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When selecting the binning region for a spectrum, I stretch the image to make the full top and bottom edges of the spectrum visible. I then set the edge of the spectrum bin to include all of the signal, even the weak parts, while trying not to include any unnecessary background.
Yes this can be particularly important if there is any chromatism or atmospheric dispersion which will change the width of the spectrum along the wavelength axis (or any residual tilt) so the fraction sampled varies. (ISIS has an optimal binning option which seems to cope with this while minimising the sky contribution. It is rather effective on faint targets, though I am not exactly sure how it does it). Similarly the sky background zones need to be set far enough away not to include any of the spectrum. Stretching also helps spot any weak spectra which might be in the selected sky background zones, a bigger problem with slitless setups.
Cheers
RobinCheck for a scratch in the mirror slit coating
If it is that, you should see it in the guider image
Cheers
Robin” why and how does reddening matter? OK, the actual spectrum does not match what we would expect – but as long as it is consistent as it is, could that not sufficient for a good ref-star? ”
If the spectrum is reliable then yes, reddening should not matter as you have shown here. 0.6 reddening is a lot though (~80% of the light in the V band is being absorbed) and I prefer my reference stars to be as “normal” as possible. If the (B-V) matches that expected for the spectral class as it should do for a star without IS extinction that gives added confidence that all is well. (Reddening of course matters and needs to be low if we are assuming a spectral type and using the Pickles spectrum)
” I use a camera with relatively low dynamic range (8 bits)”
8 bits is not really enough for serious work but are you sure your camera is only 8 bits? the ASI183MM spec says it can run at 12 bits (often stretched to appear as 16 bits in fits images by the software.)
https://www.zwoastro.com/product/asi183/I don’t have much experience of CMOS cameras but as a starting point I would suggest setting the gain so you use the full 15k well depth, expose so the maximum is around 2/3 of saturation and average many flats (eg 20-30) to beat down the noise at the blue end, not forgetting to subtract bias and dark when [preparing the master flat (The ISIS “masters” tab handles this)
The other thing to watch out with flats is stray light in the spectrograph. I don’t know what the LowSpec is like but with the LHIRES, flats at the blue end are unusable as most of the light comes from stray light, not from the spectrum !
Perhaps more significantly though is that in this case it confirms that the MILES spectrum of HD185859 appears to be ok, despite it being a blue hypergiant emission line star (B0.5 Iae) with a very high IS extinction of E(B-V) = 0.6 (I have to say it would not have been my first choice as a reference star !)
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Excellent ! The matching width and strength of the Balmer lines in your HD187362 spectrum with the Pickles reference confirms the spectral classification (I used A2v but the difference between A2v and A3v is <5%, perhaps A2.5V would be the closest match) and the matching continuum shape tells us that there is little or no interstellar extinction. We can now also confidently use HD187362 as a reference star
Looks good. My continuum is also a good match against the classifying spectrum from ATel 16743 taken a few hours earlier, here overalaid, displaced in y for clarity
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Strengthening H alpha relative to the continuum. Here is the line profile tonight at high resolution R~1500 (LHIRES III, 2400l/mm grating) in radial velocity
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One thing I like to do if possible is to take references before and after the target, particularly when the exposure time is long. For the nova Vul 2024 spectrum I used HD187362 an A1v-A3v star depending on which reference you take and with B-V = 0.07 so matching the expected colour for that spectral class. I used it with the Pickles A2v spectrum. The response changed a bit between the two measurements 2 hours apart, partly due to the decreasing air mass but mainly I suspect due to a drift in focus as I took it early in the evening and everything was still cooling down. I used the average response and the end result should still be within my normal claimed precision of +-10% 4000-7500A
Cheers
RobinAttachments:
Magnus said
“So it might be that the different profiles are captured at times when the skyglow was significantly different. I need to check this more. If it makes sense.”
Yes accurate sky background subtraction is important particularly at this time of year for me too as any remaining offset will change the resulting response curve It will work ok on the original reference star but then give to wrong result on other stars due to the difference in background offset relative to the brightness of the target. Chose the spectrum binning zone so it includes the full width of the spectrum and the sky zones so they are close to the spectrum but do not include any of the spectrum. Stretch the image to see this better eg here Nova Vul 2024 in twilight
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I seem to recall someone somewhere on the internet trying an objective prism on a Seestar but abandoning it as it affected the balance of the scope too much. There have been some tests using very low dispersion gratings which allow the full spectrum to be fitted in the field but they were very inefficient poor quality gratings.
Note this is all hearsay, third hand so don’t take it as gospel !
Cheers
RobinHi Magnus,
I am not saying that this is necessarily the problem here but with the technique generally used by amateurs for flux calibration using reference stars nearby in the sky to the target it is important to do some investigations into the stars used as a reference. This is different to photometry where others (eg AAVSO/BAAVSS) have done the hard work and prepared nice sets of reference stars, checking that they are non variable and accurately measuring their brightness. As spectrcoscopists we are not so fortunate so we work with what we can find . A rule of thumb for example is to use main sequence stars as giants are more likely to be variable. Also look to see how they are described in SIMBAD and if they appear as variables in AAVSO VSX.
HD207673 raised a number of red flags for me. SIMBAD defines it as A2ib a hypergiant star so likely to be variable. Its MILES spectrum does not look like it should for the given spectral classification possibly due to reddening, even though the data in the spreadsheet suggests E(B-V) =0. This is likely to be wrong as the SIMBAD data suggests E(B-V) = 0.4 which would confirm some reddening. Perhaps the MILES spectrum is ok, perhaps not.
Professionals take a different approach, using a small set of reliable standards which might be far from the target at a different air mass and correcting for the atmospheric extinction separately. Christian Buil advocates this method for his StarEx/SpecInti project and it might work for you. If it does it is perhaps a better method but it does not work for me as my atmospheric conditions are not stable enough and my telescope has achromatic lenses (a focal reducer) so the instrument response is not constant but highly dependent on the focus on the night.
If you were getting good results with your previous camera but not now and suspect this is a new problem perhaps the best thing to do is to try to reproduce some of your previous measurements using the same stars. This would then narrow the area down that you need to investigate.
Cheers
RobinA spectrum from last night
https://britastro.org/specdb/data_graph.php?obs_id=16646
very reddened by interstellar dustCheers
RobinThe Miles spectrum of HD207673 also appears to be very reddened so probably best avoided
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Also HD199478 is very high extinction so best avoided
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To put the problem into context I have plotted your spectra blue overlaid with the MILES spectrum. (attached). None of these stars are main sequence so are not ideal references as they may be variable. There are differences but it is not a complete disaster. Were they measured at the same air mass?
Cheers
RobinAttachments:
Hi Lars
compare with a Miles library profile of the same star, divide and smoothen the result.
Are you doing this using the ISIS “response” and “continuum” functions using the MILES spectra in the ISIS database ?
Cheers
RobinYou present a graph that is not only linear, but with values on the ordinates also much higher than my 255. I would ask you for as explicit an explanation as possible.
jpg have only 256 levels per channel (8 bit) but many DSLR including the Nikon D3000 take images using 12 bits (4096 levels) so you should be able to see 4096 levels in RAW images from your camera. I have not used the D3000 or AIP4WIN though so I have run out of ideas I am afraid.
Cheers
Robin
