Posts Made By: Roland Christen

Camera optics and telescope optics are two different worlds that have different priorities. Excellent color correction can be obtained in a telescope lens with only two elements. You cannot make a camera zoom lens that way - it will require many elements in order to perform the zoom function as well as keeping the overall length short with repsect to the focal length (in other words a 500mm FL camera lens tends to be 250mm long by using many elements with built-in barlows).

A telescope tends to be physically as long as the focal length of the lens, something that is not tolerated in camera lenses. Therefore a telescope lens (or mirror) can be extremely simple. This simplicity has another overwhelming advantage over a camera lens - it allows full resolution of the aperture, something that you will not find in any camera lens, even the most expensive high end job.

Diffractive optics have other drawbacks that make them very poor candidates for high resolution high contrast telescope applications.

Rolando

Fabulous!

I wonder if it's possible to see this from the Northern Hemisphere from a place like Mauna Kea in Hawaii or Haleakela on Maui. I was at 10,000 foot level on Haleakela and was able to see down to -70 degrees in the southern hemisphere. The air was crystal clear and transparent right down to the ocean surface at the horizon. I did see one nebula that was cut in half by the horizon line, and it seemed to be rising from the deep waters of the ocean.

Rolando

Reason is that achromats are relatively easy to make, glass is quite inexpensive, so if someone gears up to make thousands of them, the cost can be very low.

Rolando

An SCT can be as good as a Mak if the design is equal. Generally the design is not equal. SCTs are made with F2 mirrors and 5x secondaries. Maks are normally made with longer mirrors around F3 with 3x, 4x or 5x secondaries, depending on what the final focal ratio is to be. An F2 corrected sphere in an SCT will have large amount of sphero-chromatism (variation of spherical aberration with wavelength) which tends to soften the image. With an F3 primary, the sphero-chromatism of either design is very low, generally being less than 1/10 wave P-V.

The reason commercial SCTs are made with F2 primaries is so that they will fit inside the forks of a fork mount. With an F3 primary, the SCT would be way too long for a practical fork mount. Thus, in order to satisfy the needs for 99% of the users, all commercial SCTs will have F2 primaries.

One other advantage of a Mak-Cass is that it can be designed with zero coma and still have all-spherical components. In order to do that with an SCT (i.e. the Meade design), the secondary mirror must be aspherical, and the corrector power must be approximately double that of an all-spherical SCT. This causes the sphero-chromatism to increase yet again, making this type of design a bit softer still.

Rolando

Shoot galaxies with the 9.25" Celestron. The springtime sky is full of galaxies, not too many wide field deep sky objects. You can take short exposures and stack them if your mount has trouble tracking long exposures. The galaxies in Leo are quite bright and don't require very long deep exposures. Shoot at F10 for the best resolution.

Rolando

What's wrong with that? It looks lovely to me.

Rolando

A Petzval is a simple achromat doublet which has been shortened by the addition of a telecompressor element placed half way down the tube. It does not eliminate chromatic aberration (blue and red color error). It does reduce sphero-chromatism which is inherent in short focus doublets, and also can flatten the field. Longitudinal chromatic aberration is still present, same as a normal achromat. It is a way to obtain short focal ratios in a long tube (essentially the opposite of a Cassegrain configuration where you obtain long focal ratios in a short tube - another example of optical magic that is now well understood but was the source of much head scratching in times past).

Rolando

Pretty sweet! Sharp!!

Rolando

Good enough to hang up on your wall! You've come a long way.

Rolando

My tongue is hanging out about 3 yards :S

Rolando