Weather
Really clear and cool, very nice night. Seeing better than 2 arcsec, low temp around 60F, some dew so fairly humid once temp gets down near 60, very little wind. UPDATE: This is just one spectacularly nice night! Too damn bad the ‘f-ing’ AP1200 had to shit the bed.
Rig #1 – More troubles with AP1200 mount
It appears that the encoder in the declination assembly may be fried. Just a guess but it exhibits a few familiar symptoms: (taken from email sent to George at Astro-Physics)
1) I turn on the mount. The speed setting shown on the hand control is 64. If I press N or S the declination motor runs very fast for a very short period of time and then stops. The yellow light comes on.
2) This occurs whether I am using the hand control or the computer/APV2 driver, and occurs whether I have both or just one of the computer and/or hand paddle connected.
3) Anticipating one of the tests the AP guys might suggest I swapped the cables going to the RA and Dec motor assemblies. Note that when I do that the RA cable being connected to the Dec motor, if tracking is on the Dec motor immediately goes to high speed as soon as the power is turned on. So, turning the tracking off, if I press the N-S buttons the RA moves as expected. Pressing E-W causes the Dec motor to run and stop.
4) So I swapped the cables back to their normal positions. Note I also tested all of the pin-outs on the “Y” cable with a continuity tester and it seems to be just fine.
5) After swapping the cables back to their normal positions I noticed a strange behavior. I had removed the declination motor assembly and was holding it in my hand. I set the move speed to 0.5x sidereal and pressed N – the dec motor began running at a fairly slow speed (but much faster than sidereal – maybe 1 rpm). It would not stop – lifting my finger from the N button did not cause it to stop running. Pressing the N button again, however, caused the motor to run faster. In fact with each press of the N button the motor ran faster and faster. So I pressed the S button and, sure enough, the motor slowed with each additional press of the “S” button until I was able to get it to stop, then with the next press of the S button it began moving slowly in the opposite direction, again moving faster with each additional “S” button press.
6) All of the above was true whether using the hand control or the move buttons on the AP v2 Ascom GUI.
7) All the above was done using CP3 electronics (with firmware “S”). I still have the CP2 electronics that the mount came with (firmware version “E”). So I tried the same tests using that box and got largely the same results – though instead of the dec motor stopping almost instantly, with the CP2 electronics it would run for a second or two before suddenly stopping.
8) I also notice that the encoder itself is noisy – that is, if I open up the spur gear box and remove one of the gears so that the motor is not turning the encoder it is much quieter. The encoder makes a sound almost like the bearings were bad? If that makes any sense! It reminds me a bit of what the RA motor assembly sounded like before you guys worked on it, replacing motor and encoder back in May.
9) Looking at the RA and Dec in the v2 Ascom interface on the computer I noticed that even when the declination motor is running there is no change being shown in the declination readout. Moving the RA causes the RA readout to update as expected.
Of course this throws a huge monkey wrench into the WR program.
If getting this fixed costs as much as getting the RA assembly repaired I will have paid more for a used AP1200 CP2 than I would have paid for a new AP1200 CP3.
Rig #2: V1101 Aql
Ironically the mount that the AP1200 is supposed to be replacing is working just fine. Thank god! Joe Patterson urged I try hard to get as many nights of data on this object as possible, so at least having one rig operating keeps me sane! Anyway, Joe mentioned the need for good data from this longitude to fill the gap between Europe and the west US. Anyway, 75-second exposures. Got about 180 images. Out at around 1:55am.
Results from tonight’s data
Shown, below, is the light curve for V1101 from August 14/15. Pretty decent signal-to-noise, but notice the jump in the comparison star’s light curve. Two possible causes come to mind. First, it could be due to poor flat-fields contaminated with scattered light. The ~0.03 magnitude jump seems a bit much for that to be the case, especially since I can select a bunch of images from a few nights and combine them, filtering out the stars, and get a very flat background. The biggest issue with scattered light in flat fields (from what I understand) comes from the fact that the color of the flat source (in this case twilight sky) and the actual night sky are different. The reflectivity of the inner surface of the baffle tube is quite different at different wavelengths. But if that were the case I’d expect that the combined science images described above would show some sort of gradient – and something changing a few percent over a small region would be VERY evident. Which leads to the second possibility and that is variable PSF shape across the image. The technique used to reduce the data is called “optimal extraction” and uses a model PSF derived from a bright star in the image as a weighting mask. The technique is very sensitive to changes in PSF shape – so if there was, say, a lot of coma it might cause the effect seen. Again, though, a 3% difference over a small difference in location on the chip seems large to me. Perhaps a third explanation – bugs in my software? More investigation needed! Anyway, V1101 was a bit fainter tonight and that allowed the relative variability to be more obvious.
