Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 08/10/2022 - 20:01
I'm interested in exploring spectroscopy. I do not have access for direct observing. I know that the Bright Star Monitor telescopes have various instruments. I would like to explore analyzing observations from this network using RSpec; and providing them to AVSpec. Has this been done?
Thanks in advance,
Clear Skies,
Mike
NMI
You may want to reach out to Lauren, HLAA, on that. I believe she is working on that npw.
Peter
What spectrographs are there on these scopes? I remember Arne Henden and I put Star Analysers on some of the BSM scopes when we arranged a group Star Analyser purchase for AAVSO members back in 2014. They worked pretty well with the small aperture refractors but I understand from Arne when I spoke to him last a few moths back that they are not working very well with the new fast astrographs that replaced them, not surprisingly given the fast focal ratio.
Cheers
Robin
Hello Robin,
stupid question: Why would a fast optic not be good for the use of SA? Intuitively I would say with a fast optic you can extend the grating-sensor distance and increase resolution. With same exposure time you get better spectra. What's wrong about this?
Thanks,
Matthias
because it's a fast optic, the resolution gets degraded more as aberrations get much worse, with the square of the focal ratio. i guess something like different parts of the fatter light cone strike the grating at different angles, so they don't get consistently dispersed, and resolution is thus lost at all points on the spectrum apart from a single wavelength.
With conventional slitless spectrographs (ie one with collimated optics so the beam is parallel at the grating) fast optics can be useful as they give a smaller star image size meaning you need a shorter (so brighter) spectrum for the same resolution. With a converging beam setup though chromatic coma distorts the image of the star in the spectrum limiting resolution. This effect gets worse the steeper the converging angle. See example spot diagrams by Christian Buil here
http://astrosurf.com/buil/us/spe1/spectro2.htm
The effects of chromatic coma can be reduced at any given wavelength by deliberately defocussing to minimise the distortion in the dispersion direction and apparently beating the theoretical limit on resolution described there.
Cheers
Robin
Thanks everyone for your replies.
Using BSM-Berry, my understanding is that it has the following specifications:
Takahashi FS060CB 6.0cm astrograph
Paramount ME
SBIG ST10 CFW-10, unknown distance to grating
with maybe a SA100
Then
Using Tom Field's SA calculator
Telescope Aperture 60 mm
Telescope Focal Ratio f6
Grating to Sensor Distance, assume 70 (?)
Camera Pixel Size 9um
Camera Width ~1000 pix (?)
Gives three green flags
if it has an SA200
then we get a white flag for Spectrum brightness; could be improved by reducing grating distance, which is an unknown.
Given the automated BSM system, can we offset the focus for spectral filter observing?
This scope would potentially be ok. (The calculations embedded in Tom's calculator are mine). The main limitation is the available spacing. The main purpose of these scopes is photometry with filter wheels which tend to be mounted close to the sensor so smaller filters can be used. It is unlikely that the spacing is 70mm, more like half that.
This is the system Arne was discussing with me last September
"
AAVSOnet has moved from the AstroTech 65mm/70mm refractors to the Takahashi E-180 astrograph, curtesy of the Tzec Maun Foundation. While this has dramatically improved the quality of the photometry, I'm still trying to get the SA200 gratings to work in this configuration. The spectra are fuzzy, and there is significant scattered light perpendicular to the dispersion.
What I'm wondering is if I'm hitting a basic limitation of the SA200. The Epsilon astrographs are f/2.8, while the refractors were ~f/6. Do you have any opinions regarding this? Thanks!
"
The 9um pixel size is rather for this 355mm fl scope though. I suspect the image must be significantly under sampled which is not ideal for spectroscopy. (The pixel size rather than the star image size will potentially limit the resolution)
Cheers
Robin
The 9um pixel size is rather large for this 355mm fl scope though. I suspect the image must be significantly under sampled which is not ideal for spectroscopy. (The pixel size rather than the star image size will potentially limit the resolution)
Cheers
Robin
Lauren Harrington has been working with the SA200's on the AAVSOnet Bright Star Monitors to get them focussed as closely as possible. One is on BSM_TX, a Televue NP-101 refractor working at f/5.4 with a focal length of 545mm. This should work with the SA200, though being a refractor, there will be some chromatic aberration. The other is at BSM_Berry, which was updated last year to the Takahashi E-180 astrograph working at f/2.8 and a focal length of 504mm. The focal length is ok, but the prior posts mention that the f/2.8 beam is too fast for the SA200, and so resolution will be compromised. So both of those systems, one in the north and one in the south, are available for members to use. Sorry for the lack of recent information on the AAVSOnet telescopes; Lauren/Peter/George are working on new web pages that should be released soon.
An example of a grating image for BSM_TX is shown at http://images.aavsonet.aavso.org/bsm_tx/bsm_tx_220802_images/index1.html (in fact, all AAVSOnet image thumbprints are available at images.aavsonet.aavso.org if anyone wants to browse through a few million images).
Right now, those are the two main spectroscopy "instruments" on AAVSOnet. There is an eShel at OC61 that is not fully automated, and there are at least two upcoming systems that AAVSOnet will have some time on that have R=1000ish spectrographs, but they won't be available for several months. Our hope is to provide automated slit spectroscopy in the future, once the hardware and software issues are resolved.
Arne
Is the autoguiding capability of the oc61 at Mt john good enough to keep a star on slit or fiber input, or is it currently operated unguided?
When I spoke to Dr pollard recently and inquired about the echelle on the oc61, she said it needs a focal reducer and flip mirror.
Who has authority at AAvso to make decisions about the oc61 and echelle in order to get it operational? ( Does AAvso own the oc61?)
I am happy to assist and can even build a reimager to f6 with optics on hand. Tom Love has actually offered to have his fully equipped alpy600 installed to the oc61, but received no response from Dr pollard. again, it would need focal reduction from the native f16 cassegrain focus to something closer to the alpy optimal f4 focal ratio.
The OC61 telescope is owned by the University of Canterbury, and is being run by the AAVSO.
The tail end of the telescope has a flip mirror system designed by Nigel Frost, where the straight through channel is for direct imaging and the folded channel supports the eShel. The eShel channel does contain a focal reducer, and the spectrograph has a Lodestar autoguider. My guess is that you can probably do 7th magnitude objects with reasonable exposure time.
The spectrograph has not been run for a couple of years. It presumably works. I'm not positive that it has any recent upgrades. It is not supported by ACP scheduler at this point, and so has to be run manually by logging into the computer desktop.
The direct imaging channel camera is being replaced by a QHY600, which will greatly increase its performance. However, at a telescope focal ratio of f/15, it is drastically oversampled, and will only give a 14x9arcmin FOV. I looked for a commercial focal reducer that gave a 42mm corrected field, with 0.5x preferred but 0.7x accepted, and there was nothing available. Perhaps you can research and find something! We don't have a lot of room to play with, as the camera has to fit in the fork, so a multi-lens reimaging system won't work.
Arne
The ALPY 600 is a good proven simple solution as a remote low resolution faint object spectrograph due to its excellent mechanical and thermal stability and simplicity of operation (no adjustments). See the productive 2SPOT group one operating in Chile for example.
https://2spot.org/FR/index.php
https://i.postimg.cc/C1G2Tc2N/Carte-Voeux-2022s.jpg
Depending on the seeing though, reducing the focal length of the oc61 to give a star image size to match the 23um slit of the ALPY could be challenging (ie expensive!) due to potential chromatism which is very problematic when trying to flux calibrate wide range slit spectra even in relative terms.
Cheers
Robin