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Hi John, as David said this can be a big challenge. My fiber guide head has a field of about 10 arc minutes. Which is too small to reliably plate solve. Although I did manage to make this work with the slightly larger field of a LISA spectrograph.
One relativity cheap solution is to use a flip mirror which works well if you are finding by eye at the scope. Otherwise you would need to add a DIY solution to flip the mirror and another camera.
Another expensive solution is to use a dichroic mirror. I use an Innovations Foresight ONAG and a third CCD in my automated system but light longer than 7500 A is not available to the spectrograph. However, I could find no other solution to automatically finding and centering the target..
Regards Andrew
After much experimentation I have found that the most consistent way of measuring EW (for me at least) is to first rectify the spectrum to unit intensity for the continuum the use a function fit. I have found this simplest to do using SPLAT-VO. This has a modern interface and the best integrated set of tools with one exception. SPLAT allows polynomial and free hand fitting of the continuum, functional fitting of line with the standard functions for measuring the lines. It also has “flip and translate” of a spectrum for measuring line shifts. It is only for blended lines and emission lines where IRAF has better tools.
Here are four spectra of Rho Cas with 3 Fe I lines near 5572 A that I intend to monitor along with others. The spectra match to within a few percent in the continuum but are offset here for clarity. The lower two spectra are at half the exposure of the top two. The poor S/N on the first one is due to the very poor seeing at the time.
Regards Andrew
Grant, I suspect the bracket will be the least of your troubles with a dual mount. That is a big load for an EQ6 if that is the mount you intend using.
Regards Andrew
You are going to need a fast computer with a large disk Robin, and ideally some software like UFO that can read the files and auto-detect meteors for you. If not you will have a full-time job for those all to frequent cloudy nights!
However, I agree these new large, sensitive, low read noise CMOS sensors will open up all kinds of possibilities.
We have come a long way from DIY modified long exposure web cams the you worked with Robin.
Regards Andrew
If it works for you Hugh, I don’t see any good reason not to use this method. If I were using it I would just look to see if the “continuum” you are using remains stable or if it moves systematically as the line changes strength. Even this does not matter if you just wish to follow the rise and fall of the intensity.
Regards Andrew
Regards Andrew
Hi Andy, I think your approach will work well with well isolated lines where you can be sure all the flux is from the line you are interested in. Where lines are blended fitting a Gaussians (or other line profile) allows you to deblend them, at least approximately. The biggest problem I have is that there is no obvious continuum in the yellow hypergiants which is why I am attracted to the idea in the paper of subtracting the fitted profile from the spectra to give a new estimate of the continuum.
I am going to experiment with SPLAT and see if taking different starting continuum levels converges to the same value in a few iterations – I easily lose focus!
Regards Andrew
Hi Robin, yes you are right the biggest challenge is estimating where the continuum is. IRAF gets you to manually place two points on the “continuum” and then identify all lines between these two points. It then deblends the lines by fitting Gaussian ( or other functions) to the curves and reports the lines center, FWHM and eqw etc. for all the lines.
SPLAT allows a similar process but you can provide a constant for the continuum or fit a polynomial for the background. Again it has several function you can fit with.
I found a paper on DAOSPEC which is an automatic program that has a good discussion on current methods. It also introduced an interesting idea I am trying manually with SPLAT. You estimate (guess) a background and fit the lines. You then subtract the fitted lines which leaves a new local background that provides a new estimate of the local continuum and you then use this etc. etc. iterating as long as you wish.
They also argue that you don’t want the true continuum but the local continuum which will include unmodeled lines who’s average contribution you want to avoid.
The paper is here https://arxiv.org/abs/0811.2932
I think I will use an ensemble of methods to give an error estimate and then pick the one that is as robust as I can make it.
Regards Andrew
PS Given how lazy I am I shall try tp work out how to download and compile DAOSPEC or similar and let it do the work!
Thanks Andy and Robin for your kind words. I enjoy tinkering as much as using which is just as well given the near perma-cloud here. One advantage of the fiber feed is I can play with new bits without disturbing the telescope and it’s pointing.
Sorry Tony, I don’t have any plans as such I basically read all I could find and then pinched the ideas. I then drew a layout on paper and transferred it to ceramic base, drilled holes, and fiddled about until it worked. The optical layout it is similar to the Shelyak Instruments eShel but on its side. I went for high thermal mass so it is in a large plywood box covered in 2″ back insulation. It has an air to air peltier cooler controlled by a Laird Tehnologies PID controller. Once stabilised it is better than +/- 0.02 degrees so the calibration remains stable during a nights observing.
Happy to answer any specific question or help anyone trying to do similar things.
Regards Andrew
Well done Robin you have got far to good at this! You also seem to get gaps in the cloud I don’t see.
Do you leave the Alpy on all the time or do you swap with your LHIRES III? Also do you have two cameras or do you have to swap one over?
Regards Andrew
Clearly you are an astronomer the question is am I disqualified? I don’t image either I do spectroscopy and have never looked through my current telescope. I do also like being out under the night sky but I don’t have either good eyesight or dark skies.
I have discovered candidate new Be stars doing automated spectroscopy with a home built telescope and commercial spectrograph and camera – is that not enough to be am astronomer?
Regards Andrew
I would add two points. Firstly I only put a guide rail on one side. This avoids binding if they are not exactly parallel.
Secondly, the load is quite high so use wheels with roller bearings.
Good luck Andrew
As James said I would think it a combination of non stellar objects, precession (chart is for 2000), proper motion, chart errors.
Regards Andrew
I am sure I will get these sooner or later to add to the collection having used the down load atlas a lot and the guide less so.
I think I have most, if not all, the currently available “spectroscopy” books and while none covered everything my current preferred tome is Spectroscopic Instrumentation by Thomas Eversburg and Klaus Vollmann but that reflect my particular interests.
Regards Andrew
For instruments that need it they can either rotate the instrument or use an optical derotator like a dove prism to counter the field rotation.
Regards Andrew
Yes it does, I use them to create lists of frames then act on the list. For example if I have a set of images HD123_1,HD123_2 etc. Then
files HD123* > l_HD123 #create a file l_HD123 with all the images in it.
imcombine @l_HD123 HD123 combine=median #median combines all the images in the list into a new image HD123
The format of the commands is standard task input output parameters.
You can also use wild cards inline but I have not done so.
I attach my calibration script which would have a .cl extension in IRAF. I have changed it to .doc to upload. Open it in Notepad. Note that I am not using darks at the moment so just bias subtraction. I will add scaled darks to the script.
Regards Andrew
“…but professional astronomers are perfectly happy with personalised batch files processing chains akin to a DOS .bat file. Its us Windows users who have gone soft :)” Their not the only ones. I am an inherently lazy astronomer.
I have automated the capture of my echelle spectra to the point where I can just input a set of targets in a .dat file and it will work through them while I do other things. By the time I have IRAF bent to my will all I should need to do is load the data files and set of the script and out should pop the radial velocity measurements.
Some way to go though yet as I have only finished the wavelength calibration for the first 10 orders with 20 to go,
Regards Andrew
Hi Steve, glad you got it loaded. I found it is quite quick and easy to use the command line once you get into it’s style. However, I am part of the generation that used punch cards and paper tape before the Commadore PET and then DOS. Indeed the first computer I worked on had a ferrite core!
The graphics windows in IRAF are of a style of their own so you need to look at the documentation and tutorials. Again it gets easy with practice.
Regards Andrew
