First light

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Tom Love
Posts: 82
Joined: Sun Apr 21, 2019 5:57 am

First light

Post by Tom Love »

This has been a long time coming. I have been constructing a 50cm telescope in a dome, and tonight I managed my first spectrum.

The telescope is a 50cm f/4 prime focus design - the simplest possible telescope - using a lightweight mirror from Teleskop Express. Prime focus means no secondary, so it's one less surface to lose photons, attract condensation and to deal with in collimation. The truss assembly, spider, mirror mount and alt-az fork were made by a local engineering firm to my specifications using friction drives taken from a Mesu mount (which perform impeccably): http://www.mesu-optics.nl/mesu200_en.html The focuser and rotator are from PrimaLuce Lab.
telescope.JPG
This evening I used an Alpy for a spectrum of the recent nova in Vela. I fitted it with a 35 micron slit to maintain efficiency at longer focal length in typical seeing conditions, but the tradeoff is that the resolution drops to around R~450. Which is still good enough for transient classification and monitoring. The task over the coming months will be to explore the magnitude limit of the setup.

I will also be using a Lhires (with a few modifications) on this telescope.

There is some distance yet to go before I have a smooth production line for data with this equipment, but the first spectrum is an important step. And a relief finally to have got some actual data after several years of working towards this.
Nova_Vel_2025.JPG
PNV_Vel_test.JPG
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Martinborough, New Zealand. Alpy, Lhires RC12
Joan Guarro Flo
Posts: 690
Joined: Thu Nov 03, 2011 8:50 pm

Re: First light

Post by Joan Guarro Flo »

Hi Tom,

Bold design!

Are you using a Barlow lens? It seems that Alpy can work to F/5.5 approximately.

So you if you are using your 50cm telescope at F/4, you are losing a lot of light.

It's the same as if your telescope has only 40 cm in diameter, am I wrong?

Cheers, Joan.
Olivier GARDE
Posts: 1268
Joined: Thu Sep 29, 2011 6:35 am
Location: Rhône Alpes FRANCE
Contact:

Re: First light

Post by Olivier GARDE »

No, the Alpy 600 is optimized for an F/D ratio of 4, which is perfect for Tom's configuration.

See spectrograph specifications:
https://www.shelyak.com/produit/spectro ... ns-produit

Of course you have to increase the slit width to 35µm with a 50cm telescope in diameter.

Note that we also have an Alpy in Chile, which is used on a 305mm Newton telescope open at F/4, enabling us to go very far in magnitude on very faint targets.
LHIRES III #5, LISA, e-Shel, C14, RC400 Astrosib, AP1600
http://o.garde.free.fr/astro/Spectro1/Bienvenue.html
Tom Love
Posts: 82
Joined: Sun Apr 21, 2019 5:57 am

Re: First light

Post by Tom Love »

That's right - the Alpy runs at native f/4 with a larger slit. I think it might be the LISA that is optimised for f/5.5.

I'm also adapting my Lhires to run from the f/4 telescope on a fibre, which requires some messing about with a macro lens in front of the spectrograph and removing the slit entirely. I've done quite a bit of testing, and I think I can make it work. In this case it will drop the resolution to around R~6000, which is fine for my main Lhires activity of measuring RVs in close binary systems. These are mostly quite rotationally broadened and difficult to measure, so you're doing well to get any better than 5 km/s precision anyway.

It's been a week since I restarted spectroscopy: so far my faintest target is 14th magnitude, and produced nice results with a much shorter exposure than on my old RC12. But I have quite a lot of tweaking and refining to do as I push it progressively towards fainter targets, and see how far I can go...
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Martinborough, New Zealand. Alpy, Lhires RC12
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