BAA Winchester Weekend - Step 1 - Astrometrica
Thanks to Dropbox, the following link will allow you to download the five images of comet 70P Kojima that I used in the demonstration. When you open these you will see that you don’t need pretty images to do this work, but your images should be fully calibrated. In my case, I use bias, dark and flat frames to reduce the images in maxim DL. You do not need to use Maxim DL, but I do because it links well with the ACP Observatory Control software that I use to manage the equipment in Australia. I never guide my exposures, partly because I spent some time when setting up to minimise periodic error in the Paramount mount, but also because comet exposures tend to be short. The image scale on my system is 1.35 arc seconds per pixel. Given that many comets move at about this rate, I typically expose for a minute, very rarely going beyond three minutes and often quite less. The exposure lengths on the images of 70P are 150 seconds, which was possible given that the comet was only moving at 0.52 arc seconds per minute when the exposures were taken on 17 March 2015. To complete Step 1 you will need: - Astrometrica - UCAC 4 catalogue - CMC-15 catalogue (or CMC-14, the version prior to CMC-15) Astrometrica is a fantastic piece of software, a bargain at Euro 25 and available from Herbert Raab’s website at the following link: I keep local copies of the UCAC 4 and CMC-15 catalogues on my hard drive so that I can work when away from home, on the train, etc. but you don’t need to. Astrometrica will access portions of the catalogues as required if you have an internet link - just set the “VizieR” options for the relevant catalogues in Astrometrica’s “Catalogs” section (see below). When you have Astrometrica and my images of 70P, you’ll need to set up Astrometrica to be able to automatch the images to UCAC 4 and CMC-15. The following screenshots show how I set up Astrometrica (click the pictures to enlarge). If you do the same you should have no difficulty in automatching the images. When these settings are input, open the five images of 70P in Astrometrica. You now need to measure the comet’s position and brightness in each of the five images twice, once using the UCAC 4 catalogue and again using the CMC-15 catalogue, in this order. This is required to provide FoCAs with the information it requires in Step 2. So, start by selecting UCAC 4 as the star catalogue in the “Program” tab of Astrometrica’s settings and then hit “Astrometric Data Reduction (Ctrl+A)”. You may see a pop-up box saying “Reference Star Match Error” but if so just click on the button “Automatic Reference Star Match using 50 Stars” and all should be fine. The image below shows how Astrometrica looks for me following a successful Astrometric data reduction. I have used Astrometrica for a few years now and really like it, but occasionally I struggle to successfully automatch images. I have found that by tinkering with the settings, in particular the upper and lower magnitude limits of the star catalogues, I can usually get it to work. But I’ve never found a simple answer, I’m afraid you’ll have to tinker and find settings that work for your system through trial and error. The help files are very good, it’s worth spending time on these. Assuming that you have successfully matched the images to the UCAC 4 catalogue, click on the “Blink current Images (Ctrl+B)” button and you should see the comet move as each of the five images is blinked. Now, cycle through the images one by one and for each, measure the comet by clicking on it and pressing the button to the right of “Object Designation” - see image below. If all is well you will see the menu in the image below which shows 70P in the expected position (zero in dRA and dDe). If so, click “Accept” and continue to the next image until all five are measured. Once you have measured the comet’s position in each of the five images using the UCAC 4 catalogue, close the Blink image, go back into the “Program” tab of Astrometrica’s settings and select the CMC-15 catalogue, then hit “Astrometric Data Reduction (Ctrl+A)” as before to match the images to the CMC-15 catalogue. As before, blink and cycle through the images and measure the comet’s position in each of the five images using the CMC-15 catalogue. And that’s all there is to Step 1. The information required by FoCAs in Step 2 is held in Astrometrica system files, so you can progress to Step 2 (leaving Astrometrica open in the background). But if you want to check whether you have the same readings as me click on the “Send MPC Report” button at the top of the Astrometrica page. (Don’t worry - this will not send a report as the settings have not been completed). It will open a window as demonstrated in the image below. That’s the end of Step 1.
Hills Observatory: 1 January 2013 to 30 March 2020