Posts Made By: Roland Christen

Increased the contrast on this one with some detail lost in the brightest portions.

Rolando

You always manage to make that little scope sing.

Rolando

You still have the Ovision worm installed?

Rolando

Best I've seen in a long time. This cries out for a mosaic. Are you up for the challenge?

Rolando

In any achromat the manufaturer can trade unfocused blue for unfocused red. Since lenses were originally made for looking at the planets, and since planets did not have much blue, especially Mars, then it was the proper thing to do to let blue hang out, with red being well focused along with the yellow-green. Of course, there were also lenses designed for photography, and these used film that was sensitive to blue and had no sensitivity in the red at all. Therefore these lenses were designed to let the red go totally out of focus and bring the blue to focus with the green. That's the type of lens you have.

Rolando

That's a gorgeous picture. Not much light pollution where you shoot from.

Rolando

When you look at the Moon, the brightness causes your eye's entrance pupil to shrink. This produces a smaller aperture on your telescope (unless you are using very high powers). Smaller aperture = less chromatic aberration.

Your scope has lots of color error. You just don't see most of it on the planets and fainter stars because it is blue-violet and it is spread out, thus faint. On a bright star like Sirius, you will see it quite well.

You can judge the color correction of any lens in the daytime by viewing the reflections of a crumpled up piece of aluminum foil with sunlight reflecting off it. With an achromat or poorly corrected ED-apo, you will see green, red and blue colors. A mirror or a well corrected apo will show white light reflections.

Rolando

Nice transporter behind the house.

Rolando

The lens is permanently collimated and there are no collimation adjustments of any kind. In fact, even if you try, you cannot knock it out of collimation, it's simply impossible. It is also impossible for the elements to shift with respect to each other since there is no room inside the lens cell for that to happen.

Now, the reason for the elongated stars - what temperature was it when you went out to shoot? Did you bring the scope from a warm place out into a freezing night? If so, you probably had severe tube currents (tube currents = warm air rising and cold air falling inside the tube). This kind of thing will produce plumes which show up as elongated stars. Did you try looking through the scope with an eyepiece? If so, what did you see?

You can always tell tube currents by the fact that the plumes or elongation is always in the vertical direction. Why? because warm air rises. It can take an hour or two for tube currents to subside if the night is especially cold. Nothing you can do about it except to pre-coll your scope before using. Leave it in an unheated garage before taking it out.

Rolando

These are the glasses Scott makes at this time (latest list): http://www.us.schott.com/advanced_optics/english/download/positive_list_0812_for_usa.pdf

Here is the complete Abbe diagram showing all the glasses Scott makes: http://www.us.schott.com/advanced_optics/english/download/abbe_vdnd_us.pdf
There are none in the area above Vd=85. FPL53 and CaF2 have Vd=95 and OK4 has a Vd=91, so they are both higher rated ED glasses.

Schott makes the first generation FK51 glass, same as Chinese FK61 and Ohara FPL51. These glasses can be used to make ED doublets and triplets and are quite inexpensive compared to Ohara FPL53 and especially CaF2 (fluorite).

Rolando