[David Boyd]

Hi Tony,

I’m not sure I understand your question. Measuring the instrumental and atmospheric response for a symbiotic is no different to any other star in that you want to use a reference star close in altitude and azimuth to the target and for which you have a good spectrum, so ideally a Miles library star but failing that a Pickles spectrum for the same spectral type as the reference star. It doesn’t need to have a spectral type similar to your target star. That should enable you to correct your measured spectrum to the exo-atmospheric spectrum of the target, whatever type of star it is.

Apologies if I’ve misunderstood what you were asking. If so, try again!

David

[David Boyd]

Hi Tony,

That is the broad Raman scattered O VI line at 6825A. See http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1989A%26A…211L..31S&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

It was a mystery for a long time until Schmid published his paper. It’s a trademark of symbiotic stars with high excitation nebulae.

David

[David Boyd]

Hi Alun,

Congratulations, that’s a nice well calibrated and corrected spectrum. You should upload it to Andy’s spectroscopy database.

Cheers,

David

[David Boyd]

Hi Alun,

That looks good. You can now see emission lines of H-alpha, H-beta and H-gamma plus the two [N II] lines bracketing H-alpha and the dip at 5896 due to Na absorption in our galaxy. You could get three independent estimates of the redshift from the three Balmer lines. I’m guessing you haven’t applied a correction for instrument and atmospheric losses? If you did this it would remove the hump and you’d end up with a relatively flat spectrum similar to this one on NED http://ned.ipac.caltech.edu/spc1/2006/2006ApJS..164…81M/NGC_2903_nuc:S:Opt:mk2006_sp.png

Cheers,

David

[David Boyd]

Hi Alun,

If you don’t have the Alpy calibration unit, you can use the Balmer lines in your A-type spectrum to find the polynomial which maps pixels into Angstroms. You can use that to calibrate your galaxy spectrum independently of the sky background lines and use the sky sodium line as a check that everything is correct. You can then measure the galaxy redshift in km/s using the H-alpha line (delta lambda x 300,000 / lambda) which is always a satisfying thing to do with a small scope in your back garden. Once you have done the background subtraction, you will probably find a small absorption line at the sodium wavelength in your spectrum due to absorption of light from the distant galaxy by sodium in our own galaxy.

Cheers,

David

[David Boyd]

Hi Alun,

According to the NED database http://ned.ipac.caltech.edu/forms/byname.html NGC 2903 has a redshift velocity of +550 km/s which corresponds to a wavelength shift at H-alpha of +12A. It’s hard to tell exactly from your plot but that looks close to what you are seeing. The NED database also contains spectra of the galaxy which show it has a fairly flat continuum with only a few strong emission lines at H-alpha, H-beta and (probably) [N II] at 6584A, all of which you see faintly. So the stronger emission lines you are seeing such as sodium D at 5896A (which is not redshifted) may be sky background lines. Did you subtract the sky background during processing?

David

[David Boyd]

Thanks Andrew. These look very useful. Neither came up on a google search. Wrong search words I guess.

David

[David Boyd]

Do either of you know of a reference which quantifies the size of atmospheric chromatic dispersion as a function of zenith angle at sea level? A google search has not thrown up anything useful so far. All the references I can find address the issue for large telescopes on mountaintops. It is difficult to tell from Christian’s webpage what the actual size of the effect is.

David

[David Boyd]

Thanks for your comments. There may well be another reason for the effect I’ve seen but I’m struggling to identify what it is.

I think it would be difficult to detect such an effect photometrically because of focus variation between filters.

My first thought was that it was the problem Christian identified with parallactic angle as I cannot set my LISA to that orientation because the cameras which project from both ends of the LISA foul the pier when near the meridian. However I would have expected the effect of that to be worse at the blue end.

Thanks for the reference Andrew, I’ll digest that tomorrow and keep thinking.

David

[David Boyd]

I’ve experienced the same problem as John with the same sequence of Miles spectra which Robin used. The response profiles calculated for each star give an excellent match with the Miles spectrum for that star but differ from star to star. All the spectra were taken over a period of an hour at an air mass of around 1.2 with no change to the equipment including focus and no obvious change in atmospheric conditions. However, I do live about 50 miles west of Heathrow and planes are flying over at high altitude all the time. I recently asked on the ARAS Forum if anyone have noticed changes in atmospheric response due to aircraft con trails but so far there has been no response. Possibly a bit of a long shot.

I repeated the exercise a few days later and found the same variation, again different response profiles from star to star but no consistency with the first set. I now plan to experiment with varying the position of the guide star relative to the slit for the same star to see what effect this has. That will not be for a while as the forecast here is bad for several days.

I’m using a LISA with a 23 micron slit on a C11 with a focal reducer operating at f/5.5. The slit size on the sky is 3.1″ which is a reasonable match to the seeing here. These is a noticeable fishtail on the spectra due to a combination of chromatic aberration in the LISA and the FR lens but I include the full fishtail that in the binning zone and do this with the same binning zone for all the stars. I use the optimal binning option in ISIS but switching that off has no effect on the derived response profiles. It is not a problem with smoothing the response profile as the effect is much larger than any variation that causes.

Cheers, David

[David Boyd]

Hi John,

To get the value for the rms residual given by ISIS you need to calculate sqrt(sumsq(dlambda values)/6) because you have 11 lines and there are 5 parameters in the fit.

