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Posts Made By: Dan D DuBal

November 8, 2005 08:01 PM Forum: Eyepieces

Re: Safety undercuts

Posted By Dan D DuBal

I like the way you think, David. wink 8)

Cheers & best wishes.
-Dan

December 22, 2012 07:55 AM Forum: Solar System Observing

Daylight Jupiter Observation With Unaided Eye

Posted By Dan D DuBal

Hi, Darian.

Years ago, at a star party in Wyoming, I was able to "follow" (check back and confirm view from time to time) Jupiter until mid morning - perhaps as late as 10:30am, before deciding I needed more sleep. This was done at high altitude (about 9000 ft.), and under excellent sky transparency, so I'm not sure if that qualifies as "normal daytime brightness." wink If I recall correctly, the view was *not* easy. Venus was a morning star, too, and was relatively (vs. Jupiter) "easy" to find past noon.

Cheers & best wishes.
-Dan

March 22, 2014 10:49 AM Forum: Refractors

Planets through an Astro-Tech 72mm scope

Posted By Dan D DuBal

Hi, Paul.

The biggest factor in seeing planetary detail through small apertures is practice/experience. In my own 60-75mm refractors, Mars *can* show some detail, including polar cap(s) and the larger, more-contrasty features (Syrtis Major, et.al, and even polar "melt lines"). I could re-hash my own experiences viewing Mars in a good 60mm achromat and a 75mm apochromat, but it'd be easier to simply direct you to three old posts of mine (ca. 2003). If you use the Search Forums feature and input "dan d dubal" and a date range of 7/6/2003 to 7/7/2003, you'll come across my three-part "Huygens Challenge" babble. Perhaps it'll help with some perspective.

-And, yes, I still have the Jason. smile

Best wishes and regards.
-Dan

June 9, 2014 07:49 AM Forum: Refractors

AstroTech AT72 drawtube slipping

Posted By Dan D DuBal

Hi, Paul.

My understanding (certainly not guaranteed) is that the knob shown in the photo is for focus lock/unlock. There's a mechanism for focus tension adjustment via the hole "behind" that knob (above it, in your pic) - along the focuser shaft axis, in the "middle" of those four screw heads. I believe there's an allen screw in that recess. You might try the following:
1) back off the focus lock knob (make sure it's unlocked)
2) tighten the allen screw (tension adjuster) - just a smidge - checking & maintaining the focuser's ease & "feel" while you do so. (If it's too tight, you'll know.) I also recommend keeping your allen key engaged with the allen screw and noting its position (ala clock hand) relative to the scope/focus assembly, *before* you do any tightening, just in case you want to undo any adjustments you try.

The tension adjuster and focus lock do work together, so that may be all you need to bolster the focuser's capacity.

Take care, and best wishes.
-Dan

June 13, 2014 10:09 AM Forum: Off Topic Discussions

Photos For Ads Question

Posted By Dan D DuBal

Hi, Darian.

I have a "default" mode, when I take photos for ads. I simply set the camera's resolution to "VGA" (640 x 480). Thereafter, uploading/inserting to ads is fairly straightforward and "oops" free.

Best wishes.
-Dan

June 19, 2014 10:03 AM Forum: Refractors

Vintage 90mm f=1000mm Orion Skyview achromat OTA

Posted By Dan D DuBal

Hello, Saied.

A triangular fresnel pattern may indeed indicate pinch astigmatism in the system. If you've confirmed there are no light path intrusions (screws, et.al) causing diffraction, then pinch astigmatism is likely. In that case, I would expect one of the objective elements to be pinched.

Have you confirmed that the objective is the source? If you can, try rotating the cell assembly while viewing the fresnel pattern (defocussed star test). An extra pair of hands (another person) will make this easier. If the triangular pattern rotates with the objective, then you know the objective is the source of astigmatism.

It's possible that the diagonal is pinched. Astigmatic diagonals usually display an elliptical pattern of astigmatism, but a three-point pinch is still possible. Try another diagonal, or try without the diagonal.

I also recommend backing off the retaining ring to allow for the slightest rattle, then leave the ring loosened (do not snug it back against the objective). Once done, check your fresnel pattern again. If you're still seeing pinch astigmatism, perhaps the front element remains pinched - even if the rear element is not - perhaps due to a lodged spacer (not likely) or a second retaining ring (even less likely), or due to the element's fit within the objective cell (maybe lodged/pinched). If you must, back off the retaining ring a good bit more to check for "breathing room" for both elements. Hold the cell assembly "face down," lower it to a "pedestal" topped with sterile cotton or white unscented facial tissue (something scratch-free) and make sure the crown-spacer-flint stack does indeed elevate within the cell. If you confirm neither element is pinched, re-set the retaining ring (again, allow for the slightest movement/rattle).

If none of the above works, let us know.

Best wishes and kind regards.
-Dan