I have such a mask for my 16" f/6.25 dob -- but have yet to try it out. In part because I've mostly neglected double stars (and the moon) in my observing sessions. I really need to change that... ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, July 10, 2013 11:44 AM Subject: Re: [Utah-astronomy] Off-axis Aperture mask 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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