Forum Replies Created
-
Posts
-
I understand what you are saying but it goes against the grain to throw away information. Here’s my reasoning.
Suppose that on the night in question SV Ari was at V=19.45 and that I had easily enough SNR to measure it to an accuracy of 0.01 magnitude based on an extrapolation of the sequence magnitudes down below 169. I would report a positive result.
Suppose that a field star was also measured on exactly the same image at, say, V=19.61, also to an accuracy of 0.01 magnitudes. I feel I would be justified in recording it as such, if only in my own records. Note whether or not that second star is a variable is irrelevant because it is being measured at a specific point in time.
Now, a week or so later, SV Ari has faded to a true magnitude of, say, V=22.0 which is way below the detection limit. However, that same field star is still measurable on an image taken at the later date. For the sake of example, let´s say it is now measured at V 19.62 with accuracy 0.01, again using only the official sequence. It is quite irrelevant in this particular instance whether that star has truly faded slightly or whether the difference between the two measurements arises for SNR reasons. It is quite clear that SV Ari at this date is significantly fainter than V=19.6. It seems wrong to me to throw away the additional information about the limit on the brightness of the variable.
Please note, in the latter case, I would NOT be using the V=19.6 star as part of the sequence to determine instrumental magnitudes and their errors. All of that is still being done exclusively with the standard sequence through a lengthy extrapolation.
Yes, I´m quite prepared to work with Jeremy and/or the AAVSO to extend the sequence in this case and others. However, prospective additional sequence members will need to be checked that they do not vary significantly on timescales ranging between hours and years before they can be used with confidence. (This issue has already bitten me: I discovered that one of the AAVSO comparisons for V3721 Oph is an EA with minima 0.025 and 0.010 magnitudes.) My suggestion, on the other hand, requires no assumption of constancy, only that the limiting magnitude can be measured at the time of observation.
Geosynchronous satellites have been plentiful for decades now. The good thing is that their orbits are known precisely and their ephemerides are widely available so that you can take your images at times when they are absent. Further, they are confined to a small strip of the sky so there are many other areas to image without their intrusion.
The real err… persons born to unmarried parents, are the likes of the Starlink constellation. IMAO, anyway.
A NEO will also move during and between exposures but very likely nowhere as quickly and certainly nowhere nearly as bright. Both characteristics are easy to determine in software.
Yup.
I’m on their mailing list. They appear to have mitigating procedures under development.
AFAICT, they would prefer not to have to deal with it but they are prepared to do so.
One approach is that each field is imaged in several filters. A satellite will have moved between successive images, so a transient still stands an excellent chance of being detected. A problem, as I understand it, is that the calibration images (i.e. those without transients) will require much more care in their preparation.
VS observers such as myself are relatively fortunate. We already have to deal with satellite trails and cosmic ray hits but, as long as they don’t lie within the apertures around the VS and comparison stars, they don’t matter.
Those interested in precision astrometry — of asteroids and comets, for example — are in a similar position.
It’s the takers of pretty pictures who will have the real difficulties.
Perhaps more people will become interested in the measurement of images rather than in imaging per se. I’d argue that the former is more scientifically valuable but I’m a scientist and so can be expected to be biased.
I can provide my VS pipeline to anyone who may find it interesting and/or useful. The stages run as follows, where the bolded terms are my scripts, most of them in Perl, one in bash:
- Download a CCD photometry HTML file from AAVSO’s VSP page, then run it through CCD2APT which produces two files, one a source list for APT and the other a comparisons list for APT2VSS (described below). This needs be done only once per variable.
- (Optional but recommended) Check the images for plausibility — correct field, not trailed, no inconvenient satellite tracks, etc. I use the ds9 viewer.
- Put a WCS on the FITS files with solve-field from a local installation of astrometry.net. For efficiency reasons I store a pre-parameterized call to solve-field held in a trivial shell-script. That for SV Ari reads:
solve-field -O -p -L 0.25 -H 1.5 -u app -8 pos -3 03:25:03.34 -4 +19:49:52.9 -5 2.0 -z 4 *.fit
- Remove all the extraneous crud that solve-field leaves behind with clean_solve.
- Co-add if desired, using SWarp with COMBINE-T = AVERAGE
- Use APT for aperture photometry, either interactively or scripted. APT is very easy to script.
- APT2VSS converts APT’s CVS-format output into a TSV file ready for submission directly to the BAA-VSS database.
