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Posts posted by Merlin66

  1. Check out also our website:


    The TransSpec spreadsheet, by entering your parameters will give a good indication of performance. It handles both "in the converging beam" and as "objective gratings"

    Your option 1 will work, needs around 50mm grating spacing in front of the sensor

    Option 2 I think may be problematic due to the limited spacing, but an objective grating set-up would work.

    Hope this helps,




  2. I had similar issues with an older ED80.

    Three distinct notches and flaring. There were no obvious spacers visible.

    I marked the orientation of the notches on the outer surface of the cell and found they matched the position of the collimating screws. This scope had three peripheral screws which moved the front element relative to the rear element.

    These has obviously been over tightened and nipped the glass. Just an easing of each screw and allow a settling down period seemed to work.

    The notches/ flares have never returned - Oh, and the collimation is still perfect!

    • Like 3
    • Thanks 1

  3. Marcel,

    It really depends on what you want to achieve.

    The SA gratings are a great way to start the journey, I call it "spectroscopy 101"

    They allow you to acquire low resolution spectra which then leads to the calibration and processing tools you need to master. Once you are familiar with the processing you can consider analysing the results and determining what the various observed lines can tell us.

    A slit spectrograph is a major leap forward, in resolution but comes at a financial cost in terms of the instrument and the supporting hardware.

    I'd recommend one step at a time and start with a grating.

    Our webpage may help you:



  4. Valerio,

    Well done mate! Good to see you moving along. You've done well to produce your first 1D calibrated profiles.

    Capella (#1) - Your reference lamp looks a bit out of focus. The calibration is close, but I as I mentioned previously the lack of good reference lines down around 4200A reduces the accuracy.

    I'd recommend changing the scale to the more common Angstrom ( Chart Settings/ Advanced/ Wavelength unit)

    When you compare to the reference spectrum you can see the mismatch between features. (look at the region around 5200A) A longer reference lamp exposure and use of the blue lines would help.

    In the meantime if you want to study a particular region you could "nudge" the calibration. Right click on the target image, Profile Properties/calibration - at the bottom of the window you'll see a box "Lambda offset (nm)" you can enter a suitable number and see the impact on the registration of the two curves (ref spectrum/ target spectrum). Try -0.1 (one angstrom) for starters then try again to improve further. (Note: pressing Apply will show you the impact but will not "lock it in" until you press OK, giving you time to play!)

    After calibration, normalise the profile to make the numbers easier (!) Select a quiet region in the spectrum (say around 6400, 6400-6410) and set this to 1 ( Image tab/ normalise flux scale)

    Response Curve: You don't show the response curve you used....The process is detailed in the BASS User Guide (press F1 to open) Section 2.15 gives all the info you need.

    Labels/ Lines - See Section 2.26 of the Guide. Once you've labelled your spectrum you can save it for future use. I attach some label files which may be useful.

    Other than the effects of different atmospheric absorption (altitude difference between ref and target spectra), if the same set up is used it should work reasonably close....

    I think the calibration of the Aldebaran is sightly out, but obviously the registration between your Capella response and the target is well out! (Look at the bump in the target around 6000A in the target and the bump around 5700A in the corrected version could be an indicator)

    The last profile shows that your response curve is not working. If you had a good response curve the result would be a spectrum very close to the shape of the ref spectrum (for a K5III). It's always a good sanity check to compare the corrected profile against a reference profile......


    You're doing well. Once you've mastered these basics they will be used over and over again on future spectra.



    Sample TypeF.lbl Sample TypeG.lbl Sample TypeM.lbl Sample TypeB.lbl

  5. Photoshop is used to tweak the image after it has been acquired from the camera and after the pre-processing, flats darks etc.

    Programs like Astroart can control all your cameras, do the necessary pre-processing and a lot of the post processing you may need.

    Well worth checking out.


    • Like 1

  6. I’ve been using Astroart for the past ten years or so. Fabio has been very responsive to our needs and has regularly issued upgrades based on user feedback.

    I regularly use Astroart for guiding. Fabio even developed a guiding system for us in spectroscopy. Works very well, I can recommend it. 


  7. Valerio,

    BASS project has all the tools you'll need. It has a comprehensive collection of star spectra (Pickles and Miles catalogue) built-in and available for comparison.

    Before adding any more RELCO lamps, just check what exposure you need to bring out the lines down at 4200A.

    The "best exposure" is the one that gives you the maximum ADU without overexposing - best SNR. Once you have taken some target spectra, you'll get a better feel for the capabilities of your set-up and be able to establish a routine. I've gone for a sub exposure of 300s and depending on the SNR required, may stack 20 or so subs.

    I use Astroart to control all my cameras and pre-processing. This allows me to quickly do a "profile" along the sub exposure and confirm the  maximum ADU.

    Once you have a good Instrument response curve it can be used for all your spectra. On the proviso that your using exactly the same set up. There will be an atmospheric effect to consider - it's generally accepted that the reference star spectrum (an A type star for convenience) should be taken at/ close to the same altitude as the target spectrum to minimise this effect.

    To get started, and for practise in the processing, you can use the Pickles/ Miles reference spectrum, then move on to an actual reference star spectrum.


  8. Valerio,

    Ran your data through BASS Project...

    Pretty good, the resolution is around R=971 in the red.

    You can see the calibration is limited to the red region - this means the calibration is not so good in the blue. More reference lamp exposure time might pull out the emission lines around 4200A

    The #1 exposure as you can seen doesn't appear to be over-exposed and the focus is pretty good.

    The response curve of your camera is obvious, the jump around 5700A is typical! This can easily be corrected by preparing an Instrument response curve and using that to "correct" your spectrum.

    Comparing with the solar reflected light (G2v) from Mars you've recorded a wealth of lines!

    Well done.

    You just need to open and use BASS Project to replicate these results.

    Onwards and Upwards

    Valerio_mars 051020_ref.jpg

    Valerio_mars 051020_composite.jpg

    Valerio_mars 051020_composite_G2v.jpg

    Valerio_mars 051020_composite_annotated.jpg

  9. Valerio,

    Good to see your "first light" results!

    In BASS project it's easy to load all the results and compare the quality of each profile, just hide the "bad ones" and work with the "good".

    I'll have a look at your results and post my findings.


  10. I was lucky enough to spent a few years with Jimmy Barker while working in South Africa, he was a great guy.

    The scope he used was a Cassegrain with a third mirror to make a  Nasmyth arrangement. He used a “hollow” Dec shaft to push the focused beam and the focuser was mounted on the end of the fork mount.

    I have some images...

    I did some observing with it, it worked very well. Not sure how it would perform for photography.

    Well worth considering.

    • Like 1
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