Forum Replies Created
-
Posts
-
Thanks. Here’s a short movie taken a little later when it had cleared up. Still a very bright Moon in the sky.
Thanks for the reminder. That’s a great movie. It is cloudy here in Chelmsford at the moment but I did have a few gaps earlier and caught the trail. It is currently around mag 15.3 moving at 7 arcsec/min.
https://openphdguiding.org/man-dev/Tools.htm#Comet_Tracking
Although I have never used this myself…
Well, given the position, it is possibly Leda but I don’t think you could be sure unless you had multiple stacks which showed the blob moving at the correct rate. If you try this in future you will want to get a long enough series of subs that you can make two separate stacks, each of which have enough SNR to get astrometry of the object. Once you have confirmed it by its motion you can always stack the whole set to get a higher SNR. That’s what I did with this faint comet which was about the same magnitude as Leda.
Good try Paul but that’s the trouble with faint objects in busy starfields! When you say it is “in about the right place” could you quantify in terms of arcsec? Leda’s ephemeris will be very good. Normally, in cases like this, you would produce two stacks at different times and check that the object’s motion is consistent with the ephemeris. You may not have enough subs to do this in which case the identification will always be iffy.
Try this:
http://www.nickdjames.com/astrolinux/utils_ubuntu18.tgz
The tools are all dynamically linked so you will need the libraries but they are all available for Ubuntu. I think the following should get them all:
apt install libcfitsio-dev
apt install libgd-dev
apt install libnetpbm10-dev
apt install wcslib-devNick.
Paul,
I have a VM running Ubuntu 18.04 LTS. I’ll build you a version of the tools in that. Hopefully that will work on your system. If not I’ll have a look at linking the libs statically and, failing all that I’ll give you the source (with an NDA!).
Nick.
Paul,
If you don’t mind command line software I have written a set of tools which I use as part of my image calibration and processing pipeline. They are based on the cfitsio library so should read any format supported by that library. You can download them from here:
http://www.nickdjames.com/astrolinux/utils.tgz
The stacking tool is called fcombine. The normal syntax to use it is:
fcombine -N -a 611,713 -A 0 -o 0.368,236.7,1.42 200p_ofs FLI*200P*FITThis says:Normalise the median level of each image before stacking, track on a guide star at position 611,713 in the first image, PA of the image is 0 deg (i.e. north up), Offset at 0.368 arcsec/sec in PA 236.7 deg, Pixel scale is 1.42 arcsec/pix. Output file is 200p_ofs and the input files are all files which match FLI*200P*FIT. You can add -F to write as a float fits and -s to do a true average rather than a statistically clipped average. You can also use -v to get a lot of stuff about what it is doing during teh stack (otherwise it is silent). Note that the position for the guide star assumes (1,1) is the top left corner of the image (the convention most amateur software uses) rather than the bottom left (which is what pros tend to use). The FITS standard doesn’t say which is right.
Typing the command with no arguments gives a list of what it can do. For fcombine you get:
fcombine [opts] outfile infiles
By default combines images using mean with outlier rejection
-a x,y[,d] Align on guidestar
-A n[,M] Position angle of image with optional mirroring
-C Cometstack
-c n Crop to image of size nxn
-d n Resample and drizzle by a factor of n
-F Write float format rather than int16
-f file Take alignment offsets from file
-g n Apply gain of n to output
-h Do not use FITS header
-m Minimum pixels
-M Maximum pixels
-n Normalize mean levels by scaling
-N Normalize mean levels by offset
-v Verbose
-V Version
-r n Tracking and PSF radius to use (default = 5.0)
-I Scale to a FITS that IRIS can handle
-R Write output mirror reversed
-o rate,pa,pix Offset (arcsec/min)
-O rate,pa,pix Offset (arcsec/frame)
-p n Add pedestal of n
-P n Reject for PSF > n
-D n Shift step size (default is 1)
-s No outlier rejection
-S n Stacksize (all images by default
-t Align on targetRonan, I can’t make it this year but I can absolutely commend this meeting to anyone in the BAA who is thinking of going. It covers a subject which is really important to all of us, not only observers but people who value the natural environment. Newport is a great town and the skies are really dark between occasional clouds.
Amazing that you can get a spectrum at all that close to the bright core. I had my first chance to image it last night and it seems to have faded quite a lot.
Interestingly someone at ESA implies that this would have been possible in this Twitter thread but I don’t think it is. As I understand it the best case delta-V of comet interceptor from L2 is around 3 km/s. If that was all used as a prograde, in-plane impulse it wouldn’t be enough to raise the aphelion to 2au (the Earth’s velocity around the Sun is around 30 km/s).
Nice idea but the perihelion distance is around 2 au so comet interceptor wouldn’t have been able to get close. I managed to get it this morning despite the annoyingly bright Moon.
I don’t see that there is any harm in asking for PDF handouts, in the same way that we always ask speakers if we can post their videos. I can understand why some pros don’t want to do this and, if the speaker doesn’t want to have anything online then fine, but I don’t see why you would recommend that we don’t even ask (that is new to me by the way).
David. Thanks for this image. It may not look that impressive but it is extraordinary that we are now able to discover and track objects like this which have come from interstellar space. I’m sure that the pros will be doing everything they can to get a spectrum of this object in the near future. I’ve not been around to image this yet but will try as soon as I get the opportunity. It is not often that you get the opportunity to image a chunk of ice from another star system with an amateur telescope.
Hi Alex, If you or some other willing helper can collect the material I’ll find a way of getting it uploaded to the website. It would certainly be good to get it all in one place.
Now designated C/2019 Q4 (Borisov) in the MPEC here.
Mike. Yes a great weekend. Good to meet up with everyone and Armagh (City and Observatory) did us proud. I agree about the discussions in the bar afterwards. They are always most productive. Many thanks to our meetings secretary, Hazel Collett, for all the organisation and to the speakers and to everyone from all over GB and Ireland who turned up.
