So, as simple as the hiss drive is in theory, in practice, it is not quite so easy. At least for me.
It’s been a week now since the last post, when I essentially finished the rover and gave it the first field test (fail). Every day (or nearly so) has been cloudy. I tried waking up early (ran in to dense fog almost every day, or overcast skies) and I tried staying up late (that got me more clouds and rain). I didn’t get a new telescope (technically) but I was hoping the telescope gods were punishing me because what I will get out of the rover will make it seem like I got a new telescope.
Okay, superstitions aside, here’s the recap: I couldn’t sit back and do nothing, so I took the rover out during the day when the moon was still up (because of course we had clear, sunny days). I couldn’t get any tracking—when I opened the valve wide enough for me to detect airflow (the hiss) the image shot across the FOV. Flummoxed, I gave up for the day after messing with it for a while.
Field Test #2:
I tried to guess where the north celestial pole was (obviously Polaris was not visible) and lined up the rover using a compass. Luckily my driveway runs north-south, so it’s fairly easy to point towards a tree in my yard that sits about under Polaris. Anyway, once the rover was lined up, I inflated the inner tube (which only took about 7 pumps) and inserted a 25mm eyepiece. This time, following the advice I received, I stayed away from the moon. I was up early, and the sky was clear, so I tried Jupiter.
Man, it’s been too long (nearly a year!) since I saw Jupiter…
Anyway…after tearing myself away from the first blurry images (I just pulled off the cover and started viewing, no cool down—and I’m sure after a year in mothballs the scope is way out of alignment, so no collimation—I figured it was a field test of tracking, not clarity of image) I got down to testing the drive again.
Again, every time I tried fiddling with the escapement valve knob, I ended up either doing nothing, or watching Jupiter skid across the FOV (very smoothly, by the way, way smoother than by hand!). Once again I was left scratching my head.
As the baby monitor went off, signalling my daughter was awake and the end of field test, an idea suddenly hit in my head: I have been looking at this problem completely ass backwards. In my head, for some reason, I’ve been imagining the polar alignment and hinge on the east side canceling the rotation of the earth—I know, that’s’ really what it does do. But for some reason, I’ve been stuck on the mental image that in the eyepiece, as I release air from the inner-tube, because it’s canceling the movement of the earth, the image should shift in the opposite direction that it naturally travels.
Standing there int he driveway listening to my daughter talk about a penguin (yeah, no idea), I suddenly realized that because I’m thinking the telescope moves opposite the earth, I’m probably trying to release too much air to cancel the movement. After all, I opened the valve until I heard a hiss…the planet basically moved a little faster than normal. Thinking the drive would reverse that movement, I open more, only to see Jupiter sail away laughing. Again and again.
That’s when it dawned on me: the inner-tube deflating allows the scope to speed up and chase the object as it moves east to west in the sky (relative to us, of course). This is one of those few moments in life where I completely understood Archimedes jumping out of his tub. With my revelation, I suddenly realized that I was opening the valve waaaaaay too much. That’s also when I remembered reading posts about people mentioning when they were tracking with their hiss drives, they could not hear a hiss.
Alas, time was up and I had to pack it up. That left me stewing on my idea for the last few days. To feel like I’m still moving forward with this project, I took some advice from Henk on astronomyforum.net, and made myself a polar alignment tool:
This is simply a piece of tubatwo cut to about a foot in length. I then drew an angle across it on end about at precisely 43º (my latitude). Then I made sure to give one side of a two-by-one a flat edge and glued and screwed it in place. The result is a simple tool that makes sure the rover is aligned at my latitude. To use it, I simply hang it on the hinge support block so that the tubatwo is hanging vertical. Then I take a small torpedo level and slap on the side of the tubatwo. When the bubble reads true at 90º, I know the rover is pointed at 43º. That will allow me to shim the thing to the proper angle.
Along those lines, I also added a t-nut and carriage bolt with locking nut tot he front of the rover. When I need to adjust the angle to reach the magic number, I just screw or loosen the carriage bolt and voila, angle achieved. Simple but effective. You can also see the small caster I installed to make this thing fully mobile.
I also made a simple carriage bolt lock-knob that tightens through the rover bases and locks it stable when moving…no more worrying about the upper half bouncing around. Just some scrap maple drilled out to house the bolt and a t-nut on the bottom of the rover.
Field Test #3:
So, I just had to wait a few more days until last night when there were just enough big sucker holes to allow me to peer through the clouds at a star…any star…and test my hiss drive theory.
It took about half an hour of fiddling with the valves, but I finally was able to slow the transition of Fomalhaut across my FOV from 40 seconds (6mm eyepiece) to 1 minute 45 seconds (just to go from one edge to the center!).
Talk about exciting! I got about 3 minutes of observing a single object without bumpin’ and nudgin’…
I think I can squeeze more out of it as well—no pun intended—by fine tuning the valve where the air is released. I mean, I barely turned it open, and couldn’t feel any air or hear any air escaping at all. I’m thinking by playing with the amount it’s opened, I can slow things down even more.
I noticed that the telescope bounced around a bit when you focused or slewed to get on target. Really though, it settled out pretty quickly and the image was stable in less than 6 or 7 seconds. Now that I can hold the image for close to three minutes, a few seconds is totally do-able.
Also, I noticed that when the tire was really inflated, the drive automatically moved faster—the greater pressure of air inside the tube must be the culprit. As the tube deflates, the tracking becomes more easy to adjust and control.
Next time—-if I ever get clear skies again—-I’m going to time just how long I can get an object to track, and I’m going to see the total time one inflation of the tube will last. I’ve been messing around every time and letting out too much air to get an accurate reading. But now that I know what I’m doing (sort of) I think I can get a pretty good number. All this leads me to believe this will be an outstanding mod to this scope and I can’t wait to try the Frankencam on it now!
Now I can spend cloudy days thinking about how to attach the power strip from the original base, and possible the folding table, and figure out how to organize the tubes and air pump…hmmmm….