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An interesting example has just popped up showing the problems a rogue warm pixel can cause
Looks like it is flickering for a few minutes when it drops. Drifting over a hot/cold pixel perhaps ?
Robin
I don’t have any experience with CMOS (yet) but with my CCD cameras I redo them every few months. Bias frames take no time at all but getting enough long exposure darks eg 1200 sec can take all night so I just leave the camera running on a cloudy night. I then produce a defect map from these and scale the master darks depending on exposure. I find the only significant changes have been with hot and warm pixels (and with one camera a faint line defect appeared) These are not a disaster as a cosmics removal tool normally zaps them but it is better if they can be fixed at source using an up to date defect map
With spectroscopic flats it is important to redo them if you move the wavelength range. I normally do them for each observation with the LHIRES and once a night with the ALPY. it takes little time with the built in calibration units though the results with the LHIRES at the blue end are very suspect due to light leaking past the slit.
I aim to sum at least 20 exposures (30 for ALPY flats where the light level is very low at the blue end)
Cheers
Robin
Estimates from Stan Howerton last night give SN 2020rcq 12.5 V so still rising but SN 2020qxp at 14.5 so probably sub-luminous near maximum as suggested by the TNS classifiers
SN 2020qxp is now also confirmed as a Ia and the classifiers suggest it should be near maximum light in which case it should in theory be approaching ~mag 11 excluding extinction in the parent galaxy. They also compare it though with SN 2007on which was classed as “transitional” (ie with a luminosity lower than normal for a 1a but more luminous than sub luminous 91bg-types). SN 2007on was considered a good candidate for a white dwarf merger supernova event eg
https://arxiv.org/abs/1802.09469
The light curve of SN 2020qxp could be interesting
Cheers
Robin
Hi Andrew,
If you untick the box that deletes the intermediate files, ISIS saves the individual profiles in a time series (as @pro_n.fit)
Cheers
Robin
Hi John,
There was an excellent detailed commentary on the evolution of nova spectra by Prof Steve Shore during the Nova Del 2013 ARAS campaign here
http://www.astrosurf.com/aras/novae/Nova2013Del.html
He has also issued an ATel on the current nova based on amateur spectra
http://www.astronomerstelegram.org/?read=13939
Cheers
Robin
A bit closer still, same part of the sky (unfortunately). Confirmed as a 1a. Rising rapidly, heading for ~mag 11 ?
https://wis-tns.weizmann.ac.il/object/2020rcq
Robin
Thanks David,
I approached this not knowing much about it just to see what might be possible to measure but think there is other pro work like this around, though most of it seems to be concentrated on the coma rather than the tail. I’ve not seen anything like this by amateurs though. In fact I cannot recall seeing amateur tail spectra of any comet and it was quite a surprise when I recorded it as it looked so different to the coma spectra you usually see, with CO+ in place of C2. The Sodium distribution looks similar to what the PSI team got which is what I was mainly interested in trying to pick up.
https://psi.edu/news/neowisesodiumtail
How far the CN spreads surprised me. I suspect the distribution across the tail actually spreads much further even than my plot suggests. The measured distribution approaches zero steeply rather than gradually tailing off suggesting it is clipped. It likely extended beyond the ends of the slit so was clipped when I used the edges of the field to subtract the sky background.
It would be nice to measure further out along the tail but I already had to stitch 2 slit lengths together to cover the width of the tail even this close in.
Cheers
Robin
I have finally completed the analysis of my spectroscopic cross sections through coma and tail taken 2020-07-13 and added a poster to my members page
https://britastro.org/node/23655
It shows some interesting differences in the distribution of the various components which make up the cometary material.
The CN and Sodium components are seen in both coma and tail but the C2 component which dominates the coma emission spectrum is absent in the tail, replaced by the CO+ component.
The spatial distributions are significantly different for the various components. In particular the sodium emission in the coma is confined to a narrow central region but the CN component is widespread in both the coma and tail. The Sodium distribution in the tail is skewed anticlockwise relative to the dust, consistent with the presence of a separate sodium tail.
Cheers
Robin
Here is the region from H beta and redwards covering the “Fe curtain” region at R~1950 with the LHIRES and 600l/mm grating
The H beta FWHM is ~360km/s after correcting for the spectrograph resolution but there may be some higher velocities redwards in the line profiles
Cheers
Robin
Hi David,
I would not bother attempting flat correction of slitless spectra. Normal flat correction techniques do not work because each point is a mix of spectral and spatial information. (The technique used on professional setups is complex and involves generating effectively a “flat cube” for each wavelength at each point in the field.) My advice would be to take a conventional flat beforehand to check for any obvious dust donuts etc and place your target and reference star zero orders as near as possible on the same pixel avoiding any dust spots so any gross flat effects eg vignetting etc cancel.