- (Optional, every so often) Accumulate all the individual TSV files for a given variable with merge_phottsv into a multi-record file suitable for BAA-VSS submission.
- (Optional, once a month) Run all the BAA-VSS files created in a given month through VSS2TA to produce a file in the format which The Astronomer prefers.
Scripting the core of the pipeline is itself trivial.
I tried all combinations I could think of. The macros (i.e. the ones supplied by the VSS) did not then work with Libre Office.
The bulk of the images I processed were high-cadence photometry of V3721 Oph. I’m the L in CHL, the others being Phil Charles and Kevin Hills.
I’m now looking at the Virtual Moon Atlas which displays the appearance at specific times and dates — which means that the libration and illumination are depicted realistically.
The “Dark feature” is a combination of Peary (closest to the limb), Byrd and Gioja. All of these are well above 80N. The southernmost, Gioja, is at 83.346N; Peary is 88.625N.
Your “dark-floored crater” is Scoresby and the atlas shows its floor in deep shadow at 2019-12-08T22:35Z (my guess at your time of observation). The “craters” between Anaxagoras and Scoresby are Goldschmidt D and a peak for which I have yet to find a name. The pair to the north of Scoresby might be Main and Main A or perhaps Main L. The “very bright floor” and the neighbouring features on your sketch appears to be an extremely eroded crater which does not seem to have been named.
The attached image is a screenshot of the area in question as it appears on the Virtual Moon Atlas for the time and date in question. The red square marks Goldschmidt D
Hmm, you posted while I was still composing my reply so I didn’t see your annotation.
Either or both of us could be wrong. We can’t both be right.
Paul
Can you post your image here or on your member page? We may be able to help with the identification.
Unfortunately, I see no image with Chromium on a Ubuntu system. Looks like the web site not be as portable as desired 8-(
This is what the HTML looks like which, incidentally, gives the URL of the image:
Nothing obviously wrong so perhaps it’s in the CSS?
Unfortunately, I see no image with Chromium on a Ubuntu system.
Looks like the web site not be as portable as desired. 8-(
“I find it much easier not having to mentally convert black dots on a white background to bright dots on a dark background”.
I almost always display negative images for comparison with (in my case) VS finder charts. A single simple mouse click.
I don’t have this one, but I do have the full 17-volume Tycho & Hipparcos catalogue which I picked up 2nd hand for £250.
The relevance is that volumes 14-16 are essentially the MSA, though printed at a slightly smaller scale. A superb atlas.
Incidentally, if you are going for Uranometria 2000, get Vol 2 as well. Otherwise you will be restricted to the northern hemisphere. Even up in sub-arctic Britain quite a bit of the southern sky is visible. I wish I had realised that when I bought only V1 and now regret the decision to save money.
Added in edit: just bought a “like-new” copy of U2000-V2. Thanks for prompting me!
No need to stick with your copy except for sentimental reasons. Second-hand copies are dirt cheap. I’ve five different editions in the library, none of which cost me more than a few pounds. The first (1910) edition was was given to me by a friend whose astronomer father had died recently. It’s been well-used for the last century and ought to be re-bound professionally.
If you wish I can put you in contact with a guy who is down-sizing his library and from whom I bought the Tycho catalogue. He had a copy of Hans Vehrenberg’s “Photographischer Stern-Atlas” which goes down to mag 14 or so. Don’t know whether it is still for sale.
Thank you. I’ve been using APT in batch mode to process many thousands of images from high-cadence photometry runs, at least in part because it is so easy to automate compared with AIJ. Time for me to re-evaluate that decision.
Now to see whether your macro works in LibreOffice. The ones supplied by the BAA-VSS do not, another reason why APT has been the utility of choice.
Paul
With the red laser I suspect it’s thermal expansion of the cavity which changes the resonant frequency. The LED is a broadband emitter but constructive interference occurs only when the cavity is an integral number of wavelengths. Change the length and the resonant wavelength changes.
The Nd transition is at a frequency governed by an electronic transition involving a 4f orbital. Those orbitals are largely isolated from their chemical and thermal surroundings so the transition is very much narrow band which doesn’t change greatly with temperature. The pumping diode is a broadband source but that doesn’t matter as long as it emits strongly enough at the laser frequency.
In a previous life I used to play with a tunable laser — a dye laser pumped by a fixed frequency argon-ion laser in that instance. It was tuned by changing the length of the cavity.
This is how it looked in the guider 2019-12-01T17:18:12 3 hours after the discovery was announced (South is up)