spectro-aras.com

In Christian's excellent intro to the UVEX (http://www.astrosurf.com/buil/UVEX_project_us/) it states that:

The typical instrument is a Schmidt-Cassegrain, Maksutov or Ritchey-Christian telescope of 150 to 280 mm in diameter with an aperture ratio of between F / 8 and F / 10. This is an important remark and a limitation that you must absolutely consider. If your telescope is open at F/5 (a Newton for example), UVEX is not really adapted: you will obtain a spectrum but it will be degraded (loss of resolution of 30% compared to F/10 input beam for a 25 microns slit and exploitable spectral domain reduced). An aperture at F/6 is in practice the brightest configuration usable with UVEX.

Given that the input M1 collimation mirror is f/4 (25.4mm dia, 100mm FL) why can't an f/4 scope be used? Is it due to the astigmatism of the Czerny-Turner design if the full mirror aperture is used?

Cheers,
Bernard

I have to say that I'm surprised, and disappointed, that nobody has yet responded to my basic UVEX question.

Cheers,
Bernard

I don't know for sure, but I've just looked at the UVEX website, and the optical diagram shows that the first mirror is not fully illuminated by the beam from the slit, so presumably it's effectively working at something like f/6 or 8. It also shows that the second mirror is​ fully illuminated, so presumably a wider beam at the entry to the instrument would vignette at the grating and the camera, losing both light and resolution.

http://spectro-uvex.tech/wp-content/upl ... ique-3.jpg

This is just my guess - I'll be interested to see if anybody who actually knows about this can confirm (or correct!).

Cheers,

Tom

Good point Tom, which I hadn't noticed!

If the M2 'camera' mirror uses its full aperture to give un-vignetted coverage to the full refracted blue-red spectrum it implies that if an f/4 scope were used, filling the full M1 aperture, then the spectrum at M2 would be significantly vignetted. One good reason against using a 'fast' scope.

Still not clear to me why the resolution will be degraded. Maybe I should go back to the 'resolution' equation as that might reveal the reason!

Cheers,
Bernard

I think that if the camera is vignetting, then in effect you're not using light from the full width of the grating, so you lose resolution.

Hi,

The level of astigmatism aberration is proportional to the power 3 of D/F ratio.
The level of spherical aberration is proportional to the power 2 od D/F ratio.

The difference between f/5 and f/10 is considerable in terms of image quality (better at f/10 of course).

Christian

Tom Love wrote:I think that if the camera is vignetting, then in effect you're not using light from the full width of the grating, so you lose resolution.

Hi Tom,

the grating not being fully illuminated only affects the resolution if it is limited by the theoretical resolving power of the grating (R~number of lines illuminated) I dont know the specific numbers for the UVEX but generally I find this is not the case in amateur spectrographs. For example a 25mm wide 600l/mm grating would have a theoretical resolving power of 15000 but most designs using this grating have a much lower practical resolving power.

Cheers
Robin

That makes sense - thanks Robin.

Hi,

Here is a concrete example obtained with UVEX (4),slit= 23µ, grating= 600, F/D=8 the resolution power obtained is around 2500, it is lower in UV.
Cheers
Pierre

Hi Pierre,

Impressive NIR spectra!
Is the 600 grating blazed at 750nm?

Which gratings do you consider the most useful for the UVEX? I have the 300 (500nm blaze) but am wondering if I really need more.

Cheers,
Bernard