LHIRES3-LISA
NEW SET of SLITS


Abstract

Every Lhires3 or Lisa owners have a different telescope and specific weather conditions especially seeing. Is the 35 μm width slit best for everyone?

With the following study, it looks like that 23 μm slit is too much small and 50 μm is too wide. A new set of slits is then a better solution for spectrograph optimization in order to get the best resolution with enough light for everyone.

High resolution spectrographs

With Lhires3 2400 g/mm, we need light. Then most of light has to enter the slit. Or 14", 16" or bigger telescopes have big problem because star dimension at f/d 10 is wider than 35 μm. But 50 μm slit makes resolution beeing to much small for scientific works.

Here is a comparative tabular (computed with ETCL, gaving RP a bit lower than experimental measurements) of resolution power (RP) for 3" seeing sky with many telescope diameters at λ6500 Å with a 2400 g/mm grating:

Diameter (") Star's size (μm)
for 3" seeing		 RP with a
35 μm slit		 % of light out of
a 35 μm slit		 Best slit size (μm)
with enough RP		 RP with the best
slit width		 % of light inner
the new slit
8 29 13400 0 % 26 17000 89 %
10 36 13400 3 % 30 15200 83 %
12 44 13400 21 % 30
35		 15200
13400		 68 %
79 %
14 52 13400 33 % 45
35		 10800
13400		 86 %
67 %
16 59 13400 41 % 45 10800 76 %
23 85 13400 59 % 45
50		 10800
9800		 53 %
59 %

Depending on telesopes diameters, there is a need of 26 μm, 30 μm, 35 μm and 45 μm slit widths.

Low resolution spectrographs

Working at low resolution doesn't mean getting no details in spectra. Example: in emission lines nebulae, [SII] at λ7316 and λ7331 are only separated by 15 Å. With optic distortions and low focus quality in ccd chipset at each edges, such lines won't be easy to sperate in spectra.

Here is comparative with actual an new set of slit widths for resolution power (RP) and limit magnitude for an 8" telescope for an hour exposure:

Slit width (μm) RP for Lhires3
150 g/mm		 RP for LISA Mag. limit Typical target and scientific work
15 × × too low very very bright stars
19 810 1050 very bright stars
23 720 1000 bright stellar spectra, bright cataclismics... ; Balmer and He lines separation
26 680 stellar spectra, cataclismics ; Balmer and He lines separation with detailled shapes
30 590 PN, novae, cataclismics ; [SII] separation, weak lines on faint targets
35 520 750 PN, novae, cataclismics, bright asteroids and comets ; no easy [SII] separation
45 420 rather faint nebulae, asteriods, comets ; no [SII] separation
50 380 600 15.5 faint objects
100 200 500 17 very faint objects

This leads with new 26 μm and 30 μm slit sizes. 35 μm slit width isn't the optimal setup in order to easly get details in low resolution spectra.


New set of slits

For low resolution spectrography, 26 μm and 30 μm slit sizes would be necessary. And for high resolution spectrography, 30 μm, 35 μm and 45 μm would lead to better spectra SNR.
Then, the following set of slits will improve LHIRES3 and LISA setup: 26 μm, 30 μm, 35 μm and 45 μm.


How to suscribe?

Shelyak can build such new slits setup that would costs 220 € (with the optionnal slit holder) or 140 € without holder, as the actual, if a dozen commands are requested.
Send me an email if you are interested: bma.ova /AT/ gmail.com