astrophotography – Page 2 – Daniel Estévez

Imaging times for DSLWP-B eclipse observation

In my last post, I spoke about the possibility of imaging the July 2 solar eclipse using the Inory eye camera on-board DSLWP-B. After discussing the plans for the observations with Wei Mingchuan BG2BHC, we have decided to activate the DSLWP-B Amateur payload during the following intervals:

2019-06-30 05:30 to 07:30
2019-07-01 05:30 to 07:30
2019-07-02 18:00 to 20:00
2019-07-03 06:00 to 08:00
2019-07-04 06:30 to 08:30
2019-07-05 07:30 to 09:30

The camera will be used in 2x zoomed mode, which has a field of view of 14×18.5, degrees. Using the zoomed mode requires careful planning, since part of the Moon needs to appear inside the image, to help the camera auto-exposure algorithm, but the Moon shouldn’t hide the Earth.

The June 30 activation will be used to test the camera, taking images of the Moon in similar positions to those on July 2. The Earth will not be in view of the camera on this day, but these tests will serve to validate camera pointing, exposure, and satellite ephemeris errors.

The following imaging times have been proposed:

2019-06-30 05:51:20
2019-06-30 05:52:20
2019-06-30 05:53:20
2019-06-30 06:29:50
2019-06-30 06:30:50
2019-06-30 06:31:50
2019-07-02 18:56:00
2019-07-02 18:57:00
2019-07-02 18:58:00
2019-07-02 19:31:45
2019-07-02 19:32:45
2019-07-02 19:33:45

The idea for July 2 is to take an image of the Earth and Moon just before the Earth becomes hidden behind the Moon and just after it reappears. Determining these moments accurately is difficult. The Moon will be moving rather fast across the field of view of the camera, since the orbit altitude is rather low. Therefore, the timing of these events is sensitive to the satellite ephemeris and the orbit propagation algorithm. To try to mitigate this effect, we will take a series of three images spaced one minute instead of taking a single image.

On June 30, the same imaging run is mimicked: a series of three images will be taken before the Moon hides the centre of the image (this time the Earth will not be present) and a series of three images will be taken after the centre of the image becomes unblocked again.

The figure below shows the camera view prediction for the June 30 imaging run. The calculations have been done with the 20190630 ephemeris from dslwp_dev.

We note that the second run of three images seems a little early. Wei is doing his calculations with STK and apparently he is getting slightly different orbital predictions compared to my predictions done in GMAT. We haven’t tried to study these differences, but this gives an idea of how sensitive the imaging times are to ephemeris and orbital propagators. Hopefully the series of three images will account for orbital errors. Additionally, after doing the test run on June 30, the results can be compared with the orbital prediction and the imaging times for July 2 can be modified slightly if necessary.

The figure below shows the camera view for the July 2 eclipse imaging run. The 20190630 ephemeris have been used for this plot also. We have the same effect, where the second proposed imaging times seem somewhat early.

Since this time the Earth is also visible in the image, it is convenient to plot the “Earthrise view” plot, which I have used on other occasions. This shows the angular distance between the Earth and the Moon rim, so it can be used to determine if the the Earth is hidden by the Moon (negative distance) or not.

As we can see below, according to my GMAT prediction, the Earth will not be visible in the images around 19:30. It seems these should be taken a few minutes later. However, Wei has obtained different results with STK. In any case, these imaging times can be corrected based on the results obtained on June 30.

The plots in this post have been made in this Jupyter notebook.

DSLWP-B and the solar eclipse

On July 2, there will be a total solar eclipse observable from parts of the Pacific Ocean, Chile and Argentina. This gives the opportunity to image the eclipse with the Inory eye camera on-board DSLWP-B, the Chinese lunar orbiting Amateur satellite. Wei Mingchuan BG2BHC has already started planning for the eclipse observation, and I have run my usual calculations using the 20190618 ephemeris from dlswp_dev.

The main interest in trying to do an imaging session during the eclipse is to photograph the shadow of the Moon on the surface of the Earth. The camera doesn’t have a large resolution, and the Earth looks small in the image, but perhaps it will be possible to distinguish the shadow clearly.

Besides this, it is also interesting to try to get the Moon in the image, as it has been done in other occasions. This not only gives a more interesting picture, but also helps the camera auto-exposure algorithm by providing a large bright object in the field of view. Past attempts to image the Earth alone have all yielded over-exposed images. It turns out that the orbit of DSLWP-B is ideal to image the eclipse, partly by chance and partly because of the nominal satellite attitude.

Recall that the camera of DSLWP-B is always pointing away from the Sun, because the satellite aims its solar panel towards the Sun. Since DSLWP-B orbits the Moon, this means that the Earth will be in the centre of the camera field of view whenever a solar eclipse happens. However, the satellite could be at any point of its orbit. It might happen that the Moon is between the satellite and the Earth, hiding the view, or, more likely, that the Moon is outside of the field of view of the camera.

