Discovery Truss 12.5" f5


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Subject: Re: vote by xxx.xxx.160.184
By: Bernie
In Reply to: Anonymous (xxx.xxx.160.184) (Original Vote)
Date: 06/01/2002 06:48:54 pm PDT
>The first thing you notice about Discovery telescopes is their poor customer service. They never seem to be sure what their production schedule is or when a product ships. This 'scope was over 3 months later than scheduled, despite claims during the remaining 2 months that the 'scope had been shipped, which wasn't true. At least they are easy to get a hold of, and want to be helpful.
>
>The 'scope arrived VERY well packed. No way anything is going to happen to the 'scope during shipping. Everything is packed in oversized boxes and filled with expanding foam.
>
>The really nasty surprise came when the primary and cell were opened. The primary is shipped in the cell, and there were several sheets of tissue paper taped over the mirror surface, with the tape going all the way around the mirror, isolating it from contaminants... you'd think. In the box was a large quantity of SAND. And upon removing the tissue from the primary, I discovered sand ON THE SURFACE OF THE PRIMARY. It was even stuck to the surface of the extememely dirty mirror. The whole mirror seemed to have been sprayed with Pam or some such oily substance. Some sand grains had to be picked off the surface of the mirror, most slid off. Total volume of sand on the surface was about 1/8 teaspoon. In the end, though, the mirror cleaned up fine, and, miraculously, showed no signs of sleeking or scratching. Amazing considering the amount of sand present! Discovery had little to say, other than that they figured the sand came from the inside of the steel tube cell, left over from sandblasting. Still, the fact that the sand was on the surface of the taped and sealed mirror indicates that it was present when the mirror was packed.
>
>The mirror cell needed refinement to allow collimation. First, the collimating bolts hit the primary, and had to be filed to allow clearance. Second, there was no provision for centering and holding the mirror in the cell, unless the mirror clips were really tightened down, which could easily introduce astigmatism, even in the 2" thick mirror. The mirror is now held centered in the cell with globs of silicone. The cell is a 3 point non-floating suspension. Probably OK for a 2" thick mirror, but a flotation cell would be better. Lastly, the center dot on the mirror was 1/4" off center and had to be replaced.
>
>The secondary spider assembly was not flat on the top and bottom. This is a pain if you wish to align the focusser axis with the spider axis by threading a bolt through the spider with the secondary removed, ala "the Dobsonian Telescope" by Kriege and Barry. If the secondary doesn't offer a flat surface, then the bolt will not follow the optical axis of the scope. It's fairly important to be able to do this to allow accurate collimation, which is very critical even at f5. This was fixed by filing the spider surface flat.
>
>The scope otherwise went together fine. The hardware is not captive, meaning lots of parts to keep track of and not drop in the grass. No tools are needed.
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>The bearings are the classic ebony star and teflon. EXTREMELY smooth and VERY light touch, too much so for my taste; the scope would leave the target when changing eyepieces. The Azimuth bearings were way to light and a little tippy due to the too-tall center pad. This had to be sanded down to allow the 3 main pads to take more weight. Without this adjustment, the rocker tipped back and forth on the central pad.
>
>The worst offense was that the 'scope turned out to be top heavy even without the finder attached. This is the worst case, as counterbalancing a top heavy 'scope requires 3 to 4 times the offending weight at the rear of the 'scope. So, adding the included 8 x 50 finder was out of the question. Normally, finder weights are considered when the 'scope is being built. This 'scope was ordered with the 8x50 finder, and should have balanced with it present, let alone without it. This issue was fixed with "virtual" counterweights; bungee cords rigged on the mirror box and rocker to take up the excess weight. A functional, though poor fix. Normally this shouldn't have to be done to a commercial Dob until you add the 8 x 50, Telrad, AND a Panoptic EP...
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>The 'scope is very stable, and well constructed and finished, though not furniture quality; the finish is rough to the touch and the parts are stapled and glued together. The staple holes are filled, at least.
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>Collimation is easy and tool free. The secondary has thumb screws.
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>The secondary is perfectly sized. Big enough to allow a 5/8" fully illuminated field, yet small enough to have only a 20% obstruction.
>
>The mirror star-tests all right. Diffraction rings are not the same on either side of focus, indicating over or under correction, I can never remember which. However, images focus very well anyway. I've seen VERY few mirrors that have identical diffraction rings in and out of focus, so this mirror is par for the course. Things focus fine up to 250X. Past this, poor seeing has likely been the culprit in limiting further power. So, in the end, I can't speak very quantitatively about the mirror, other than it doesn't show any severe aberrations.
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>The focusser is a JMI Crayford, low profile, and smooth. Finder is functional though has significant coma, making all the edge of filed stars fuzzy and crescent shaped. This is bad enough to make star hopping difficult; especially if field stars are dim, and stars only focus near the very center of the field. I have much better luck with Celestron and Meade 8 x 50's. So, I'd get the 'scope with a Telrad and purchase the 8 x 50 from another company.
>
>All said, the 'scope is very nice. Gave it a 7 for the modifications, out of balance, dirty mirrors and SAND, mediocre focusser, and delivery time.




Just a note.....ALL of the premium dobs usually take about 2 to 4 months longer than they quote...My Obsession took an extra 3 months to receive. Three other club members waited 4, 4, and 3 months extra to receive there Starmasters....and they give their scopes a 10....Please rate the scope...NOT nit pick the companies delivery time. If that was the case the Starmasters, Starsplitters, Obsessions, and the Astrophysics stuff would rate a 3!!!


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Subject: Re: vote by xxx.xxx.160.184
By: Anonymous (xxx.xxx.208.41)
In Reply to: Bernie (Original Message)
Date: 02/08/2003 06:18:26 am PDT
Well, sorry, but I don't feel that telling someone the truth about inflated delivery times is "nitpicking" the company. As a potential Discovery purchaser, I want to know.

Also, anyone who doesn't find anything wrong with a scope raises questions in my mind. Rating a scope a 10 puts a bias on most of the rest of the review.


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