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. On Wed, Jul 10, 2013 at 11:44 AM, Chuck Hards <chuck.hards@gmail.com> wrote:
Debbie, any central obstruction will increase diffration. Energy is robbed from the central portion of the Airy Disk and redistributed into the rings. Most of it in the first ring, less in succeeding rings. The larger the central obstruction, the more diffraction. This translates into "bloated" star images in the eyepiece. Refractors have no obstruction. Achromatic refractors send some uncorrected color into the rings, but this is generally less than what a central obstruction does. APOs are the best, with their nearly perfect color correction.
Spiders will also add diffraction, but double-star observers can sometimes use this to advantage by rotating the tube or secondary cage such that companion stars are between diffraction spikes. The first ring in the Airy Disk is still bloated from the central obstruction, however.
Smaller apertures are generally less affected by poor seeing, but this is somewhat of an optical illusion. The larger the aperture, the smaller the Airy Disk.
Using an off-aperture mask effectively turns your large Newtonian into an unobstructed off-axis reflector. This eliminates diffraction effects from the central obstruction and spider. Be sure to locate the mask between the spider vanes. Sometimes, it does help, on some objects, but then you are losing the resolution of the larger aperture. So you see, it's a trade-off.
Go ahead and try it if you are curious. You have a TeleVue refractor also, don't you? You might want to compare the reflector stopped-down to the same aperture as the refractor, and compare the view in both scopes. Adjust the magnification on the refractor with a good Barlow so it's effective focal length is about the same as the reflector.
On Wed, Jul 10, 2013 at 9:16 AM, Debbie <astrodeb@beyondbb.com> wrote:
Anyone used an off-axis aperture mask for double stars? Here is a quote from CN forums. I'm thinking a 6 inch off-axis mask would be perfect to try some good doubles.
Debbie
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