Off axis performance for slit spectroscopy

[MattJenko]

Is it important to have good off axis performance for a scope for doing slit spectroscopy? I am assuming that with a slit, you are using on axis light to focus on the slit, so does it really matter if the rest of the field is not flat or has various aberrations?

[andrew s]

Off axis performance does not matter provided you use close to the center of the slit. If you are using a reflective slit with the common duel doublet transfer optics to the guide camera it will introduce more aberrations that most standard telescope designs.

Regards Andrew

[Merlin66]

The slit is used on the optical axis...

Off axis aberrations etc are not usually an issue.

 

[MattJenko]

Thanks. An example would be choosing a C11 for doing this. There would be no intrinsic need to go for the Edge version over a classic version, and another would be using a flattener for an RC etc. If that is the case (which I thought and hoped) then it makes the scope choice a little easier (and cheaper!).

[andrew s]

Correct the C11 is just as good. However, the key parameters are matching the focal ratio of the telescope to that of the spectrograph and matching the focal length of the telescope to the slit width required for the resolution you are after. There are only a few commercial spectrographs available and they specify the focal ratio and the resolution they provide at a given slit width. What are you thinking of doing?

The site Ken linked to in his signature has an excel spreadsheet which which you can explore options. There is also lots of good information here http://www.astrosurf.com/buil/spectrographs.htm and here http://www.threehillsobservatory.co.uk/astro/astro.htm

Regards Andrew

 

[MattJenko]

Thanks Andrew. The reason I chose the C11 was the native F10 which matches the stated optimal ratio for the LHiresIII which I am tempted by. I am a member of that yahoo group, I'll have a dig around in the files to find it!

[andrew s]

This should be the file https://xa.yimg.com/df/astronomical_spectroscopy/SimSpec+V4_3.xls?token=9watPHhUWKsyK62ioHUyK3CYlvW4uzjSa0YY7OXvmyXvXEtsMeWoQRmiYQMZ56dRXFLZUZEJ01eNSmmCFE5L3mfO0EM9jC0ud4a07JTzWNg1&type=download

It just downloads it. The C11 and the LHires III are a good match.

Regards Andrew

[Merlin66]

The SimSpec spreadsheet is now available for download from the website.

http://www.astronomicalspectroscopy.com/instrument.html

I and many others successfully use the C11 for spectroscopy. I have a Spectra-L200 with a lodestar guider and ATik314 camera. Using a NEQ6, CdC, EQMod and AstroArt for acquisition. Slit guiding with PHD2 or AA6.

 

 

 

[robin_astro]

Yep the C11 is my workhorse too with a focal reducer to match spectrograph focal ratio where needed

1.  With the LHIRES III at f10

2. With the ALPY at ~f5

3. With the Star Analyser at ~f5

Robin