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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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posted in wrong thread- deleted
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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
RobinI strongly recommend using RAW images. jpg are not useful for science.
How are you measuring the pixel value in the image? Is this the value in a monochrome image made from the colour image? If so, check that none of the RGB channels in the colour image is saturated. (for example the G channel could be saturated but not the B and R channels and you would no know this from the mono image) This could also give the non linear effect you are seeing.
Cheers
RobinHere are the linearity results for my Canon 350D DSLR.
I measured the average intensity in an evenly illuminated area of the flat images with exposures from 0.5 to 25 seconds. I used the RAW images, separated into the three colour RGB channels. The camera is linear in all three channels up to the saturation limit (12bits=4096)
Cheers
RobinAttachments:
It looks like that link is not to the latest version. Here is a link to the page on DSLR photometry the AAVSO site which I guess should have the current version
https://www.aavso.org/dslr-camera-photometry-guideCheers
RobinWhat is a soft that allows me to quickly read the PVs of a .RAW frame?
I have not used DSLR for this kind of thing for a long time but I used ISIS to decode RAW into 3 channel fits. The AAVSO DSLR
Observing manual also has some suggestions (page 25)
https://www.aavso.org/sites/default/files/AAVSO_DSLR_Observing_Manual_v1-2.pdf
but there are probably other image processing programs which can also do the jobCheers
RobinIt looks like the camera is applying a gamma correction. Is this in the jpg images ? If so the RAW images might be linear
Cheers
RobinHi Alex,
The lead author gives their e-mail address, so you could contact them for clarification.
Yes I have done. We’ll see what comes back. Fig 4 has error bars which are clearly nowhere near 50x wider for the SPAD data which if the 50 fold figure is correct, implies the uncertainty is determined by factors other than the SNR. At 50ms sample rate I would expect scintillation to be a significant factor in the uncertainty, though there seems to be perhaps some interesting correlation in the fine detail between the two measurements during egress which cannot be due to scintillation.
Cheers
RobinFascinating technology that SPAD array. I don’t understand though why the SNR for the SPAD array shown in fig 4 is so much lower than for the CMOS camera. They say it is 50x lower and attribute it to the low 3.5% fill factor but by my calculation, that alone should only give an ~5x reduction of SNR, not 50x ie sqrt(1/0.035)
Cheers
RobinThanks Peter,
Yes if you unset the orientation eg by clicking the icon at the bottom of the image frame it does as you say then display the image but with the original orientation. It is most peculiar as as moving the mouse over the greyed out image to locate the high counts where the stars are does confirm it actually performs the reorientation and you can display the brightness histogram etc but it just refuses to display the image for some reason. I guess there must be something odd about the image that it does not like. It is a pity as my intention was to give it a plate solved spectrograph guider image and overlay it on a DSS/SDSS image to check the position of the slit is correct for objects which are too faint to see in the guider exposures. I will reread the manual and perhaps try some other images.
Cheers
Robin
