Forum der Vereinigung der Sternfreunde

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:
--
"Very funny Scotty... now beam down our clothes!"
http://tech.groups.yahoo.com/group/astr ... =322612425

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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MAY THERE ALWAYS BE STARLIGHT ON THE PATH

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

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:
--
"Very funny Scotty... now beam down our clothes!"
http://tech.groups.yahoo.com/group/astr ... =322612425

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:


--
"Very funny Scotty... now beam down our clothes!"
http://tech.groups.yahoo.com/group/astr ... =322612425

Hi Ken,

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