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We might be talking about the same thing, but in case not.
You would expect the graph to change once you are outside the linearly region and then into saturation. Once a star becomes saturated you are not detecting all the photons that land on the chip.
Cheers,
Andy
Hi Simon,
The important point is once a pixel is saturated it won’t count any more photons. It is possible that if a tiny number pixels are just above saturation or in the non-linear region then that non-linearity won’t be obvious. There will be lots of pixels contributing. However, it is a risk and you would not be able to justify your result if you were trying to extract a reliable magnitude.
Looking at your graphs, they show a slightly different behaviour in the linear region. It appears to be a steeper slop, followed by a little bit of a wiggle.
If saturation is a problem as you want longer exposures then you can slightly defocus the star. This will spread the photons over more pixels while keeping them within the linear region.
Cheers,
Andy
I can’t offer a perfect way to do this, but you may be helped by looking at this another way.
The photometry aperture containing the light of the supernova will itself contain background light from the host galaxy. That disc containing the light of the supernova will be huge in terms of the galaxy, covering many millions if not billions of stars. Hence the trick is not to subtract a sky background without the galaxy, but to have an annulus that is representative of the background light you are collecting with the supernova photons.
Andy
Hi Mike,
The best way to keep appraised of upcoming webinars is to look at the bottom left of the BAA homepage, where the next 4 meetings/webinars are listed. There is also a full list of meetings and webinars on the BAA meetings/events page.
https://britastro.org/meetings
While the postponed meetings get in the way a bit, the webinars for the next 2 weeks are showing in the bottom left of the homepage.
The next few meetings/webinars are also usually listed in the monthly BAA newsletter. Though with the rapid changes of the past few weeks it has not been possible to know about all of the events sufficiently in advance to get them on to the newsletter, which is why we said to check the events page in the last newsletter.
Sending out bulk emails is a tricky topic. If we send too many we get complaints, and if we send too few we get different complaints, and we have to be careful to keep within data protection regulations.
Best wishes,
Andy
Hi James,
Can you let us know what part of the VSS pages are not working for you, so we can try to replicated the problem? It would also be useful to know which browser and operating system you are using as these behave differently.
I found I had to clear the cache to get the VSS pages working in Chrome, but they worked first time in Firefox and MS Edge.
Thanks,
Andy
The BAA web server was upgraded over the weekend. As Jeremy indicated, I suspect it was some kind of cached data in browsers that needed to be refreshed.
Andy
Thanks for pointing this out Jack.
On a similar topic, the BAA is fortunate to have been given permission to broadcast a webinar for A Special Image for the 30th Anniversary of the launch of the Hubble Space Telescope this Friday 24th April at midday. More details here:
https://britastro.org/node/21425
Andy
Hi Kevin,
Good to hear you are finding worthwhile pursuits during the lockdown. Those are interesting results.
If I am reading those plots correctly, you are seeing a 10%-15% difference at the red end of your instrument response when using either Castor or Regulus. That is quite a substantial difference. Assuming the instrument response from the 2 stars were calculated on the same night and under similar observing conditions, were the altitude of the stars similar? If not then the differing effect of atmospheric extinction with altitude might be the cause, though even then I am surprised by the size.
Another possibility is somehow the slit isn’t being uniformly illuminated by blue to red light. This might happen due to atmospheric dispersion. The atmosphere can start to split the light of a star just like a very weak spectrograph, so you capture more blue or red light depending on where the slit is placed on the stellar source. You could get a similar result with telescopes that contain lenses if they don’t bring red and blue to the same focus.
A way to test for this would be to compare results between a wide and a narrow slit. A very wide slit should capture all of the starlight even if it has been chromatically dispersed by the atmosphere or lenses. While a narrow slit might record more blue or red light as you move from target to target, or across a single target.
Just thoughts, and I’ll be interested to hear what others think.
Assuming there is no reliable way to correct for this, then I agree cropping the spectrum is a good approach.
Keep safe and well,
Andy
According to SIMBAD it is of A2Vn C spectral type. I also found reference to a debris disk in Wikipedia linking to a paper by Wyatt et al 2007 https://arxiv.org/pdf/astro-ph/0703608.pdf.
So it is a main sequence A-type star and the paper quotes an age of 300 Myr, though I think that comes from another paper (I’ve not read the paper in detail, just skimmed a few bits). I am surprised a main sequence star could have a debris disk as I thought this was associated with young stars. If I’ve understood the paper there are collisions in the debris disk generating dust and an IR excess. Perhaps the variation in luminosity is due to variable obscuration by the dust?
Interesting how such a well known and prominent star can throw up surprises to us, well spotted Alan!
