Well we are back in OC after attending the APS April meeting in Denver were we heard many good discussions including the successful imaging of the black hole by the Event Horizon Telescope (EHT) collaboration, as covered in our post of April 14, 15, 16 and 17, and now wanted to review some black hole horizon physics and also try to image M87 ourselves using the new light weight Sky Watcher mount.
The figure below, found in the book "Exploring Black Holes" begins to look at how the paths of light rays approaching a black hole can be distorted and bent and even go into orbit around a black hole. This bending of light, or more generally, gravitational lensing, works just as well with galaxies and halos of dark matter, but the figure illustrates the concept. Also, in the case of the recent EHT image of the M87 black hole event horizon, some of the light that we see was produced by interaction right at the event horizon and surrounding accretion disk and not necessarily from remote stars.
|One example showing gravitational bending of light from a distant star (Exploring Black Holes (Source: E. Taylor and J. Wheeler, 2000)|
This illustration is just one of many diagrams in the book. The diagrams are supported by much of the theory and calculations associated with the physics of general relativity covered by, mainly, the Schwarzschild metric of black holes. This metric, using just algebra and a touch of calculus, can be used to calculate the distortion in space and time around a compact gravitational object like a star or even a black hole. The book "Exploring Black Holes" is a gentle way of getting into the calculations of GR. There is even one chapter on the effects of black hole spin.
|Exploring Black Holes (Source: E. Taylor and J. Wheeler, 2000)|
Ok, ok, so enough of the astrophysics, it's time to do some observing with a new lightweight goto mount! Ever since OCA Craig Bobchin told us about the new SkyWatcher mount (see the blog entry of April 5, 2019 ), I wanted to get one because my goal is ease of setup of the mount and being able to go outside and within 5 minutes being able to goto any object then up in the sky. My friends (and your friends too) at Amazon had one on my doorstep very quickly after just one little click on my IPAD. I elected M87 as the first object to go after in this trial use of the new mount. I especially wanted to see if the M87 jet could be imaged with this new setup?
|Lightweight Sky-Watcher goto mount (Available from Amazon and other retailers)|
Now if you are a serious astroimager, this mount might not be for you, but for the occasional astroimager, and more often someone who just wants to go outside and make a quick light curve measurement of some star for AAVSO, for example, I hoped that this mount might be the way to go. So here is the mount with the Nikon Coolpix camera, with superzoom lens with 24-3000mm focal length, mounted on the Sky-Watcher AZ-GTi mount.
|Superzoom Nikon camera mounted on Sky-Watcher mount (Source: Palmia Observatory)|
So, at the first available clear night, I carried the mount outside, with just one arm, and carried out the camera with the other arm. The hand controller for the mount is an app that runs on your SmartPhone. The mount uses the time and location from your phone to know where it is and the app prompts you for several available alignment steps. I elected to use a 3-star alignment and the app let me choose three visible stars. After alignment, which took just a couple of minutes, I immediately slewed the scope to M87 and took some images of what I hoped would be M87.
This 30 second image was taken with maximum camera focal length set to 3000mm. That focal length setting is pretty impressive if the photo quality turns out ok. The field of view is 39 x 29 arc minutes (as determined by astrometry, see below). Hey, hooray, there is a fuzzy little bit of light in the left center of the image.
|M87 shows up dimly, center left in this 30 second, 39 x 29 arcminute image (Source: Palmia Observatory)|
But is that little bit of fuzzy light M87? Yep, the image below shows the astrometry for that image, courtesy of Astrometry.net. So the goto capability of the Sky-Watcher was pretty close to right on, at least in this first trial use.
|Astrometry of Nikon Coolpix image shows M87, center, left (Source: Palmia Observatory)|
Just in case you are interested, I blew up the image of M87 and you can see it below. Hmm, it doesn't look like we will be able to see the jet in this image and in fact it is not very detailed compared to other images I have seen.
You can see some star streaking, so the alignment is not exact, and the sky is quite bright due to city lights. So, the Nikon Coolpix camera limitations are now also becoming apparent. The exposure setting is limited to 30 seconds and even in the bulb setting, it is limited to 60 seconds. I didn't attempt a longer exposure because I didn't have a remote shutter release for the camera. The other drawback is that for long focal length settings, Nikon only allows ISO = 100. True , you can adjust the ISO to higher values, but only for lower settings of the focal length.
We can also see, now, why the advertisements for the Nikon P1000 always describe the focal length, 24-3000mm, as "focal length equivalent." The "equivalent" term is key and refers to a comparison with a full size 35mm image and when you go through the details ad compare the focal length of the P1000 image to the image taken with the 600mm DSLR, you can see that the "equivalent" focal length is really only about 1800mm. Compare the P1000 field of view of 39x29 arcminutes to the DSLR field of view of 2.19 x 1.46 degrees.