Cheers
Robin
The mirror slit does have a couple of drawbacks too though. Bright stars are normally guided when on the slit using the overspill. (Typically the slit width is chosen to match the star FWHM so there is always some light which does not pass through the slit) The resulting image is “Hamburger” shaped though, split by the slit, which some guider programs struggle with. Also because unlike an air slit, the light has to pass through glass this can limit measurements at the UV end of the spectrum. (The beam spliiter has the same problem though.) The original not for profit LHIRES kit had jaws lovingly hand polished by volunteers but that would be expensive to do commercially. Christian Buil has been experimenting with reflective air slits (from the base ALPY module) for the UVEX, along with using all reflective telescope optics and a different glass for the camera window to reach a remarkable 3200 Angstroms. Beyond that atmospheric ozone absorption becomes an issue.
http://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2277&start=10#p12459
Cheers
Robin
Here is a link to the handbook. (dated feb 2016)
https://www.sxccd.com/handbooks/Handbook%20for%20the%20SX%20Spectrograph.pdf
The spectrum images there show very bad astigmatism, which is claimed to be an advantageous feature ! (It is not for serious use) though the spectra in the video did not look as bad as this so perhaps some improvements have been made.
Robin
I don’t believe the lack of remotely operating spectrographs is necessarily due mainly to the lack of suitable instruments. There are spectrographs such as the ALPY and DADOS which need no adjustment (In fact the SX spectrograph should not need adjustment in normal use with a suitable size camera really) and others like the Eshel which is capable of fully remote operation but you see very few operated this way. Remote operation of slit/fibre fed spectrographs is much tougher than imaging, particularly if you want to automatically acquire targets. (Andrew will be able to advise on that) Spectroscopy also needs a lot of telescope time which is expensive on rented telescopes.
Cheers
Robin
Hi Grant,
There are several good reasons why beam splitter guiding was abandoned in favour of the more expensive mirror slit guiding option by all the other manufacturers (SBig, Baader and Shelyak). Focusing and positioning the star on the slit is absolutely critical. You cannot guarantee long term the splitter image will remain parfocal and in the perfect sub pixel alignment with the slit you need and realignment is problematic working effectively blind, particularly with a spectrograph like this showing astigmatism where you have no idea if the star is in focus on the slit for best throughput. (Terry stated in the video that the XY slit position they supply with the spectrograph could not be guaranteed and the user would need to tweak this to place the star precisely on the slit but this is far from straightforward in practise.) The symptom is low throughput which was seen by users of the LHIRES copy designed by Ken Harison for example and eventually replaced with a mirror slit guider in the commercial version. (This also failed commercially for other reasons). Additionally when guiding on field stars, these need to be 2-3 magnitudes brighter compared with a mirror slit because of the 10% split
Cheers
Robin
Hi Grant,
Dont get me wrong, it would be good to have some competition for Shelyak who dominate the market currently and reflective optics are indeed a big advantage, mainly due to the lack of chromatism (ask David Boyd about his LISA) than the extended UV response, but designing one suitable for astronomy rather than for bench use is tough. The one Maurice reviewed was far from free of astigmatism though which limits its use. The field distortion looks suspect too in that video. The length of the slit is very short but the calibration lines were significantly curved even over that short distance, fine for a bench instrument but you are going to struggle to produce spectroscopic images of comets like this for example with this instrument
https://britastro.org/comment/8942#comment-8942
Availability of calibration lamps and guider modules is a given these days. The calibration lamp is just the same fluorescent lamp starter discovered by Swiss amateur Richard Walker and adopted by Shelyak for all their instruments and all other other offerings currently use a mirror slit guider rather than a beam splitter which has practical alignment issues and was abandoned by other manufacturers (This was touched on in the video where it was said that the user would be expected to tweak the guide position to position the star on the slit, critical for throughput and potentially challenging when you cannot actually see the slit in beam splitter designs so it remains to be seen how this will work in practise)
I did not intend criticising the instrument. Difficult really since I have not used one, though I do see some potential issues and did ask various questions about it when it came out which remain unanswered because of the lack of experienced users publishing results. (The limited knowledge of the dealer/customer? in the video was embarrassing). I agree with Eric, we are lacking an in depth practical appraisal by experienced spectroscopists. Contrast this with how much we already know about the design and performance of its direct competitor out in commercial form later this year.
http://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2577
For example
https://www.shelyak.com/wp-content/uploads/SMSW2_Buil_UVEX.pdf
http://www.spectro-aras.com/forum/viewforum.php?f=45
Cheers
Robin
