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Autor:	Filipe Dias [ 12. März 2011, 13:44:35 PM ]
Betreff des Beitrags:	Littrow design and sensor size question
Hi, In a Littrow-design spectrograph, what would be the dominant aspects that could limit the size of the image (spectrum) formed on the sensor? (what would limit the choice for sensor size?) What would limit a Lhires to 7~8 mm of spectrum ? (I think that is the official number)

Autor:	Daniel Sablowski [ 12. März 2011, 18:58:07 PM ]
Betreff des Beitrags:
Hi Filipe It depends on the aberrations of your system (not only on the camera lens). The dominating factor is the image curvature. Also the chromatic focal shift is important. Both should be as small as possible over the image. The chromatic focal shift is a general problem for a spectrograph which cover the whole visible. Also by rotating the grating. So, image curvature is the determining aberration, if there are no off-axis aberrations. If the systems works under strong off-axis, astigmatism will be important. But such a bad system would only work well if you do not need to rotate the grating. But this depends also on the spectrograph mounting. In the case of a CT spectrograph the astigmatism is constant over the whole spectra and spherical aberration determines the R. If there is a system under development all aberrations are important in general and for the sice of the ccd image curvature is most important. I hope this was a slightly helpful

Autor:	Filipe Dias [ 13. März 2011, 14:51:15 PM ]
Betreff des Beitrags:
Hi Daniel, Is the image curvature you mention from the optics inside the spectrograph? It could possibly be caused or made worse because of the light not going through the objective/collimator parallel to the optical axis.. Does it make sense that by replacing a small doublet (working as objective and collimator) by a larger optical assembly (possibly a photographic objective) of the same focal distance that the image could be better corrected? And what about distance of the grating to the objective/collimator? I imagine that the closest they are, light goes less "off-axis" from the optical axis, however it goes in at a slightly greater angle.. Would problems increase if the focal length of the collimator/objective would be longer or shorter?

Autor:

Daniel Sablowski [ 13. März 2011, 20:39:01 PM ]

Betreff des Beitrags:

Hi Filipe

Zitat:

Hi Daniel,

Is the image curvature you mention from the optics inside the spectrograph?

Yes.

Zitat:

It could possibly be caused or made worse because of the light not going through the objective/collimator parallel to the optical axis..

I think, I do not realy understand this.

Zitat:

Does it make sense that by replacing a small doublet (working as objective and collimator) by a larger optical assembly (possibly a photographic objective) of the same focal distance that the image could be better corrected?

Yes! Optical systems have some importante parameters. One of thies is refractivity. In general aberrations will be reduced, if the refractivity of the system is spread to many surfaces. So, an objective with many lenses is better corrected. It is always a quastion how good a system must be corrected. In the case of a littrow as an astronomical spectrograph a doublet is good enough for the most applications.

Zitat:

And what about distance of the grating to the objective/collimator? I imagine that the closest they are, light goes less "off-axis" from the optical axis, however it goes in at a slightly greater angle..

Should be small as possible. In the case of a littrow not so good realized.

Zitat:

Would problems increase if the focal length of the collimator/objective would be longer or shorter?

The aberrations increas in general if the focal lenght decreases, specially spherical aberration. But focal length and aberrations must always be treated in context with the diameter of the optic. The F-number is more meaningful. Small F numbers, fast optics are in general worse corrected.

Dateianhänge:

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Autor:	Filipe Dias [ 14. März 2011, 01:26:02 AM ]
Betreff des Beitrags:
Thank you Daniel! Very informative.

Autor:	Daniel Sablowski [ 14. März 2011, 19:47:06 PM ]
Betreff des Beitrags:
No problem Filipe

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