Forum Replies Created
-
Posts
-
Back in the early days of webcam planetary (and DSO) imaging. (~15 years ago) I wrote a little IR FAQ based on my experiences and those of others on the QCUIAG yahoo group.
http://www.threehillsobservatory.co.uk/astro/IR_FAQ.htm
The results achieved these days far surpass what I was getting back then though and I am out of touch with current thinking. The basic reasoning there should still be sound.
Of particular interest may be the test mentioned there I I made on Saturn with and without IR block which showed lower noise if IR was included, no loss of resolution and that it was possible to correct the colour caste caused by the IR leaking past the RGB filters.
The colour camera will almost certainly already have an IR blocking filter so you would need to find out how to remove it to test this out
Cheers
Robin
Hi James,
I would recommend checking out the tracking capability of the the Alt Az mount you are considering. This is much more demanding for Alt Az mounts as, unlike equatorial mounts where all that is needed is a single axis running at siderial rate, both axes have to run simultaneously at varying speeds under computer control. For low cost mounts you may find this rather than field rotation limits the useful exposure time. I have a vague memory of a low cost Alt Az mount (Celestron I think) which could be run in equatorial mode using a wedge which might be a better solution but this would depend on the mount drive software being able to cope with this configuration.
Cheers
Robin
A bit detailed for a forum discussion but ideally could the database fields be read from the fits header? That way the need to enter all the info for each spectrum, with possible transcribing errors could be avoided and any search could then be done based on header information (eg with this you could potentially search for say “all spectra of a particular object covering H alpha at a resolution > 5000 and an SNR > 100” )
re the display of spectra, I wonder if any of the code used this VdS spectroscopy group database might be useful. I have contacts there if interested
http://spektroskopie.fg-vds.de/index_e.htm
(follow the link on the side to “database”)
Robin
Since in principle it is possible to put any data into a fits file there is no fundamental reason why any spectrum profile could not use the fits format. The big advantage is that all the observation and reduction details can then be contained within the fits header provided apropriate keywords are included. The BeSS standard is a good starting point of course but perhaps the header could be generalised and extended to include further useful keywords/flags etc, for example to indicate things like the type of object, level of data reduction etc. Provided the keywords required by BeSS were still present, compatibility with BeSS (or potentially any other database which used a subset of the included keywords) could be maintained (Note also that the BesS database system actually rewrites parts of the the header, ignoring some keywords and filling in others based on the information in the submitted header ie the BeSS input standard header is slightly different from what is held in the database) The “BeSS standard header” generated by ISIS software for example has already been modified in this way with added keywords for the observatory coordinates for example to make it suitable for the simple but more general database (really just lists of spectra currently) already being developed by ARAS.
Robin
Nice catch Bill !
I am wondering if your fireball spectrum is actually of the cooling afterglow after the remnant had passed on rather than the fireball at maximum, which would explain the change in the spectrum and the lack of OI/NI lines in the IR. If you look at the previous frames in the video where the main flaring took place, most of the fireball spectrum is saturated but the OI lines can be seen carrying on straight through (see attached). This would be analagous to this fireball spectrum published earlier this year (recorded in 2012) for example
http://arxiv.org/ftp/arxiv/papers/1503/1503.06400.pdf
figs 2 and 3
Cheers
Robin
Hi Steve, Andy,
Provided you have good lines to work with it is fairly straightforward to measure radial velocities to a precision 1/10 of the resolution or better. (As I plan to show at the workshop in a couple of weeks) so an ALPY with say 10A resolution could measure a 1A shift or 50km/s at 6000A as a reasonable target.
You can go much, much more precise than this though if you use an off telescope fibre fed spectrograph and correlation techniques as is done for exoplanets. eg Buil measured to 50m/s (yes metres!) using a 0.6A resolution echelle spectrograph which I think works out at ~1/500 of the resolution
Cheers
Robin
Just checked Cartes du Ciel (software similar to Stellarium). It reports for Polaris as of today
Apparent RA: 02h52m45.576s DE:+89°19’29.12″
Mean of the date RA: 02h51m08.460s DE:+89°19’50.32″
Astrometric J2000 RA: 02h31m52.656s DE:+89°15’50.60″Robin
As it happens – Francois Teyssier has just updated his reference star finder for low resolution spectroscopy
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1227
It finds the potential reference stars nearest in altitude to your target and now will even plot a trajectory so you can see how the relative altitude changes with time
It now includes all the MILES A and B stars (which have professionally measured spectra) as well as a long list of bright main sequence A and B stars with low interstellar extinction and published spectral classification. These should have spectra similar the generic (eg Pickles) spectra for the particular spectral type but they do not have published spectra so you need to be a little careful using these, perhaps at some point comparing them with a MILES standard for example to confirm they are typical of their type.
I think measuring a few MILES stars of different spectral types is good practice if you are starting out or testing new equipment as any deviations between the measurement and the database version can be seen and followed up to improve one’s technique. There are some examples here where I did this using the Star Analyser and ALPY
http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm
I find getting the blue end below ~4000A perfect particularly difficult for a number of reasons:-
the sensitivity of the instrument drops off sharply
any errors due to atmospheric extinction are greater
any problems from selective sampling of particular wavelengths by the slit due to for example atmospheric dispersion or chromatic aberrations in the optics are greater
it is difficult getting enough signal from the (halogen) flat lamp to keep the noise level down
the increased crowding of the Balmer lines towards the Balmer jump makes it more difficult to get a perfect match between measured and reference spectra.
