Focuser and reducer for Lisa and Meade

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David Cejudo
Posts: 45
Joined: Mon Apr 23, 2018 9:01 pm

Focuser and reducer for Lisa and Meade

Post by David Cejudo »

Hello.

I am planning to buy the Lisa spectroscope for my 12"LX200 telescope.

Will it be advisable to buy a dedicated focuser for the set up?
Or will it be enough with the Meade microfocuser that came with the telescope? I guess a temperature compensating focuser would be the one to get?
Is there any specially recommended focuser?

Also, for Lisa i guess i will be needing a focal reducer for my f/10 telescope. Any recommendations on this one?

Thanks.

David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
Ken Harrison
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Location: St Leonards, Australia
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Re: Focuser and reducer for Lisa and Meade

Post by Ken Harrison »

Not a LISA but I did use the Spectra-L200 on the Meade 12”.
I found the Moonlight 10:1 micro focuser worked OK, but I had the focuser motor fitted later (#1219??) it worked well for me.
I’d suggest the x0.63 reducer. Pretty popular and for spectroscopy can work well.
Hope this helps.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer
http://www.astronomicalspectroscopy.com
Robin Leadbeater
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Re: Focuser and reducer for Lisa and Meade

Post by Robin Leadbeater »

Hi David,

Most focal reducers are achromats and can be a big problem for spectroscopy due to chromatic aberrations at the blue end which makes getting a reliable instrument response difficult.
See here for more information
http://www.astrosurf.com/buil/isis/lisa_ri/index_en.htm
http://www.astrosurf.com/buil/dispersion/atmo.htm
They are best avoided if possible but may be necessary in your case to match the star image size to the slit.
If your seeing is very good you might be able to run without one for stellar spectroscopy. At 1.6 arcsec seeing the standard 23um slit would be a good match to a 300mm f10 but the lower focal ratio would still be an advantage for extended objects. My seeing is typically 3 arcsec so I need to use one with my ALPY and a C11. I see significant chromatic aberrations with my Celestron reducer though.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
Posts: 1926
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Re: Focuser and reducer for Lisa and Meade

Post by Robin Leadbeater »

Here is an extreme example from 2 nights ago of the effect of focus drift when using an achromatic focal reducer. My CII is an aluminium tube model and shows large drifts in focus with temperature. I started an observing run by measuring a reference star. The temperature of the telescope dropped significantly during the run so by the time I remeasured the same reference star ~2 hours later, the focus had drifted significantly. This does not matter so much with a fully reflective telescope. The spectrum stays the same. You just lose some light because the star overspills the slit. Because the focal reducer is achromatic though the shape of the spectrum changed completely because the wavelengths in the star image which pass through the slit are different. Note how the ratio between the two spectra (red) is an exact copy of the focus error curve for an achromatic lens

Robin
_hd123299_focus_effect_20190122.png
_hd123299_focus_effect_20190122.png (9.04 KiB) Viewed 10128 times
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
David Cejudo
Posts: 45
Joined: Mon Apr 23, 2018 9:01 pm

Re: Focuser and reducer for Lisa and Meade

Post by David Cejudo »

Hi, Robin.

Thanks for your answer.

If the spectral response changes with focus position and this one drifts with temperature, i guess a solution would be a focuser that automatically changes position with temperature. A TCF from Optec, for example. I use one in my other telescope for photometry and Works pretty well.

Is that correct?

Also, an apochromatic reductor would be a good idea, though expensive, i guess. Is there any known apochromatic reductor that would work for LISA?

Thanks.
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
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Re: Focuser and reducer for Lisa and Meade

Post by Robin Leadbeater »

Hi David,

Sorry TCF systems and APO focal reducers are outside my budget so I dont have any experience but would obviously be a good thing. Using a standard achromatic focal reducer and an SCT is certainly not a show stopper though and is a very common setup among LISA/ALPY users here. You just have to be aware of the issues and work round them, for example by taking reference stars at start and end of an observation. If I were choosing a scope for spectroscopy now, I think I would go for a fast telescope with a carbon fibre construction and fully reflective optics matched to the fast low resolution (ALPY) spectrograph and use a Barlow to match to the slower high resolution ( LHIRES) spectrograph where the spectrum range is limited so chromatism is less of a problem.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Joan Guarro Flo
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Re: Focuser and reducer for Lisa and Meade

Post by Joan Guarro Flo »

Hello,

Here Christian using one very acceptable solution, http://www.astrosurf.com/buil/lisa3/test.htm.

I've used these focal reducers in my two telescopes and I'm happy with they, I think that is a good option...

Cheeres, Joan.
Robin Leadbeater
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Re: Focuser and reducer for Lisa and Meade

Post by Robin Leadbeater »

Hi Joan,

The Baader Alan-Gee is still an achromat though so shows the same problem, as Christian later demonstrated
http://www.astrosurf.com/buil/isis/lisa_ri/index_en.htm

With achromatic telescope optics it is important to keep the same focus between target and reference

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Joan Guarro Flo
Posts: 611
Joined: Thu Nov 03, 2011 8:50 pm

Re: Focuser and reducer for Lisa and Meade

Post by Joan Guarro Flo »

Hi Robin,

I agree with you, this isn't a ideal solution, but I think that in Christian's explanation, the 4th picture, example B it's a good compromise solution.
All start with the telescope, for this work one Newton is better than a catadrioptic, and following with one spectrograph of mirrors, without lenses...

Friendly, Joan.
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