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I see his re-discovery has been suitably recognised with a certifcate
https://twitter.com/astronomerstel/status/976139340182179841
Hi Peter,
I suspect the ripples could be because the spectrum is at an angle. (The camera response can also cause ripples but they tend to be broader than what we are seeing here.)
You can orientate the grating with the camera off the scope. Look at the sensor through the grating and rotate the grating until the images of the sensor produced by the grating all line up.
Cheers
Robin
EDIT – Peter, not John. (senior moment!)
Hi Tony,
If you are interested in building a compact Littrow design using a camera lens you might find this design of Christian Buil’s interesting if you have not come across it already.
http://www.astrosurf.com/buil/us/spa/test.htm
You need a pretty good lens for this short focal length fast f ratio design to work well. He talks about the choice of lens, particularly the need for it to be compact enough to be able to mount the grating close enough to avoid vignetting. He used a Nikon lens (210 Euro in 2002) but perhaps you could try the 50mm f1.8 Olympus lens which was used with the famous Olympus OM1 SLR camera and can be picked up second hand at a good price.
For testing on a simple scope, how about a cheap small aperture Newtonian? Off axis performance is poor but you only need a good image on axis for slit spectroscopy and because there is no chromatic aberrations will be much better than a cheap refractor or camera lens
Cheers
Robin
Hi Grant.
I also run with a home made dew shield unless it is too windy. This keeps things clear all night except once when the dew shield fell off !
Robin
Hubble resolved stars in the nearest galaxies and measured their distances using Cepheid varibles in the 1920’s and together with Slypher and Humason established that the universe was expanding in the 1930’s. By 1967 the Big Bang was firmly established as the prevailing theory since the measurement of the cosmic microwave background by Penzias and Wilson in 1964
According to the spectrum on TNS
https://wis-tns.weizmann.ac.il/object/2018pv
This is a type Ia and ngc3941 has a distance modulus of mag 30.3 so theoretically it should top out around mag 11. The spectrum shows signs of high extinction from the host galaxy though (strong insterstellar Na line) so probably will struggle to reach that
Cheers
Robin
I’ve just realised that you have to use a Barlow lens with a DSLR dont you because of the back focus? In that case the 5.2 um pixel size entered in the general tab will definitely not be correct.
Looking at your screenshots Ha alpha is at 3383 and H beta at 1623 so the effective pixel size will be 1.774um which ties in with the 3x magnification factor quoted for the DSLR Barlow in the ALPY instruction manual. I suspect this is the main cause of the problem
Robin
Having accurate geometric corrections can also influence how easy ISIS finds locating the lines. You mentioned a 3 deg tilt but the screenshot shows 1.5 deg. Is this correct? Also is the smile correct? (measured using a strong line in the lamp spectrum)
Cheers
Robin
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
