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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 NGT events in the Orbital Event file (obev...) indicate. Originally, we dealt with this by factoring 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 lasts 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 with time as the orbit evolves.

This year, at the request of XRT and EIS, ISAS has added in calculations of XTW, or X-ray Twilight, in addition to optical night (NGT). It's very helpful to have these extra calculations, and we are road-testing with EIS to see if the predictions do a better job than the default 10-minute buffer around NGT that we used to use.

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 was the length of night, including twilight, which cannot easily be calibrated out.

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.

3. How much time is available for observing during eclipse season?#

The eclipses have their maximum duration around 20 June each year. The EIS CO is provided with the times for "XRT twilight" each orbit and, for 20 June 2009, the duration of XRT twilight was 30 minutes. The guideline for EIS COs is to begin the EIS plan 2 minutes after exiting XRT twilight. The Hinode orbit is 98 minutes, therefore the smallest possible useful observing period during the eclipse season is 66 minutes.

For study designers it is thus useful to make your study last 66 minutes or less in order for it to make best possible use of the EIS data allocation.