Posts Made By: Roland Christen

You will not get a hand corrected lens at the price point of this telescope. These lenses are all manufactured in high production using machine polishing on high speed plastic laps. The lenses are not made with pitch with any kind of final figuring. Typically some tweaking of overall correction is made by adjusting the thickness of the airspace. If the set is undercorrected, the lens cannot usually be nulled beyond a certain point. If overcorrected, then incresing the spacer thickness will allow spherical nulling, but will also change the color correction a small amount.

The images you posted are a bit out of focus to determine the amount of spherical error, but I would guess the lens is somewhere around 1/2 wave or worse.

When you take out-of focus images, take 5 sets. Two images far out like the ones you did - these will show zones. Two closer to focus with perhaps 2-3 Fresnel rings showing - these will show spherical error and whether astigmatism and coma are present. Finally, one at focus to show any residual scatter, color error and other defects that occur at actual focus.

Rolando

That is nice! Suicide doors and windshield that cranks out. Great stuff!

Rolando

It's a twist on an old idea. Dialyte scopes are as old as the first achromat - going back 150+ years (invented by Plossl around 1850). They were first proposed as the answer to the problem of getting good flint material in large sizes. Back then one could obtain good crown glass, but in order to make an achromat, one needed an equal size flint glass of high quality. Problem was, nobody made a good high quality flint (even Alvan Clark used to complain bitterly about the junk flints that he had to work with). The Dialyte tried to solve this by sung a full aperture crown up front with a small aperture flint some distance down the tube. These scopes did sort of work, but fields were narrow because the design has inherent lateral color.

The modern dialyte has been updated using computer design software, so now one can do this using a number of carefully spaced small aperture lenses mounted at several places inside the tube. The front element is a simple inexpensive crown singlet. All color correction is done down inside the tube.

Another variation of this is the Schuppman medial telescope which uses a similar concept of full aperture singlet with sub-aperture correctors. Most of these designs are long focus with limited field sizes, but for visual planetary they would be well suited.

Rolando

I am not sure that he could walk away with this one so easily

Rolando

Really really nice!

Rolando

>>I guess the "hand figured" optics claim by Celestron isn't a joke!>>

It is not surprising that an achromat is well figured. Achromats by their very nature have the most forgiving of all optical designs and the easiest to make. It is a fact that the easiest optic to make will inevitably be the most precise, because one needs to really screw up in polishing to get anything less than perfection. These lenses are not "hand figured" but simply polished accurately on modern polishing spindles. The 4 curves of an achromat are very shallow and do not cause any alignment problems. With the standard crown and flint elements and simple spherical surfaces the result is essentially a perfect spherical null at one wavelength in the middle of the visual spectrum. This is really quite easy to do with modern equipment, and quite hard to screw up (but can be screwed up if one pushes the envelope).

All other optical systems from the simple Newtonian to the Cassegrain, SCT, Apo refractor, RC, Dall Kirkham, etc., have much more complicated elements that must be manufactured to a very high standard in order for the entire package to work effectively.

Rolando

Type of glass used does not produce color or the lack of color in the stars. Processing is the key. If you had attended the Advanced imaging Conference in San Jose, you would have heard talk after talk about how to process CCD images for this or that effect, color of stars, color of nebulae, etc, etc, etc. These guys were using the most expensive and sophisticated equipment including high end Apos, RC scopes, and various astrocameras, in some cases costing in the 6 figures.

The lament is always the same - how do I get the best results from my equipment. Not one of them would ever think of attributing it to the type of glass (unless of course it was a lowly achromat, which was never even considered by these guys). Like in real-esate where it's location location location, in the case of astrophotography it's processing processing processing.

P.S. FPL53 is a very high transmission glass of exceptional purity which will never impart the slightest coloration to the image. BSL7 or BK7 is likewise the same. Transmission over the photovisual spectrum of these two glasses exceeds 99.9% in a 10mm thick piece. There is no absorption or difference in transmission from deep red to deep violet. They are both extremely pure water-white glasses. Fluorite has no advantage in this regard over either of them, and Fluorite needs a mating element in any case.

Rolando

Take a picture of the Pleiades with a digital camera and you will see what this scope is.

Rolando

>>Omega Centauri: http://jjd.pbase.com/gregbradley/image/92706659/original
The Horsehead: http://www.pbase.com/gregbradley/image/92757065
Eta Carina: http://www.pbase.com/gregbradley/image/92758083 >>

Those are some really nice results. You've gotta try some H-alpha too.

Rolando

I would suggest that a lens is apochromatic if the Strehl ratio is above a certain level for a certain wavelength range.

For a visual scope I would call it a visual apochromat if the Strehl were above 90% from C to F (wavelength range 656nm to 486nm). Furthermore, the theoretical Strehl should be above 99% for a mid-wavelength somewhere between d and e (588nm to 546nm).

For an imaging scope I would call it apochromatic if the Strehl were above 80% for a wavelength range from r to g wavelengths (706nm to 436nm).

These are both extremely strict limits which are not easily reached with compromised designs. What these limits do is to define the theoretical maximum sharpness of the lens over a particular wavelength range, rather than the absolute color correction.

It may surprise some that an optimized aplanic 12" F8 SCT may appear totally colorfree, but it does not meet the definition for an apochromatic imaging scope since the Strehl ratio is below 50% at 436nm in the blue-violet. Being a mirror scope with zero-power corrector, it does not have any primary or secondary chromatic aberration, but it does have sphero-chromatism, which introduces a certain amount of softness into the image at the ends of the spectrum.

In summary, the color correction is composed of many individual components, but the Strehl ratio for a given wavelength range will determine the sharpness of the final image and the snap at focus, the contrast and definition of detail on a planetary disc.

Rolando