Well, it spent nearly a month in transit over the Christmas break from the US to NZ but my second hand L200 has arrived!
It suffered a tiny bit of damage on the journey, the neon bulb and shutter had broken away from the bar used to rotate it in front of the slit. Fixed with a bit of araldite.
The previous owner has done a really nice job of the construction, far better than I would have been able to I believe it's a 4th generation model, with an Edmunds slit assembly and a Vixen flip mirror with a beam splitter. I will probably order the reflective slit upgrade once my bank balance recovers a bit.
The previous owner made an experimental 2400 line grating assembly, it will be interesting to see if this works in practice.
Very rough first light images attached, just slapped my DSLR on, no real focus or slit width tuning but ... rainbows!!!
Malc,
Glad it arrived!
Your neon spectrum shows the issue of chromatic depth of focus..the lines at the rhs ( around Ha) are not too bad but you can see the focus goes soft towards the blue....
Such is life!
Onwards and Upwards.
*Finally* got to put some starlight through my L200 last night! Just working with an unmodified DSLR and no guiding to try and keep things simple initially, using a 600 l/mm grating. Slit was at around 40 microns, scope is Canterbury Astro Soc's 0.4m RCX400.
Unfortunately I totally misjudged exposure times and consequently the SNR is very low for most of my exposures.
Also my first run at reducing the data has given me a very dodgy wavelength calibration, about 10 angstrom out in the blue end. Whether this is a limitation of using the neon lamp or user error I'm not sure, I'll keep cracking at it and see where I get to.
I'm thinking of popping an energy saver bulb in the observatory and slewing towards it for a rough and ready calibration until I get something better sorted, is that feasible?
Despite all of that, there is some data in there, see attached V1369 Cen spectrum, no instrument correction applied. P cyg absorptions are visible in Hb (focus off for Ha), and velocities look good when I fudge the zero point to correct the bad calibration.
I ended up with the Filly Argon for blue side calibrations....the mercury line in a typical fluoro can be used, also Robin has shown with VERY long exposures of the neon faint calibration lines can be seen in the blue....
I assume you also have a beamsplitter guider? That will be the next thing, it will allow you to close the slit a bit more and improve the resolution - obviously at the cost of even more exposure time.
Congratulations on first light!
Onwards and Upwards.
You shouldn't need terribly long exposures to get a good S/N. Without guiding the star may not have spent long enough on the slit.
What software are you using to calibrate?
You shouldn't need terribly long exposures to get a good S/N. Without guiding the star may not have spent long enough on the slit.
What software are you using to calibrate?
Hi Terry,
Yes, I think the star was drifting off the slit fairly quickly. I was having to perform a fairly shoddy process of getting the star on the fully open slit at zero order, then gradually closing the slit and slewing backwards and forwards to try and keep the star on the slit centre, then moving the grating to the required position, taking a neon, then taking lights. I think by the time I got to the last step the star was already drifting off.
I had the slit aligned in RA because we have quite a lot of PE on the mount, but in hindsight the dec drift, which is small but measurable, would take the star off the slit never to return. If I align the slit in dec then at least the PE will take the star on and off the slit.
But really the answer is to start working on guiding and using the societies SBIG rather than my DSLR. Another couple of cans of worms
Calibration was with IRAF It uses a polynomial to fit the wavelength solution and I don't think it works too well when there aren't calibration lines right across the image. I've ended up applying a linear dispersion based on features in the spectrum which gives me a good fit.
Malc,
You'll find a hellava difference when you guide.
If you use the beamsplitter make sure you also load and set-up Al's Reticule. A bit of a fiddle to initially set up, but once it's done - acquisition and guiding is MUCH easier.....
Yeh, I've read through the docs for getting Al's Reticle running with the beamsplitter. One thing that concerns me is the set up seems to be quite involved. A disadvantage I have is that as the scope I am using is not mine I need to set up and tear down each session. I'm also not sure about back focus with the beam splitter in place.
I think I will need to remove the visual back but I'm not sure what other adapters the scope has available.
I think I will probably need to order the reflective slit upgrade asap. Hopefully that will minimise the setup time.
malc,
I should have said with the modified flip-mirror guider and Al's reticule once set up it doesn't change.
You can remove the spectroscope/ guider and replace without additional calibration.
It's only when you remove and replace the guide camera you may need to re-calibrate the "virtual slit" position.
The beam-splitter adds 30mm to the backfocus requirement i.e. goes up to 85mm.
Thanks Ken, that could work. I'd need to reconfigure the case, or maybe even get a new one, to fit the whole shebang in but I could live with that. Still I think the ultimate solution is the reflective slit.
I'm not sure what other mods the guy did on the Spectra-L200, but the beam splitter usually just screws into the front T thread - no mods/ changes to the actual housing.....
I believe it's a 4th generation model, with an Edmunds slit assembly and a Vixen flip mirror with a beam splitter. I will probably order the reflective slit upgrade once my bank balance recovers a bit.