Also the Nikon P1000 does not support deeper imaging with longer exposures than 60 seconds, even in Bulb mode. Also, you can't set ISO higher than 100 in the longest focal length setting. Both of these limitations are real limitations for going deeper. Of course, all of you serious astroimagers already know that one good step would be to observe from a more dark sky site with a larger aperture device with longer exposure times and even stacking of images. Hmm, yep, that is true, but I'm not ready to go down that road yet!
|Cropped image showing some of M87 structure (Source: Palmia Observatory)|
Ok, so that was that first night, and the next night was clear too, so this time I switched to another DSLR with a 600mm telephoto lens and the Sky-Watcher mount. In this non-flash iPhone photo, you can get a sense of the amount of background city and house lighting.
|Trying out the Sky-Watcher with 600mm telephoto lens on DSLR (Source: Palmia Observatory)|
For the attempt with the 600mm telephoto lens, I only used the 2-star alignment process and for that reason, or other possible reasons, which we will go into later, the star trailing was much worse. Yes, the goto function got the camera pointed near M87, but the tracking was off and we can see star trails. In this image, which has field of view of 2.19 x 1.46 degrees, you can see star trails in this 60 second image that are about 2 arc minutes long. So the Earth rotation rate of 15 degrees per hour, which is 15 arc minutes per minute of time, is not being completely compensated for.
|Star trails and very dim, stretched M87, in this 60 second, 600mm, image, (Source: Palmia Observatory)|
Before looking into why the tracking was not that good, the above image was uploaded to Astrometry.net, to make sure that M87 was in the field of view. Yep, there it is, so the 2-star alignment was sufficient for the goto function.
|Astrometry of 60 second, 600mm DSLR image shows M87 (Source: Palmia Observatory)|
So was the less than ideal tracking in this example due to just using the 2-star alignment, instead of the 3-star alignment process, or due to other factors, including "operator error"? The 61 second exposure below shows the background stars around Castor, which was one of the selected alignment stars. Hmm, the tracking in this photo seems to be pretty good; not much star trailing!
|And yet, at alignment star, Castor, tracking looks good in this 61 sec, 600mm image (Source: Palmia Observatory)|
So, all in all, I was quite happy with the performance of the Sky-Watcher mount. The mount and the Nikon Coolpix P1000 both have their limitations when it comes to serious astroimaging, but for the occasional astroimager and the astronomer wannabe with a high requirement for ease of carry and setup, they both might be quite worth while. Below, I try to summarize some of the lessons learned with this trial use of two new pieces of equipment.
2. The image goto capability seemed to be pretty good even with just 2-star, rather than 3-start alignments.
3. The tracking capability, during this initial run of observations, was not super good, but it is not clear if further "operator" attention to details wouldn't result in better tracking. The key point is that the mount works fairly well for a mount that can be carried outside with one hand.
4. If you are the more serious astroimager with a permanent or mostly permanent scope setup, you probably will not be happy or satisfied with the AZ-GTi. On the other hand, if your main concern is not tracking accuracy, but ease of setup and transportability, the AZ-GTi is a pretty good deal.
5. The observed tracking performance of the Sky-Watcher mount can probably be improved by correcting for some "operator error and/or laziness." I only relied on eye ball assessment of the tripod leveling, even though the built in levels showed the mount was level, the initial starting position of the optical axis was not perfectly level. Also, since a camera only attaches to the mount with a single 1/4-20 screw, there can be misalignment between the desired 90 degrees and the elevation axis. But all of these potential errors can be corrected by the operator if he/she were only to use their level. Maybe on one of the next clear nights, we can see if these actions improve the performance.
6. The Nikon Coolpix P1000 camera, with superzoom between 24mm-3000mm, worked fairly well, given the limitations of its small photo sensor, which means that the wide range in focal lengths is given in equivalent terms as if the P1000 had a large photo sensor, like on a real full image 35mm DSLR. So, compared with my Canon T6 DSLR, with 600mm telephoto lens, the angular resolution of the P1000 was, on an equivalent basis, operating with a focal length setting of 3000mm, really about the same as 1800mm equivalent, when compared to the DSLR APS-C sensor size.
7. The Nikon Coolpix P1000 camera has serious limitations for astroimaging also in that the ISO setting is limited to 100 for long focal lengths and the available exposure settings are limited to less than 60 seconds.
Until next time,
Resident Astronomer George
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