View Full Version here: : Spectra L200 first light
09-09-2010, 09:34 PM
I have completed building my Spectra L200 spectrascope from Ken Harrison and had a frustrating first light with it last night.
I haven't got any way to change the focus of the camera yet so the camera is rigidly attached to the housing with extension tubes. The focus is reasonable but needs tweaking.
I connected the L200 to my VC200L after I had aimed it at a beta Pavo as a first test star. After trying for ages to position the star on the slit I realised that I couldn't achieve focus with the focal reducer in place. I removed the focal reducer and was able to get what I thought was the star on the slot. I had a good image of the star so then tried to image the spectrum. I seemed to have no luck even with 120 sec exposures. After about an hour I decided to try a brighter star so aimed at alpha Pavo (peacock).
I then realised that I had had the original star off the end of the slit and was trying to get the spectrum of the diffraction spike.:screwy:
The spectroscope is quite a bit heavier than my normal CCD camera and I didn't try to rebalance the scope. The end result was that the mount wouldn't move in dec at guiding speeds. Faster was OK. This made it difficult to position the star in the slit. I finally succeeded to get a single exposure of part of the spectrum of peacock a bit after midnight. As I had to work today I called it quits at that time.
The spectrum has worked well.
The slit was at about 50um I think.
The camera is my old ST9E with a 512x512 array of 20um pixels.
This gives a measured resolution of 1.61 A/pixel
I have identified some of the lines in the spectrum but I'm not sure of all of them.
Even though it was frustrating, it was a successful night.
Of course it is raining tonight so there isn't any follow up yet.:shrug:
09-09-2010, 09:47 PM
Haven't got mine yet... but I'm looking forward to it. I appreciate your frustration, but you have achieved excellent progress so far.
09-09-2010, 10:03 PM
Good results first time out....now you appreciate the issues of guiding on a slit!! ;)
Looking at the spectrum...
Alp Pavo is a B2IVc binary; nothing in ELODIE
Kaler "Stars and their Spectra" has a nice comparison spectrum of Gam Ori (B2 III) on p188.
The main lines (besides H) are He I at 4009/4026/4144/4387/4471/4687
SiII at 4129
MgII at 4481
You should record the micrometer setting and use this to "re-set" the grating back to this region the next time you image.
The spectrum is noisy, but OK for a single exposure (what duration?) when you stack a few more you'll get a major improvement. John S. is achieving snr>100 with the pre-production Spectra-L200.
Onward and upward.
09-09-2010, 10:52 PM
It was a single 60sec exposure. Looking at your numbers there are certainly He 1 lines at 4026, 4387 amd 4470 visible.
Not sure about others though.
10-09-2010, 12:32 PM
I assume that you could set the Spectrometer to achieve 0th order image, that way to get an actual image and focus and put the target in the centre - Yes? I'm hoping to get one of these spectrometers and do some asteroid spectroscopy through my 14"
10-09-2010, 02:58 PM
Yes. I can image the 0th order image. The problem though was that when I image such bright stars my ST9E camera instantly blooms. I tried putting my Bahinov mask on and was able to use this to focus.
You have to focus 2 different things. The star image needs to be focussed on the slit and the CCD needs to also focus on the slit. At present I can easily focus the star using the normal scope focusser. To focus the camera requires a second focusser between the camera and the body of the spectroscope. I have rigidly connected the 2 together at present so my ability to focus the camera is limited to using different length connecting tubes with T threads on each end. I can make a fine focusser on my lathe but am still thinking about a design for it -Some sort of threaded rings that go inside each other that can be locked and have T threads on each end.
10-09-2010, 05:01 PM
For Asteroid spectroscopy you could use a wide slit configuration.
The imaging camera/ slit focus can be done during daylight and the supplied O rings allow up to 0.5mm of fine focus.
Fainter objects could make use of the lower dispersion 300 l/mm grating.
10-09-2010, 05:38 PM
OK, I'm confused. How can you be focused on the star AND on the slit?
10-09-2010, 05:52 PM
Each line in the spectrum is really an image of the slit at that frequency. So you need to set the camera up so the slit is in focus. Then if you open the slit fully wide, and set the grating to the 0 order spectrum (star image) you can then focus the scope... the star image will appear in focus to the camera when it is focussed on the slit. Clear as mud? If you think about the light rays as a vibrating string... antinode at the objective of the scope, node at the slit, antinode at the grating (and lens) and node at the camera. (not sure if that helps actually...)
Once focussed, close up the slit, and go to the 1st order spectrum and you're away...:) (if only it were that simple:P:lol:).
10-09-2010, 06:25 PM
The spectroscope imaging of the the slit is independent of the telescope.
The Spectroscope should be set up so that the imaging camera is focused on the slit. When pointed, with a narow slit opening, towards the Sun or a bright sky the resulting spectrum should appear clear and show numerous absorption lines.
Once the camera is focused on the slit, it should NOT be re-focused or touched during the aquisition of the target star.
The spectroscope is now placed on the telescope in the focuser and the image of the target star / asteroid/ nebula is brought to focus using the telescope focuser. This position will coincide with the focal plane of the slit.
The imaging camera is focused on the rear of the slit and the target star focused on the front of the slit.
12-09-2010, 11:56 PM
Some more results
The images below are all of Peacock (Alpha Pavo).
Each is a stack of 5 x 60 sec exposures and I have tried to normalise the graphs. They show the spectrum from UV through to IR. I haven't quite got the framing correct yet and there is a little bit ot the spectrum missing between each frame. I have been moving the micrometer adjuster by 0.4mm between frames but this is leaves gaps. I will try with 0.3mm so there will be a bit of overlap.
All of the H lines are visible except H beta which I missed between the first 2 frames.
The deep line in the last frame is from atmospheric water.
13-09-2010, 01:31 AM
I'm impressed once again with your early success.
The SimSpec V3 spreadsheet (available on the Y! group astronomical_spectroscopy) will make it easier to determine the spectral coverage of your CCD.
Once you have calibrated the micrometer head you can just "dial in" a central wavelength.
13-09-2010, 07:07 AM
Wow! That's excellent Terry! I can't wait to get mine going:D.
I painted mine yesterday - Silver Hammertone:).
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