Posts Made By: Dan D DuBal

Depends on the designs (and the particular type of ED) in question.

Calcium Fluorite ("U" before "O") is a crystal which has optical properties which are condusive and appropriate to apochromatic design.

The efficacy of any particular "ED" glass (and there are many types which fall under the generic "ED" category) will depend on its characteristics *and* the objective design into which it's integrated (i.e., the characteristics of the mating element(s)).

Yes, there are a couple/few "synthetic fluorite" glasses which are, essentially, optically equivalent to Calcium Fluorite. However, there are other factors which must be considered when a lens designer/manufacturer makes those decisions (i.e. Calcium Fluorite or ED/SD/UD glass) -- namely, cost & supply at that particular given time.

As far as apochromatic performance goes...
There are objective designs utilizing Calcium Fluorite which *can* be "bettered" by other designs utilizing ED/SD/UD glass. Likewise, there are designs utilizing ED/SD/UD glass which can be "bettered" by other designs utilizing Calcium Fluorite. Regardless of this fact: for most folks in most scenarios, differences to the eye from one excellent apochromat to another will likely be extremely miniscule (if not invisible).

I wish there was a "cleaner" answer, but lens design (the "recipe," if you will) *is* more important than a given element (or "ingredient").

Best wishes.
-Dan

Are you familiar with the star dangling just "below" the end of the Big Dipper's handle? The end of the handle is marked by Alkaid, and the star below it is 24 Canes Venatici. Keeping those two stars in mind, try and get your bearings using the image at the bottom of this page:

http://www.ne.jp/asahi/stellar/scenes/object_e/m51.htm

Light pollution, of course, will hinder M51's visibility. However, the galaxy's "core" should remain discernible (though likely dim and indistinct, if sky glow is significant). Start with a lower magnification and get familiar with the brighter field stars shown in the photo -- recognize the area where M51 *should* be -- and you should be able to find it.

Best wishes and luck.
-Dan

Let's not mince words -- the "sequels" STANK. :-)

"Highlander" is a good one. Interesting use of water throughout the movie (if you hadn't already noticed). I also get a kick out of Connor's (Lambert's) garbled underwater laughter.

Certainly tough to convey some of the story's themes via 90-minute action movie, though. Better suited for a novel, I think. Haven't read Garry Douglas's novelization, so I have no idea if it's worthy.

Cheers.
-Dan

Jovian transit shadows are easily seen in my 75mm refractor at ~80x (and likely lower).

By all means, TRY.

Best wishes and luck.
-Dan

I agree and avoid the term when able. However, you got me thinking about my own use of the term "fidelity" -- hardly quantifiable in its own right, that's for sure. Maybe I tend to use it in a sort of "general/relative/overall" context.

Back to "sharpness:" if I had to relate what the term means *to me* (and only me), I might fumble/babble my way through concepts which entail...
-accuracy of image/wavefront
-"invisible" (or practically so) scatter
-lower image magnifications

Cheers & best wishes.
-Dan

ROOKIE?!?

I shudder to imagine what we'll see from you once you're gain more and more experience! ;-)

Outstanding image, Bob. PLEASE, keep at it.

Best wishes.
-Dan

:-|

Sure puts things into perspective.

How often are we triggered into an angry growl by such "major catastrophies" as a late newspaper or a neighbor's loud stereo?

Best wishes.
-Dan

Hi, Martin.
What a great looking telescope. You've obviously taken great care of your SP-C8, as its clean and shiny condition indicates. May you continue to enjoy our universe's wonders for years and years to come.
Best wishes, and thanks for sharing.
-Dan

I myself would expect a good classic 114mm f/9 Newtonian (no corrector) to perform better than the shorty. Your reasoning is sound -- simpler (if not purely *possible*) collimation, better image fidelity, easier to upgrade (better modularity).

The corrector may very well decrease aberrations inherent in the short-focus *spherical* primary, but the f/9 primary of the long-focus Nexstar is already at an advantage -- it has fewer aberrations to begin with.

There are plenty of good 114mm f/9 Newtonians out there -- whether they're branded by Tasco, Orion, Vixen, Celestron, Meade, or others. Yes, there are also plenty of mediocre (or worse) examples, too. These scopes have been around for decades. I have *not* heard of many good short-tube 114mm f/9 or f/8.8 "corrected" Newtonians. They may work just fine (I notice Orion offers one, and their satisfaction/return policy is excellent), but I just haven't heard from their owners and users.

If the choice were mine, I'd stick with the classical 114mm f/9.

Best wishes and luck.
-Dan

The Astele 95 utilizes the standard SCT rear port coupling, so the short answer is YES -- a 2-inch visual back (or diagonal) CAN be mounted to the scope.

What I'm not certain about is the scope's inherent moving-primary focus range -- whether or not a 2-inch set up will allow for visual focus at infinity. Experienced owners should have some insight, here. Perhaps a test may be performed: try a 2-inch SCT diagonal, or install a 2-inch extension between standard diagonal and eyepiece & see if scope can reach infinity focus (&/or focus *beyond* that point). The extension should approximate the "length" of a 2-inch set-up.

Assuming 2-inch focus *can* be reached, there are yet two other points to keep in mind:
1. Such a set up *will* yield a vignetted image plane -- *significantly* vignetted (image will be dimmer off axis).
2. Such a set up *may* result in aperture stop-down (primary baffle may intrude upon light cone & reduce the scope's working aperture). Both the design/dimensions of the primary baffle *and* the inherent moving/focus range of the primary will determine whether or not stop-down will occur.

Enough babble from me. Let's hope some owners can offer more insight and experience.

Best wishes.
-Dan