- This topic has 25 replies, 6 voices, and was last updated 1 year, 8 months ago by
.
-
Posts
-
I’m a big fan of ASTAP! It’s by far the fastest and forgiving for platesolving, it’s great for photometry of variable stars and it can also do comet photometry among a whole load of other things. You can also output in BAA VSS format now.
I’ve been looking into using Comphot recently and the big problem was not having the MZERO FITS header keyword set in any of my files. I’m not a fan of Astrometrica – although I did eventually get it working. It does have some useful features but it feels a little old and clunky to me. I feel ASTAP is more up-to-date and certainly going places (as well as being free).
I’ve recently been speaking to Han K who wrote ASTAP and he has now added in the calculation of MZERO. Simply process: add your lights, calibrate using darks / flats etc., platesolve (in 0.2s usually), set the photometry aperture to Max (this is required) and then Ctrl-U (the Calibrate photometry command) and MZERO is created. So you can then use the output files for Comphot. You can also stack on the stars or the comet, as needed by Comphot.
I haven’t enough experience to know if the values and outputs created are good enough so it would be useful to know what other more experienced comet people think. Han is of the opinion that the catalogues that ASTAP uses are more accurate than UCAC4 that Astrometrica uses. Again I don’t know…
You can also do comet photometry directly from ASTAP and I and a few other Astronomical Society of Edinburgh members have been using this approach successfully (we think 🙂 for a while. You can see a quick 5 minute video of how to do it here: https://www.youtube.com/watch?v=BEjcSm_cZx8
Very straightforward and quick. You can also measure the angular size of coma, tail and direction.Currently it uses a rectangular aperture which is not ideal for coma brightness measurements but Han will be adding a circular one shortly. It would be great if others who know more could advise on this.
I think it would be great to have another tool for comet photometry and I think ASTAP could be a worthwhile addition. What do you think?
More info on the ASTAP site: https://sourceforge.net/projects/astap-program/
I can’t comment on ASTAP but can say that APT (A Photometry Tool by Russ Laher) considers circular apertures to be a special case of arbitrary elliptical apertures. This may be an appropriate way of enhancing ASTAP if the code is going to be uprated anyway…
Elliptical apertures are extremely useful for phometry of galaxies. Might they also be for cometary comae?
Thanks for the link to sourceforge. I will check out its contents.
Mark – Thanks for signalling that ASTAP might be suitable for comet photometry. Please pursue this and see where it leads.
Paul, yes – circular apertures are just a special case of elliptical ones, which can also be adjusted for any tilt angle. Interestingly, there is also another form of photometric aperture used by pros called the ‘capsule aperture’, which is like a rectangular aperture but it has a hemispherical form at each end of the narrower sides of the rectangle. It is used for the photometry of trailed images.
Richard: thanks for revealing the term “capsule aperture” to me. I may well contact Russ and suggest that he consider adding them to APT.
For only slightly trailed images (major/minor axis < 2, say) an elliptical aperture works very well – as measured by comparing the results for untrailed subs and circular apertures with their trailed counterparts of the same field and a carefully chosen elliptical aperture. If you examine some of my entries in the VSS database you will find some examples of elliptical aperture photometry.
Richard: thank you for revealing the term “capsule aperture” to me. I may well contact Russ and suggest that he consider adding them to APT. He has been very welcoming of suggestions for enhancement in the past.
For only slightly trailed images (major/minor axis < 2, say) an elliptical aperture works very well – as measured by comparing the results for untrailed subs and circular apertures with their trailed counterparts of the same field and a carefully chosen elliptical aperture. If you examine some of my entries in the VSS database you will find some examples of elliptical aperture photometry.
There’s a conversation on the ASTAP forum about adding a circle / ellipse and how to measure the sky background. Perhaps others who know more about comet photometry than me (most people!) could advise and join?
https://sourceforge.net/p/astap-program/discussion/suggestions/thread/b1c1be085c/
Han has added the option to measure inside a circle / ellipse now in the development version (right mouse drag while holding down the Ctrl key).
Thanks
MarkI will get in contact soon. From the GUI point of view, APT works rather well in setting the ellipse parameters.
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.
I’ve put a pair of green channel stacks of C/2022 E3 here:
https://nickdjames.com/comphot/Examples/2022E3/
The non-offset stack has been plate-solved but please re-do that with whatever tool you use. The offset stack is a sigma clip at the comet’s offset rate. It will have an identical zero point to the fixed stack.
Have a go at measuring the comet’s magnitude and coma diameter using these images and whatever tool you prefer. For consistency please use Gaia DR2 or 3 G magnitudes. It will be interesting to see what results we get.
Interesting – I see there is a fairly bright star mixed up with the inner coma.
I can do absolute photometry on the coma using a capsule aperture.
I have first subtracted stars manually from within the coma using SalsaJ on the non-offset FITS image.
Using AstPhot32 I can go up to an aperture 200 pixels across and many hundreds of pixels in length.
This way I would not use the offset stack, which I am suspicious of because the stars are not completely eliminated.Can you suggest what size aperture encompasses all of the coma – similar to what the eye would record?
I can make sure I go beyond that with the multi-aperture photometry.Richard
Richard – Thanks for having a go at this challenge. Comphot estimates a coma diameter of 29.6 arcmin from these images. The attached file is one of the diagnostic plots that it produces and this one shows the diameter that it has determined. The estimator basically generates a set of circular rings centred on the photocentre and keeps going out until the level in the ring falls to less than 0.8*sigma above the median sky level where sigma is the standard deviation of the noise in the background image. This threshold is rather arbitrary but it seems to work reasonably well.
BTW this green frame is one of the components of the final image here:
https://britastro.org/observations/observation.php?id=20230123_023135_df98af0b86eb5907
The colour image nicely separates the green coma from the whitish dust tail.
Attachments:
Hi Nick
I’ve had a go using ASTAP. It should be said that the two files you provided are not in a form we would normally use for ASTAP so this can’t be accurate.
I get mag. 5.6 and coma of 20′. There’s no way I can see a coma diameter of 29′ in the data using ASTAP.Measuring the coma diameter is always tricky and I’m sure Comphot will be better at that with the techniques it uses.
It would be interesting to see if the files produced by ASTAP are suitable for use with Comphot though. It can stack on comet or stars, platesolve the star stack and also provide MZERO.
MarkWell that’s not true now. If I stretch the levels to extremes I can get a coma width of about 29/30′.
Attachments:
Mark,
Your 5.6 is a little brighter than comphot’s estimate which was 5.8 but that is not a bad agreement for a big comet. The full output from comphot is:
10×10 20×20 30×30 40×40 50×50 60×60
10.07 9.00 8.41 8.06 7.79 7.58 # Counts
10.07 9.02 8.48 8.11 7.84 7.62 # Median annuli
Total integrated magnitude: 5.78 (radius 891.1 arcsec)
COMPHOT: 1.4.6 2023 01 23.052 5.78 29.70 18.47 23.50 19.02 3076.2 5.50 Astrometrica 4.12.0.448 Nick James C_2022_E3__ZTF_ G_2022_E3_o_0000.fitThese measures are pretty much in the middle of the estimates on COBS:
Could you provide a step-by-step description of the approach that you used with ASTAP please?
re to Grant,
If light is spread on many pixels you can adjust the aperture circle see documentation p76; step 4 adjust the aperture
Should answer extended sources?
Regards
Peter
- You must be logged in to reply to this topic.