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Hiss Drive: What am I doing wrong?

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#1 VikingRaider


    Mariner 2

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Posted 23 October 2013 - 11:33 AM

Okay...so finally got the rover I built (here's the post about that) out under clears skies! Too bad it was 10:30 this morning... But I take what I can get. The moon was still up so at least I had a target to try this puppy out.

Unfortunately, it performed less than stellar. I think. See, I am all kinds of confused on how this thing is supposed to operate now. My understanding was that you build the hinge on the eastern side of the platform, angled to point at Polaris (generally, the NCP specifically) at an angle to match your latitude (in my case 43.8*....I used 45* to make construction easier, then figured I could shim it in the front to bring it down a degree or so). With the inner tube inflated, you get your target in the eyepiece and slowly release air---the platform rotates around the hinge counteracting the movement of the earth.

What happened during the first field test? Well, I locked on to the moon, (about 45 minutes before setting so it was low in the west) and let big flooded basin Grimaldi drift through the eyepiece (12mm, from my perspective, the target moved from left to right across the FOV). After a minute or so, I reset the FOV so Grimaldi was on the left again, let it work it's way towards the center of the FOV then started to release the air a tiny bit. I couldn't hear any hissing (but with cars passing buy and house construction a few doors down, I'm not surprised...sure is a lot noisier during the day!) but I also couldn't tell if the moon was shifting any less. It seemed to kind of pause, but not really. So I opened up the valve until I heard air escaping and watched Grimaldi slide quickly across the eyepiece in the same direction it had when I had the valve closed (from left to right)! I reset and tried again, opening the valve all the way this time to see what happened. The moon sped (very smoothly!) from left to right, as if time was sped up.

So there I am, scratching my head in the driveway, trying to figure things out. If I have done this correctly, shouldn't the moon go the opposite direction, from right to left in the eyepiece when the drive is engaged (and opened "too much")? How else could the hiss drive (or any drive for that matter) counteract the earth's rotation?

Then I had a sinking thought...what if I put the hinge on the wrong side? Almost every picture I've seen on the internet (okay, I think it was every picture) shows the drive set up just like mine---ie, facing north, the hinge is on the right (east) side. So what gives? Well, I tried a little experiment and turned the thing around and had the hinge angle point south this time (it is on wheels, after all, it took only a second to turn around).

The result? Instead of the moon gliding past left to right, when I opened the valve, the moon went up in the eyepiece. So odd.

I'm trying for the life of me to wrap my mind around this---and I've been out of the hobby nearly a year now, so can someone please slap me upside the head and knock these cobwebs out? Something doesn't add up here.

For the record, obviously, I couldn't have been properly lined up on Polaris (or the NCP) since it was daylight. I used a compass (well away from the OTA) to find North and lined it up with that by sight. I also tried this setup pushed a little to the east and west of what I thought was north, just to see if it made any difference, and there was no noticeable change in the target's progression across the FOV. I also tried putting a block of wood under the front of the rover to change the angle from 45* to...something less. I tired with and without the block shim, again, no noticeable change.

Perhaps if I tried it at night time there would be a greater indication of the things I was doing having some sort of affect. Perhaps at night I would get a better alignment---okay, I know at night I'll get a better alignment. But that said, shouldn't there be at least a hint of this thing counteracting the "normal" progression of a target through the eyepiece FOV???

So confused. Can any of you astronomical gurus out there throw me a clue? I'm fully prepared to look like a fool---after all I built the dag-gum thing and can't figure out how to use it lol :scratchhead:

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