NOW: Will soon show some plots. Until then some theoretical numbers.
Bandpass (limited by slit width) is around 1.25 km/s throughout the observable wavelength range. (This number is simply the product of the dispersion and the slit width.) Thus the theoretical effective spectral resolution is around 240000, a more refined analysis shows that it should be at least 200000.
Wavelength window with a Megaplus 1.6: around 0.9 nm at 400 nm, 1.8 nm at 800 nm.
Dispersion is around 15 mm/nm at 400 nm, 7 mm/nm at 800 nm.
The spatial scale should be close to the usual scale given by the telescope and its reimaging optics since the Littrow system gives 1:1 imaging. Thus the relevant numbers can be found here.
A Megaplus camera at standard gain (0 dB gain for 'old' cameras with
control box which is equal to 
dB gain for 'new' or 'i' cameras)
saturates at about 290 ms exposure time in the 630 nm continuum at
solar disk centre.
The grating can be rotated to allow the desired wavelengths to the exit ports. The spectrograph should always operate near its blaze angle.
NOW: Calibrations and routines for quick wavelength tuning are in the pipeline. In the meantime, here are some numbers from which wavelengths should be possible to find with moderate precision.
Infall angle  [deg] |
Newport angle [deg] | remark |
| 0 | 89.7 | 0th order back towards slit |
| 90 | -0.3 | Grating surface parallel to optical axis |
| 63.4 | 26.3 | Blaze angle |
| Port | lambda [nm] | order | Newport [deg] |
| A | all | 0 | 89.2 |
| B | 396.4 | 57 | 27.0 |
| A | 557.6 | 41 | 24.7 |
| A | 557.6 | 40 | 27.4 |
| B | 557.6 | 41 | 25.7 |
| B | 557.6 | 40 | 28.5 |
| A | 630.2 | 36 | 25.6 |
| A | 709.1 | 32 | 25.5 |
| A | 709.1 | 31 | 28.9 |
| A | 709.1 | 30 | 32.1 |
The grating constant is 79 grooves/mm. The focal length is 1500 mm.
Note that the cameras can be moved sidewise so there will never be a calibration table or program with 0.01 deg or even 0.1 deg precision. The observer must always be prepared to identify lines using a spectral atlas.
NOW: There is an IDL program to help finding and combining wavelengths. Start IDL on the server machine (number six) and give the command littslideplot. Necessary routines in idl/litt and idl/atlas.
Note that it is always possible to observe a wavelength at another order, away from the blaze. There is a definite price to pay in that efficiency goes down, but this could make more wavelength combinations possible.
There is a number of filters suitable for order sorting. At shorter wavelengths the requirements are such that they must be custom ordered. At red or NIR wavelengths one may use standard filters that manufacturers have in stock, if the wavelengths are suitable. Custom-ordered filters take several months to manufacture.
NOW: We will prepare some notes to help with filter specifications.
Here is a list of the filters. Clicking on the wavelengths gives the transmission curves as pdf files. /bf NOW: not yet!
There are of course suitable slit-jaw filters among the imaging filters too, but the ones listed here were ordered specifically for this purpose and are stored together with the spectrograph filters. As can be gleaned from the pass-bands, the requirements are easier at longer wavelengths and some filter could be useful both for order-sorting and slit-jaw imaging.
| CWL/FWHM | approx. useful | Manufacturer, remarks. | designated |
| [nm] | range (untilted) [nm] | A=Andover, B=Barr | slit-jaw filter |
| 396.83/2.1 | 396.0-397.5 | B, CaII H | |
| 457.20/2.7 | 456.5-458 | B,  Mg I line, in steep gradient |
A 457.26/1 and |
| A 458.05/1 | |||
| 538.45/4.1 | 537-539.5 | B, CI | A 532.1/0.9 |
| 557.89/4.4 | 556-559.5 | B, Fe Doppler | |
| 589.50/2 | 589-590 | A, Na D, 25 mm diameter | |
| 615.40/5.1 | 613.5-617 | B, FeII mag. | |
| 630.08/5.3 | 628-632 | B, Fe magn., telluric | A 632.99/1 |
| 656.24/4.3 | 655-657.5 | A, H- |
|
| 710.30/8.3 | 708-713 | B, Fe Doppler | A 694.44/0.8 |
| 715.72/4.1 | 714.5-717 | A, 25 mm diameter | |
| 772.80/10 | 770-776 | A | |
| 777.52/6.8 | 775-780 | B, OI | |
| 782.97/10 | 780-785 | A | |
| 792.46/10 | 790-796 | A, no good central plateau | |
| 851.51/9.8 | 848.5-854 | A, CaII | |
| 867.99/10.1 | 865-871 | A, CaII | |
| 871.42/10 | 868-875 |
The filters can be used at slightly shorter wavelengths by tilting them. Transmission plots are stored with the filters. Please put the filters back in their boxes which are marked 'spec' or 'slit-jaw' and put the boxes back in the green drawer. Otherwise imaging people may take them and destroy them.
Figure 2 shows the slit holder. It can rotate along a vertical axis which passes through the slit. Thus the reflection from the slit plate can be directed towards the desired direction without changing the slit position. The central part of the holder can rotate in the plane of the reflecting slit plate to change the slit orientation. The reflective slit plate is interchangeable.
![\includegraphics[width=0.9\textwidth]{figs/slitholder.eps}](img6.png)
|
The Littrow lens movement and the grating rotation is controlled via a Newport control unit placed on the shelf above the spectrograph. Keep this turned off when all adjustments have been made.
Figure 3 shows the control unit. There are buttons and displays on the unit for manual control. Unit 1 is the lens, unit 2 is the grating. Note that the motors must be turned on with the buttons to right of the display. After turning on the unit, hit the "home" buttons for calibration. There should be a manual on the top of the unit, but operations are rather intuitive. A blinking "E" in the corner of the display indicates an error report which can be accessed via the MENU button and arrow keys. Error reports are given rather liberally even for such things as trying to move a motor while it was turned off. Thus most are not very interesting.
The Newport unit can be controlled via a computer interface which disables manual control when active. Start the interface by clicking the SPECT icon on the desktop control panel. If the control unit is turned off or not connected there will be a complaint. The stages will be calibrated at startup and go to their preset starting positions.
The lens can be set to three predetermined positions of which one is the focus position where it should ALWAYS be when observing. The grating can be controlled either by typing in a rotation angle or by clicking on arrow buttons.
NOW: The spectrograph control is connected to royac19.
The spectrograph is not perfectly stable. Long test runs show small movements of the spectral lines. Further work may identify the causes and come up with remedies in the form of stabilising hardware or introducing reference spectra.
The slit will be rotating relative to the sun. The only way to counteract this is by scanning to make the effective area studied broader. Consider running the telescope in FLIP mode in the afternoons to decrease the rotation.
![\includegraphics[width=0.9\textwidth]{figs/newportcontrol.eps}](img7.png)