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Discovery DHQ 6" Dob



Quite some time back I sold off 2 commercially made 8" reflectors ( f/4.5 and f/6) and two home assembled 6" ( f/8 and f/10) reflectors because it was becoming too much trouble to move the scopes to and from their basement storage area for observing sessions. These scopes were replaced with a number of smaller instruments, refractors and Maks, that increased my observing time and ease. I fell in love with their tack sharp images, quick cooling time ( Maks excepted), and lack of tube currents; but after a few years I re-caught some aperture fever. I decided again to obtain a 6" newt and after deliberating several choices, including the build/buy option, I opted to purchase a Discovery DHQ 6" model with the standard plate glass miror.

The scope arrived quickly after placing the order and was received in good condition. Assembly was simple and took about 30 minutes after unpacking the boxes.

Mechanics and design:

They are some nice touches to the Discovery scopes. The ground boards have a nylon bushing to prevent enlargement of the central bolt hole for the az axis. In addition there is a center telflon pad, bored through to accept the bolt, that absorbs the weight over this part of the mount. Azimuth motions were good right from the box , but I found that a single slim washer from a milk jug placed over the pad improved the motion to excellant. A washer cut from a thicker jug proved too be too much - so it was pretty close to perfect right from the factory.

The focuser had some of the smoothest motion I had ever experienced from a non-premium model. This appears to be the same model they use in their 8" scopes. I could detect no image shift. I did note that the focuser tube came quite close to jutting into the optical path. This might present a problem with certain eyepieces but did not occur with the several I use the most.

I ran into some problems with the altitude bearings. The use of the adjustable balance feature is simple and straightforward but the alt motions were far too smooth. I was not excited at the prospect of having to readjust the balance point whenever I changed the altitude. But then I noticed the scope was not going to hold its position near zenith regardless of how the bearings were adjusted. The motions were so smooth that the weight of the focuser and finder alone were pulling the scope over every time the scope got near zenith. I pulled the teflon pads from the rocker box and repositioned them to gain more friction but the results were the same. To cure this problem I spent $12 at the local hardware store. This went to the purchase of some "s" hooks, two door handles ( which I was going to buy anyway), two small turnbuckles, and two tension springs. I attached the door handles to the sides of the rocker box to assist in transporting the assembly. I then affixed the springs to the alt bearing bolts using "s" hooks on one end and attached the turnbuckles to the other end of each spring. The turnbuckles can then be attached to the door handles using their hooks. I locked the alt bearings at what I felt was their single best balance position with an average eyepiece. After hooking up the springs the balance problem was gone, gone, gone. And no need to shift the adjustable bearings with eyepiece changes. Terrific! (I contacted Discovery about this problem and they basically "blessed" my solution but did not comment on whether this problem was noticed on any other 6" scopes).

The mirror cell allows for rapid cooling and easy collimation but I noticed that it held the mirror about 1/8" off center from tube axis. (Discovery said that at f/8 the system is less sensitive to collimation errors. True - but...). I cannot see an easy fix for this but there are some potential work arounds I have in mind. For the moment it will have to stay that way.

The secondary required adjustment and this took about two hours. I had to fabricate a "right angle" wrench to hold the nut behind the spider since I did not have any existing wrenches that would fit inside the tube. On other scopes I used to use an old set of ignition wrenches, but Discovery uses a 3/8" rod and no wrench in my tool sets that was small enough to fit in the tube was big enough to hold this nut.

Finder:

The 6x30 was quite acceptable. No problems with focus.

Supplied eyepieces:

Acceptable. Not terrific, but better than the "MA" types I've seen provided on some competing models. It's also nice to get two focal lengths standard with the scope. And Discovery will allow focal length substitutions.

The Field Test:

On a night of good seeing, after allowing the scope to adjust to ambient temperature for about an hour, I was able to get a decent airy disk with a single observable diffraction ring at 200x. The scope easily split Epsilon Bootes and Epsilon Lyrae. Pi Aquila ( 1.5" separation) was also cleanly separated at this magnification. All the images "snapped" into focus - a quick test for decent optical performance. The moon also showed some nice detail although the image was "rolling" as the seeing conditions were changing. Boosting the magnification to 240x did not get any improvement in the images simply because the seeing conditons were degrading as the evening wore on. I did note that tracking was not a problem at this magnification - the scope motions were smoother than any commercial dob I ever used ( after my slight modifications).

Bottom line:

A very good deal for the $299 price. The performance of this scope was as good or better than any smaller commercial dob I have used in my 25 years of observing. Discovery gave me a pretty darn good mirror. The problem with the altitude bearings might be very frustrating for a beginner - but was easily and inexpensively fixed with my spring additions. Maybe Discovery should produce a one page instruction set showing how to do this if the same problem might be present on for other 6" scopes. I suspect this might not be the case for their larger scopes because the tube weights will be more than than that of the 6" model.



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