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I am not sure if the transmission grating (grism) used in the direct vision spectrograph you are using is sensitive to polarisation though.
Transmission gratings are not sensitive to polarisation so that is one thing less to worry about 😉
https://www.edmundoptics.co.uk/f/transmission-gratings/13496
Cheers
RobinOne thing you could do without having to calibrate is to compare the spectrum of the antitwilight sky with that in different directions. Just divide the two spectra to see the difference (Adjust the exposure so the intensity is similar to reduce problems with non-linearity of the camera and be sure to switch off any automatic colour correction in the camera).
Note though that some reflective gratings respond differently depending on the polarisation which changes depending on the direction in the sky. This is an example of the response of a reflection grating. S and P are for the two directions of polarisation. You can also see why you need to consider the grating response as well as the camera QE if you want an absolute flux calibration
https://www.optometrics.com/assets/M466Sales.pngI am not sure if the transmission grating (grism) used in the direct vision spectrograph you are using is sensitive to polarisation though. This shows typical curves for some transmission gratings but it does not show any effect of polarisation.
https://www.optometrics.com/assets/ARTx.png
The one in your spectrograph will probably be similar to the 600l/mm curveCheers
RobinYou can approximately calibrate the spectrograph in relative flux using a tungsten lamp of known temperature which will have a black body curve but this will not include the effect of the atmosphere. If you then measure the sun at two different elevations you can then also estimate the effect of the atmosphere. You will need to calibrate the spectrum in wavelength first though
Cheers
RobinNote that even if you know the QE of the camera, this is not enough to flux calibrate the spectrum as there are many other effects between the sun and the camera sensor. The atmosphere and the grating efficiency are the main ones. You need a known light source to flux calibrate spectra. For astronomical spectroscopy this is done by measuring a star with a known spectrum as described in my document here
http://www.threehillsobservatory.co.uk/astro/Relative_flux_calibration_20221222.pdf
but that would not be straightforward with your setup.Can you describe in more detail what you are trying to measure about the solar spectrum ?
Cheers
RobinThe only caveat with these high reflectivity dielectric mirror coatings is that the reflectivity can plummet outside the visible range, particularly into the UV below 4000A. Something to keep in mind if doing spectroscopy at that end of the spectrum.
Only 10 Lyrids on my NW facing camera last night 21/22nd, though cloud was patchy and completely clouded over by 02:00 UT so probably missed the peak
I just thought it was suspicious to get 3 meteors so close together and parallel within a 30 second period.
Specular reflectance flares from satellite constellations perhaps ?
Cheers
RobinIf it is gas from an eruption it will be interesting to see if it turns into a dark (absorption) feature in UV images once it reaches the sunlit area
Robin
European institutions including British will need to step up to fill some of these gaps.
There are other important resources like this in Europe like CDS (SIMBAD etc) for example, and others which form a global “astro-ecosystem”. This is less to do with money and more to do with politics and cultural shifts in the US though of course.
The controlled constant temperature that cooling provides also means that calibration frames can be re-used
Robin
Interesting graphics from NASA in this article. The accretion disc appears to be around the wrong star, with the erupting T CrB occuring in the outer regions of the disc. Wow!
I did a reverse image search and tracked it back to where it was repurposed from in 2010
https://svs.gsfc.nasa.gov/20184
not that it makes it any better but at least we now know who was to blame !They actually do also have a decent animation linked from that page showing the accreting system but not the outburst
https://svs.gsfc.nasa.gov/20393/EDIT the explosion is shown in a further animations down the page
I see at least Space.com put a question mark at the end of their headline and quoted Jeremy’s scepticism (Also the lack of any evidence in the RV data to support the presence of a suggested third body to trigger the outburst on certain dates.)
https://www.space.com/the-universe/stars/hold-onto-your-hats-is-the-blaze-star-t-corona-borealis-about-to-go-boomThe downside for spectroscopy though would be the built in 430-690nm “astro” filter. Does it also have an unfiltered setting do you know ? (If not, for dedicated astro use an obvious hack would be to remove the terrestrial “VIS” filter like with DSLR)
The dual band OIII/Hbeta +H alpha filter setting could be interesting for wavelength calibration thoughCheers
RobinHi Andy,
It is perhaps also interesting for spectroscopy. I see from the specs that the sensor is larger than the one in the SeeStar 30 which means that unlike the Seestars it could potentially be used directly with a Star Analyser recording the full spectrum in one shot (not including the zero order, though that plus the blue end of the spectrum could be captures separately.) The equatorial mode would also avoid the problem of the spectrum fanning out due to field rotation which is seen with the SeeStar. The tiny 2 micron pixels means the image is probably well over sampled which should be good for avoiding artifacts with the colour sensor.
Cheers
RobinThe specs on line say it has an (impressively large) 9.6v 45Ah internal battery, good for 10 hours operation
I suspect 45Ah may be a typo and the battery is actually 4.5Ah even though the 45Ah figure appears everywhere on different sites.
Compare for example with the effectively identical Celestron branded version, here where a space has been used instead of the decimal point (9 5v 4 5Ah)
https://altairastro.com/celestron-nexstar-evolution-925×22-sct-6537-p.asp
That would be ok for the claimed 10 hrs just tracking (at ~0.5A) but a lot of slewing would take more power. What life do you typically get with the internal battery ? (If the internal battery is 4.5Ah then say 3 x that time with the 17Ahr power tank would seem reasonable given that you don’t want to run down lead acid batteries too low to prolong their life)Cheers
RobinThe specs on line say it has an (impressively large) 9.6v 45Ah internal battery, good for 10 hours operation eg
https://www.firstlightoptics.com/alt-azimuth-astronomy-mounts/sky-watcher-fusion-120i-synscan-mount-and-tripod.html
so if that is correct, a 17Ah external power pack is not going to increase the run time dramatically. How long do you want to run for?Cheers
RobinHi Giovani,
The dust donuts are almost in focus so if they are not on the sensor I think they must be on a surface close to another focal plane in the system. Can you post full frames (not cropped) of both DSC02817 and DSC02819 please ?
Cheers
RobinI did a thing for the BBC news 6 & 10pm yesterday where they wanted a live image of a planet
Yes ! Heard your name , looked up from the observatory computer screen and saw a live image Of Jupiter on the 10 o’clock news. Was that a live feed? That has to be some kind of first hasn’t it ?
Ken,
Dark, bias flat and sky background subtraction (based on an interpolation using regions above and below the spectrum) is normally all handled by the spectrum reduction program
Cheers
RobinNot sure where the discrepancy arises but the minimum will be somewhat resolution dependent if the line is not fully resolved.
OK so the difference between my measurement and the BASS2000 spectrum is not just due to a difference in resolution. See attached comparison where the BASS2000 spectrum has been spline filtered to ~ match the resolution in my spectrum. The discrepancy is still there and looks to be due to a zero offset so I need to check back on how I did the background subtraction.
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