Locating targets

Forums Spectroscopy Locating targets

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    Kevin Gurney

    I have now acquired my first spectrum (with SCT) and processed in Demetra (beta) – see my uploaded image on my profile page

    The target (psi 1 Aur) is fairly bright so probably not difficult to get good data here, but  I was pleasantly surprised to see a good match with a reference spectrum for another K5 star  (HD47914 – Miles dbase) as shown in the wavelength-range-restricted plot. There are differences in general shape however…so maybe instrument response needs more work.

    Anyway, at the recent workshop, Francois noted that one issue is locating your target star. While I have some experience in locating diffuse objects (galaxies, nebulae) I didn’t appreciate how hard locating stars can be! I dont have a permanent rig so have to align any finderscopes to the main optic axis each time. The limitations of using a guide camera and reflective slit as your main target location image soon became apparent… 🙂   I was using  a camera on a finderscope (as well as using a visual spotter scope too). Is this what people generally use? I found the quality of images from the guide cam was not up to plate solving (I usually manage OK in Sequence generatr pro).. Any tips on locating target stars? But right now, I think I will have to get some kind of daytime target set-up to align finderscopes and main image..



    Andy Wilson

    Hi Kevin,

    For locating targets when polar alignment may be a bit off, I suggest star hopping. Start with a bright star that cannot be mistaken for anything else, then move towards the target star, if necessary locating and centering some other stars of intermediate brightness along the way.

    The guide image in spectroscopes tends to be of lower quality due to the transfer optics, but it is a lot better than it used to be. I find that I can sometimes solve my guide images with Astrometry.net, but not always. I’ve also taken to storing a guide image, so that I can always go back and check if the spectrum turns out to be unexpected. Observing variable stars can be a bit easier as there will often be a BAA chart and/or and AAVSO chart. With these I find it straightforward to confirm that I’ve got the right star.

    That is a great first spectrum. I agree something does not look right with the continuum, but it can take some trial and error to get the response profile working well. Robin’s suggestion of observing 2 Miles stars, and using one to create the response profile to correct the continuum on the other is very good. Especially if you choose A or B type stars which have simple spectra so it is easier to see what is going on with the response profile. The correspondence between the features in the spectra shows that your wavelength calibration is working well.

    Best wishes,


    Kevin Gurney

    Hi Andy

    Thanks for advice – yes, I must make sure to save guide images…

    Also, can you point me to Robin’s ‘2 star’ method please?



    Robin Leadbeater

    Hi Kevin,

    Here is the reference I think Andy is referring to.


    A MILES A star is used as a calibration reference and then to verify the quality of my observations and data reduction, other MILES stars were measured based on the calibration from the reference star and compared with what they should look like. 

    Tony Rodda

    I followed Robin’s method of capturing a series of Miles stars just for practice and instrument response/calibration.

    It works well. (Unless, like i did on one occasion, you misidentify a comparison Miles star.  That leads to some head scratching! But reinforces Kevin’s point).

    i particularly like the Miles database excel spreadsheet for finding suitable Miles stars.  Very useful.

    Kevin, I don’t know if I understand your description above correctly but are you guiding through a guide-scope/camera separate to the Alpy guide module?

    If so, I managed to do this down to (above) mag 12 target but only with a fixed setup and considerable patience.  It was also prone to wind, bumps, etc.  A QHY5LII through a 50mm finder/guider would plate solve in SG Pro.  But I then had to manually point to the target for the last few pixels.  I couldn’t get SG Pro to be that accurate.

    I now follow accepted practice.  A Lodestar or G3 on the Alpy module.  Can’t plate solve but the field is big and bright enough to find a star.  Actually, I’m at the point where I can find/see far fainter than I can capture usable spectra.  I use the QHY/Finder as a rough ‘first-pass’.



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