Forum Replies Created
-
Posts
-
<p class=”wp-dark-mode-bg-image”>I was wondering if the BAA held any copies of Alan Heath’s autobiography-“Memoirs of an Astronomer”? Neither Amazon, Waterstone’s,nor Summerfield Books (the specialist natural history booksellers) have it. Thanks, Rob Duffy.
I have a copy but not a duplicate.
It could be loaned out to anyone willing to promise to return it and to pay postage in each direction (or to travel to CB22).
Paul
<p class=”wp-dark-mode-bg-image”>Is there anything like JPL Horizons anywhere else?
Some of us have been discussing the provision of globally distrubuted backups on https://bsky.app/ where you may like to subscribe to #astronomy.
It’s not actually that difficult to store relatively static data like the abstract service. With my resources alone I could contribute a terabyte of storage and half a gigabit per second bandwidth. Dynamic data such as provided by the JPL and MPC sites are harder because of the need to install the requisite software.
I use cooling all the time, partly from habit but motly because my major interest is photometry where the thermal noise reduction is welcome if not essential. This is especially valuable for exoplanetary transits and precision photometry below, say, 17th magnitude.
<p class=”wp-dark-mode-bg-image”>Haha. (Apart from this) I haven’t seen much April Fool’s Day content across the media. Serious times, I guess. David
How would we know the difference from other days? There’s enough fake news swilling around as it is.
Your choice, but IMO it is difficult to have too much overkill.
If it is within your weight and financial budget go for something over-specified rather than anything less.
Paul
I am potentially interested, depending on price. The information below may help others to express an interest.
According to https://www.amazon.de/-/en/Atlas-Planetarischen-Nebel-Objekte-zwischen/dp/3949370153/ the German edition of the first volume is €69.90 with shipping to Spain of €5.77.
At today’s spot rate the sterling price would be £58.58.
Paul
JPL Horizons gives detailed ephemerides for essentially everything known in the Solar system. I use it for inner moons of the planets. For outer moons of JSUN and asteroids MPC is used.
<p class=”wp-dark-mode-bg-image”>Wonderful to see this photographic record before the jet age and commercialised eclipse tours. Travellers dressed in their finery during the long adventure, rather than in today’s casual wear.<br class=”wp-dark-mode-bg-image”>
I say chaps, how about playing quoits at the Winchester Weekend or the Christmas Meeting? 1900s dress code not obligatory.I have a croquet set somewhere …
Craig: that is pretty damn good!
To be honest, I thought you would have difficulty with a 100mm aperture, given that recommendations tend to start at 200mm, or 150mm at a pinch. You surprised me!
Your data looks entirely sufficient to make a scientifically valuable measurement. Don’t worry about the predicted time to be different from your measurement. That is the whole point of the exercise: to test the predictions. Please report it so others can learn from it.
I am impressed, and please keep measuring other transits with either or both of your telescopes.
Paul.
P.S. You should boast about this one in other astronomical fora such as SGL, CN and Bluesky.
Ah, I forgot about chromatic aberration. 🙁 Too much time spent with reflectors. That said, my Dilworth despite being similar to a classical Cassegrain though with spherical mirrors, has a series of achromatic transfer lenses. I have never noticed any chromatic aberration when imaging.
Again, the best way is to suck it and see. If the unfiltered images are not too bloated (and they may well not appear larger at all), set the photometry aperture perhaps just a little bit larger to compensate and measure the same field with the same exposure on the same night (i.e. consecutive images) filtered and unfiltered so that the sky conditions are as similar as possible. Use the version which gives you the better SNR.
As always: SNR and cadence is what is important; aesthetics doesn’t get a look in.
Good!
Sounds promising.
Note that a luminance filter removes the near-IR which, if present, gives an unusual colour balance. You are (or should be) not interested in what it looks like but only how many photons you have available. My advice is to run unfiltered in future and let the IR photons through so that they may contribute to the overall signal.
<p class=”wp-dark-mode-bg-image”>I took a calibrated 2 min sub, debayered and extracted the green channel (OSC camera).
Something worth remembering: if all you are interested in is the change of brightness as time passes, it matters relatively little what filter(s) you are using.
If you extract all four Bayer layers (1xR, 2xG and 1xB) and then add them together you will have more signal and relatively less detector noise in both star and sky. If all channels have equal signal, which they won’t, the SNR will be increased by a factor of sqrt(2) or approximately 1.4 which is well worth having.
