Posts Made By: Roland Christen

Parallax

RC

>>Y has a periodic error on a 78 second cycle. A very pretty Sine Wave. >>

It appears that you have a bad gearbox on your drive motor. The gears are very tiny inside this gearbox, and if they are not meshed properly or if they are not manufactured with a proper tooth profile, you will get a cyclical error that is many times faster than the worm periodic error. Unfortunately, there is nothing that you can adjust to fix this.

RC

http://www.company7.com/library/zeiss/notes.html

>>I've never been clear whether or how a powermate is better than a barlow - if (big if) the barlow does not cause vignetting in an eyepiece, I don't understand why a 4-element powermate would give better star images than a 2-element barlow.>>

There is much confusion about Barlows and their effect on eyepieces. Let me see if I can add to this confusion

A Barlow increases the size of the virtual image by redirecting the incoming convergent rays of light from the objective lens or mirror. A 2x barlow will double the size of the image, so that the original field of 1" diameter (to give an example) becomes an image of 2" diameter some distance further back from the original focus point.

In order to achieve this 1" to 2" size increase, the original rays must diverge outwardly. The amount of divergence is inversely proportional to the focal length of the Barlow. What this means is that the rays will diverge mildly with a long focal length Barlow (one with a fl = -5") versus a shorty Barlow (fl = -2") where the rays will diverge rather rapidly with a large angle. To see what I mean by divergence angle, draw a 2" field and a 1" field separated by 5" versus one separated by 2" and connect the outer parts of the field with straight lines (see attached picture).

Normally, Barlows have fairly short focal lengths ranging from -1" to -2". This allows the eyepiece to come to focus close to the original point and uses less focuser travel. However, a short focal length Barlow with its large diverging light cone means that the outer rays diverge far enough to miss the edge of a typical eyepiece field lens diameter, so the Barlow appears to cause vignetting. It is in fact the result of too small a field lens in the eyepiece, not some stopped down effect of the Barlow itself.

A 4 element Barlow uses a positive achromat lens near the final focal plane to bring the divergent rays back to parallel so that the field lens of a typical eyepiece can accept the ray bundle without vignetting. The 2 element Astro-Physics Barcon achieves a similar goal by having a long focal length (-5" or -127mm) to minimize the divergence of the outer ray bundle. Most eyepieces can tolerate a small divergence, but begin to show heavy vignetting with short FL Barlows. Plossls are the worst in this regard because they actually have field lenses that are too small even without a Barlow's divergent ray bundle.

Roland Christen.

There is a Chinese marketeer who is looking to sell a batch of these particular optics wholesale - quality unknown, for a fairly steep price. I don't think you have seen the last of this fluorite. I bet it will surface at some point as a tube assembly by a 3rd party with price tag between $1200 and $1500.

RC

Dance of the filaments.

Roland Christen

The material is called Fluorite with the U before O, not Flourite. There is no flour in it. There is fluorspar in it. It was named for its ability to fluoresce (which means it can glow in the dark).

RC

>>I have a Lumicon enhanced 2" diagonal that I believe is a dielectric mirror. >>

An enhanced diagonal is NOT a dielectric mirror. Enhanced means that the aluminum coating has one or more thin layers of silicon monoxide (or other oxide) added to the underlying aluminum layer. It will still sleek or scratch easily if you are not super careful when you clean it.

Roland Christen

>>When I check collimation on my refractor(TMB APO triplet) I get the doughnut with more or less concentric rings inside and at the center the hole or dark spot. >>

You're checking collimation with what? On a star? With an eyepiece? With a Cheshire? I'm confused.

RC

Right hand side at 1/2 size

Roland Christen