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Hi Denis,
The results will appear in the report of the AGM in the February Journal. I am seeing if I can get hold of a copy to add to this forum thread.
Thanks for pointing out the ‘Who’s who’ Peter.
Best wishes,
Andy
Hi Ron,
I also own a 10″ Altair Astro (GSO) RC. I understand that Altair made some kind of modification to the GSO RCs before selling them. I think it was something to do with the mirror cell but I don’t know exactly what. They have stopped selling them now.
I’ve done collimation once and it was a bit of an ordeal. I expect it is one of those things that once you have done it a few times then it gets much easier. I’ve found it to be very stable and appears to hold collimation well. Though as I am also using it for spectroscopy, I don’t worry too much about having the collimation absolutely spot on as this won’t affect the spectrograph resolution, just the throughput and so SNR.
I also upgraded to the Feathertouch Focuser and found it to be rock solid. I’ve not bothered with a motorised focus as I am in the observatory and I can tweak the focus if needed. Though I find the whole telescope and focuser to be extremely stable and so I rarely need to adjust it, except due to seasonal temperature changes.
If you get a 16″ then I’ll be very interested to hear how you get on with it. I wonder about one day upgrading to a larger GSO RC. Though for me the choice was also about a compromise between matching the focal ratio with my LHIRES III spectrograph, and keeping the focal length short so that I have a reasonable field of view in the guide camera.
Best wishes,
Andy
Hi Colin,
The shop has now been updated so you can order 2016 and 2017 Journals.
Apologies for the delay.
Andy
Hi Colin,
I am very sorry that no-one has contacted you. Normally someone would respond within a few days at the most and so for some reason I don’t think your message could have been received. If everything works as it should then you should automatically receive a confirmation email from the website.
I will check the stock availability of the Journals and then get the shop selection updated. I’ll let you know when this is done.
It is also worth knowing that when you are logged in you have online access to the entire Journal archive. Journals from recent years can be viewed from the Publications -> Journals menu. The full archive of all Journals in pdf form can be access from Publications -> Downloads -> Journals Archive.
Best wishes,
Andy Wilson
BAA Systems Administrator and Web Content Editor
Hi John,
I’ve also had this problem. I found that not only the mounting, but also the focuser and mirror flop caused problems. I’m not sure whether you have an extra focuser for fine focusing your C8. I used a Crayford focuser on a 14″ Meade for fine focus without moving the mirror. The weight of the spectrograph and 2 cameras caused the focus tube to shift as I slewed, making it nearly impossible to point accurately.
I am using a different setup now (10″ F8 RC) but I found several points that could help you, 2 already mentioned by Andrew and David.
– If you can afford to setup an electronic finder scope then that can be a big help. Otherwise just a good finder so you can easily and accurately point your telescope to bright stars.
– Star hopping from nearby bright stars. Start off with an obvious bright star on which to sync your telescope. Then jump to bright but progressively fainter stars nearer your target, re-syncing your telescope on each star, until you reach your target.
– I used a flip mirror between the spectrograph and telescope so I could use an eyepiece to check where the telescope was pointing. I found this was a big help but not without its difficulties. I could just about get it so the spectrograph and eyepiece were in focus at the same time. A couple of things to bare in mind. First, this moves the spectrograph and cameras further from the back of the telescope, increasing the moment arm on the telescope and mounting. Second, if you use a Crayford or Rack and Pinion focuser for fine focus then this increases the strain on the focuser.
I would expect finding objects to be much easier with your 80mm refractor as the field of view will be larger. As long as the focuser and mounting are up to the job.
Cheers,
Andy
Hi Andrew,
I’ve also been automating equivalent width calculations recently. I suspect you will take a different approach to me, but I thought I’d share my experience.
I am trying to measure the EW of lots of lines in lots of spectra automatically. I’ve not gone down the path of fitting polynomials or Gaussians. Instead:
- Work out the local continuum.
- Sum up the flux from the spectrum between 2 predetermined wavelengths for the start and end of the line.
- Calculate a dummy flux based on my local continuum level between the 2 wavelengths.
- Calculate my absorption flux by subtracting the summed flux from my dummy flux
- Finally calculate a width based on this absorption flux and the height of my local continuum.
As in the other posts, I also find working out the local continuum level to be the tricky bit. To deal with varying levels of noise, I take the median continuum of a few wavelength bins either side of my start and end line wavelength, then take the average of the continuum at my start and end wavelengths.
That is an interesting paper, I’ll have to give it a read.
Cheers,
Andy
Hi Peter,
I think you are using a slitless Star Analyser, in which case I wonder if small tracking errors could be the cause. If the star moves or gradually drifts, then so would the spectrum and that would blur the final stacked spectrum. It does appear that the star at the zero order position just below 50 pixels in your first plot is located at a slightly different position between the stack and the single image.
