Re: Fw: OT: Testing a Telescope Mount Shake & Level with a Home Made very long, ultra precise "Bubble Level"
toggle quoted messageShow quoted text
You might want to add some food coloring to the water for a better visual sighting.
From: ap-gto@... [mailto:ap-gto@...]
Sent: Saturday, July 19, 2014 1:12 AM
Subject: [ap-gto] Fw: OT: Testing a Telescope Mount Shake & Level with a Home Made very long, ultra precise "Bubble Level"
Somewhat OFF TOPIC, but I thought someone might want to test for any shake on their pier or tripod attached mount.
You need an accurate bubble level to see what gradually happens , and a professional quality one can be expensive. This is about making a much better one for less than a couple of bucks. I hope this description is sufficient, but photos can be provided.
I had some nagging suspicions about the stability of my slightly overloaded AP-900 mount, and wanted some way of accurately measuring any “potential” shake of the entire mount and scope, or even a vertical shift after a meridian flip.
Actually, my immediate concern originated in trying to remedy a 23 “arc-Minute” RA shift in the image, after a meridian flip, on a very carefully PemPro-2 “polar wizard” aligned mount – to less than 10 arc-Seconds on both axes. The shift could have been caused by the OTA (likely) being non-orthogonal to the mount’s DEC axle – or alternatively, a shift of the entire telescope as it changed sides, and leaned over by about that degree, causing an STL image to shift, exactly and repeatedly, right to the very edge of the STL-11000 FOV. A precise torpedo type bubble level might eliminate one of these two possibilities – or at least attest the stability of this overloaded system.
I figured using a standard “carpenter’s bubble level” wouldn’t show much, since its typical bubble tube is far too short (less than an inch), so it hits its end points or marker divisions, before much happens. I wanted something with a very long bubble travel, and extreme sensitivity to indicate any tilt of the entire telescope, as it moved around. This might be caused by its soft footing, or any number of “imperfect mechanical links”. Ideally a bubble level should be as long as twice the distance from the mount centre to the saddle, which it might hit in swinging around on a meridian flip. They don’t make these, so I decided to make one myself.
*** How potentially shaky is my entire setup?
My 85-lb RCOS truss RC-14 scope, plus 60 lbs of cwts., sits on a Losmandy HD tripod, its pipe legs standing on cinder blocks buried in the lawn, its leg extension knobs are locked down tight using a rubber-strap wrench, with three large SS Losmandy knobs attaching an AP (LM2AP) mount adapter and a Flat Surface Adapter on top of all that. Finally, the four AP-900 mount’s fork attachment screws are attached to an AP (RPA) Rotating Pier Adapter underneath it, with its own semi-firm brake screws.
It all seems potentially problematic, almost like a “Cirque Du Soleil” flying circus, with so many connections to the OTA superstructure, possibly leading to micro-instability for astro photography.
So I decided to test the entire system as a unit. Initially, I intended to buy a $100 specially purpose, precise, machinist’s bubble level, normally used to level a lathe bed. But even then, although the accurately machined six inch version was a bit longer than a standard carpenter’s level and weighed a few pounds, it still had an air bubble and overall leverage span I felt was too short to show enough “incremental motion” at the start of any mount tilting.
*** SOLUTION – Home Made extreme precision “Torpedo Bubble Level” for $1.50 ***
This consists of just two, 25-cent screw-end protector rubber caps (Home Depot), and a “one-dollar”, Plexiglas 13-inch long “Serological Pipette”, commonly used in chemical or medical testing labs or schools, (must be the 25 ml capacity), with lots of mm marker divisions, normally for the fluids being measured. Found mine in a local “surplus store” – normally a disposable supplies item, easily found on the web.
The tube is about a half inch diameter, so it is easy to read. I sucked up enough water into the Pipette, to create a long enough, and sufficiently fat air bubble for my intended precision, then capped the tube ends. I rested one end of this bubble level on the AP-900 Azimuth Adjuster knob (finger detent), and propped up the other end on a “few coins” slid along underneath, resting on the adjuster, until the bubble sat dead level. Then I executed several meridian flips, and watched for the bubble to gradually drift from center along its 13-inch length of tube ...
Well, it did not. It remained dead center, no matter how fast I slewed the mount from side to side, or did meridian flips.
So, this “extreme length bubble” helped me determine that my mount setup itself is rock stable, doesn’t shift around, even with all those screws, bolts and connections. However, the OTA truss structure itself, may be another matter.
Slipping in a brass shim for the mount cradle, if needed to compensate for any OTA parallax, under such an elephant, could prove challenging :-)
As an after thought, to winterize the fluid making it easier to see, I tried replacing the water in the tube with green mouthwash – too foamy, and broke up into several smaller air bubbles. Then tried some red “liqueur”, but the sugar content turned the bubble into a “sluggish wave”. Turpentine might work better, and shouldn’t freeze & burst the tube, if I use it later, as a mount axes level, taped into its D-Plate or saddle channel.
Just an idea ... in case you need a really precise home made bubble level.