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What’s new and different on a 10” Hardin Dob?

Posted by Rod Kaufman   07/08/2005 12:00AM

Purchasing a new scope can be a daunting experience and many amateurs have reported losing sleep over the myriad of choices available today. Upon deciding to purchase a Hardin 10” dob as a “grab and go” scope, I contacted Astromart sponsor Steve Fisher of “Up In The Night” and my angst at purchasing a new scope was reduced by several magnitudes. Steve not only offered to cover the shipping charges but personally delivered the scope to a FedEx terminal thereby minimizing the “truck-time” needed to deliver the scope. This method of assistance reduced the possibility of breakage during the delivery process from Utah to California. After only two business days via FedEx ground delivery, two boxes arrived at my doorstep.

Initial inspection revealed no evidence of breakage of the optical tube assembly and the supporting base. Assembly was accomplished in a few hours with some improvements noted in comparison to earlier models. The loop tethers used for attaching the altitude tension springs to the base were problematic in early models since they broke after repeated use. Two pull rings now attach the tension springs to the base with relative ease. The second modification is notable in its’ absence: a metal plate formerly used to seal the bottom of the tube has been eliminated thereby insuring quicker cool-down times of the primary mirror. The mirror cell now has a central aperture that allows for the attachment of a fan whereas the previous design required offsetting a fan if attached directly to the cell. The passive cooling provided by the well-ventilated cell appears to reduce the difficulties encountered with tube currents as well.

The first light experience posed two major challenges for the scope: mechanical and optical performance. I again chose to use resources available through the Astromart to assist me in the evaluation of the optical performance of the scope. First, I acquired a ronchi eyepiece with a barlow lens to view the resulting parallel lines on polaris. Each of the four lines appeared straight without deviations at the edge. Second, I had also acquired a second primary mirror of the same manufacturer of the scope and I chose to have the latter evaluated with a complimentary analysis provided by Astromart sponsor Woden Optics. While the ronchi eyepiece provides a qualitative assessment of the optical performance of the system, a quantitative analysis is also required. Woden Optics determined the peak to valley correction of the primary to be 1/6 wave with a respectable strehl ratio of 0.959. While Woden stated the primary could be refigured to a higher level of quality, it would require a very experienced observer to discern any resulting differences in performance. Woden further described the primary as “more than useable.” I substituted the primaries in the scope and my subjective impression indicated the stock primary in the scope was essentially similar to the optic evaluated by Woden. On some nights, I felt the stock primary might have been slightly more corrected than the optic evaluated by Woden but on other nights they appeared to be about the same. (As an aside, I also had acquired an 8” GSO primary and an evaluation by Woden Optics demonstrated essentially the same results as noted for the 10” mirror.) My overall conclusion of the optical performance of the samples tested is quite favorable with a continuing trend of improvement.

In evaluating the mechanical performance of the scope, I decided to track Jupiter continuously for several hours noting subjective factors such as observer fatigue and ease of motions in altitude and azimuth. Observer comfort is, of course, very subjective and not quantitative. However, I found I could view comfortably without undue fatigue for a few hours at a time. Altitude motions are quite smooth with this scope. The azimuth bearing does require some adjustment to obtain the proper “feel” and the placement of small tabs of felt or Velcro on the baseboard may be tried as a quick fix for this problem. A locking nut for the bolt that secures the baseboard to the groundboard should be use to prevent loosening of the bolt during revolutions of the scope in azimuth. The overall motions of the scope are relatively smooth so a beginner should not have difficulty in applying the proper force while veteran observers can modify the motions to their satisfaction.

Although I didn’t note marked issues with the time required to cool the primary mirror, I recommend the addition of a cooling fan if anticipated reductions in nighttime temperatures are significant. Turning to another resource of the Astromart for advice, veteran observer and supporter Floyd Blue suggested a muffin fan should be applied to a fabricated plate and attached to the bottom of the tube. I chose to forgo the plate and attached a muffin fan directly to the rear of the cell with tabs of Velcro applied to each corner of the fan. Although rated at 12 volts, a muffin fan can function well with a 9 volt battery so I attached the leads of the fan to a 9 volt connector available from Radio Shack. The fan can function with the battery attached to it via two Velcro tabs without a cord leading to a heavier 12 volt battery source. Thus the fan and its’ power source are self-contained and can run while observing without the worry of tangling a power cord at the end of the scope. I found I could run powers up to 150 or so without any undue vibration with the fan. Reasonable cooling of the primary occurred in about 25 minutes in warm summer nights.

My recommendation is positive for this telescope and I find it suitable for novice and avid amateurs as it combines portability and sufficient light-grasp to satisfy most observing situations demanding transportation to dark sky locations. I wish I had a similar scope when I began observing thirty years ago and it’s a nice secondary scope to use when I don’t feel inclined to wheel my 16” truss dob from my driveway to the front yard or to a dark sky location. I have seen some very pleasing views of the messier objects with the Hardin scope and its’ planetary performance has been more than adequate. Although some relatively minor modifications may be required to meet personal requirements, this scope should provide years of service.