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Hi Lars,
Rspec is designed to be easy to use but has limited functionality.
BASS is the next level up. It has a lot more functionality and is a lot easier to use than say ISIS. ISIS is pretty much the gold standard in spectral processing, but is the hardest to use.
I suggest starting out with BASS as it is free, and see how you get on.
I also suggest starting simple. The A-type star is used to response correct the spectrum. This is to remove the bulk effect of the Earth’s atmosphere, and to adjust the bulk profile of the spectrum for the telescope and camera sensitivity. Note this doesn’t remove any spectral lines, just large scale features that affect the shape of the continuum. While this is important for getting a correct continuum, I wouldn’t bother with your first few spectra unless you really want to. I took spectra for 3 to 6 months before attempting a response correction.
When you are ready to do a response correction, then you will need to choose an A-type star at about the same altitude and on the night of your observation. The altitude of the star will affect the response correction as when you look lower down you are looking through more atmosphere. Also, atmospheric conditions can affect the response, so a response taken on one night may not work on another night.
Most important of all is to keep it fun! Don’t worry about getting things exactly right at the start.
Best wishes,
Andy
If anyone is interested then I would not let age hold you back.
I am fortunate enough to have returned to university in my late forties to do a PhD in astronomy. While I have seen more years than most of my fellow PhD students, I’ve met plenty older than me who only started once they retired.
Andy
I’ve been in contact with the BAA Office and they have confirmed there has been a bit of a delay to the Journal this month. They should be arriving with people any day now, noting the BAA Office haven’t yet received their box of copies, to their home addresses as they are of course working from home at present.
Paul & David, I was sorry to hear you had problems with your Journal deliveries. I’ve done a little investigation with the BAA Office to check there is no ongoing problem.
David’s appeared to be some kind of delivery problem. He’s had no changes to his address and his address was in the list sent to the printers in August.
There was a minor problem with Paul’s address that we didn’t spot. Basically an issue around the country not matching a known Royal Mail Zone causing it to fail. We discovered this after the December Journal print run and have implemented some changes to avoid this problem in future. The Office are now able to produce a list of any problem addresses before doing the Journal run, so they can fix them prior to generating the mailing list to the Journal printers. It only affected a handful of members, I think less than 6, who were sent their Journals separately by the Office. We should have spotted and fixed this sooner, so our apologies.
Best wishes
Andy (with my BAA Systems Administrator hat on)
Hi Daryl,
Thanks for pointing that out. It was a mistake I made when updating the meetings/webinars for the first half of this year earlier today. I accidentally unticked a settings that promotes meetings from the events page onto the front page.
Best wishes,
Andy (with my Web Content Editor hat on)
Hi Andrew,
In that case it would be best to create a zip containing everything, or a choice of zip and another compression format.
It is easy to add one or two files, but each file requires its own download page, hence it makes sense to have a single file with everything. It will also be easier for people to download a single file.
Best wishes,
Andy
Hi Andrew,
I agree this sounds useful, thank you.
I may be the person who helps to get these onto the website, so my initial thoughts are about what the files are like. Are there one or many, and what file type are they?
The E&T pages sound like a useful place to put these, although the Drupal software on which the website is based can sometimes make things like this a little tricky. They might need to go in the Publications->Downloads->Sections->E&T area of the website. A backup plan would be to put them on the BAA Spectroscopy Database pages, though the main website would be better as long as it doesn’t get too messy.
Best wishes,
Andy
It is worth noting that while the G magnitudes are excellent, there are known small issues for bright blue stars and faint red stars. Details can be found in Riello et al 2020.
The bright blue stars are G<13 and BP-RP<0.1, appear to have an ‘anomaly’ of up to a few mmags.
The Gaia web pages provide Python to correct the systematic in fainter G magnitudes, G>13. This is not a major problem and in most cases will be very small except for redder stars. For example it can reach about the 1% level for about BP-RP>2, though at the extreme red end can reach 2.5%. I’m using a few million Gaia sources, and in my sample the official correction adjusted 1% of the stars by 1% in G. There is mention in the paper of a separate table in the Gaia archive for corrected G-band photometry, though I’ve not seen that yet and I wonder if it will appear in the coming days.
There also appears to be parallax zero point bias by magnitude, colour and position (Lindegren et al 2020). Again a Python script is provided to calculate this bias.
If anyone is interested in the Python corrections, then the source code can be found on the Gaia webpages here: https://www.cosmos.esa.int/web/gaia/edr3-code
This is not intended to take away anything from the Gaia results, which are amazing. It is usual for a major survey to have some small issues to be aware of, especially noting this is just an interim data release.
Andy
I am glad to see no one was hurt. As sad as it is to loose Arecibo there are huge new projects in radio astronomy. The one that springs to mind is the Square Kilometre Array. I am sure there is a trade off between building and operating new observatories, versus keeping old ones operational.
Andy
Hi Peter,
Many thanks for pointing this out. I did a quick search of recent images and found an idential image by Peter Goodhew FRAS. I have corrected the page as I am sure this was an innocent mistake, possibly even a browser autocomplete when this was promoted to picture of the week.
Our apologies to you and Peter Goodhew FRAS.
Andy
Thanks for sharing this Callum. I heard of this spectrograph a few years ago but didn’t know they were adding automation.
It is great that SX are entering this field though I didn’t see anything that tells me how well the spectrograph performs. The idea looks great, but they really need it in the hands of someone who knows how to produce good quality spectra to see how well it performs in the real world. It could be fantastic but they only showed the most simple of results, a bit like demonstrating a new telescope by taking single images of the Moon without proper processing.
I would want to know how stable is it on the back of a telescope in an imaging run. It appears the design should be very stable but theory and practice don’t always match. I also want to know what the resolution is as that defines what you could use it for, along with how that matches to CCD pixel size. Interesting that the camera can be shifted across the spectrum though I’d want to know how much of a spectrum you can fit in a single exposure, and how stable that movement is. It has to be absolutely rock solid, otherwise things like wavelength calibration and response correction needed to create good quality spectra would fail. If someone could obtain spectra of more challenging targets and do the full end to end process including response correction that would go a long way to demonstrating they have a high quality product.
I do not agree that they are coming in cheaper than their competitors. There are only a couple of modular spectrographs I can think of, most come as complete units. Of the modular spectrographs one is cheaper and the more expensive one is a top end spectrograph designed for large research telescopes. They appear to be coming it at about the same price or possibly more expensive than complete unit spectrographs. That is not to put their spectrograph down, just I did not agree with their comments on the price. This might not be a negative as they have automation, though they are not the only spectrograph with automation capability.
I don’t mean to be too negative as I think SX produce great CCDs and accessories, and I want this to be a great spectrograph. I am just hesitant as this is their first foray into spectroscopy and I’ve not seen any results that demonstrate the quality of the spectra it produces.
Cheers,
Andy
Hi Jack,
I hope you don’t mind but I thought I’d ask what problem it is you are trying to solve.
I remember you were having issues with wavelength shift in your spectrograph. If this is the problem then I don’t think mirror flop in the telescope is likely to be the cause.
Of course if you are having trouble with shifting position of the star when you cross the meridian then that would be fixed by solving mirror flop.
Cheers,
Andy
Hi Jack,
You don’t need high end equipment to do photometry so that should be fine.
What really matters is making sure you do flat fields, and that is the same whatever equipment you use.
Cheers,
Andy
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
