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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,

_________________
Fil.

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