UVEX3

[Thalestris24]

2 minutes ago, Merlin66 said:

Louise,

Think of the reflective slit plate as an off axis guider... but where the pick off mirror is large and on the optical axis so it picks up a guider FOV as big as the mirror/ guider chip.

In the middle of this "pick off mirror" we have a very fine gap (the slit) say around 20 micron wide. We need to focus the target star into this slit gap and still be able to guide for as long as required. The starlight then goes through the slit and into the spectrograph....the light is collimated (made parallel) and reaches the grating which disperses the light into the spectrum.

This spectrum is then refocused (after the grating) by the imaging lens (or the collimator in the case of a Littrow) to present a spectral image to your imaging camera.

Hope this helps.

 

 

Yeah, I've figured all that out now

Louise

[Merlin66]

Louise,

Although not the best solution the beamsplitter guider worked reasonably well.

We developed a "virtual" slit which could be registered to the actual as a means of acquiring and guiding on the target star.

A Vixen flip mirror body was used:

http://www.astronomicalspectroscopy.com/files/Vixen mod.pdf

Al's reticule was used...

AlsReticule33.zip

[robin_astro]

Hi Louise,

Christian Buil has a nice page which goes through the design of a classical spectrograph here

http://www.astrosurf.com/buil/us/stage/calcul/design_us.htm

The Shelyak  instrument probably most similar in concept to the lowspec is the LISA, which is also a classical spectrograph design, though the better LISA optics give better performance at the UV/IR ends and allow it to be used on larger aperture faster telescopes. The lowspec is more versatile as you can change the grating to alter the resolution. 

The LHIRES is a Littrow design in which the light passes through the same lens on the way to and from the grating. This was originally an open source design by Christian Buil and others and was sold as a not for profit kit by the design team. (Mine is one of these) The Shelyak company was then set up to offer it commercially.  You can still find all the drawings etc here but it is not really practical to build one yourself as a one off, though some people have.  

http://www.astrosurf.com/thizy/lhires3/index-en.html

Cheers

Robin

Edited October 12, 2019 by robin_astro

[Thalestris24]

1 minute ago, robin_astro said:

Hi Louise,

Christian Buil has a nice page which goes through the design of a classical spectrograph here

http://www.astrosurf.com/buil/us/stage/calcul/design_us.htm

The Shelyak  instrument probably most similar in concept to the lowspec is the LISA, which is also a classical spectrograph design, though the better LISA optics allow it to be used on larger aperture faster telescopes. The lowspec is more versatile as you can change the grating to alter the resolution. 

The LHIRES is a Littrow design in which the light passes through the same lens on the way to and from the grating. This was originally an open source design by Christian Buil and others and was sold as a not for profit kit by the design team. (Mine is one of these) The Shelyak company was then set up to offer it commercially.  You can still find all the drawings etc here but it is not really practical to build one yourself as a one off, though some people have.  

http://www.astrosurf.com/thizy/lhires3/index-en.html

Cheers

Robin

Have seen all those, thanks!

Louise

[CharlieM]

On 12/10/2019 at 12:13, robin_astro said:

Hi Louise,

Christian Buil has a nice page which goes through the design of a classical spectrograph here

http://www.astrosurf.com/buil/us/stage/calcul/design_us.htm

 

Cheers

Robin

I hope you don't mind the newbie questions but I notice on that page that Christian is using a lot more lenses than i have seen in the DIY designs like the Starex. Is there a reason , for example, that he uses a single achromat for a collimator in the Starex, but numerous lenses on that page ? Was it just a matter of convenience ?

[robin_astro]

8 hours ago, CharlieM said:

I hope you don't mind the newbie questions but I notice on that page that Christian is using a lot more lenses than i have seen in the DIY designs like the Starex

That is a good and interesting question.  This was one of his earliest spectrograph designs and in that case he just used standard SLR camera lenses which have many elements to correct for various aberrations, important for making good sharp images across the field at low focal ratios rather than specifically for spectroscopy. You can use simpler (smaller and lighter) achromats in spectrographs eg the StarEx, LowSpec, LHIRES all use achromats but they do have drawbacks such as not working well at low focal ratios and suffer from chromatic aberration, particularly noticeable as a shift of focus at the blue end of the spectrum. The ALPY also uses achromats and works well down to f4 but they are specially designed lenses that Christian designed to minimise chromatism in the particular application.  There are commercial designs that use multi-element camera lenses like Shelyak's LISA and eShel (A good sharp image over a wide field is particularly important in echelle spectrograph designs) but even there chromatism at the blue end can be problem even with high quality camera lenses. Spectroscopy is a severe test for chromatism and most professional spectrographs use all reflective optics. Shelyak's UVEX is an example of this but they are difficult to design and align to be free of other aberrations.

Cheers

Robin 

Edited April 26, 2023 by robin_astro
various typos

[CharlieM]

1 hour ago, robin_astro said:

 lenses which have many elements to correct for various aberrations, important for making good sharp images across the field at low focal ratios rather than specifically for spectroscopy. You can use simpler (smaller and lighter) achromats in spectrographs eg the StarEx, LowSpec, LHIRES all use achromats but they do have drawbacks such as not working well at low focal ratios and suffer from chromatic aberration, particularly noticeable as a shift of focus at the blue end of the spectrum.  

Ok. I take that to mean aberrations caused by the spec itself and not the scope. I'm using an RC8. I'm a little confused as to which design to settle on now. If a single lens is going to be so much of a problem, perhaps I need to look at the UVEX as opposed to the StarEx, which was my plan forward ??

[robin_astro]

2 hours ago, CharlieM said:

If a single lens is going to be so much of a problem, perhaps I need to look at the UVEX as opposed to the StarEx, which was my plan forward ??

If you are relatively new to spectroscopy and are looking for a home build I would go for the StarEx. The optical kit for it from Shelyak uses specially designed lenses which perform better in the blue than standard off the shelf achromats and it is a good match for the f8 RC8. The UVEX is more an advanced design for experienced observers, particularly if looking for the best performance in the UV but it  is tricky to get working well.  

Cheers

Robin

[robin_astro]

Here are some examples just posted on the solex/starex  forum of some spectra at both high and low resolution with a StarEx and an f5 Newtonian so quite demanding for the spectrograph optics. The results look good even down to  below 3800A in the UV and up to 9500A in the IR

https://groups.io/g/Solex-project/topic/un_session_au_newton_200f5/98516943

Robin

Edited April 26, 2023 by robin_astro

[CharlieM]

3 hours ago, robin_astro said:

Here are some examples just posted on the solex/starex  forum of some spectra at both high and low resolution with a StarEx and an f5 Newtonian so quite demanding for the spectrograph optics. The results look good even down to  below 3800A in the UV and up to 9500A in the IR

https://groups.io/g/Solex-project/topic/un_session_au_newton_200f5/98516943

Robin

I lost track of Tony Rodda. He , as I understood it, was going to send me his modded files for the Starex cause I did buy Ken's Ovio slit wheel and don't want it to go to waste. I was going to make Paul's LowSpec but changed my mind. I guess I'm on hold until I hear from Tony. I hope he hasn't forgotten me. If I could find editable versions of the StarEx, I might just do my own mods for the StarEx.