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Hi David,
I’ve just spotted your latest results on your personal page. These are really nice, The H alpha emission line in Gamma Cas shows up very well
Cheers
Robin
Yes that was a good result on SN2018gj at mag 14.5 by Etienne Bertrand using a standard ALPY. I see there is now a professional confirmation of type II too
http://www.astronomerstelegram.org/?read=11172
Looking at the weather forecast my next clear spell is likely be Saturday evening when I am down at Burlington House talking about…. supernova spectroscopy ! If it had been a type Ia it would probably have got pretty bright, perhaps mag 12 but as a type II, maximum will be less certain and probably lower.
Cheers
Robin
Hi David
There’s no substitute for good seeing to get decent resolution with the Star Analyser but with bright targets you can try using short exposures and select, align (on the zero order or sometimes better on a prominent line if the stacking program will lock onto it) and stack as the planetary imagers do. This helps with dynamic range when using video type cameras as well as generally improving sharpness of the spectrum but the problem is that that the variations are wavelength dependent. (hence the twinking colours seen with bright stars in bad seeing) so the spectrum can still end up blurred.
Cheers
Robin
Hi Hugh,
The SA 100 should be more linear than the SA200 because of the lower dispersion angle but I think the wavelength errors you are seeing may be more to do with the zero order not quite being in the expected place (ie at 0 Angstroms) ie it is more of an offset error than a non linearity. I have seen this to a greater or lesser extent from time to time and never really managed to nail down the cause. (Something to do with atmospheric dispersion perhaps ? What does a linear fit between the shortest and longest wavelength Balmer lines look like for example ? At the end of the day though wavelength calibration of a slitless system and the transfer of that calibration to other targets is always going to be somewhat approximate.
Cheers
Robin
Jim Ferreira’s website also has some nice examples of Star Analyser targets to whet your appetite while we wait for the skies to clear
http://www.lafterhall.com/spectroscopy.html
Cheers
Robin
Some additional words from the master. No not me! but Christian Buil who’s work with a simple school lab diffraction grating got me into this game.
The theory and early experiments.
http://www.astrosurf.com/buil/us/spe1/spectro1.htm
Star Analyser review and tips
http://www.astrosurf.com/buil/staranalyser2/evaluation_en.htm
http://www.astrosurf.com/buil/staranalyser/obs.htm
http://www.astrosurf.com/aras/staranalyser/userguide.htm
(Dont worry at this stage about the wedge prism he uses, it is available as an accessory but only improves the resolution of the SA100 slightly and makes life more complicated for the beginner.)
Robin
Hi Kate,
Large variations between sub exposures can be a result of a seeing/scintillation affecting the guiding. (The star is jumping in and out of the slit). Guiding on very bright stars can be problematic. You need a well exposed (ie not saturated) star image to guide on but to avoid saturation the exposures end up being very short. The guider then makes too frequent adjustments on seeing variations. PHD is also prone to hunting about the slit position when guiding using the overspill from the slit. (PHD tends to lock onto the brightest half of the split star image rather than the centre of the star) Using one of the alternative algorithms in PHD eg hysteresis or slow pass filter can sometimes help in these cases.
Cheers
Robin
Hi Kate,
How do the pixel ADU counts in the raw spectrum images compare ? ie are the counts in the spectrum image much lower in the noisy spectra? If so we can discount some sort of problem with data reduction.
If the counts in the noisy spectrum images are lower, are they lower in all the sub images or is there a lot of variation between images?
Cheers
Robin
My “cheap and cheerful” electronic finder. A Philips webcam mounted afocally on the back of the C11 finderscope, displayed in WXastrocapture and viewed remotely via remote desktop
Cheers
Robin
I have a similar setup though the guide camera sensor is a bit bigger (ICX 415AL) I have the C11 on an EQ6 mount on a tripod running EQMod and CdC and the alignment/pointing is pretty poor to be honest. However I use a webcam clamped on the back of the standard guidescope which allows me to see stars to ~mag5. (Ihave a little red LED hanging in front of the finder to illuminate the cross hairs). If I centre the finderscope on one of these near to the target, it easily places them in the guider field where I recentre and sync. I then slew to the target which should be visible (for faint targets or crowded fields I use a DSS image for reference) If not, I sync more nearby bright stars so EQmod can triangulate or occasionally have to star hop in from the bright stars using CdC and the guider image. I also use the same system successfully at f10 with the LHIRES where the guider field is even smaller.
Cheers
Robin
Hi Kate,
I have one but have not used it “in anger”. To produce a spectrum calibrated in absolute flux, two sets of spectra are taken, one as normal in the narrow slit, and one effectively “slitless” using the wider part.
The narrow slit spectrum has the fine detail and accurate wavelength calibration but only includes the fraction of flux which happens to pass through the slit so cannot be used to measure the absolute flux. The wide slit spectrum has lower resolution and poorer wavelength calibration but includes the total flux from the target. By combining the two a fully flux calibrated spectrum can be produced with accurate wavelength and good resolution. Christian Buil’s page here shows how to do this using ISIS.
http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm
and an example of it in use (in French)
http://www.astrosurf.com/buil/alpy600/photometric_slit.htm
The conventional photometric brightnesses can then be calculated by integrating the spectrum over the wavelength range of the standard photometric filters.
More commonly in the amateur realm, the reverse is done ie a spectrum is produced as normal using a narrow slit and then calibrated in absolute flux using brightness measurements obtained using photometric filters.
