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Note that the calibration assistant uses both calibration lamp lines and Balmer lines, which is explained in more detail in method 3 of the above link, so both sets of lines have to be clear in the spectra and identified correctly by ISIS. Checking the individual errors for each line can help with finding which lines are causing the problem.
Cheers
Robin
Cheers
Robin
Hi John,
Are you using the calibration assistant method described here ?
http://www.astrosurf.com/buil/isis/guide_alpy/tuto_en.htm
If the rms errors are large in the wavelength calibration. It is usually because ISIS is not finding the correct lines in the lamp spectrum. To get this to work correctly the entered pixel size is critical. Because of tolerances in the ALPY this is not identical to the actual pixel size. You can calculate the pixel size to be entered by measuring the distance in pixels between H alpha and H beta in a hot star. The pixel size is then 3123/no of pixels. You may still need to do some fine tuning though to get ISIS to lock onto the lines correctly. Also the lamp spectrum needs to be well exposed and in good focus.
If you are having problems it can be worthwhile taking a step back and not using the calibration assistant but following the instructions here which has more information on the automatic wavelength calibration function and includes information on troubleshooting problems (method is 2 for the calibration module but the whole thing is worth reading to understand what is going on)
http://www.astrosurf.com/buil/isis/guide_alpy/resume_calibration.htm
If fact I generally use this method rather than the calibration assistant
Once set up, I find the automatic calibration reliable and much easier than finding the lines manually but it can be fiddly initially to get the pixel size correct.
Cheers
Robin
I am not sure if selecting the sun automatically forces the mount to track at solar speed though. You may need to select this elsewhere in the hand controller menu.
Robin
The sun is not automatically included in the list of targets for safety reasons. You need to set it up first. Here are some instructions, yours may be similar
“Note that to include the Sun in the list, you must first access the Menu button (UNDO back to the “Press ENTER to begin alignment” prompt), select Utilities and then Sun Menu. This is a one-time setting and will be stored for future use. ”
from
http://www.nexstarsite.com/Book/Updates/SolarSystemAlign.htm
Dont forget to keep the main scope and finderscope covers on of course !
Cheers
Robin
Stability can be designed in though. Looking at the ALPY for example, the design cleverly transfers the load of the camera round the key (very light weight) optical components, leaving them carrying no load which means no measurable flexure.
Robin
Don’t forget thermal stability either. The coefficient of expansion of ABS is about 4x that of aluminium.
I think 3D printing is a great prototyping tool but after using spectrographs with rather poor and excellent stability (LHIRES and ALPY respectively) I look for the highest stability possible in the design and construction of a spectrograph.
Cheers
Robin
No. Whichever direction you look in, you are seeing the universe as it was at an earlier time. This is true for all observers regardless of their location. (ie the aliens are seeing us as we were, not as we will be.) There is also no evidence that the universe has an edge or a directionality such that we could be “looking out or in”.
Most of the component cost will be in the etalon but of course the cost of components is only a small part of the cost of running a sustainable business. The PST does seem to have gone up disproportionately though (I think they used to be around half that price new when I bought one second hand for £250 a good few years ago. The pound was much stronger against the dollar back then mind you) Imagine how much the warranty claims to fix all those PST with deteriorating coatings must have cost them though!
Robin
Thanks Bill,
The reason I asked was that I was surprised to see that for the Geminid for example, the continuum has the same value across the wavelength range, given that it would be expected to be the product of the typical instrument response and the presumably black body curve of the thermal contribution to the spectrum. This continuum would also be expected to vary in shape for the different speed meteors depending on the temperature.
An alternative approach would be to plot the flux calibrated spectra with the continuum component subtracted which should show the relative intensity of the emission lines directly
Cheers
Robin
Robin
Hi Bill,
These appear to be relative to a “continuum” equal to 1 are they normalised relative to some measured continuum or has the continuum just been applied to the uncorrected emission spectra as an offset ?
Robin
I have just got round to watching David’s Sky Notes which I missed as I had to catch my last train north.
SN2018gj imaged there by David Swan and Peter Carson is interestingly another supernova discovered and confirmed spectroscopically by amateurs. (Patrick Wiggins and Etienne Bertrand respectively)
Patrick Wiggins first put the news of his possible discovery out on the supernova discussion forum where he was advised to upload it onto the Transient Name Server post haste !
https://groups.yahoo.com/neo/groups/isn_chat/conversations/messages/1047
I was clouded out but realised it was potentially bright enough to be recorded spectroscopically using a standard ALPY 600 so I put an alert out on the ARAS forum which Etienne picked up.
http://spectro-aras.com/forum/viewtopic.php?f=5&t=1938
The confirming spectrum was taken just 9 hours after the discovery
Robin
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
