Sorry, I mis-spoke. Resolution is proportional to the inverse of the aperture, not the inverse of the aperture cubed. From: Brent Watson <brentjwatson@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, July 10, 2013 3:39 PM Subject: Re: [Utah-astronomy] Off-axis Aperture mask Debbie, As Chuck stated previously, the secondary obstruction causes light to be moved from the Airy disk to the difraction rings. What is not stated is that as this light is removed from the Airy disk, it actually makes the Airy disk appear smaller than it would otherwise be. This means that the Airy disk from an obstructed telescope is smaller that that of an unobstructed telescope. This can actually help in splitting double stars, particularly when the second star does not coincide with the diffraction rings of the first star, as is the case on many tight doubles. How big is that Airy disk? Dawe's limit can give you an approximation. If you take the aperture of the telescope and divide by 4.56, you will have a good idea of the size in arc seconds of the Airy disk radius for an unobstructed telescope. The obstruction will shrink this size a bit. The real equation states that the resolution of a telescope is proportional the the inverse cube of the aperture. The above relationship (Dawe's limit) just happens to approximate it pretty well for our size optical systems. As you can see, the larger aperture obstructed telescope has two things working in its favor. The first is the smaller Airy disk from the increased aperture, and the second is the reduction in size of the Airy disk because of difration from the obstruction. So, why use an off axis aperture stop? The reason is that it is much more difficult to get the light path through the atmosphere to be steady for a larger aperture instrument. I am sure you have noticed that the number of nights you have difraction limited seeing with your dob is far less than the number of nights you have difraction limited seeing with your refractor. I can not authoritatively give you a number, but it seems like the number of nights I had good seeing with my 22 inch was about equal to the inverse cube of the ratio of the diameters betwen my dob and my refractor. (1/((22/8)^3) , or about a factor of 1/22. That is a rough estimate, and is only a SWAG. The off axis aperture stop will make it so your dob's aperture only six inches, and so you will have good seeing through the atmosphere more often. As you can see, it is really a situation where you are managing the light path through the atmosphere. Your dob should produce good double star images on those nights when you have equivalent seeing, but those nights will be much more rare. Just my opinions and observations, FWIW.