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| Reflections from needles as virtual slits https://forum.vdsastro.de/viewtopic.php?t=3064 |
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| Autor: | Ken Harrison [ 01. November 2009, 10:58:54 AM ] |
| Betreff des Beitrags: | Reflections from needles as virtual slits |
I'm reviewing the use of reflections from needles as virtual slits. HD Suiter in his "Star testing astronomical telescopes " - Appendix F gives formula for the size of the "glitter" point. I've also found ( thanks Robin!) Maurice Gavin's original 1981 BAA article on needle reflections. http://www.astroman.fsnet.co.uk/needle1.htm I know that Norbert Reinecke in his 2006 "Homebuilt medium resolution Littrow-Spectrograph" mentions the use of the needle reflection to get a comparison spectrum into the field. My question is: The needle really only seems to have been used in slitless spectroscopes for solar observing. The calculations show that an input beam from a f6 system on a 1mm needle could give a virtual slit of 30 micron which could be usefull for stellar work. Do I therefore infer that the reflectivity of the needle is to low to give a usable result...??? Has anyone had experience using needle reflections for stellar work. Ken On 30/10/2009, Thomas Hunger wrote: Zitat: Hallo MitstreiterInnen,
es ist nunmehr 9 (!) Jahre her, dass die "Einführungsschrift" veröffentlicht wurde. Sie hat sich bewährt. Seither hat sich aber Beachtliches verändert. Daher wäre es an der Zeit, eine zweite, überarbeitet und erweiterte Auflage zu machen. Der Inhalt soll dabei nicht nur die gerätetechnisch neuen Richtungen aufzeigen, sondern auch - und das halte ich für ganz wichtig - Objektklassen darstellen (was ist möglich und was ist sinnvoll). Wer sich also berufen fühlt, an diesem Projekt mitzumachen, möge es mir bitte kundtun. Ansonsten werde ich mich penetrant noch an den einen oder die andere wenden Und noch ganz wichtig: ein Feedback zur aktuellen Ausgabe ist mir auch willkommen (Stichwort: Lernkurve). Danke schon mal und beste Grüße Thomas -- "Very funny Scotty... now beam down our clothes!" http://tech.groups.yahoo.com/group/astr ... =322612425 |
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| Autor: | Norbert Reinecke [ 01. November 2009, 19:57:53 PM ] |
| Betreff des Beitrags: | |
Hi Ken, no, I didn't try stellar work with a reflective slit. I see the major and principal difficulty in the fact that you are always faced with two extremely different F# looking with your collimator lens perpendicularly at the needle. In one plane the F# of the telescope will be unchanged while in the the second perpendicular plane the F# will be drastically increased since the needle acts like a strong barlow-type cylindrical lens. In case of bright sources this is of no relevance while in case of starlight you loose much too much light in dispersion direction. In case of my selfbuilt spectroscope I obviously had no such problem since the needle was used just for the calibration path equipped with a strong Ne source. Best regards, Norbert |
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| Autor: | Norbert Reinecke [ 01. November 2009, 22:05:38 PM ] |
| Betreff des Beitrags: | |
Hi Ken, sorry, I need to append a correction to my previous posting: F# of course is strongly decreased (insteaed of increased) in dispersion direction. There is - what so ever - no way to change this situation of the totally unequal F# in both planes and hence it will not be possible to do stellar work in my mind. Regards, Norbert |
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| Autor: | Ken Harrison [ 02. November 2009, 17:45:39 PM ] |
| Betreff des Beitrags: | Reflections from needles as virtual slits |
Norbert, Thanks for the reply. I understand the limitations and feel you're probably correct.... not a workable solution for stellar work, but OK for solar. On 01/11/2009, Norbert Reinecke wrote: Zitat: Hi Ken,
sorry, I need to append a correction to my previous posting: F# of course is strongly decreased (insteaed of increased) in dispersion direction. There is - what so ever - no way to change this situation of the totally unequal F# in both planes and hence it will not be possible to do stellar work in my mind. Regards, Norbert MAY THERE ALWAYS BE STARLIGHT ON THE PATH -- "Very funny Scotty... now beam down our clothes!" http://tech.groups.yahoo.com/group/astr ... =322612425 |
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| Autor: | Ken Harrison [ 02. November 2009, 17:50:01 PM ] |
| Betreff des Beitrags: | Reflections from needles as virtual slits |
On a similar but different issue...what about the use of a cylinderical lens to produce a "virtual slit" like Fulvio Mete's spectroscope?? Is there any reason this has not been picked up by the "mainstream" amateur constructors?? http://www.lightfrominfinity.org/spettr ... ellare.htm On 02/11/2009, Ken Harrison wrote: Zitat: Norbert, Thanks for the reply. I understand the limitations and feel you're probably correct.... not a workable solution for stellar work, but OK for solar. On 01/11/2009, Norbert Reinecke wrote: Zitat: Hi Ken,
sorry, I need to append a correction to my previous posting: F# of course is strongly decreased (insteaed of increased) in dispersion direction. There is - what so ever - no way to change this situation of the totally unequal F# in both planes and hence it will not be possible to do stellar work in my mind. Regards, Norbert MAY THERE ALWAYS BE STARLIGHT ON THE PATH -- "Very funny Scotty... now beam down our clothes!" http://tech.groups.yahoo.com/group/astr ... =322612425 -- "Very funny Scotty... now beam down our clothes!" http://tech.groups.yahoo.com/group/astr ... =322612425 |
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| Autor: | Robin Leadbeater [ 04. November 2009, 14:58:40 PM ] |
| Betreff des Beitrags: | Re: Reflections from needles as virtual slits |
Zitat: On a similar but different issue...what about the use of a cylinderical lens to produce a "virtual slit" like Fulvio Mete's spectroscope??
Hi Ken,Is there any reason this has not been picked up by the "mainstream" amateur constructors?? As far as I can see, it suffers from the same problem. It makes the source look narrower in the dispersion direction by acting as an extremely powerful focal reducer. This means the spectrograph cannot collect all the light from the resulting very fast beam and so it is very inefficient. Spreading the light out in the other direction may make visual observing of the spectrum easier but it lowers signal/noise when using a camera. It is better for S/N to keep the spectrum as narrow a possible. Robin |
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