The total eclipse is observable between 18:01 and 20:45 UTC, with the maximum happening at 19:23. The two figures below show the positions of the Moon and Earth within the field of view of the camera. As explained above, the Earth is near the centre of the image during the eclipse. In the bottom figure we see that the Earth is hidden by the Moon until 19:27.

Therefore, it seems that this is an optimal chance to image the eclipse. The Earth will emerge behind the Moon very near to the eclipse maximum. Since DSLWP-B is orbiting at a lower altitude in comparison with other imaging sessions, the Moon will move rather quickly through the camera field of view and disappear in a matter of 10 minutes.

Thus, my recommendation is that instead of taking images every 10 minutes, as it has been done in other occasions, a smaller interval of 2 minutes is used instead. A series of 9 images starting at 19:20 is shown in the plot above as green lines. This gives good coverage of the eclipse and the Earth appearing behind the Moon.

The figure below shows the simulation of the view in GMAT. Note that the field of view of the camera is smaller than what this image shows.

Astrometry with DSLWP-B camera

In my last post, I spoke about the images taken by DSLWP-B, the Chinese lunar orbiting Amateur satellite, during the first week of June. One of these images was the picture of stars shown below, taken on 2019-06-07 08:00 UTC.

Image 0xD5, taken on 2019-06-07 08:00, downloaded on 2019-06-07 09:30 – 09:45

Although it may seem that this image is not very interesting in comparison with the other awesome images of the Earth and the Moon, the stars that appear in the image can be used as a reference to compute where the camera was aiming. In a previous post, I used an image of stars to compute the field of view of the camera. In this post, I will assess the accuracy in the camera pointing. Currently, there exist only two images of stars taken by DSLWP-B, so it is interesting to try to study these as much as possible.

Report for DSLWP-B June imaging

In my previous post, I spoke about the opportunity to take images of the Moon and Earth using the Inory eye camera on DSLWP-B during the first week of June. All the tentative plannings for programming the image taking and downloading the images listed in that post were eventually made final, so the observation runs have been done without any modifications to the schedule.

On June 3, a series of 9 images with 10 minutes of spacing was taken starting at 03:05 UTC. This gives a nice sequence of the Earth hiding behind the Moon and reappearing. One of the images was partially downloaded during the same 2 hour activation of the Amateur payload on June 3. Several of the remaining images were downloaded between June 4 and June 6. On June 7, the station of Reinhard Kuehn DK5LA, which is normally used as the uplink station, wasn’t available, so a single image outside of the Moon series was downloaded using Harbin as uplink station.

This is a report of the images taken and downloaded during this week.

November DSLWP-B images of the Moon and Earth

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In previous posts, I have already spoken about the chance of DSLWP-B taking images of the Moon and Earth during the beginning of November. The window to take these images was between November 6 and 9. This window included the possibility of taking an Earthrise image, with the Earth appearing from behind the Moon.

The planning for the activations of the Amateur payload made by Wei Mingchuan BG2BHC was as follows.

7 Nov 2018 08:13 to 7 Nov 2018 10:13
8 Nov 2018 09:40 to 8 Nov 2018 11:40
9 Nov 2018 12:00 to 9 Nov 2018 14:00
10 Nov 2018 14:00 to 10 Nov 2018 16:00
11 Nov 2018 13:30 to 11 Nov 2018 15:30

On November 7, from 8:13 to 9:33 UTC, a total of 9 images with 10 minutes of spacing between each would be taken. These images would be downloaded during the activations on the next days. As usual, an image would also be taken when the Amateur payload powered up on November 8 to 11, but the main focus was on downloading the sequence of images taken on November 7. This is a complete report of the images taken and downloaded.

Download of DSLWP-B Moon and Earth pictures continues

In a previous post I spoke about the images of the Moon and Earth taken by the Chinese lunar-orbiting satellite DSLWP-B between October 6 and 10. Some images taken during these days hadn’t been downloaded yet. The activities have continued during this week by downloading the remaining images and taking and downloading new images. This is a report of the images downloaded between October 14 and 19.

Report of the DSLWP-B Amateur observations of the Moon and Earth

The Chinese microsatellite DSLWP-B has been in lunar orbit since 25 May 2018. This satellite carries an Amateur radio payload which includes a small 640×480 CCD camera. The JPEG images taken by the camera can be transmitted using the SSDV protocol at 125 bits per second in the 70cm Amateur satellite band.

Update 17:00 UTC: Wei comments that the camera sensor is CMOS, not CCD, and it has 2592×1944 pixels. The image is resampled to 640×480 to save memory and bandwidth.

The orientation of the camera is fixed: the camera is mounted looking in the opposite direction of the solar panel, which is usually kept pointing directly to the Sun. Therefore, the camera is usually looking directly away from the Sun. The possibility of imaging celestial bodies such as the Moon and the Earth depends on the relative positions of these and the Sun.

During the first week of October there was a new Moon, which implied that it was possible to take images of the Moon and the Earth, as I have described in this post and this other post. This is a report of all the images taken and downloaded during the observation window.