<p class=”wp-dark-mode-bg-image”>Right so I was woken at 3am by the youngest sprog and couldn’t get back to sleep, so I did a load of internet searching and reading around SNR, AstroImageJ and aperture photometry (totally normal behaviour, right?).
<p class=”wp-dark-mode-bg-image”>Stumbled across this useful looking calculator…<br class=”wp-dark-mode-bg-image”>
https://mirametrics.com/sn_calculator_mvn.php<p class=”wp-dark-mode-bg-image”>Plugging in the pertinent data, or estimates of, and it thinks using a 102mm refractor operating with 2 minute exposures, and assuming my minimum acceptable SNR to be 500, then it looks like I should be aiming for stars of at least mag 12.5 with a 2% transit depth.
<p class=”wp-dark-mode-bg-image”>Edit: Plugging in numbers for my 300mm/1500mm Newtonian says it should be good down to mag 14 ish with a 2% depth.
Perfectly normal behaviour, IME.
Your numbers sound about right to me and consistent, though not identical, with what I posted earlier.
Your noise will depend greatly on your sky brightness, of course. Don’t expect as good results from a brilliantly moonlit Bortle-7 location as a new moon B4.
I would recommend installing a proper photometry package (I use and recommend APT, Aperture Photometry Tool) but there are several others available. Any of them will give a good measurement of the SNR of any given star in any given image.
Incidentally, and this doesn’t apply to exoplanetary transit work, stacking many unsaturated images in addition mode will increase the SNR by a factor of the square root of the number of images. By adding the images you preserve the photometric accuracy and let the counts per pixel exceed the detector’s saturation limit.
Craig: that FOV is comparable to my 400mm, which is 16.45 * 13.18 arcmin, or 0.27 x 0.22 degrees. I have not yet had any problem finding a suitable comparison star but, to be fair, stars do become much more common at fainter magnitudes.
Once more: suck it and see is my recommended approach. Remember that you can do all this sort of experimentation at your convenience – you do not need to wait until a transit is imminent. If you do so for all the stars which you are likely to observe over the next few weeks you can choose which telescope to use as appropriate.
Gary: I don’t understand.
I remain logged in for days or weeks at a time. Perhaps we use different browsers: Firefox under Ubuntu here.
I have no personal experience with a 102mm aperture but my 400mm can do useful work on a 1% depth at 13th-14th magnitude. Simple scaling says that you have 1/16 the collecting area, which corresponds to three magnitudes in brightness.
Accordingly, I expect that you may have a chance with HAT-P-20 but it’s not certain. Certainly well worth giving it a try.
Regardless, it’s easy enough to test the capabilities of your equipment. Choose a star of a particular magnitude and see how long an exposure is require to reach a SNR of at least 500 and preferably 1000. If it is a couple of minutes or less, and preferably less, you can measure a transit of depth 2% or better. Repeat with fainter stars until you determine your limiting magnitude.
Good luck!
Paul
SWarp (https://www.astromatic.net/software/swarp/) is the only stacking program I use most. It works exceedingly well.
I believe that SWarp is available for the Mac. (I have never needed to look for it before but here it is: https://ports.macports.org/port/swarp/ )
I use a local installation of astrometry.net to put the WCS on my images. Toddle over to https://astrometry.net/ for links to both the on-line plate solver (“use”) and how to drive the local installation (“download”). I already knew that a MacOS version of the local installation exists.
FWIW, I use “ANSVR” to put a WCS on images taken with Maxim DL under Windoze for the purpose of a rough and ready indication of the centre of teh field for synchronizing the mount’s idea of its pointing location to the telescope position. Never tried stacking with SWarp under that OS but often use Maxim DL’s stacker to see when enough subs have been taken to give a good enough SNR for photometry. AN and SWarp are used subsequently to do a proper job.
I would love to have them but La Palma is even further for collection from Falkirk than is Cambridge.
Let’s hope you find a good home for them.
As of two hours ago:
Knud Jahnke @knudjahnke.bsky.social
The closest edge of the fire perimeter is currently 500m from JPL. But wind speeds have decreased and wind is not blowing towards JPL anymore.
https://www.fire.ca.gov/incidents/2025/1/7/eaton-fire
in response to my post on Bluesky yesterday.
