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I seem I have a set a hare running here!… but it is interesting.
If the computer scientists can get worked up about ‘travelling salesmen’
https://simple.wikipedia.org/wiki/Travelling_salesman_problem
then why cant we solve the ‘travelling astronomer’ variant? 🙂
But Tony has a serious point so we should give it some thought….
Kevin
“One plea though… Can we do the midlands area?”
If you stick a pin the ‘centre of mass of England’, its in Leicestershire 🙂
Kevin
I now have four observations and thought I detected some regular, cyclic activity. I made a gif in ISIS to try and draw this out..
https://giphy.com/gifs/astronomy-spectroscopy-be-stars-jTd7ZILZzsqrcNecVn
(just right click, save and view locally – the website is a huge distraction!)
Paul has the data…
Kevin
Hi Tony
I have just managed to get gnuplot and ISIS working for the first time…
The rules of engagement appear to be:
(i) You have to have version 4 (I have 4.6.6)
(ii) You must install in C:gnuplot
(iii) You need full control of that folder (i.e. set the permissions ‘full control’ for users)
(iv) You need teh .gnu files distributed with the ISIS gnuplot extras (like std.gnu)
I think v4 should give you a pgnuplot.exe? (v 5 doesnt)
Kevin
Hi Andy
I would especially welcome your 2nd and 3rd suggestions… (advanced processing and setup). I also think Robin’s idea is a good one, as we then have something concrete to nail down the problems with, by cross comparison.
I believe I might have been foolsih enough to volunteer to help at some stage 🙂 so I will honor that one if needed.
Cheers
Kevin
Robin said: ” I think for relative flux calibrated H alpha spectra, most of the time IR correction of a narrow wavelength range at high resolution is a waste of good observing time.”
Yes – this had been my assumption, albeit based on a hunch rather than any evidential basis…
Uisng ‘precedent and custom’ as the criterion, my encounters with the litertature seem to show rectified data in almost all cases.
Cheers
Kevin
Hi Robin
Once again – thanks for a full and detailed response!
I had tried with a MILES star comparison but, as you say, it should really be done with something at high res. Interstingly though, I did get an ‘instrument response’ which looked typical of that shown on CB’s page.
If I were to take spectra some of the ISIS database reference stars, could I go back and make IR’s to retrospectively apply them?
Cheers
Kevin
Hi all
I have now used my Lhires with the ‘engineered shim, guiding mod’ (aka washer for tilting the mirror). My guide image looks a bit more like Andy’s second image above – prior to that it looked a bit like the first… Its now much easier to guide. I use slit guiding in Prism.
I can vouch for the light leakage problem (ingress not exit). This isnt a problem I guess, unless you try and do some of the daylight solar experiments they suggest (that’s how I first discovered it). I have now used judicially applied electrical tape (reduced but not eliminated light entry entirley). There might be a problem if you had excessive glare from a laptop similar?
I also managed to get something ‘out of the box’. But I am finding going past the mag 4.5(ish) barrier is not trivial… The SNR goes down appreciably, but I am hoping the easier guiding will help here.
I think the Lhires is an early design and was made as a kit. It has this feel about it, whereas the Alpy is built like a tank…
I would also be up for a weekend on this topic. I will be at the meeting on 29th.
Cheers
Kevin
… and the fit of the piston shaft into the retainer is not as tight as it might be, so some play there….
Anyway. I’ll see what Francois says…
In the meantime, I have some split washers of just the right thickness if anyone wants one 🙂
Kevin
…. (contd) but (to clarify again) I can get a good spectrum in the top half of the image. My question is whether what I see is ‘normal behavior’ under mirror movement
Maybe a face-to-face at the late September meeting if we are all going?
Kevin
Hi Robin
To clarify this – the effect I am talking about is brought about by moving the main mirror on its axis using the set screw in the base. In spit of my guiding difficulties (now resolved! – see below) I still had the star in the centre of the slit. I simply had to have the slit well off to one side in my guide image to get the centre into the sweet spot zone
Kevin
I managed an hour outside last night and tested the mod on the guide mirror – much better! There is now a ‘sweet spot’ in the middle of the guide image (rather than at one edge) and I think the image as a whole looks generally a bit better. Recall I tilted the guide mirror forward using a washer between the mirror mount and the frame. I should get in touch with Francois I think…
I know that early versions had three degrees of freedom on the guide mirror (two tilt and back-and-forth). The reduction to one with the piston design has thrown some of the ‘baby out with the bathwater’ in my view..
Incidentally – adjusting the main mirror angle still has the same effect as described in this thread (move image across sensor and change its height) – my tweaking the grating has made no substantive difference.
