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Blue-end disparity.

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Anthony Rodda's picture
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Blue-end disparity.

A couple of weeks ago Robin mentioned a 'drift' of blue end traces (away from library spectra).  I'd seen that in mine too but not to any great extent.  Then, last weekend, I saw a considerable drift, see attached.  

There's quite a bit of deviation throughout the plot (and I should have filtered the reference to make it easier) but the blue-end deviation is very noticeable.

Is this bad technique at the scope or bad processing?  I'd thought I had decent guiding and I can't see anything in my processing to account for it. Very frustrating.

I also seem to remember a slide deck (from the BAA-Shelyak workshop?) containing slides explaining how different positioning of the star on the slit leads to distorted spectra.  That is, sampling different red-blue parts of the disc.  There were some good examples.  Can anyone remember that slide-deck location or have a copy?

Regards

Tony

Robin Leadbeater's picture
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Blue end disparity

Hi Tony,

Except for a mismatch in the strength of the lines, probably due to a mismatch in resolution, the disparity at the blue end does not look that obvious to me from here. What does the fractional error look like if you divide one by the other and smooth the result?

I (and others) have also found getting a good flux calibration below 4000A tough. Most of the sources of error were probably been covered in this thread.

https://britastro.org/node/9700

and Chrisitan Buil's site covers chromatic aberration, atmospheric absorption and the effect of position on the slit

http://www.astrosurf.com/buil/dispersion/atmo.htm

I suspect the best (though time consuming) solution when after highest accuracy in the continuum shape at the blue end is to use a spectrophotometric technique using a very wide photometric slit to determine the shape of the continuum and combine that with the resolution of a narrow slit spectrum eg

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

David found using a wider slit gave better results, though the reason for the improvement was not clear in this case

https://britastro.org/node/9199

Cheers

Robin

Anthony Rodda's picture
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Many thanks Robin.

I might consider a wider slit or indeed the Shelyak photometric combo slit.

Regards (and thanks once more for your time).

T

John Simpson's picture
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photometric slit

Tony,

I've been using the photometric slit for a few weeks now - plus the references [Robin quoted these in an earlier post]. It has been a great help in my case - just showing how bad my guiding has been [basically I need to replace my mount]. Example for HD205021 attached.

Regards

john s

Anthony Rodda's picture
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Thanks John.

Regards T

andrew.j.smith1905's picture
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Bad guiding

I would not assume it is bad guiding John. Bad guiding can be a benefit as the whole of the stars PSF gets sampled (more or less) while good guiding may just sample the brightest (often green) part of the spectrum as seen in the guide camera. The narrower the slit the more this becomes true!

Regards Andrew

Anthony Rodda's picture
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PHD2 fooling around!

Yes Andrew.  I realised that after a few attempts at 'tinkering' with PHD2!

At first, on guiding on the target star, I noticed a few zig-zags in the PHD trace as it hunted for the star in the slit.  I tried to train that out with an increased hysteresis value but that just caused decreased 'reactiveness' either side of the slit and led to worse guiding (it would).

So I went back to my original values when I realised that it was, as you say, sampling the whole disc by default and thereby negating (to some extent) the chromatic aberration. 

I wouldn't say 'bad guiding is good' but "iffy" guiding certainly has it's benefits!

I still guide on a nearby star given the opportunity.

Regards

T