30 July 2019 at 12:04 pm #574369
I have just obtained an ALPY 600, having abandoned my earlier intention of building a spectrograph myself(!).
On first test, using the SHELYAK Barlow and my Canon 1100D camera, I got a resonable first attempt and everything seemed promising. Subsequently, I carried out further tests but found the spectra as can be seen in my posted images.
Does anyone know what has happened, how I can correct it and how I can prevent it ever happening again?
I haven’t had the business part of the spectrograph apart but have taken off and replaced everything that is supposed to be moveable according to the pdf manual.
Hoping for some help,
Attachments:30 July 2019 at 6:02 pm #581216
It is almost certainly the grism holder that that has come loose and is rotating relative to the slit. (It seems to be a quite common problem. Not sure why but I suspect Shelyak dont always tighten them up properly when the leave the factory as I have never had one move on its own after tightening. It is easy to fix though. The grim is held in the core module by an allen headed grub screw(s?). You view the sky spectrum for example visually through the core module, and rotate the grism until the spectrum is square and tighten. It is worthwhile getting it as precisely square as possible otherwise the ISIS software automatic wavelength calibration can fail. You can see a picture of the assembly here (page 4)
Take care if you slacken the grub screw off too far ,there is a spring underneath the grism assembly that holds the collimator lens in place which can launch everything if you are not careful!
Robin31 July 2019 at 10:22 am #581218
Yes, it’s certainly the grism mounting (mine is slightly different from your description, BTW). I have three Allen keys provided but have so far only used the thinnest – I’m wondering what they are for. It is possible that the thread of the grism holder is vulnerable to cross-threading (I had trouble resetting it: I won’t be touching it again in a hurry).
I did the best I could to put it right and have taken another solar spectrum off the Cumbrian clouds. I found it much harder to get a decent focus this time, though. I’m not sure why that is. Perhaps the grism isn’t quite flat now?
Thanks for the help. As a first real project, I intend to compare the spectrum of Vega as found by my ersatz 100 l/mm filter ring setup and as found by the ALPY.
David31 July 2019 at 12:27 pm #581220
Are you “local” then? I am in North Cumbria near Cockermouth
Interesting I was not aware they had changed the design Is the grism holder threaded now?. I have 4 grism holders (the one from my original beta model ALPY 600, the one I used to build the ALPY200, one I got through the BAA supported program and one I bought a few months back for another experiment.) All just have a plain barrel with stepped groove for the grub screw. This is the component I mean
Robin31 July 2019 at 3:17 pm #581221
This is how the sky spectrum looked when my ALPY arrived (one of the first batch in 2013) and how it looked after adjusting the Grism (full size in attachment)3 August 2019 at 1:15 pm #581226
This is a second go – the previous attempt just vanished when I tried to add an attachment.
Yes, I’m local. We’ve met at the Cockermouth Astronomical Society.
The grism holder now has a screw thread (not a good one IMHO) and so I guess has the tube it fits in. There is no locking screw (very much a retrograde move). The holder seems to come loose easily. It was loose just now when I checked it and I’m sure it was tight when I put it away.
I attach a copy of my last solar spectrum. I seem to not be focused very well (but the loss of detail caused by the size limit on image uploads to the forum has reduced the precision a lot as well).
David Perkin.3 August 2019 at 1:18 pm #581227
The forum posting system is getting a bit tedious. What happened that time?3 August 2019 at 6:31 pm #581228
I don’t get to CAS very often these days, mainly when they need a speaker.
That is a curious design change. The big advantage with the ALPY is that provided everything is tightened up, it is a very stable instrument.
(The forum compresses large images to a smaller size but if you upload them as an attachment rather than an image, the original size and quality is maintained. It is also fussy about the file type. In any case 2MB is the maximum size allowed)
Robin4 August 2019 at 11:17 am #581229
I’ve sent you an email
Robin14 August 2019 at 6:27 pm #581263
As Robin said, we continued the discussion by EMail.
I would like to thank Robin publicly for his help and advice.
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