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If you want to filter the reference spectrum first as i suggested in the tutorial in the other thread “ALHENA WITH A STARANALYSER”
https://britastro.org/node/11586
https://britastro.org/sites/default/files/attachments/Correcting%20for%20instrument%20response.pdf
then the spline filter function in Visual Spec can be found at “Operations”, “Spline Filter”
it is also a good idea to roughly rescale the spectra first before calculating the instrument response so you dont have to deal with very large numbers. You can do this by selecting a region of the spectrum using the cursor (mouse left click and drag), then right click and select “normalize”. The result is the spectrum is averaged to 1 over the selected region
Cheers
Robin
Hi Peter,
This video on the Visual Spec website goes though all the steps in detail to calculate and use the instrument response
http://astrosurf.com/vdesnoux/video/reponse.avi
Cheers
Robin
Another area to look at is the grating holder. I noticed one of mine was much worse than the other. One of the holes in the metal plate that the brass rod ran though was slightly oval and the springs on both of them were rather weak and were probably not taking up all the play in some orientations. I put an extra turn on both of them which improved things. If you have the removable grating type holder, check there is no slack causing the grating to move. (It needs to be just tight enough to clamp the grating but not too tight otherwise you risk distorting the grating which shows up as astigmatism)
I have also heard tell of one case where the mirror came lose (It was stuck with double sided tape in the original kit, not sure if this applies to the factory built versions) Also check that the screws hold the mirror mounting (both the adjusting screw and the pivot) are done up tight too.
Also the position of the neon relative to the slit affects the position of the calibration lines so check it is centred over the slit and comes to the same place each time.
If your calibration lines move back and forth as you rotate the doublet check that the lens is seated correctly in its holder. (The original kit had two fibre washers, one either side of the lens. I removed one so the lens seats directly against the metal holder on one side, Similarly I removed the plastic foot from the micrometer so there is direct metal contact which stopped some cyclic movement in the lines as the micrometer was adjusted.
All this relates to the early kit though and may have been improved in later versions.
Robin
I have run this way for over 10 years using cheap second hand computers running XP Pro in the observatory. They die after a few years in the less than ideal environment but I just replace them for little money. (I installed the 3rd one last summer). Recently I have taken to running it on a separate network to the house internet so I can keep the speed up and guarantee security with XP. (It is quite heavily loaded with guidescope camera (1s frame rate), guider (1s frame rate) and spectrograph imaging camera, mount, focuser and spectrograph controls)
Robin
That’s good news. I see measurable flexure with my LHIRES even with everything done up tight. It was one of the early kit ones so perhaps I should take it all apart and rebuild it !
Hi Alun,
Unwanted movement of components is a significant issue in spectrographs and the the tolerances are very small. (1/5 pixel (~1um) shifts are easily detectable.) The LHIRES design is particularly prone to these problems. Have you checked by aiming the spectrograph in different directions (NSEW,Zenith) and watching the position of the lamp lines that your mod is not introducing any additional unwanted instability ?
Cheers
Robin
Hi Peter,
I have marked the O2 Telluric band at 7620A on your image. There is nothing useful beyond that as it overlaps with the 2nd order spectrum. You can fit the spectrum including this line and the zero order horizontally across the image at your current spacing.
Cheers
Robin
Hi Peter,
If the distance from grating to sensor is 23.5 mm you should be able to easily fit the useful part of the spectrum (0 to 7000A) horizontally across the camera field (If this is a mono camera without an IR blocking filter the spectrum will continue further into the IR up to perhaps 9000A and there will be a faint overlapping second order spectrum beyond that but the part we need is just 0-7000A.)
Cheers
Robin
The best distance of the SA200 from your camera sensor for your setup would be about 30mm but you can use up to 40mm and still fit the spectrum and zero order horizontally. See the attached calculator output. Note that this camera has large pixels for your short focal length which means your images are probably under sampled (~3 arcsec/pixel) This can also give problems with ripples in the spectrum as the thickness of the spectrum may be narrower than a single row of pixels.
Robin
Hi Peter,
Moving the grating closer to the camera shortens the length of the spectrum. You can calculate the effect using the on line calculator.
http://www.patonhawksley.co.uk/calculator/
You may find that you cannot get the SA200 close enough to the camera sensor when mounted on the camera nosepiece. The SA200 is designed to be used close to the sensor, for example in a filter wheel, not on a camera nosepiece. The best model to use for this application is probably the SA100. You can use the calculator to decide which model is best
Robin
I see his re-discovery has been suitably recognised with a certifcate
https://twitter.com/astronomerstel/status/976139340182179841
Hi Peter,
I suspect the ripples could be because the spectrum is at an angle. (The camera response can also cause ripples but they tend to be broader than what we are seeing here.)
You can orientate the grating with the camera off the scope. Look at the sensor through the grating and rotate the grating until the images of the sensor produced by the grating all line up.
Cheers
Robin
EDIT – Peter, not John. (senior moment!)
Hi Tony,
If you are interested in building a compact Littrow design using a camera lens you might find this design of Christian Buil’s interesting if you have not come across it already.
http://www.astrosurf.com/buil/us/spa/test.htm
You need a pretty good lens for this short focal length fast f ratio design to work well. He talks about the choice of lens, particularly the need for it to be compact enough to be able to mount the grating close enough to avoid vignetting. He used a Nikon lens (210 Euro in 2002) but perhaps you could try the 50mm f1.8 Olympus lens which was used with the famous Olympus OM1 SLR camera and can be picked up second hand at a good price.
For testing on a simple scope, how about a cheap small aperture Newtonian? Off axis performance is poor but you only need a good image on axis for slit spectroscopy and because there is no chromatic aberrations will be much better than a cheap refractor or camera lens
Cheers
Robin
Hi Grant.
I also run with a home made dew shield unless it is too windy. This keeps things clear all night except once when the dew shield fell off !
Robin
Hubble resolved stars in the nearest galaxies and measured their distances using Cepheid varibles in the 1920’s and together with Slypher and Humason established that the universe was expanding in the 1930’s. By 1967 the Big Bang was firmly established as the prevailing theory since the measurement of the cosmic microwave background by Penzias and Wilson in 1964
According to the spectrum on TNS
https://wis-tns.weizmann.ac.il/object/2018pv
This is a type Ia and ngc3941 has a distance modulus of mag 30.3 so theoretically it should top out around mag 11. The spectrum shows signs of high extinction from the host galaxy though (strong insterstellar Na line) so probably will struggle to reach that
Cheers
Robin
I’ve just realised that you have to use a Barlow lens with a DSLR dont you because of the back focus? In that case the 5.2 um pixel size entered in the general tab will definitely not be correct.
Looking at your screenshots Ha alpha is at 3383 and H beta at 1623 so the effective pixel size will be 1.774um which ties in with the 3x magnification factor quoted for the DSLR Barlow in the ALPY instruction manual. I suspect this is the main cause of the problem
Robin
