poor to support that magnification, is a tedious chore at best. Last night the on-axis mask won a place on my accessory A-list.