Posts Made By: Roland Christen

Here is a quote from another newsgroup from a perplexed individual:

"ED is like the Cabbage-patch doll industry! Keep the price high by creating shortages and mystery.
One can buy a 4" achromatic telescope for $200-$300 or lens set for lot less but now replacing one of the lens element to ED glass, price of the scope or the lens set goes up 10x. Gee!!! "

Why is an ED scope more expensive? My guess is that you can get a cheap 4" for $200 - 300 because it has cheap construction and cheap lenses that are not really well made. Nobody bothers to make a cheap ED with junky construction and poorly figured optics. I mean, why bother? In actuality, a well made achromat refractor will cost 1/2 to 2/3 that of an ED, not 1/10 if construction is the same quality. Whatdaya think? Is there a market for el-cheapo ED?

Rolando

Imaging or visual?

Rolando

I used a 14" Mak-Newt which had zero cooling problems, even though it had both thick corrector and fairly thick mirror. In fact, it was cooled down and ready to go as soon as the sun set and had none of the thermal plumes and swirling that I saw in a 16" Dob at a friend's observatory.

Having made Maks up to 10" myself, and used a 14" Mak-Newt with Russian optics, with none of the dreaded cooldown problems, I believe that most of these problems stem from the way the tube assemblies are constructed, not from the basic optical design. The corrector itself is basically a zero power window, so no amount of thermal distortion will affect the wavefront to any significant degree. I have actually taken a warm corrector from an 8" Mak out to the scope, which was at freezing temperature, placed it on the front of the scope, and measured the amount of correction change. Except for an initial 5-10 minute thermal plume, there was no change in the optical figure of the telescope.

Rolando

>>2) should you take luminance at 2x2 and rgb at 3x3? >>

It's all about signal to noise. That is fixed, regardless of binning, and is primarily a function of aperture.

Some say there is no gain to binning except for reduction of download time and small reduction of download noise. You can try this experiment. Try a 10 second shot binned 1x1, then 2x2. Then take the 1x1 binned image and resize it to half size to equal the 2x2 binned image. Is the signal to noise the same? I suspect it would be for non-ABG cameras, but in ABG cameras there might be an advantage to the binned image.

A similar issue arises whether or not to telecompress to attain more signal with a faster F-ratio. If it's true that signal to noise is constant and varies only with aperture, then there would be no gain by telecompressing. In fact, you lose some resolution if you compress too much. My own criteria for compressing is to use it only to achieve a wider field, if the object does not fit at prime focus.

Rolando

>>Can the problem be the power supply >>

I have tested EM 10 mounts here and find them to be very accurate in drive rate. The voltage supply will make no difference in the rate. It is always nice to have a stable voltage supply with no spikes or line noise.

In most cases of perceived wrong drive rate, it turned out to be simply a polar alignment problem. The star will drift east or west depending on whether the polar axis is set too high or too low in altitude. Short of doing a classic drift alignment, try raising the polar axis if the rate seems too fast, or lowering it if it seems too slow. In some cases I found that people were quite surprised that it was not a tracking problem, but a simple polar alignment problem.

Rolando

It is very simple for you to find the source of these three spikes. If they originate in the objective plane, then simply point your telescope to a bright star (use Polaris since it won't move during the test). Place the star in the middle of the eyepiece field. Now remove the eyepiece and place your eye in its place. Look up toward the objective and you will see the lens filled with light from the star. It should be evenly illuminated all around. Any obstruction around the periphery(spacer, cement or other) will be instantly visible as a dark shadow. When you can see these shadows, try shining a flashlight into the objective from above and looking again from the focuser end to see if you can see the cause.

Judging from the even illumination of these shadows, i would guess that it comes from either the spacers or the pads on which the lens rests. It is not as likely that the lens is being pinched, but strange things can happen with optics.

Rolando

You are probably not polar aligned (Alt axis too low). See my response in the CCD imaging forum.

By the way, the easiest way to measure PE is to offset the azimuth axis and let the star drift in Declination. Then you can see the wiggles on a time exposure and measure the error in pixels. Are you using PemPro by any chance to measure the PE?

Rolando

Your work is superb!

Rolando

The term ED might imply that it is fully corrected for color error at all visible wavelengths. However ED does not mean anything about correction at all. ED glass CAN be used to correct color error, but does not necessarily do so on its own.

Full color correction comes from a combination of ED glass and crown glass, either in doublet or triplet configuration. If ED glass is combined with Flint glass or Crown-Flint or other similar types, you will have more or less color error. Only crown glass mates can achieve true apo performance over a wide wavelength range.

Why do manufaturers use ED glass with flint or crown-flint and not the ideal crown glasses? After all, a crown glass is actually purer, clearer, less prone to striae and less costly than any crown-flint or flint glass. They do this to achieve fast focal ratios without messy aspherization, or to keep the cost of fabrication down because flint glass makes for easy manufacturing and looser tolerances. The tolerance level will need to be as much as 10 times tighter when you use a crown mate with ED glass, but the color correction achieveable this way can be as much as 10 times better.

Rolando

Yes, the Abbe designation is a very specific design. However, because it is highly prized by knowledgeable amateurs, there is a definite pressure in marketing departments of competing companies to use this name. To my knowledge, Zeiss has never published the design of their latest versions, but since they decided to abandon the amateur market, there really is nobody there to mind the store and perhaps raise a ruckus.

I have full sets of the Zeiss Abbe Orthos, and I can tell you that none of the ones called "Abbe Orthos" come anywhere close in performance, on-axis or off-axis.

Rolando