Cheers
Robin
This document showing how a range of slitless (Star Analyser) spectra were corrected for instrument response using a MILES standard A0v star might also be of interest
http://www.threehillsobservatory.co.uk/astro/SA100_miles_instrument_response.pdf
from this page
http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm
The results are pretty good, though some small errors can be seen at the blue end due to atmospheric extinction which was only partially corrected for. Using matched reference stars at the same elevation as each target star rather than a common reference would have improved this.
Robin
Hi Bill,
You wrote..
> In the basic library with Visual Spec the spectrum of Vega, for example has a sharp cut off then some oscillating components.
There may be something wrong here. The 5A/pixel Pickles A0v spectrum in the Vspec library (file a0v.dat) shows a smooth decline in the continuum at the blue end as the Balmer lines merge towards the Balmer jump and a relatively flat continuum beyond that.
You can see it here for example where I use it to calculate the instrument response of my star analyser setup
http://www.threehillsobservatory.co.uk/astro/Spectroscopy_BAA_VSS_workshop_2008.pdf
in slide 38
Are you using a different spectrum at very high resolution perhaps ?
Note that correcting just using the CCD (or photdiode) response will not give a good result as the response of the grating (and the atmosphere) is also significant here
Robin
Background subtraction errors are also a common souce of instrument response problems in slitless spectroscopy and are worth looking out for. These tend to be most pronounced at the blue end where the instrument response drops off sharply so small zero errors have a large effect.
Robin
Yes flux calibration of meteor spectra, even in relative terms is going to be tough. The instrument response can be corrected for using a reference star and provided atmospheric conditions are stable, even the atmospheric extinction con be accounted for by taking reference stars at different elevations and applying a correction based on the elevation of the meteor (The usual simpler technique of chosing a reference close in elevation to the target to cancel atmospheric extinction is problematic here of course !) A big problem though is likely to be flat field effects which can be severe with these sorts of wide field systems. A normal flat field correction does not work with slitless spectrographs. The usual workround is to place the reference star at the same position in the field thus cancelling gross flat field errors but again this is tricky here, particularly if the spectrum is generated by integrating along the track. Measurements of reference stars at the apropriate locations in the field the night after the meteor observations could perhaps be used though. Measuring standard star spectra with the star positioned at different points in the field and comparing them would give an idea of the severity of the flat field problem.
Cheers
Robin
Looking for auroral lines could be particularly interesting as you could use higher resolution to enhance the contrast between any continuum spectrum background. (like imaging with narrowband filters) You would still have to watch out for auroral lines from our own atmosphere and subtract them of course.
Cheers
Robin
Hi Andy,
Hmm.. getting an even enough flat might take some arranging. I think you would need to go back to a flat light source in front of the telescope rather than rely on the internal flat lamp, to take into accout any vignetting before the spectrograph. You could perhaps test it though by taking a daylight sky spectrum which over the small field should give the same counts independent of position along the slit, once flat corrected.
Robin
With a long slit orientated appropriatelyand a suitable image scale you might be able to get all you need in one shot (Well 2, one exposed for the bright side) You would need a flat field illumination source which is nice and even in the slit direction so the subtractions work accurately.
Funnily I was just reading Paul Abel’s beginners article on Earthshine and musing about measuring that spectroscopically, perhaps mounting the spectrograph behind a camera lens to get a sensible image scale to match the slit. Much easier than Ashen Light though !
Robin
I have been runnng with my “ALPY 200” low resolution faint object spectrograph configuration again over the past few weeks and have been able to work my way down about a mag fainter to around 17.5.
http://www.threehillsobservatory.co.uk/astro/spectra_46.htm
I recorded the spectra of these two supernovae (asassn-15fa and asassn_15fz) a few weeks after discovery so they have evolved significantly compared the original spectra taken soon after they were discovered. The match with spectra of supernovae at a similar age is still clear though.
The supernova contribution is already much lower than the sky background which is significantly limiting the signal/noise. I am not sure if more aperture or darker skies will be needed to go much lower, probably a combination of both.
Robin
Hi Stan,
A couple of other safety tips.
Keep the front cap on the finder scope or make another filter for it from the offcuts of solar film
Be sure to put the filters on before aiming at the sun and remember to swing away from the sun before taking them off. (Seems obvious I know but I forgot once when packing up some time after an eclipse that the mount was still faithfully tracking the sun and within a couple of seconds of removing the filter the eyepeice cap had a nice neat pinhole melted through it! I keep it as reminder as to why you should never look at the sun through an unfiltered instrument.)
Robin
Hi Nick,
There is currently no centralised international database but the object specific database BeSS for professional and amateur spectra of Be stars makes an excellent model for such a general database.
http://basebe.obspm.fr/basebe/
This is a professionally funded high quality system with moderation of submitted data and a mentoring service for amateur contributors run by pros and amateurs. There were thoughts to expand it but it would need funding. I have been talking with AAVSO for many years about this (including advising several of the various working parties there have been over the years) and the people behind BeSS have offered the complete system to AAVSO to use but they appear to want to go their own way against the advice of other experienced amateurs in this area and I have currently given up on them.
In the meantime the most complete source of amateur spectroscopic data is the ARAS organisation who have been archiving spectra submitted to them on a range of objects
http://www.astrosurf.com/aras/
(although unofficial, ARAS is where most serious amateur spectroscopists can be found and they are currently the main force driving up standards in this area)
I plan to talk more about how ARAS is functioning at the upcoming VSS meeting using Nova Del 2013 as an example
Cheers
Robin
