Black wheel weights installed on upper cage strut.
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I've often heard that a Dobsonian telescope is really never quite finished. In an effort to uphold that tradition, I am continuing work on the TK-13. On this page I will document six projects: balancing the telescope, installing digital setting circles, portable power, upgrading the focuser and adding a light baffle to the upper truss assembly. I know, that's only five... the sixth just involved reaching for my wallet: a new observing chair.
Balancing the TK-13: Once I finished building the telescope, and replaced the original truss poles, I noticed an imbalance of approximately four pounds. The upper cage would have to be modified to accommodate this imbalance. The addition of a Telrad and a 9x50 RACI finder scope reduced the imbalance to less than three pounds.
I started out simply - attaching an ordinary ankle weight with Velcro to the top end of a couple of the truss poles. This solution was quite effective, however not very elegant. I still employ one of these weights during setup time in the field, to balance the scope while I collimate the optics.
The next step was to design some manner of adding weight to the top end without distratcing from the overall aesthetic appeal of the instrument. I attached 24 ounces of wheel weights, painted black, to one of the upper cage struts. The telescope is now balanced for use with most 2" eyepieces (the 20mm Nagler still presents a problem), but still a little light when using lighter occulars.
A virtual counterweight system is being investigated.
Digital setting circles: Knowing that I wanted to add both digital setting circles and an equatorial platform, my decision ultimately came down to a choice between Sky Commander and Argo Navis. I arrived at these two systems based on two important factors: the ability to disable the computer's sidereal clock and to be able to pause the system while the platform table is reset. Sky Commander can accommodate both of these requirements; Argo Navis will soon have this capability. The Argo Navis computer looks a bit more modern than the Sky Commander, but it's hard to argue with the Sky Commander's track record. I have seen many Sky Commanders at star parties, I have yet to see an Argo Navis. The Argo Navis has a much larger database, but I also plan to interface either one with my laptop running SkyMap Pro.
 Sky Commander computer and manual. |
 The complete Sky Commander kit. |
 The altitude encoder installed. |
 The azimuth encoder installed. |
 Sky Commander computer mounted on mirror box. |
 A view from above. |
Portable power: Having a portable source of power was a necessity with my SCT; and so it has become with the TK-13. The need for power is primarily due to the fact that I have grown fond of taking my laptop into the field when I observe. Having SkyMap Pro, Starry Night Pro, Lunar Map Pro, Cartes du Ciel, JupSat, Mars Previewer and Vickers Deep Space Atlas at my disposal is quite comforting. And let's not forget that the equatorial table needs juice too!
When designing my power supply, I decided that I did not want to lug around a flooded battery - so I decided on a gel cell. I wanted the battery to be able to provide two nights of usage between recharges, so I settled on a 70 Amp-Hour unit. I have two 12-volt DC outlets and a 7-outlet power strip (with a 300-watt Xantrex inverter) connected to the battery. Also wired up is a quick-connect cable for my Deltran Battery Tender Plus charger. Industrial strength Velcro keeps everything in place in the Igloo Ice Cube cooler.
 But it's portable power: 7 AC and two 12V outlets. |
 Inside: a 70AH gel cell, 300-watt inverter, and wiring. |
Focuser Upgrade: After using the TK-13 for nearly a year, I have determined that I would like to utilize a two-speed focuser. The standard Phase 4 focuser is a solid performer, but I have a desire for fine control at higher magnifications. To that end I placed an order yesterday for a low-profile Starlight Instruments Feathertouch focuser (1½" draw tube travel). I ordered it from Astrosystems, who will also supply a custom mounting plate for the TK-13. After a week-long backorder, the new focuser arrived today. The replacement, and subsequent mechanical alignment and optical collimation, took about 90 minutes. Now, if the clouds clear away, I'll have a look at something other than distant power lines!
Optimistically, I left the TK-13 assembled in the garage, and by 19:00 it appeared that tonight would not be clear. So, I hurriedly wheeled the scope out into the driveway, seated my 12mm Nagler into the focuser, and aimed at the Moon. The motion of the Feathertouch is the smoothest I've ever experienced. The fine adjustment (10:1 ratio) is a joy to use. Though not quite optimally positioned for observation, I couldn't help but leisurely meander down the rille known as Rima Birt, across the bowl craters Birt and Birt A, on to the Straight Wall. Here I decided to add my TV Big Barlow, increasing the magnification to 250x. This is where the focuser really shines, as features snap into focus with a slight turn of the 'golden wheel'. As the clouds rolled in, I had just a moment to visit Copernicus at the edge of the terminator. I can hardly wait for a clear night!
 Standard Astrosystems Phase 4 Focuser. |
 Starlight Instruments Feathertouch Focuser. |
UTA Light Baffle: Occasionally I find it advantageous to block ambient light trespassing from a nearby 'insecurity' light. The baffle serves to cut stray light, and improve contrast, by extending the upper truss assembly opposite the focuser. My baffle is constructed from a piece of black Kydex roughly 12" x 30" (0.028"). It is secured by four small pieces of black industrial strength Velcro. The inside of the baffle has been sprayed with flat black paint. All in all, a simple project that pays high dividends.
 The baffle from the outside... |
 ...and the inside. |
Observing Chair: If you've spent any amount of time at all at the eyepiece you know the importance of a good observing chair. Quite a few amateur astronomers - especially novices - spend most, if not all, of their observing sessions hurriedly racing from one object to another. My guess is that this behavior is attributed to a combination of wanting to see as much as possible and an unrefined sense of observation. Once one progresses from this stage they come to realize that 'seeing as much as possible' becomes more important with each object. Quality versus quantity. No matter what type of telescope is used, the observer always sees more when they are comfortable. And the best way to be comfortable at the eyepiece is when one assumes a seated posture. Enter the observing chair.
My first observing chairs were those located on my patio, and they worked out fine when using an SCT in my backyard. But they weren't very portable, they weren't adjustable, and they weren't very comfortable. As I do when confronted with similar dilemmas, I turned to the internet and began my research. I soon came across the Denver observing chair, named after the city of its inventors. These guys made the plans freely available, and I soon had another project. After about $40 spent at Home Depot and a couple hours in the garage I too had a Denver observing chair. The chair served me well, receiving upgrades and improvements over time. First came the padding of the seat; next were notches to secure the seat at different heights.
 My Denver chair from the front... |
 ...and the back. |
As I contemplated the next two modifications of my Denver chair, I also entertained thoughts of just -- gasp -- purchasing one! I wanted to make my chair more portable - the wide front 'foot' occupied a good bit of space in the vehicle. And, I wanted a greater range of height adjustments. The first could be accomplished easily, while the latter was impossible. So, I returned to the internet to investigate commercially available chairs. In keeping with the design of the TK-13, I decided I wanted a wooden chair. The Catsperch looked great, but Jim's backlog was over a year. He does sell plans, but by now I was convinced that I would let someone build me a chair. That's when I discovered Starbuckets Telescopes on Astromart. A quick email to Neil Carroll resulted in an order for one of his chairs made from hickory.
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© 2002-2004 Peter Argenziano, all rights reserved.
Page last updated on 17 June, 2004