Modifications inspired by using Brutus at the 2006 Winter Star Party
The biggest mod I decided to do was to cut down the front of the rocker box, since the lower truss tubes hit this whenever the scope was lowered in elevation to ~25 degrees or so. While these low look angles are obviously not the best windows of performance for a large Dob, not being able to access them at all prevented me from looking at Omega Centauri through this scope at WSP-- which was one of the reasons I'd queued up early for a prime beachfront observing site! I also found this lack of low elevation capability prevented from doing things ranging from Messier Marathoning to looking at the inferior planets with the scope. I debated whether to cut "scallops" out of the scope front to accommodate the truss tubes, but decided it would be neater to just cut straight across. Since I potentially was reducing lateral stability of the rocker box a bit, I compensated by installing 4" steel corner braces inside the remaining frontal board surface; for good measure, I added a pair to the rear board, too.
On the left below is the scope at its lowest elevation before the mod; on the right is the scope post-mod
I actually took off too much wood the first time around, and ended up being able to point the scope below the horizontal (which is useless), in which position the mirror box was so super-elevated that the trunions were slewed so far back as to be off the front set of rocker box Teflon pads. This was a fatal condition, since there would be no feasible way of lifting the mirror box/assembled scope up single-handedly and putting it back on the track. I ended up putting a 1 1/2" strip of the cut off front board back on, and securing it with mending plates, furniture glue, and wood putty. Now the scope can't go below ~5 degrees (which is a safe lower limit!), but there is absolutely NO chance of pulling the scope off the rocker trunions now.
Rocker box before modification Rocker box after modification During modification (I used a router for the cuts)
The rocker box with internal braces, and 3+ coats of fresh spar varnish
OTHER MODIFICATIONS
Moved the finder scope and remounted it below the focuser on the secondary cage. This means that the highest the observer needs to go on a stepladder is to reach the eyepiece for actually viewing, rather than finding the object. (This did put the finder below the center line of gravity, causing the balance of the scope to be affected to the degree that I need to use a 5 lb weight when my heaviest eyepiece -- the Nagler 31 in the Paracorr-- is used at low look angles.)
Installed a Velcro "clip-less" mirror mounting system similar to that on "Natasha" for use at star parties (when it is anticipated that the scope will not be moved) or in critical (Mars-type) observing. The mirror clips can be pivoted "down" for secure transportation of the scope, and "up" for viewing, with the assurance that the ~12 square inches of Velcro have enough resistance to lateral sheer forces to hold the mirror in place even if the wind blows the OTA to a nearly horizontal elevation.
Sanded the Teflon pads (to enhance the micro-channels and break stiction) and put fresh graphite on the altitude and azimuth bearings
Put 2-3 fresh coats of spar varnish on all of the wooden surfaces, since the old coating had deteriorated over the past ten years. Water now beads up nicely-- but the additional coatings have caused a bit of friction between the rocker box/mirror box, which even the fresh graphite (see above) hasn't quite fully overcome. This makes the scope about "average" to move in terms of elevation difficulty, whereas previously I had lubricated it to the point that at the Winter Star Party it elicited comments from seasoned observers to the effect that they had never used a Dob at these high powers (820X) for planetary observing that was as easy to manually track with as Brutus. Time and use will help dissipate this unwanted friction. (UPDATE: this additional unwanted friction *did* dissipate with a few weeks of use.)
Put on reflective/thermal insulated wrap around upper shroud surface. I had done something comparable with closed cell foam on my 16" Dob "Natasha" to insulate the optical path from the observer's body heat (see Thermal Effects). At WSP, Attila Danko (of "Clear Sky Clock" fame) commented that he had done something comparable, but used material like those aftermarket inserts for automobile windshields that block light/heat buildup the during summer. He noted that this reflective material was much less prone to dew buildup than foam. I invested in a one which covers the upper half of the light path-- the part closest to the observer-- and the half of the circumference of the shroud closest to the eyepiece.
You can see the shroud in the image above.
Put transparent plastic corner protectors along the front and back edges of the rocker box to prevent the Ebony Star laminate from being peeled away from the rocker box when loading/unloading the scope from my van. (There is a lip to the vehicle, and given that I can't see the leading edge during the loading operation, this seems like a prudent way to prevent these surfaces from being pulled apart by repeated scraping.)
Replaced the 2" wide nylon mirror sling with 3/4" wide Teflon coated steel sling. Nylon webbing is both temperature and humidity sensitive, and will stretch/contract with changing atmospheric conditions. This is not much of an issue during a single observing session (although I guess it could be... but I would expect to need to recollimate any because of the changing mirror figure if temperatures dropped fast enough for the sling to contract!). What I found at WSP, though was that my "perfect" (startest tuned at high magnification) collimation from one night would be skewed by the primary moving up to 1/2" during the following day!!! I therefore replaced the nylon webbing with a 3/4"+ wide steel mirror sling-- a slightly beefier version of that used on Natasha (Mirror cell issues). Since I have read in the ATM literature that metal can bind on the mirror's edge as the mirror cell is lowered in altitude, causing astigmatism, as on Natasha I opted to coat the sling and the mirror's edge both with Teflon tape. (Teflon on Teflon has essentially zero friction-- to 4 or 5 decimal places.) The "proof of the pudding" of this tweak will be if the new sling a) is astigmatism-free b) holds collimation from night-to-night at star parties. If it helps the scope to hold collimation when being moved in and out of my garage for routine observation sessions, so much the better!
