Forum der Vereinigung der Sternfreunde

Hallo Freunde,

der Farblängsfehler eines Objektives oder Kollimators spielt im Zusammenhang mit dem Bau von Echellespektrographen eine wichtige Rolle.

Angeregt von unserer Diskussion habe ich mir überlegt, wie ich den Focalshift des Dubletts in meinem Lhires messen kann. Ich habe ja in meinem Lhires (wie auch Berthold und der bei der WR 140 Kampagne verwendete Lhires von Wolfgang) ein 200 mm Dublett mit 40 mm Durchmesser (F# 5) von Edmund, dessen Fokalshiftkurve in der Firmendokumentation angegeben ist ( http://www.edmundoptics.com/techsupport ... 741ink.pdf ).

Nun, ich habe den Lhires bei etwa 5800 Angström fokussiert und bekam eine minimale FWHM der Neonlinien im Zentrum des Spektrumausschnitts von ca. 23,7 Mikrometer. Dann habe ich Neonspektren bei unterschiedlichen Gitterstellungen aufgenommen, die Linien identifiziert und eine Kalibrierkurve aufgestellt (siehe Excelsheet in der Anlage). Parallel dazu habe ich die FWHM der Neonlinien ausgemessen. Die Aufweitung der FWHM durch den Fokalshift ergibt sich aus der Differenz FWHM_Messwert - Minimalwert und nach Multiplikation der Aufweitung mit 5 ( = F#) erhält man den gesuchten Fokalshift. Die gemessene Kurve ist in der beiliegenden Grafik dargestellt. Die Kurve deckt sich relativ gut mit den Herstellerangaben.

Wenn ich im NIR (> 700 nm) oder weiter im Blauen (< 500 nm) ernsthaft messen will, dann muss ich den Fokalshift durch eine Neufokussierung neutralisieren. That's life.

Hello friends,
the wavelength longitudinal error (focalshift) of an objective or a collimator plays in connection with the building of Echellespektrographs an important role. Lively of our discussion I considered myself, how I can measure the focalshift of the doublet in my Lhires. I have in my Lhires (like also Berthold and with the WR 140 campaign used Lhires of Wolfgang) a 200 mm doublet with 40 mm in diameter (F# 5) of Edmund, whose focalshift curve is indicated in the firm documentation (http://www.edmundoptics.com/techsupport ... 741ink.pdf).
Now, I focused the Lhires with approximately 5800 angstroms and got a minimum FWHM of the neon lines in the center of the spectrum windows of approx. 23.7 micrometers. Then I took up neon spectra with different grating positions, identified the lines and set up a calibration curve (see Excelsheet in the annex). Parallel to it I measured the FWHM of the neon lines. The expansion of the FWHM by the focalshift results from the difference FWHM_Messwert - minimal FWHM. The multiplication of the expansion with 5 (= F#) results the focalshift. The measured curve is represented in the enclosed diagram. The curve covers itself relatively well with the manufacturer data.
If I want to measure seriously in the NIR (> 700 Nm) or in the blue (< 500 Nm) , then I must neutralize the focalshift by a new focusing. That' s life.

Liebe Grüße
Lo

_________________
Herzliche Grüße / best regards

Lothar

https://lotharschanne.wordpress.com/

Hello Lothar,

I have a similar effect with the original LHIRES doublet, particularly at the blue end.
http://www.threehillsobservatory.co.uk/ ... copy_7.htm
(My figures are 2x because I changed the camera distance for these measurements, not the collimator position)

The blue end is a particular problem at low resolution as the focus cannot be maintained over the full width of the CCD.
http://www.threehillsobservatory.co.uk/ ... tra_28.htm
See the "fish tail" on the Vega spectrum.

C'est la vie

Robin

Hello Robin,

ok, that are the borders in using achromatic doublets.

In your website "calibration of Lhires" is a link "see here for details". It doesn't work. The helical focuser is interesting because you are able to focus without opening the spectrograph and adjusting the doublet.

Bye
Lothar

_________________
Herzliche Grüße / best regards

Lothar

https://lotharschanne.wordpress.com/

Hi Lothar,

I do not use the helical focuser now. It was a home built system using a camera lens body with the lenses taken out. I will find the missing page so you can see it. It worked but it was not rigid enough and it was too long to fit when I bought my ATIK camera. I think the idea might be worth some more investigation using a better engineered focuser though.

Cheers
Robin

The link now works
http://www.threehillsobservatory.co.uk/ ... copy_8.htm

Cheers
Robin

Lothar,
On my 200mm Littrow, I use the Baader helical focuser with the quick change adaptor to allow various cameras to be mounted without disturbing anything.
There's a write up in the files area of Astronomical Spectroscopy forum
http://tech.groups.yahoo.com/group/astr ... ctroscope/


 
On 10/08/2009, Lothar Schanne wrote:
--
"Very funny Scotty... now beam down our clothes!"
http://tech.groups.yahoo.com/group/astr ... =322612425

Hello Ken,

I cannot read your file xxx.mht. What is that type .mht?

cheers
Lo

_________________
Herzliche Grüße / best regards

Lothar

https://lotharschanne.wordpress.com/

Hi Lothar,
once you have signed in at YAHOO as a member of the spectroscopy group, you will be able to open the document easily.
Regards,
Norbert

Hi Lothar
MHT or MHTML is a MIME HTML "web archive" format file of saved web
pages usually saved by the Internet Explorer. This format has the ad-
vantage that it stores all scripts and images with the HTML text in a
single file:
http://en.wikipedia.org/wiki/MHTML
MIME is the Multipurpose Internet Mail Extensions that can store multi-
ple parts of emails with different types in a single email:
http://en.wikipedia.org/wiki/MIME


If you cann't or don't want to install the IE try to open it with MS Word,
an other word processor that can read MS Word format files, or the
Opera browser (you can also try to open it with Firefox, Safari and Kon-
querer). If you don't have these you could also install just one of the
MS Office Viewers (like e.g. the Word Viewer) and open it with these.
If you run Linux, use the kmhtConvert utility to convert MHTML format
files to the KDE Web Archive format.
If nothing else works, you could try to rename it to EML or an other
email format extension that supports MIME and import it into browsers
or email client programs that support MIME somehow. This should make
at least the texts in the MHTML files readable.

Clear skies
Wolfgang

--
Wolfgang Renz, Karlsruhe, Germany

Hi Wolfgang,
das hat schon mal geklappt.
Danke
Lo

_________________
Herzliche Grüße / best regards

Lothar

https://lotharschanne.wordpress.com/