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11”-14” Telescope Recommendation for Alpy 600 & Lhires III?

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#1 thedesperaterun

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Posted 20 February 2021 - 12:22 AM

Hi all,

 

I’ve decided to pull the trigger on an Alpy 600 to really start getting into spectroscopy (moving forward from my SA100 & CCD-Telephoto lens rig).  I know Celestron SCTs are popular; should I stick with that? Would the EdgeHD variant be worthwhile? Eventually I would like to move to the Lhires III, so finding a telescope that is as optimal for both is key. I want to buy once, cry once here. Thanks for any and all help. 



#2 robin_astro

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Posted 20 February 2021 - 07:44 AM

I have both these instruments which I use with a Celestron C11.  SCT are perfect with the LHIRES which is optimised for an 8-12 inch f10 scope and where you are normally looking at a narrow wavelength range where defocussing due to chromatism is not not important.

 

With the ALPY it depends on how critical you are.  I manage to get good results with the ALPY using it with a focal reducer to match it to the fast f5 optics of the ALPY but an SCT is not ideal for fast spectrograph optics and a wide spectral range though due to the chromatics aberrations, mainly from the reducer.  This makes it more difficult to get an accurate flux calibration (shape of the continuum) which can change dramatically with focus due to the selective sampling of wavelengths at the slit. This means you have to be very careful that nothing changes between measuring the target and reference. (The focus of my old metal tube C11  is not particularly stable with temperature either, new carbon fibre models might be better.)  More on the effects of chromatism (both from the telescope optics and the atmosphere) on Christian Buil's page here

 

http://www.astrosurf...ersion/atmo.htm

 

Most people use what they have or chose a telescope based on other requirements but  if starting from scratch specifically for spectroscopy I would go for a fully reflective design or perhaps a good quality APO refractor with a decent aperture though that would be much more expensive. See Christian Buil's page here for examples of telescopes which are optimum for the ALPY (in french but google etc translate well) 

http://www.astrosurf...ractor/test.htm

and what can be done with a small aperture quality APO

http://www.astrosurf...ope/spectro.htm

 

Cheers

Robin


Edited by robin_astro, 20 February 2021 - 07:46 AM.

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#3 robin_astro

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Posted 20 February 2021 - 08:04 AM

 Would the EdgeHD variant be worthwhile?

With spectroscopy you are only concerned with having a sharp image on the optical axis so good edge correction is not important at all. This means that simple Newtonians are ideal for fast spectrographs like the ALPY, giving maximum aperture at minimum cost, though physically mounting and balancing the spectrograph and ensuring sufficient back focus is more challenging. And as with all imaging applications the mount is equally important. 

 

Concerning aperture, this is not as important as I you might imagine as with larger apertures you can end up losing light from the larger star image overspilling the slit. The optimum aperture depends on your seeing. The standard ALPY slit is 23 microns so this is the optimum star image size (FWHM) you should be aiming for. With the LHIRES you can chose your slit width to match the star image size though as you increase the width you lose resolution. The design figure is for a 29um slit I think though I use a 35um slit with my C11 and typical 3 arcsec seeing

 

Cheers

Robin


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#4 thedesperaterun

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Posted 22 February 2021 - 11:06 AM

Ahh, I actually just sent you an email to your hotmail account. I am, indeed, wanting a spectroscopy-specific setup, so his page will be helpful. Thanks a ton for your replies. 




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