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Cylindrical Lens With a Grating?

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Cylindrical Lens With a Grating?

At the AAVSO meeting last week we toured the GOTO telescope lab at Appalacian State U.  The had a low-res spectroscopic setup on their telescope that used a grating and a cylilndrical lens mounted in two separate slides about three inches apart.  They said that students could slide those into the beam like similarly mounted lenses and do some slitless spectroscopy without a large change in the focus.  

I know that one of the rough things about the Star Analyzer is that you need a large amount of focus travel if you want to try mounting it in your filter wheel.  I wanted to see if anyone on this forum had experimented with using a grating and cylindrical lens combination on their rig and how it worked out for them if they had.

Spectroscopy Books


Hi John,

There are several books out on amateur spectroscopy. A Google search should show most of them. My new book published by Springer should be out this summer or early fall. 

One excellent free source of information is Richard Walker's Spectroscopy Atlas

I am currenty awaiting delivery of my ALPY 600 and Guiding Unit. I am working on a fiber optic interface using a star diagonal mirror that may allow the ALPY 600 Basic module to work without the Guiding Unit. If I succeed it will be written up in detail in my new book as well as some practical information on the ALPY 600.


Roger Pieri
Cylindrical Lens

Hi John,

The introduction of the SA100 in a converging beam doesn't change a lot the focus point (Just the difference of index / thickness of the glass against the other filters ?)  The usual issue is that the distance between the wheel and the sensor is too short to achieve an optimal dispersion and resolution. Optically speaking I don't see how a single cylindrical lens would change it.

Such lens could be used to enlarge the trace of the spectrum without affecting the focus of the lines. In "visual" spectroscopy this should make the spectral lines much more visible to the human vision (vernier effect). In spectrography, on sensor, it's not very useful to spread the photons along spectral lines as we normally have not enough flux... Could be with DSLR, just to avoid undersampling by the Bayer structure... 

Clear Skies !

Roger (PROC)

Robin Leadbeater
cylindrical lens and guiding the Alpy

Yep the cylindrical lens is to widen the spectrum vertically for visual observign to make it easier to discern the lines but you need a fairly big scope to be able to see much visually given that the spectrum is typically 6 mags fainter than the star image and spreading it vertically will make it fainter still. 

Re guiding the Alpy spectrograph. I dont follow  the logic behind fibre feeding what is already a light weight spectrograph but if money is tight there are potential alternative DIY solutions already out there to the excellent but rather high cost Alpy mirror slit Guider.   How about for example using a modified flip mirror ahead of the base Alpy 600 unit with a beam splitter in place of the mirror and viewing the reflected portion of the beam with a camera as Ken Harrison developed for the L200. (A dichroic hot mirror might even be used instead of a conventional beam splitter to guide on the unused reflected IR while allowing the visible light to pass to the spectrograph)  A further development of that could be to use the redundant flip mirror mechanism to move a diffuser screen up against the back of the beam splitter and feed light from calibration and flat lamps via ports on the bottom of the flip mirror, doing the job of  the Alpy calibration unit as well.

(BTW If anyone is interested, my Alpy 600 saw first light today

(only a cloudy sky spectrum though)



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