Another check you can do is to compare the first and last image in the stack, or a selection of images in the stack.
I think there are several ways to align individual spectrum images before combining them in a final spectrum, either pre or post extracting the spectrum from the image, though I’ve never done this myself. That could potentially be used to fix tracking errors after the event. Though in general the better the tracking the better the results.
Cheers,
Andy
That is a major accomplishment Andrew! To both build an echelle spectrograph as well as create the processing pipeline. I look forward to seeing your future results with this impressive instrument.
Andy
Looks fantastic Tony! Ready in time for the long Winter nights. All we need now is plenty of clear skies.
Cheers,
Andy
It is worth noting that all the spectroscopy software packages I have used do sky background subtraction. It is one of the standard processing steps when you usually select regions above and below the spectrum.
Hi Peter,
Nice you are starting to get results. You can pick out the strong hydrogen absorption lines of this A-type star, from right to left H-alpha, H-beta and H-gamma are clearly visible with a hint of H-delta at about 4100 Angstroms. You also have an atmospheric telluric band just short of 6900 Angstroms.
Best wishes,
Andy
Hi Alun,
That is a nice set of results showing different H-alpha profiles.
While Vega has a very simple spectrum, I do like it as it is such a great illustration of the luminosity class, which in the case of Vega is a V for dwarf. As the star has a smaller radius, so the surface pressure is higher, leading to pressure broadening of the spectral lines.
Once you get used to doing darks and flats then you will find them a doddle. Particularly with the Lhires III inbuilt flat lamp.
Cheers,
Andy
Hi Peter,
The neon bulb would have to be placed in front of the slit.
Typically you either have the neon lamp inside the spectrograph in front of the slit, or outside of the telescope so the light goes through the telescope optics and then the spectrograph. The advantage of having it shine down the telescope is you do not have to worry about the lamp position too much. If it is inside the spectrograph, then as I understand it the neon lamp must be carefully positioned so that the light follows the same path as the light coming from the telescope. Otherwise a shift in position of the lamp would cause an apparent but unreal shift in wavelength.
Though there were no light leaks, was the room completely dark or was there an opportunity for other light to pass into the spectrograph optics? Alternatively someone may have a spectrum of a similar lamp, in which case they may know if it also has a broad spectrum component.
Best wishes,
Andy
Hi Peter,
Interesting project.
At a guess I would say that you have a light leak as a neon bulb should only generate discrete emission lines, no continuum. It is possible that there is a problem with the neon bulb, but I think a light leak is more likely.
You could try taking 2 exposures of the same length, one with the neon bulb turned on, and the other with it turned off.
The other effects that can cause an offset are dark current and bias offset, but they should be roughly uniform, not showing the hump you seen in your spectrum.
Cheers,
Andy
Hi Jack,
You can measure the resolution using BASS, as per section 9a of the BASS tutorial I wrote (I think you have already done this):
You can also create a .png, .jpg or .bmp of your spectrum graph at any time by selecting:
Chart -> Save Chart to file
These will be small compared to the original fits file.
Best wishes,
Andy
There are a couple of other factors which will change how the chart looks.
First it will depend on which filter, filters or unfiltered were used when acquiring the image. I suspect the AAVSO chart is showing magnitudes for a V filter, so you would expect similar results when using a V filter on your camera. However, unfiltered or RGB would cause the stars to appear at different relative brightness. For example very red stars will typically appear much brighter in an unfiltered image.
The other factor is variable stars. You will occasionally come across other stars in your field which vary in magnitude by a sufficient degree to look different. Indeed great care has to be taken when producing charts for variable star measurement, to ensure that non of the comparison stars are themselves variable.
Nice image by the way 🙂
Best wishes,
Andy
Congratulations on the new magazine Ernst. Great to have another spectroscopy publication!
Eric, you can try translating it with Google Translate. Type ‘Translate’ into Google. Follow the link and upload the document. Then select German to English. You’ll find you need to switch back and forth between the Google translation and the pdf as the table and figures are missing from the translation.
Best wishes,
Andy
Hi Matt,
First of all welcome to the BAA 🙂
Robin has already given you some excellent advice on spectral typing so I won’t bother adding any more.
I own an Lhires III and I find it an excellent spectrograph. It took me several months of practice to get to know it well and tweak its performance, but I was able to get good spectra of bright stars very quickly. While a star analyser is great to start off with, as well as always being a good tool for fainter targets, there are some things that will be easier with the Lhires III. For example wavelength calibration, flat fielding and simply having a slit removes the potential problem of nearby stars contaminating a slitless spectrum. Though of course you do need a good mount to be able to position the star onto the slit and to hold it there accurately.
Best wishes,
Andy
