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| Littrow spectrograph for F/3.9 Newtonian https://forum.vdsastro.de/viewtopic.php?t=2815 |
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| Autor: | Filipe Dias [ 28. Februar 2009, 00:12:16 AM ] |
| Betreff des Beitrags: | Littrow spectrograph for F/3.9 Newtonian |
Hello, The idea came to mind to try to build a spectrograph. And I wanted to set a few constraints: To have less than 56mm back focus To be usable in F/3.9 and F/9.9 (Newtonian and Cassegrain focus) I was thinking of building Littrow type. In order to cope with F/3.9 at the slit, the objective collimator need to be fast. I was thinking in getting a 50mm diam. 150mm F.L. (F/3) doublet, from Edmund Optics, for less than 100 euro. The sensor will be a bit large: 9 mm in the dispersion direction. I was planing on having a 7x7mm prism (40 euro) to take the light from the slit into the doublet. For 7mm, on an F/3.9 scope, the end of the prism must be less than 54.6mm away from the slit, so there is no vignetting. A 50x50mm grating should cost less than 150 euro... My question is whether the 150mm doublet, at F/3, will need the light coming from the prism to be very close to the optical axis of the doublet. Does such spectrograph sound interesting? Thank you, |
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| Autor: | Sander Slijkhuis [ 02. März 2009, 20:10:25 PM ] |
| Betreff des Beitrags: | |
Hi Filipe, I am not sure, but I expect that the color correction for F/3 will be problematic. If you plan to build a high-resolution spectrograph, then you are nearly working monochromatic and it may not be such a problem. I do not know how bad off-axis performance (Coma) of the doublet is, but I would not expect miracles. If you really intend to go this way, ask more info from Edmund. But I would strongly consider to use a 2x Barlow on the F/3.9 port of your instrument. In that case the F/8 and F/10 beams from your Newton/Cassegrain focus will be similar, the abberations are much constrained and you can build a spectrograph that is optimised for both ports. No need to be concerned because of chromatic abberations of a Barlow, it will be less than those of the F/3 doublet. Cheers, Sander |
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