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Eclipse Effects#

What happens when Hinode goes into and comes out of night#

1. There is Hinode night and Hinode twilight!#

The Earth appears bigger to EIS than to the other two instruments on Hinode because of the EUV attenuation of the Earth's atmosphere (be thankful for it). So we see longer "night times" than the Orbital Event file (obev...) indicates. In fact, we have to factor in about ten minutes before the official NGT_ENTRY and the same after the NGT_EXIT in those files to take account of the period when EIS suffers the effects of the Earth's atmosphere. Night proper typically lasts about 17 minutes, but factor in the ingress and egress ("twilight" times), and the whole thing last for about 24 mins each orbit (i.e., quarter of the orbit). And that's based on the first couple of days for which NGT events were predicted. These figures may evolve.

2. Pointing mode#

When Hinode goes into "eclipse", the pointing mode changes from gyros-plus-UltraFineSunSensors to gyro only (because it can't see the Sun any more!). In this case, there was prediceted to be a drift in the pointing. This gets corrected some time after Hinode comes out of night.

In the one case I've looked at where a mode change occurs (from gyro to gyro+ufss), the repoint takes of order 15 seconds. This was taken from the shift seen in eis_l0_20070509_074627.fits, a HH---N03 raster, with nominal 5-second cadence (more like 6-and-a-bit). This repoint happened between 07:53:27.275 (expo start time, admittedly) and 07:53:47.117 (expo start time). The corresponding NGT_EXIT happened at 07:45:30, so there is just over 8 minutes until the repoint due to mode switch.

Since the few datasets I've looked at so far (all from the same date, from the merged telemetry) indicate that EIS twilight isn't over until about 10 minutes after NGT_EXIT, this repoint delay is not such a worry for us. The bigger worry is the length of night, including twilight, which cannot easily be calibrated out.

Nights typically last 16 to 17 minutes in the EUV (no transmission), with the total time that obscuration effects happen (either twilight or true night) clocking up 23 to 24 minutes per orbit (or a quarter of one orbit). That's now, too. It'll be interesting to see how serious it gets in the deepest part of eclipse season.

For now, I'd recommend leaving 10 minutes on either side of the NGT markers in the OBEV file if you want untainted data, or just steering clear of data within 10 minutes of those markers.

One use for these data might be to use them as dark current calibration. There is effectively no input to the detectors (unless the 3p-1s transition is strong in geocoronal emission) so anything we see will be stray light.