Hi Nick,
I remember I had problems parking my Astro-Physics mount with the SkyX, so I wonder if you problems might be related to parking. As the mount doesn’t actually need to be parked I simply slew to a safe position and then turn it off. Of course your pointing problem might be something different but I thought I’d mention it.
Other possibilities to check are whether your location is set to come from the hand controller or the computer.
I’ve never pointed at Polaris as it is not a good star to use for syncing the telescope position. You have good latitude information but the longitude precision is about as bad as it could be as a tiny movement gives a massive shift in longitude.
Good luck, I’m sure you will get there.
Andy
It should be remembered the BAA has not been silent on this matter. Article from last year.
https://britastro.org/node/18560
Andy
Hi Nick,
I successfully use Windows 10 with MaxIm DL, the Sky X and an Astrophysics mount. There are no specific tips I can give other than sometimes things don’t work first time when I change my setup, though with trial and error I’ve always got things working.
Good luck with your changes,
Andy
It is a great publicity idea for the RAS and astronomy.
Anyone involved in publications will know how difficult it is to spot every mistake and it is impossible to please everyone with the layout and language. I know mistakes in our own BAA publications often don’t get picked up until after they have gone to the printers, or been published on the website, even when several people proof read them.
An astronomers telegram has been issued as Betelgeuse started to brighten again in mid February. This appears consistent with the existing variability cycles though a deeper minimum than usual, probably due to overlapping cycles. It is still interesting and useful to observe with both photometry (visual and electronic) and spectroscopy. To verify the brightening continues as expected and to obtain more data on the behaviour on this interesting star.
There is no problem storing the photometry from every exposure, in fact I think that is what the majority of observers do. For faint targets, deciding whether and how many sub-frames to combine is always a tricky choice.
Cheers,
Andy
Hi Paul,
I think I understand what you are saying about the database, but I will just explain what is held to avoid possible confusion, including anyone who may be following this thread. Where the observer provides the instrumental magnitudes and uncertainties/errors of the variable and comparison stars then this data is recorded in the database. Thus, if someone wants to recalculate the magnitude using different reference magnitudes, or to exclude one of more of the comparison stars then this is possible. It does not hold information on non-comparison stars, e.g. every star in an image, nor does it hold data on background counts etc. We only require the Julian Date to be submitted, noting HJD and BJD can be calculated, and as a general rule data that can be calculated from existing data is not stored. The exception to this is the derived magnitude, as different observers could calculate this by different methods, and it is desirable to hold what the observer calculated.
As you are doing, it is a good idea to store images including the calibration frames. Then if a recalculation is required, or there is a query from a researcher, it is possible to delve into the detail.
There is no easy or right/wrong answer on whether to submit individual images or combined images. Individual images allow researchers to choose how to combine data (within the limitation of not having the raw images to hand), while the observer doing this can make life easier for the researcher. Also, the observer will be most familiar with their setup and the images. Personally I would suggest combing images if your results are dominated by the noise.
Cheers,
Andy
Hi Paul,
If I have understood what you are proposing, then it should not be done for submissions to the database as a single same image would contribute to multiple measurements/observations.
My understanding is you would first combine images 1-9 to get a good signal, then images 2-10, and so on up to 9-18. This is of course simplified as you would have many images. Image 9 would be used 9 times, and so the same photons would contribute to 9 observations and that would be wrong.
You can do this kind of thing in an analysis, as long as what has been done is stated. Basically it is a smoothing function. It just should not be used for submitted observations to the database. This leaves the choice of whether or not to smooth with the researcher.
Best wishes,
Andy
Hi Paul,
In my opinion an uncertainty of 0.02 magnitudes is excellent on this target. The uncertainties in the CRTS photometry in the VSS Circular phase diagram and other VSS observers appear to range between roughly 0.03 to 0.15 magnitudes, so you are doing much better.
The lower SNR observations are still useful as statistical data can be extracted from many observations, and they confirm U Leo is not doing anything different. Though as always the higher the SNR the better, so I am sure your efforts will be greatly appreciated.
Andy
Hi Paul,
In answer to your questions.
You should use the “CV” filter where you take unfiltered photometry and calibrate using V filter comparison star magnitudes. This filter is standard in the BAA and AAVSO and is there exactly for this purpose.
It is common to add images if the SNR is very low, or to take longer individual subs.
It appears the AAVSO have a sequence for AT 2019xim. It is fine and common practice to quote AAVSO sequences when submitting to the BAA Photometry database when a BAAVSS sequence is not available or if it is not appropriate for digital photometry. They have far greater resources available to produce sequences. Just quote the chart reference, e.g. AAVSO X25107PN or X25107PN.
Best wishes,
Andy
