Aperture Mask Question??

I had a question concerning aperture masks. I like viewing double stars occasionally and really love the views I get from my refractors, but there are times when I only set up my 10” dobsonian reflector. I understand that larger aperture resolves more, but when it comes to the aesthetics of double star observing, I prefer the unobstructed view through my refractors. Just yesterday, I made an aperture mask out of cardboard for my reflector. I cut out (4) 3” diameter holes. My test subject was the double double in Lyra. Without the mask, I could easily spit the double double. The split was clean, but the stars were bright tight fuzz balls that sparkled with the turbulence. The cleanliness of the split would appear and disappear. After adding the mask, there was an improvement, but the stars still looked like tight fuzz balls. At this point, I closed 3 of the 4 holes. Wow! I thought I was looking through one of my refractors. The double double appeared as little round dots. I could even discern the split as lower powers. Vega at higher powers appeared as a round dot with the airy disk circles around it. Why did the 4 hole mask not work as good as the 1 hole mask? Do the 4 holes have to be perfectly dimensioned with respect to each other and mounted perfectly? I plan to make a second mask tonight with only one hole, but make that 1 hole as large as possible without exposing the spider vanes. Thanks in advance for your replies.

Travis

With one hole you've created a smaller, unobstructed aperture. With four holes you've simply modified your full aperture, adding a large diffraction producing obstruction in the incoming light path. Essentially you have four very large and oddly shaped spider vanes. You've also eliminated you nice round entrance pupil.

Clear skies, Alan

Travis:

Depending on the size of your secondary, you might be able to sneak by with a 3.5" hole.

Jeff

Most of the improvement comes from the reduction in seeing error. It changes with (D1/D2)^(5/6), which means that 3" aperture has 2.7 times smaller average error than 10". That is like difference between 1/2 and 1/5.4 wave PV of spherical aberration. Very obvious. In effect, by reducing the aperture you switched from poor to very-good seeing.

Vla