Forum Replies Created
-
Posts
-
Hi Mark,
Astrometrica may be old and it would be a lot better if Herbert Raab open sourced it but it is the current standard for amateur astrometry and many comet and asteroid observers use it. It is worth the effort to learn and it can use a wide range of different catalogues including UCAC4 and Gaia DR2. Comphot was designed to work with Astrometrica since most observers will have already done astrometry on their frames using it and it provides a fast way of getting a measurement from stacks of frames that have been used for astrometry.
The main problem with photometry of comets is that the magnitude is fairly strongly dependent on the estimated coma diameter. Comphot uses a rather arbitrary, but at least objective, way of estimating this. For brighter/larger comets you have the additional problem of needing to remove the contribution of stars in the photometric aperture. Comphot does this by assuming that the coma is circularly symmetric. Remember that the magnitude is the magnitude of the coma, not including the tail and so a circular aperture is best since the coma is generally pretty spherical. You generally don’t need an elliptical aperture since any elongation is probably the tail.
There are a number of tools out there to do accurate comet photometry. Thomas Lehman’s AIRTOOLS is a good example. It is great to see that ASTAP is being used for this too and I’d be happy for people to use any tool that they want as long as they get consistent results.
It would be good if we could all have a go at measuring a reference image with different tools and see what results we get. I’ll dig out one of my C/2022 E3 images that we could use as an example.
Peter Pravec derived a rotation period of 0.105hr (6.3 minutes). I had pretty rubbish conditions last night but this is a lightcurve folded onto that period for two runs separated by 12 minutes. It fits quite well.
Attachments:
Yes, this NEO is very well placed for us to follow this weekend. The original orbit had a very close approach tomorrow but the latest orbit on Horizons gives a miss distance of 176,000km tomorrow night. Tonight it is around 640,000 km away and around mag 14.
https://britastro.org/observations/observation.php?id=20230324_205728_032144a771899a47
That’s a great video. The Merry-go-round observatory must have been a pleasure to use.
Starlight Nights is one of my favourite books. It book evokes the sense of an era long past and Peltier’s love for astronomy comes out of every page. Astronomy and the world were both very different then.
Gary – No dodgy hills around here that I know of, but my local Church and the trees around it are rather brightly lit. Here’s one of Jupiter and Venus from 1835UT tonight (Feb 25).
Attachments:
It would have to be rather weird software since I would think that MJD would almost always be stored as a double. I suppose in some odd cases it might be stored as an uint16_t but then it would break at 65536, not 60000 and I have no idea why anyone would do that.
John Mason send me a video of the event from Barnham, West Sussex. I’ve put a processed version here: https://www.nickdjames.com/meteor/2023/202302/2023cx1_20230213025922_jmason.mp4
This was shot using a Sony A7s and 20mm lens at f/4.
The bright fireball recorded off northern France at 02:59:20 this morning was the cause of these flashes. The object that caused the fireball has now been officially named 2023 CX1:
https://minorplanetcenter.net/mpec/K23/K23CA3.html
It is the seventh time that an object has been discovered just prior to entry. The previous ones being 2008 TC3, 2014 AA, 2018 LA, 2019 MO, 2022 EB5 and 2022 WJ1.
Unfortunately weather over most of the UK was poor but people along the south coast saw it.
https://fireball.imo.net/members/imo_view/event/2023/937
John Mason saw it from Sussex and has sent me a video. I saw the flashes through thick cloud from Chelmsford at 02:59:21.
Details of the astrometry so far.
Attachments:
Hi Alan,
The program isn’t that clever! You need to tell it the X,Y coordinates of a guide star and search radius using “-a x,y,r” (this is used to align the stacks) and then the comet offset rate and image scale using “-o rate,pa,scale”. You can also tell it the image orientation if it is not north up using “-A pa”. Have a look at the autostack script to see how I have automated this. Alternatively you can get all of these things manually. A valid example command line would be:
fcombine -C -N -a 1006,221 -A 1.37 -o 8.86,190.2,5.49 outputfile inputfiles
In this case -N says to normalise all of the input images to the same sky background before stacking, the guide star is at 1006,221 in the image (using the default search radius of 5 pixels). The image PA is 1.37 deg, the image scale is 5.49 arcsec/pix and the comet motion is 8.86 arcsec/min in PA 190.2.
If you just type fcombine with no options it will list the options that it can accept.
Hi Alan,
The “could not allocate output buffer” message occurs if the program can’t allocate enough memory to do the stack or if the star and/or comet offset pixel-shift lists are incorrect. Can you post the exact command line that you are using so I can see where it might be going wrong.
It looks as if I can’t attach scripts to the posts here but you can find it here:
https://nickdjames.com/astrolinux/
Nick.
You can provide the image coordinates of the guide star manually or use something like SourceExtractor to select one automatically. I don’t use a Paramount but it tracks the guide star frame by frame so you would need to have pretty awful PE for it to lose lock. The search radius is configurable and if some of the subs are trailed you can set a PSF threshold which is used to reject images from the stack.
As an example of how it can be used the attached script uses fcombine to automatically stack stars and comets. The STAR option is simple. The COMET option is a bit more complex since it uses a local version of astrometry.net to platesolve so that it can get the offsets right.
Grant,
It’s command line so will run fine under WSL.
It is indeed C.
It only translates images. There is no need to rotate with an equatorial that has decent polar alignment. Translation is by integer pixels. There is no sub-pixel interpolation since that would be bad for photometry. Alignment is done by centroiding a reference star.
Paul – That is why I have never liked fork mounts although the price I pay is meridian flips. The comet was really nice tonight despite the bright moonlight. I get it to be 5th mag now. Here is a picture of my telescope effortlessly pointing near to the pole taken while it was taking the picture in the inset. I don’t get many chances to take a picture like this!
Attachments:
David. Yes the ion tail is very active again. Your images are excellent. Your fast RASA is ideal for this.
Gideon van Buitenen has produced a really nice simulation of the comet’s appearance which you can find here:
http://astro.vanbuitenen.nl/resources/C2022E3_simulation.gif
It shows the PA of the ion tail swinging around very rapidly over the next few days.
Interesting. I let my iTel subscription lapse since their kit was unreliable and the cameras were mostly old and/or not well calibrated. If things improve I might be tempted back. I did try Skygems but many of their instruments are offline or broken at the moment. Does anyone have any suggestions for good, reliable, high-quality remote observing. I would prefer pay-as-you-go since I don’t use it that often.
A very nice event. I got it on two cameras from Chelmsford:
https://britastro.org/observations/observation.php?id=20230111_000436_356d03a9d87988cd
There is a video from a fireball camera in Lanzarote which shows a bright and slow fireball northwest of the Canaries at 23:18:50.
https://twitter.com/RedSpmn/status/1612727698048311297
Drop time was 23:09 so this is around 600s downrange. Virgin Orbit stated that the vehicle reached 4.9 km/s which is a long way short of orbital velocity but fast enough to ablate on re-entry so this is consistent with re-entry of the second stage and payload.
If anyone else has contacts in the Canaries please ask if they saw anything.
As Grant says, putting stuff in orbit is still hard. Falcon 9 makes it look easy but Space-X has had a lot of practice.
