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Posts Made By: Dan D DuBal

July 6, 2003 11:15 PM Forum: Solar System Observing

Huygens Challenge Revisited (pt. 1)

Posted By Dan D DuBal

Inspired by Ron B[ee]’s “Huygens’ Challenge,” & being a devout goof when it comes to pushing the limits of small-aperture scopes, I decided to point my early-60s vintage Jason (Tasco) Explorer 60mm refractor at Mars & give a good look-see at the Red Planet.

I juggled three different apertures with the Explorer: full aperture, of course, plus an effective 40mm, provided by the objective cap’s aperture mask. I also borrowed my Pentax 75’s objective cap whose mask yields an effective aperture of 50mm. With the scope’s inherent 800mm focal length (f/13.3), the 40mm & 50mm masks yield effective focal ratios of f/20 & f/16 respectively.

Stock eyepieces included two “Huygens” (20mm & 12.5mm) & a 4mm “Symmetrical Ramsden.” Believe it or not, the Ramsden (though certainly not symmetrical) is actually a decent eyepiece. Its housing has a telltale “circle-T” logo; maybe that’s a factor. None of the stocks is a “true” representative of its parent design. The Ramsden’s eye relief is far better than it should be, & both Huygens employ field stops which restrict their fields (thus increasing eye relief). However, other aspects of their designs seem “correct” (elements’ spacing & relative focal lengths, position of image plane, etc.).

In addition to the stock eyepieces, I fed the Explorer three others from my bonus menu: a Celestron 18mm Kellner, Takahashi 7.5mm LE (1.25-inch barrel/sleeve removed), & Pentax 6mm Abbe. With this group of eyepieces, the Explorer yields magnifications of 40, 44, 64, 107, 133, & 200.

Well, let’s dive right in. How did this vintage “trash” scope & its stock “garbage” eyepieces perform?
(Continued...)

July 11, 2003 08:54 AM Forum: Equipment Talk

Re: Odd star test

Posted By Dan D DuBal

Hi, Ed.
Your description -- center bright vs. center dark (on either side of focus) -- indicates either over- or undercorrection (depending on which side of focus showed bright & which side showed dark).

How many fresnel rings did you see when defocused; do you recall? And what magnification did you use? Prism diagonal or mirror? While we can't estimate the degree of spherical aberration based on those parameters alone, it may provide some insight as to the "relative" correction of the scope.

I'm going on very little, obviously, but I'm an optimist: assuming you *are* seeing spherical aberration, I suspect your refractor is still doing fine. A typical good-quality refractor sample of recent/current vintage (i.e. ~1990+) will likely display s.a. in the realm of ~1/5 wavelength. This is still quite good. And at that level, the bright center/dark center difference can be easily visible.

Couple other aspects of the fresnel pattern you might look for &/or ask yourself:
-Are the rings uniform and evenly illuminated?
-Any ring(s) stand out as being odd or "out of place" in terms of brightness (relative to a neighboring ring)?

And the IN-focus diffraction image:
-How many diffraction rings visible -- one? maybe two? lots?
-How bright is the airy disc, relative to the first diffraction ring -- *much* brighter? about the same?

You're welcome to keep picking our brains, and I'll be happy to relay any hunches I may have (based on your descriptions).

Best wishes.
-Dan


July 17, 2003 05:49 PM Forum: Maksutovs

Re: TEC 6"

Posted By Dan D DuBal

I, myself, would not spend the extra $$ for quartz. (Incidentally, this would be a quartz *mirror* option -- not just a coating.) These are not large-aperture mirrors, so the thermal advantage of a six-inch quartz primary (over Pyrex) is minor. When ambient temperature is fairly stable (or not changing quickly), and the scope is at or near that ambient temperature, there will be no visible difference between views delivered by equivalently superb quartz-mirror and pyrex-mirror TEC 6s. And "equivalently superb" is exactly what TEC will deliver.

Others would gladly pay the extra $$ for the quartz option.

Don't hesitate to present any questions you have to the crew at TEC. Ed and Yuri are highly respected, honest, sincere, and personable.

Best wishes and luck, by the way. The TEC6 is a sweet scope, indeed. I've had the pleasure of looking through two of them. If I didn't already have an MK67, I'd probably have a TEC6. Perhaps I'll end up with a bigger TEC...

