· Get a Cloudy Nights T-Shirt · Submit a Review / Article    Click here if you are having trouble logging into the forums Cloudy Nights - Official Forums of the 2009 International Year of Astronomy Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User   You are not logged in. [Login] Forum Archives · Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar Equipment Discussions >> Binoculars Previous   Index   Next   Threaded   Pages: 1 GlennLeDrew Pooh-Bah Reged: 06/18/08 Posts: 1267 Loc: Ottawa, Ontario, Canada Tasco extra wide angle 7X50 tidbits       #3398808 - 10/19/09 07:38 PM Edit   Reply   Quote Quick Reply   I have a dismembered Tasco 7X50 which boasts a 578' @ 1,000 yd FOV (11 degrees). This is one of those Japanese classics from the 70's. Here are a couple of details: - The prisms have front and rear clear apertures of 27mm. These are the largest 'standard' Porros I've encountered. The coatings are a thin, yellowish-brown tint. - The prism aperture which is shared by both Porros, i.e., the middle one of the three, is smaller, it having an estimated 19mm diameter. It appears to be sized so as to just cleanly pass the on-axis light cone. This is to better intercept stray light and help supress off-axis aberrations/defocus, and it *possibly* helps to reduce ghost images of out-of-field sources (such as Mr. Bill's Fujinon 10X50s create). - The objective focal length is 165mm, the f/ratio being f/3.3. Interestingly, this is identical to the Bushnell Xtra-Wide objectives I currently use in my home-made Mk II. But whereas the Bushnell was originally spherical (sort of!), the Tasco is clearly not. Indeed, instead of the expected undercorrected spherical aberration, I see *overcorrected* SA. This is a clear sign that either by accident or design these objectives have been aspherized. - The larger-than-expected field diameter is afforded by the use of a weak focal reducer. This single-element meniscus is mounted right to the prism shelf at the rear aperture. (In another variant of this same bino, the focal reducer is attached, via a barrel extension tube, to the eyepiece.) Without the focal reducer, the true field would be 10 degrees (obj. f.l. = 165mm, field stop diam. = 29mm.) So we see that the reducer has an effect of about 10% in increasing the field to 11 degrees. (And this is the same ratio deliverd by the reducer in the Bushnell Xtra-Wide and following versions.) - The eyepiece field stop diameter is 29mm, two millimeters larger than the prism aperture. The edge-of-field vignetting is due more to the eyepiece design than the slightly large field stop. -------------------- Home-made 11X50 right angle bino, 8.1 deg. FOV Modified 26X100 bino, 3.5 deg. FOV Home-made Mk II RA bino, using interchangeable objectives and eyepieces My Gallery Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas. Post Extras:       GlennLeDrew Pooh-Bah Reged: 06/18/08 Posts: 1267 Loc: Ottawa, Ontario, Canada Re: Tasco extra wide angle 7X50 tidbits [Re: GlennLeDrew]       #3398941 - 10/19/09 08:48 PM Edit   Reply   Quote Quick Reply   I should add that when I use the term 'standard' Porro, I mean an assembly which has both prisms of the pair identical. This is to differentiate from the much less common form where the initial prism catching the light from the objective has a tapered or stepped planform, being wider at the first aperture. This is to utilize the full objective aperture when its f/ratio is short and/or the rear prism aperture-to-field stop distance is not small enough. An example of a tapered prism is to be found in the Zeiss 8X50 Nobilem. My Celestron 25X100 has stepped prisms. -------------------- Home-made 11X50 right angle bino, 8.1 deg. FOV Modified 26X100 bino, 3.5 deg. FOV Home-made Mk II RA bino, using interchangeable objectives and eyepieces My Gallery Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas. Post Extras:       pcad Carpal Tunnel Reged: 01/17/05 Posts: 1767 Loc: Connecticut Re: Tasco extra wide angle 7X50 tidbits [Re: GlennLeDrew]       #3398972 - 10/19/09 09:07 PM Edit   Reply   Quote Quick Reply   The M19 also has graduated prisms. The objectives are f/3. -------------------- Peter Telescopes 25 - 318 mm Binoculars 15 - 88 mm Microscope 50x - 1000x Post Extras:       Simon S sage Reged: 01/07/07 Posts: 386 Loc: Crawley West Sussex UK Re: Tasco extra wide angle 7X50 tidbits [Re: pcad]       #3400543 - 10/20/09 04:24 PM Edit   Reply   Quote Quick Reply   Do you have photographs Glenn? -------------------- My binocular collection recent first http://www.flickr.com/photos/binoculars/ My binoculars in Alphabetical order http://www.flickr.com/photos/binoculars/sets/72157613812824211/detail/ Post Extras:       GlennLeDrew Pooh-Bah Reged: 06/18/08 Posts: 1267 Loc: Ottawa, Ontario, Canada Re: Tasco extra wide angle 7X50 tidbits [Re: Simon S]       #3400821 - 10/20/09 06:46 PM Edit   Reply   Quote Quick Reply   Well, Simon, I suppose I could snap a couple and add them to my gallery. I didn't think to before because I'd thought that the descriptions of what are basically reasonably well known components would tell the story. But if and when I do I'll let you know.... -------------------- Home-made 11X50 right angle bino, 8.1 deg. FOV Modified 26X100 bino, 3.5 deg. FOV Home-made Mk II RA bino, using interchangeable objectives and eyepieces My Gallery Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas. Post Extras:       Simon S sage Reged: 01/07/07 Posts: 386 Loc: Crawley West Sussex UK Re: Tasco extra wide angle 7X50 tidbits [Re: GlennLeDrew]       #3402187 - 10/21/09 01:24 PM Edit   Reply   Quote Quick Reply   Thanks Glenn... -------------------- My binocular collection recent first http://www.flickr.com/photos/binoculars/ My binoculars in Alphabetical order http://www.flickr.com/photos/binoculars/sets/72157613812824211/detail/ Post Extras:       Pages: 1 Previous   Index   Next   Threaded   Extra information 8 registered and 8 anonymous users are browsing this forum. 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