Kevin
Hi again
I’ll combine my replies to Robin and Andy here…
Andy said “ As I understand it you are moving the spectrum down the slit,”
No – I am changing the angle of the main mirror using the set screw on the bottom. I try and use a central slit position at all times. As you swing the mirror back and forth, you get the images I showed.
On grating holder… Today I tried the screws – they were as tight as they could be… Mmm.. (Thanks to Andy for posting the diagram of which screws to adjust on http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=997&start=20)
I took this opportunity to adjust the grating for uniform image placement over wavelength range. It wasnt bad before – a change of about 10% of vertical sensor size in going from Ha++ to zero-order image – but I got it down to 5%. They were tiny adjustments…To do this I used the Shelyak recommended hi-tech approach – piece of card with a gap in it, placed over the mirror slit – but it works for this. At the end, I just cinched the screws tight and didn’t drive them all the way home.
Robin noted that the total flux should stay constant. Logically it should, so let me explain what I am measuring. I am taking the final summed image from Isis, then just doing a windowed stats measuremnt of the spectrum image. But I think Andy also reported a change in total flux with this kind of manipulation…?
I am waiting for abreak in the clouds to try all this with a real star… It could be my slightly mis-oriented prism (and/or some overtightening of the grating clamp) was causing some artefactual thing here…
Kevin
And for comparison here is my spectrum image in the top half..
It doesnt seem quite as good as yours – I may well have a look at the grating clamp screws…
Kevin
Hi Robi
sorry – I am out of sequence with my replies – you note “A wide spectrum, even if it does not affect the total flux is not ideal as you have to sum more rows, introducing more noise from the camera and sky background.”
I think thats what I found (and optimal binning doesnt play nicely).
It looks like you dont see the progressive vertical widening as you move the mirror (so the image goes down the sensor)?
I think Andy saw this?
Looking at the geometry, the narrow spectrum appears to be when there are smaller angles involved in the projection between mirror, grating and sensor. So might you expect some spreading as you increase these angles? My spectrum when at the top of the image is quite narrow… I have not tried loosening the screws on the grating holder – it sounded scary 🙂 But maybe I should try and re-tighten to get a feel for how this works
Kevin
Hi Robin
“Images of the star on the slit can be deceiving if the star is over exposed as the overspill looks worse than it is.”
Yes – I try and stretch the guide image so that I see only the central core of the star. Without doing this I do see a bloated disc [Incidentally, I am using Hyperion Prism as my frame-and-focus/guiding/ software now… I like it]
With my 35mu slit, Isis reports a resolution of around 15K. This compares with the 23mu slit R of just over 17K – about a 10% loss. Inspection shows that the fine atmospheric lines are not quite as deep but I’m not worried about that..
By the way, do you know how Isis calculates R? It seems to do it from the image/spectrum and not from parameters as such
cheers
Kevin
I have now experimented with the main mirror adjustment, see below…
[For some reason my original 640×163 image has become vertically squashed, but I think you get the idea…]. Red is to the right.
Anyway – the important things are:
(i) The spectrum gets wider (more height in Y-axis) as it goes down the sensor.
(ii) As well as this, the overall integrated flux gets bigger – an increase of around 50% from the two extremes explored here. I am assuming the final Isis image has removed the background and so the flux is signal, rather than background+signal. (You might expect total flux to increase just because its a bigger area…)
(iii) However, the spectrum is not so good in the larger-flux, lower-in-image version. In particular, it doest work well with the optimal binning option; there is a strong reduction of blue end amplitude. You can recover the blue end by unchecking ‘optimal binning’ (in ISIS) but – a) I don’t think this is good practice (optimal means… optimal I guess?). b) there is some loss of resolution (narrow atmospheric lines are not quite as deep).Assuming some steady progression of behaviour from the version on the left (spectrum in top half) to that on the right, I think I will be aiming for something like the left hand image on my jpg below… Here its about 1/3 way down.
Kevin
Hi Robin
Many thanks for lead…
I looked at the yahoo group posts and spent the afternoon getting to grips with Francois instructions… I could see that, in my current configuration, the slit didn’t apper central in the image of the guide scope aperture; it was way off…
However, to correct it, I had to tilt the whole mirror assembly forward slightly which was effected using a washer on the top screw. I set up earlier but its clouded out here now so, unfortunately, I will have to wait for a proper field test.
On the matter of the position of the main mirror: I carried out several experiments last night and hope to get back with proper analysis later…But preliminary thought are that the best throughput (total flux) appears to be with the spectrum in the lower half (as you suggested) but it does increase the height of the image. However I think this might not matter – some careful examination of results required.
In any case, having explored the construction of the ‘business end’ more carefully, I have a better understanding of the light path. I think perhaps they should make you assemble them after all!
cheers
Kevin
Hi Robin