David

[David Boyd]

Robin,

That’s correct. If you add 1 to the CCD pixel positions (which are listed as x= in the Go window) before calculating the 4th order polynomial in Excel, then you get the coefficients ISIS displays in the Go window. However the calibration line dlambda residuals shown in the Go window are computed using a 4th order polynomial based on the original CCD pixel positions.

David

[David Boyd]

I use the File mode option under Spectral calibration on the General tab in ISIS 5.7.0. I give it a .lst file with a list of the calibration lines to use in the fit. ISIS then does fill in the calibration fit coefficients in the Dispersion window. It also highlights the 3rd order radio button in that window although the fit is 4th order.

One thing I have noticed is that these coefficients do not generate the correct residuals if you use them in a 4th order polynomial with the measured pixel positions for each calibration line listed in the Go window in the Go tab. If on the other hand you make a 4th order polynomial fit to the measured pixel positions and the correct line wavelengths using eg Excel and then use these coefficients to calculate the residuals for each line, you get exactly the residuals listed in the Go window. These coefficients are not the same as the ones listed lower down in the Go window and in the Dispersion window.

I have no idea what is going on there and where the coefficients listed by ISIS come from but they are not the coefficients of the mathematical fit which produces the residuals listed for each line in the Go window. Perhaps this is a consequence of using File mode calibration? I am only mentioning this in case anyone tries to use the coefficients listed by ISIS to reproduce the residuals and finds they get different answers.

David

[David Boyd]

Hello,

 

For me 2016 has been the best year as far as useable nights for observing is concerned since 2007. I work out my observing statistics as a percentage of the number of nights on which I was available to observe, so not on holiday, away from home or whatever. I managed to observe on 121 out of 267 available nights (45.3%). Best months were August and September, both with over 60% while May, October and November were over 50%. January was the worst with only 28%. In total 18950 images were recorded and measured during 609 observing runs, and 2856 spectra recorded during 144 runs.

 

Clear skies for 2017 – my last night of 2016 is distinctly overcast!

 

David

[David Boyd]

Hi Steve,

With a Miles library star you are comparing your spectrum to an actual spectrum of that star so your response correction should be accurate. If you just pick an AxV or BxV type star, take its spectral type from a catalogue and compare your spectrum with a Pickles library spectrum for that type of star, you are assuming that the catalogue type is correct and that star is a normal star of that type. This is not always the case – catalogues can be wrong and some stars are peculiar.

So using a Miles star at the same altitude and as close in azimuth as possible to the target is the best approach. This is not always possible as there is not always a convenient Miles star so sometime you just have to use an A or B type star but remember your response calibration might not be as good in that case.

David

[David Boyd]

Hi Steve,

Your UV Aur spectrum is very similar to one I took of this star back in Feb 2015 which is in the ARAS database at http://www.astrosurf.com/aras/Aras_DataBase/Symbiotics.htm. Looking back at my records, I used HD39357 as a reference star then but today I would check for a suitable Miles star. There are several more recent spectra of the same star in the database so you could download these to compare with your current spectrum.

I haven’t observed V1261 Ori myself but I see there is one recent low resolution spectrum in the ARAS database worth comparing with.

David

[David Boyd]

Hi Kate,

It looks like you are giving ISIS the wrong wavelengths for some of the Balmer lines so it is giving large errors on those lines.

David

[David Boyd]

Tony,

My comment to Steve was in relation to his f/7 apo refractor. For an f/10 SCT, I think you will definitely need a focal reducer with the Alpy.

The issue of focal length and positioning of various makes of focal reducers is a bit of a minefield. I have seen several figures quoted for the focal length of the Meade 0.63x focal reducer and even that there are two versions on the market, one made in Japan and one in China with different focal lengths. I have no idea if that is true. In my experience, the best approach is to believe nothing and measure it yourself. To get a reasonably accurate value, put the focal reducer at the back of any scope, point it at a distant object on a bright day and measure the distance from the position of the lens in the focal reducer to the point where the image is sharp. Call this F2. Do the same without the focal reducer and measure the distance from where the lens was in the focal reducer was to the new focus. Call this F1. It will be larger than F2. The focal length of the focal reducer is then FL=(F1*F2)/(F1-F2). If you don’t get a sharp image without the focal reducer you may need to rack in the focuser a bit and try again.

The focal reduction factor RF you have just measured is F2/F1. It will vary as you rack the focal reducer in and out. Every focal reducer is designed to operate optimally at the position where the reduction factor RF is the quoted value, 0.63 in the case of Meade. If the measured focal length is FL, the optimum distance from the lens in the focal reducer to the image plane, F2, in this case the slit of the Alpy, is given by F2=FL*(1-RF) = 0.37*FL. You may need to insert spacers to get as close as possible to this optimum spacing.

As a general comment, you want to keep the Alpy as close to the scope as possibly to reduce the possibility of flexure so go with the focal reducer with the shortest focal length provided there is enough space between the focal reducer lens and the Alpy slit to achieve the optimum spacing. Needless to say you should also use a high quality achromatic focal reducer in a situation like this.

Hope this makes sense. It is based on my experience and has worked for me.

David

[David Boyd]

Hi Steve,

My inclination would be to run your setup without a focal reducer to avoid adding more chromatic aberration and see how that works in practice. That also avoids the problem of having to get the spacing just right for the focal reducer. As a general rule, I like to keep things as simple as possible consistent with getting good results.

David

[David Boyd]

Hi David,

I also have an SX H694 and run this at -20C all year round. It seems to hold that temperature within a few tenths of a degree, even on warm summer nights.

Regards,

David

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