Here is an image of the steel sling, with some of the Teflon
tape affixed.
Finally got the DSC's working. Despite trying a wide range of settings, I was unable to make the original cables work with my Sky Commander Mk 4, nor would the Sky Commander's own cables work on Brutus at WSP. I was ready to give up and buy a new set of encoders-- which would have been too bad, since the 4000 tic ones currently installed were decent enough. I tried the original cables one last time-- and they worked! Maybe I'd been using a bad software setting the first time... UPDATE: DSC accuracy finally working-- most of the time. Further testing and conversation with "Sky Engineering" (maker of my Flash 4 DSC unit) led me to put the unit into a simplified mode (fork-mount) for testing. This pinpointed mechanical issues with the altitude encoder mounting, which I fixed.
Replaced the steel focuser set screw with a nylon one to minimize marring and scratching of the eyepiece barrels
Purchased a "Little Giant" four step ladder with a handle at the top. While the two and three step ladders I already had are fine for me, anyone shorter needs a taller ladder at zenith. And there is a liability issue about inviting people to use a short ladder with no hand-hold on which to brace themselves, which the Little Giant neatly solves. (Of course, this does mean I have three ladders to transport to the field now!)
Here is the "Little Giant" ladder beside the existing three step (cut down) step ladder.
This is not strictly speaking a post-WSP mod, since I purchased it before going to WSP and used it there, but I bought a "Telegizmos" scope cover. I knew that leaving the scope set up for a week in the Florida wind and sun would require better protection that sheet plastic. I was planning to buy a "Desert Storm" cover-- the de facto standard for Dobs-- when I read an encouraging review of the Telegizmos product. According to their web site (http://www.telegizmos.com) "The new TeleGizmos Solar Scope Cover is a multi-layer, polyethylene material with a fully aluminized surface. The heat reflective ability of the cover is extremely high with a material strength equal to a multi-layer light weight tarp. The material was originally developed for NASA as an insulator in spacecraft. It is very strong, light, tear and puncture resistant and is highly superior to Mylar in durability." I found it worked great out of the box, and the manufacturer was a pleasure to deal with. (I had ordered the cover from Anacortes-- the sponsors of Astromart-- but it fell through the cracks in a computer upgrade. Telegizmos stepped in to drop ship me a unit to ensure it reached me before WSP.)
A picture of the Telegizmos cover on my scope, secured by the nylon compression straps that came with it.
Added padded lugs added inside the mirror box lid (see above), to prevent it sliding around when the scope is moved while assembled
Added a second set of handle positions. I had carefully engineered the handles so that they pointed slightly below the horizontal, to maximize my leverage when getting the scope up the ramps and into and out of my van at nearly waist height. This worked-- but I discovered over the months of using the scope for sidewalk astronomy, the downside of this was that I had to bend very low to set the scope gently on the ground under normal conditions. So low in fact that, one night when I was tired, I managed to pull a muscle in my back. (This was unexpected, given my level of conditioning and the amount of weight-lifting I do). When the scope was apart for this round of mods, I added a higher set of mounting holes (backed by threaded T-nuts, of course) which allow the handles to be a 4" higher-- above the horizontal-- for everyday use. I can quickly move the handles to the "van loading" setting as needed. My back appreciates the difference!
See the two sets of holes at the lower right
Here the pole in the foreground is in the "low" position; the background pole is in the "upper" setting
Other than perhaps someday adding tracking (a quad-geared Dob Driver 2 I already have on hand), and transferring the Clement Focuser from Natasha (the Nagler 31/Paracorr combination visibly deforms the orientation of the Starlite focuser's drawtube... and must therefore affect the optical path) I've pretty much run out of things to change on the scope. (Well, I do have this idea for a 6" jumbo finder for the mirror box... <g>) It's pretty much a "state of the art" 24" scope that I believe comes close to the performance of the Kennedy-equipped 24" StarStructure and Webster models (which cost over three times as much!) while being customized from top to bottom to my observing tastes. Is this the end of my ATM'ing days-- because I've now reached telescopic nirvana??? ;-)
Assorted shots of Brutus in action:
My son Alex used Brutus in his science fair project comparing bino to monoviewing.
Here is his Mom as one of the test subjects.
Me using Brutus with our Boy Scout troop from a dark sky site.
(Is my balding spot really that big!?!)
