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I 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
RobinHi Peter,
Yes that is what I am doing (also the icon at the foot of the image frame) It appears to work but the image is just greyed out. lets see if I can attach an example
Hi John,
I would say 8 inch would be enough to break into this game. Claudio Balcon who is now the leading amateur in this area started successfully using a 0.2m aperture with his similar home built 200l/mm grism slit spectrograph, though he uses a larger 0.4m aperture now.
https://www.wis-tns.org/search?&page=1&classifier=Claudio%20Balcon
(The ones before August 2023 are all with the 8 inch)I work down to around mag 17.5 normally but there are quite a few candidates that get to around say mag 15.5-16.5 before being classified which should be within reach provided your skies are not too badly light polluted. An 8 inch at f7 is a similar focal length as my C11 running with as standard 0.63 reducer at f5.5 so if your seeing is similar it should match the same 23um slit I am using.
These are my classifications, most with the ALPY200. I tend to do more follow ups than initial classifications these days but the most recent 2 are official classifications, one at ~mag 17 and another ~mag 15.5 with strong contamination from the galaxy
https://www.wis-tns.org/search?&classifier=leadbeaterYou’ve probably already seen it but I did a Youtube presentation on it for Shelyak when they brought out the commercial version
https://www.youtube.com/watch?v=0L6LLn9HjUY&ab_channel=ShelyakInstrumentsTVCheers
RobinHere are typical spectra of M5iii in red (R Lyr) and A0v in blue (Vega) with a typical sensor QE curve (black) overlaid, first at equal V magnitude and then with the correct relative magnitudes assuming RR Lyr Vmag =4.0.
R Lyr is ~ as bright as Vega in the I band but should still look significantly fainter integrated over the response of an unfiltered image. (Unless perhaps close to the horizon where atmospheric extinction would reduce the difference)
Cheers
RobinHere it is in a 1 sec exposure with my finder last night. (Fuzzy with this unfiltered fast achromat due to the high flux in the IR)
Cheers
RobinAttachments:
today
tomorrow ! I am often told I dont know what day of the week it is, there is the evidence !
Another bit of eclipse viewing stupidity. Apparently Amazon US are refunding people (using the dealers’ money) who bought SeestarS50 smart scopes because it might be dangerous to view the sun through it as the solar filter does not comply with the AAS filter advice !
https://www.cloudynights.com/topic/917434-seestar-warning-email/Hope you have clear skies today over there
Hi John,
This applies when using a standard star spectra eg Pickles in place of an actual measured spectrum and assuming the published spectral type is correct. It depends how accurate you need to be, particularly at the violet end. Francois Teyssier’s Reference Star Finder spreadsheet default setting gives a warning for stars with E(B-V) >0.05 and an alarm >0.1. If you have ISIS you can estimate how much effect it has using the extinction tool on an A0v Pickles spectrum for example. Attached is the error with E(B-V) = 0.1. I suppose you could even correct for the effect, though that is perhaps pushing the use of stars with published classifications but no published spectra a bit far
If you have a star with a known as measured spectrum like the non dereddened MILES stars for example the extinction does not matter in theory of course, though I would avoid using the MILES stars with very high extinction. Although it was I who first advocated the use of MILES stars back in 2011,
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=191
I have fallen a bit out of love with them as only a few are ideal as reference stars as many are potentially variable.An alternative is the larger MELCHIORS set of spectra which are at much higher resolution so could also be used with higher resolution spectrographs like the LHIRES for example.
https://www.royer.se/melchiors.html
SpecInti software has a script in its toolbox for extracting them in a form readable by the usual software (note there is a typo, a missing _ in the English version of the script)
http://www.astrosurf.com/buil/specinti2/specINTI_toolbox2_en.pdf
and the STAROS campaign website also has an on line extraction tool
https://search.staros-projects.org/Cheers
RobinAttachments:
Brad Schaefer talking enthusiastically about T CrB on BBC World Service “Science In Action”
https://www.bbc.co.uk/sounds/play/w3ct4sdvRobin
Spectra from 2SPOT and Tom Love now posted in the ARAS forum
http://www.spectro-aras.com/forum/viewtopic.php?p=17753A spectrum from 2024-03-12 by the 2SPOT team (amateur remote echelle spectrograph in Chile)
http://www.spectro-aras.com/forum/viewtopic.php?t=3122Very red continuum with very intense, very broad H alpha
I have released the collar. When tightened then slackened a fraction of a turn the micrometer is easier to turn.
Then I think you have found the cause of the problem. You can slacken it right off. If it turns freely then, there is no overhaul needed. I find the micrometer setting does not move on its own in normal use but if you want to be 100% sure you can always clamp the locking collar back down after you set it at the wavelength you chose. No need to lock it with the grub screw though. That is really for use in an industrial machinery where the micrometer might be used as a set stop which must stay in position under vibration etc
If the locking collar is engaged and you have the locking screw loosened, it could be worth trying to release it first. (Screw anticlockwise) I just tried mine and when the collar is engaged it can be quite tight to then disengage. Try it with your spare, engaging and disengaging the locking collar does not affect the calibration.
Re lubricant the internet suggests clock oil for micrometers. (not silicone which creeps and could end up on the optics)
