Posts Made By: Doug Peterson

Aries yahoo site is moribund, Astrobuffet doesn't answer emails and the site has been static for two years. Are these things gone forever? Anyone have one they want to sell me?

Anyone have the link to the fellow who builds these, just reviewed here on Astromart? I know he used to be an Astromart sponsor, with his own web site offering up to 11"F5 achromats in his distinctive wood tubes.

Apparently these are the first schmidt-cassegrains to come out of the far east. I just received mine today. The good news: the star test and overall quality are in line with the American made units of recent vintage, which is to say pretty good, the same as my 5 and 8, and almost as good as my 9 and 11. The spherical abberation correction is very good, which is the the main event. I suspect we will be seeing all the rest of SCT optics production moving overseas.

takahashijapan.com

The Japanese Ultima 8x56 and 9x63 were two of the finest binoculars around. The 10x50s had an edge of field problem that prevented them from reaching the performance of the others in the series. But in general the series offered premium quality at a reasonable price, as long as you were happy with a 50 degree field.

The new Chinese Ultima DXs at a lower price offer some potential for quality at an even lower price. The 10x50s have a wider field than the others so I had to see if the quality could be maintained with the 65 degree field.

Unfortunately, the edge of field is not very good. Fujinons these are not. On the plus side, the optics are very clean, multicoated all the way down the line, and sharp over about 40 or 50 degrees. Curvature of field and astigmatism out there at the edge is typical of inexpensive wide field binos.

Personally I prefer the original leatherette finish to the new rubber armoring.

I have seen two numbers, 31% and 35% for the secondary obstruction on this telescope. Sometimes the primary baffle on cassegrains can push deeper into the light cone and increase the obstruction over that which would be normally the result of just the secondary housing diameter. That is why the best way is to measure the cental obstruction at the exit pupil of a low power eyepiece, using a magnifying reticle. Anyone done this?

On the PBS series they looked at computerized goodies like Canon's Image Stabilizing binoculars, loved em, Orion's computerized dobs, hated 'em for problems setting up and missing pieces, Meade ETX125, thought was kinda neat. As you might expect the coverage was superficial.

In attempting to remove the U-channel type dovetail mounting rail on my Mak, I see that it is held to the tube by 5 rivets of some type. It looks like it is not going to be so easy.

Has anyone had any luck removing one of these rails? The soft metal is easily damaged by overly agressive clamping.

Having become exasperated by Celestron's inability to recollimate C90s at a hundred bucks a pop, I decided to try it myself.

First you remove the tripod block and then remove the small screw underneath, which is the stop preventing the rotating half of the barrel from coming off. If you have the Astro fork version, you can remove the setting circle and pull the scope and integral altitude bearing out in one piece; two screws removed then separate the bearing from the tube.

The 3 screws on the rear cell merely plug the holes; presumably Celestron uses longer screws during the collimation operation, which involves a silicone adhesive. Celestron told me the mirror is bonded to a plate, so inserting 3ea. 8-32x3/4" thumb screws I can reach the plate (or the backside of the mirror) and make minor tweaks which stresses presumably 3 pads of silicone. Since the amount of adjustment required is small, only a tweak is required.

Nevertheless astigmatism appeared. A spring clip around a depression in the primary baffle bears against the mirror front surface, protected by a thin ring gasket. Tilting the mirror against the clip must be causing the astig.

Next I made a rubber mask to protect the mirror and used a dental-pick type tip to pull the spring clip up the primary baffle tube a few mm, where it reseated itself in a 2nd depression. Now only the adhesive is holding the mirror in place for adjustment purposes, yet it is still positively restrained, the mirror will never escape the clip. This should be sufficient: Meade used only double sticky foam tape for years to hold secondary mirrors in place on SCTs and newtonians.

I tweaked the collimation dead-on with the thumb screws. These C90s were actually figured pretty well in terms of overall wavefront quality. Only surface roughness in the corrector aspheric front surface (these are Gregorys, like Questars) and chromatic abberation marred the image quality in the 3 or 4 units I have owned over the years. The Gregory configuration has strong edge of field coma inherent in the design, particularly with the ultrafast F1.75 primary, hence a small shift in collimation spells disaster. I have seen either primary decollimation or secondary baffle drift on virtually every vintage C90 examined.

So far my fix is working fine. The next step would be to put an O-ring in the groove meant originally for the spring clip, and re-do the adhesive: use the screws to drive out the primary, create 3 new pads of adhesive, replace, preferably letting the adhesive cure live during an artificial star test.

Anyone else have collimation experiences?

Everyone assumes these are from the same factory, with some detail differences (2x Barlow in the 3.8 and 5), etc. Apparently the Parks Gold series is also the same. Anyone know who the manufacturer is?