Cheers.
-Dan

July 17, 2003 06:04 PM Forum: Star Parties

Nebraska ponderings

Posted By Dan D DuBal

To those fine Astromarteers who will be attending the NSP:

Yes, NSP is dark-sky country, and much of your valuable cargo space will be gobbled up by larger-aperture scopes and their accessories (after family members, of course). However... might any of you be bringing along a 76- or 78mm Takahashi? I never leave my Pentax 75 behind, so I'm curious about the possibility of slipping behind the wheel of an FC, FCT, or FS. I've yet to have the pleasure of viewing through one.

Cheers and best wishes.
-Dan

August 2, 2003 06:38 PM Forum: Deep Sky Observing

Small aperture -- Veil nebula

Posted By Dan D DuBal

Yet more proof that NGC 6992 is a fairly easy target, given favorable conditions (i.e. L.M. ~6.5 or better, plus reasonable dark adaptation)...

I'd previously seen the eastern arc of the Veil in a pair of Canon 8x32 roof-prism binoculars -- this was at 9000-ft altitude under a ~6.5-mag. Wyoming sky in 2001. By no means was the arc obvious or easily seen; my familiarity with the field certainly helped (I knew where to look). I got the impression, at the time, that an even-smaller binocular *would* indeed show the arc.

This past week, at the Nebraska Star Party, I tried the same experiment with another small binocular. Conditions were very similar -- ~6.5-mag. sky, good dark adaptation. This time, I used a pair of 8x25 Nikon roofs. Again, the view was modest -- a rich field of countless stars, though not very bright or dazzling. I chuckled after noting that, yes, the little Nikons did indeed manage NGC 6992 -- dim as it was. Though the nebula was very subtle, I wouldn't say it was at the threshold of visibility. I think there's more room before "threshold" occurs. If I had to ballpark, I'd think the eastern arc of the Veil *should* be visible in binoculars with sub-20mm apertures.

Next time I experiment, I'll try aperture masks on a few of my binoculars and see just "how low" I can go.

Cheers.
Dan

October 25, 2003 10:26 PM Forum: Off Topic Discussions

The Marlins did it again...

Posted By Dan D DuBal

Congrats to those spear-snouted fish from Miami.

I've always pulled for the underdogs, regardless of the sport venue. That's likely why I'm not rabid about a single favorite team. It's more fun that way (at least for me!).

Cheers, everyone.
-Dan

December 8, 2003 09:14 AM Forum: Celestron

Re: How do you say Celestron?

Posted By Dan D DuBal

...as in "celestial."

Best wishes.
-Dan

January 30, 2004 11:27 PM Forum: Off Topic Discussions

Re: Pats-vs-Cats predictions?

Posted By Dan D DuBal

Looks like you and I are among the minority, Jim!

I have *always* pulled for the underdogs.
I pulled for the Bengals -- *both* times.
I simply *knew* the Broncos would beat the Packers.
I grinned and chuckled when the Redskins counter-punched and smothered the Broncos.
I would have *loved* it if the Patriots had smacked the Bears in the mouth and shut everyone up (not that I imagined they had a snowball's chance in south Texas, of course).

Nonetheless, I feel the New England Patriots are simply playing at a higher level than their nearest NFL competitors. I've watched both Patriots squads (defense & offense) -- play by play, player player -- and I've often boggled at how..."precise" and "definitively" they perform. Football is a team-critical endeavor, and the Patriots, I feel, are simply the better team. The BEST team. The Titans were right there with New England -- at least for a while -- but the injury bug knocked Tennessee back a bit (just enough).

I think it was after week 9 when I found myself convinced that the winner of Super Bowl XXXVIII would be either the Patriots or the Titans. I was also convinced that the NFC representative -- whether Philly, St. Louis, or Carolina -- had little chance of beating the AFC victor.

Enough babble. Here's my guess:
New England Patriots 27
Carolina Panthers 13

Alternate score -- maybe a couple more field goals: 30-16

Regardless of the outcome, I'll have a grin on my face, a stuffed belly, and a nice cold barley pop in my fist. :-)

Cheers and best wishes.
Dan

February 25, 2004 06:49 AM Forum: Off Topic Discussions

About 40 hours...

Posted By Dan D DuBal

...since the last entry in the Off Topic forum.

Might this be a "waiting for Greenspan" phenomenon? :-)

Cheers and grins.
-Dan

March 8, 2004 03:18 PM Forum: Bad to the Bone Autos

Ya catch Leno's monster?

Posted By Dan D DuBal

Check "My Classic Car" dot com (episode guide -- #9001).

Here's a taste...

Cheers and torque.
-Dan