EDIT added a link
However…. I have just discovered an apparently undocumented feature (ten years late!) that on the 350D if you select mirror lock up AND self timer mode, only one press is needed. (Mirror goes up, 3sec delay, exposure starts) so a standard intervalometer should do the trick even on the 350D (and perhaps other models too?)
Hi John,
That’s interesting. On my 350D the mirror stays down after an exposure so always needs a double press of the shutter release at the start of each exposure. Perhaps it is connected with live view which the 350D doesn’t have. I rarely use my DSLR for astronomy but have just written a little program to do this for my BBC microbit, mainly for fun.
Cheers
Robin
I’ll see if I can dig out the program but it ran on XP via USB to the camera and ISTR there was no support for that when I moved to Win 7. Yep a couple of 555s would probably have been my choice back in the day. (a quick pulse to flip the mirror up then a couple of seconds delay to let things settle before starting the main exposure) but Arduino, Raspberry Pi, BBC microbit etc feeding a relay or optocoupler would do the job. I am surprised the cheap after market remote timers dont have a lock up facility but the ones I had a quick look at don’t seem to eg
https://www.amazon.co.uk/Neewer%C2%AE-Shutter-Release-Hasselblad-PowerShot/dp/B004FKYBJM/
I had a program on the laptop that operated the shutter automatically on a sequence including mirror lockup each time so did not need to manually operate it. It should not be too difficult to make up a little battery operated circuit to do the same thing though, closing the shutter remote contacts in a sequence
Hi Bill,
My venerable 350D has mirror lock up so I suspect the 1000D will have. (buried in the setup menu,first press of the shutter moves the mirror,second press operates the shutter). It depends on focal length of course but the vibration was very obvious at 300mm fl when I forgot to enable it. Though so as you say unlikely to be the cause here unless the meteor was right at the beginning of the exposure.
Robin
Wow £11k is certainly a lot of money for something that does not exist. I wonder if they accept bitcoin 😉
Hi Hugh,
(The same answer as Andrew but with a bit more detail,read and forget if you like!)
Your measurements are showing nice qualitative trends but you have to be a bit careful when interpreting the EW of emission lines.
EW works well as a measurement of absorption line strength because absorption is normally just a proportion of the continuum, so provided you can decide where the local continuum is, the EW gives you a good measurement of the line strength. (eg even if the star changes in brightness, if the absorption is constant, the EW stays the same.)
Emission lines are different as they normally come from a different source than the continuum so are independent of it. This means the EW value of an emission line makes less sense as we are measuring it relative to something not connected with the emission line. This is OK provided we know that the continuum flux is constant (or at least how it is changing, so we can correct the EW to give a true measurement of the line strength) Otherwise the EW results can be deceiving. Classical novae are a good example. If you plot the EW of H alpha as the nova evolves it looks like the line is continuously getting stronger. In fact though this is mainly due to the continuum falling away and for much of the time the actual line strength is constant and even decreasing at times.
If we look specifically at your VV Cep spectrum, the hot star is now fully eclipsed so the continuum is that of the cool star photosphere and the emission comes from somewhere else (possibly an extended region (disc?) associated with the hot star but there are many possible sources). The variations in the continuum around the emission line will be due to a blend of the many absorption lines in the cool star spectrum and we don’t see the true continuum at this resolution (It will likely be somewhere along the high points of the spectrum). The reference points you have chosen will be somewhere in the absorption lines so will only be fixed during totality, assuming no variations in the cool star spectrum. Outside totality, the reference points will rise closer to the continuum level as the hot star reappears and the cool star absorption lines lose their relative strength. The continuum flux will also increase to that of the two stars combined. Both these will affect the EW measurement even if the emission line strength actually remained constant.
All is not lost though if you want to make an absolute measurement of the way the emission line flux is varying, as all the necessary information is available.
What we would first need to do is to convert the spectrum to absolute flux. This could be done using the available measurements of photometric brightness around the time of the spectrum. Once that has been done, we can work in absolute flux rather than relative to some poorly defined and varying continuum.
The next step would be to remove the cool star component. We could try this now by subtracting a reference spectrum of a star of the same type, adjusting it to match the intensity of the absorption lines, but probably the best way to do this would be to wait to around mid eclipse when the hot star and any circumstellar material should be hidden and use this as a template. Once this is subtracted, we should be left with the flux calibrated spectrum of the uneclipsed components, probably dominated by Balmer emission lines with their actual intensities measurable directly.
Cheers
Robin
I’ve only really measured EW manually but found it very sensitive, particularly in noisy spectra, to deciding exactly where the continuum goes (linear, spline fit, what order to use ? etc) and where the line meets the continuum for the integration limits. How does IRAF do this? In your screenshot it looks like it is fitting gaussians to separate merged line components. ( I remember something similar in SPLAT) Is this part of the EW measurement?
Robin
Yes there are a few 3D printed spectrograph designs floating around, (though none for a high resolution Littrow that I can recall) The current one generating interest on the astronomical_spectroscopy yahoo group is Paul Gerlach’s LOWRES
https://groups.yahoo.com/neo/groups/astronomical_spectroscopy/conversations/messages/13914
https://www.thingiverse.com/thing:2455390
Christian Buil has also been dabbling with 3D printing for his experimental UVEX spectrograph
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1773
Robin
