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Posts Made By: Ron Oehlert

December 21, 2002 02:39 PM Forum: Telescope Making

REFRACTOR COMA

Posted By Ron Oehlert

Mayhaps a bit late, But check on the direction of lopsidedness as seen in the out of focus rings. If it is the same on both sides of focus the objective needs collimating. If the bunching flips 180* inside and outside of focus, the lens itself is to blame. Assuming 127mm aperature, this still should be a fine RFT. Ron

January 4, 2003 12:37 AM Forum: Equipment Talk

parks newtonians

Posted By Ron Oehlert

A long time ago, in our galaxy, Parks was a supplier of quality ATM parts, including their famed fiberglass tubes that they also supplied to Cave Optical (the best!). I purchased eyepieces from them and drooled for one of their tubes. When Cave folded (mostly due to it's owner's ill health) Parks took up the flame and marketed what appeared to be identical Astrola's; Parks mirrors held up as high quality. I would assume this remains so today, the scopes look the same. Ron

January 5, 2003 07:29 AM Forum: Equipment Talk

Star Test Question

Posted By Ron Oehlert

That could also indicate overcorrection. But remember that this test is extremely sensitive to any anomaly, and you may have a fine mirror. TDE also displays a fan of rays around bright objects in focus (kind of like you drew the Sun when in elementary school), more than the expected spikes from diagonal support. A ronchi grating inside of focus will show straight lines (3 or 4) with perfect optics when star testing a complete telescope, they will bow inwards with overcorrection, and have a hook with TDE. They bow outwards with undercorrection. Also give your mirror an indoors scutiny similar to a foucault test, except your eye becomes the source/knifedge. Merely place your eye ~twice the focal length from the telescope mirror looking down the tube (which has been tiped near horizontal). Use a broom handle or chairback (something) to steady your head/eye. Manuvering your view you will notice the mirror blacks out and displays a fine shadowgram. Use more than one procedure to assess your mirror's quality. Ron

January 8, 2003 09:37 PM Forum: Equipment Talk

Vigneting with 2X/3X barlow

Posted By Ron Oehlert

I use an older (much older; was known as #124 but is ~same as later #127) Meade 2X-3X variable Barlow (26mm clear aperture) used with 1.25" eyepieces and 6" f/8 newt. My 1.25" format 32mm Type I Konig w/27mm diameter field stop vignets with this Barlow. But my other eyepieces with field stop diameters of 23mm or less do not vignet when Barlowed. These eyepieces are 27mm Jaegers Kellner, 20mm Erfle, 12.5mm ortho; they provide a nice power range with a few duplications and always good eyerelief. Ron

January 11, 2003 12:52 AM Forum: Telescope Making

Barlow magnification

Posted By Ron Oehlert

It's the most accurate way. The actual multipling factor depends on the distance of the eyepiece's focal plane (it's field stop) from the Barlow lens itself. Different field stop positions with respect to the shoulder of the eyepiece will yield different multipling factors for given eyepieces. You can vary the "power" of any Barlow simply by changing this distance; with a single Barlow this can be accomplished by sliding the lens inside the tube- thus a variable Barlow. I use felt tube inserts to line the holder tube for baffling, a different one for different powers. Without the eyepiece in place, as for photography, the multiplication factor can also be varied by changing the distance to the film plane. Your 2X and 3X teleconverters could be so labled because they worked with fixed measurement distances with regards to the camera/camera lens. So by reasoning since the 2X teleconverter multiplys less at the same given distance than the 3X one it has a longer focal length, and it could also be used at 3X with a longer mounting tube. And the 3X in this same longer tube will multiply even more. Ron

January 11, 2003 01:54 AM Forum: Telescope Making

Barlow magnification

Posted By Ron Oehlert

One other note, if you cut a slit part way down one (or opposite) side(s) of your mounting tube at the eyepiece end, you can obtain a friction fit to accomodate eyepieces. Vertually all focusers, diagonals, and barlows in earlier days used such a method and I prefer this to set screws especially for gloved hands. Ron

January 11, 2003 06:42 AM Forum: Deep Sky Observing

Omega Centaurii from 38 north

Posted By Ron Oehlert

When the weather reports show a dry line thru your location and extending into neighboring states, such a night is in store. Telescopic views even well above the horizon can be extraordinary. Ron

January 15, 2003 06:00 PM Forum: Equipment Talk

Low power eyepieces for new scope

Posted By Ron Oehlert

Now that I know for sure what scope you are using these in, I suspect the vignetting I experienced in my 20mm is due to use in slower systems. Does "yours" appear to be in a common 25mm ortho body with labeling imprinted into the side? Circle V on barrel. Since my lens set is akin to a Goerz ortho instead of a classic Erfle, I assume "Erfle" is more a generic WF term in this case. Goerz' mid-1920's design used a meniscus field lens and was used in binoculars. So I assume the field lens here is better suited to a fast focal ratio whereas classic Erfle's do perform well with slower f ratios. In any event, my field stop reduction is reversible and I do enjoy the orthoscopic performance, with wider than normal AFOV. Ortho's and a 32mm type I Konig in the 1.25" version satisfy me also, and I remember well the complete lack of distortion in an old common 12mm K- it's only fault was ghosts. Scope is 6" f/8 Newt. Ron

February 7, 2003 08:41 PM Forum: Telescope Making

Looking for Tube End/Dust Cap

Posted By Ron Oehlert

I'm using plastic pail lids with a knob attached in the center (fender washers re-enforce this attachment point), they slip into the tube. Paint cans, dry-wall paste, etc come in various sizes and have flexible sealing edges. I got mine for free at a construction site and peeled the material residue off them. Also plastic tupperware style covers in kitchen supplies sections of dept. stores provide a snap-on fit if you find the correct size, I'm using one of these as well. Ron

February 18, 2003 06:18 PM Forum: Equipment Talk

PowerMate and sharpness

Posted By Ron Oehlert

If the Ultima doesn't work for you/your scope, I doubt the powermate will either. There is no reason a decent barlow per se should degrade the image. Have you compared an equivalent focal length eyepiece to the barlow/eyepiece combo in your scope or checked collimation? I routinely prefer a barlow combo to the equivalent eyepiece alone for highpower planetary scrutiny due to the increased comfort factor. I always used a barlow in my past ALPO research. Ron