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Thread: SA 200 grating

  1. #1
    Join Date
    Sep 2006
    I live in Massachusettes; Observatory in New Mexico

    Default SA 200 grating

    Does anyone use an SA 200 grating and if so, how do you do focussing? I've had no success using FocusMax as the spectra confuses the algorithm.


  2. #2
    Join Date
    Aug 2014
    Northern Vermont


    Hi George,

    Surprised you have not gotten any responses yet, but I almost never look at this part of the forum -- maybe that's common?

    I don't know of any auto-focus routine that will work with the Star Analyzer in the filter wheel. I use a CDK-17 with the SA_200 in the wheel, and I manually focused on the spectrum, using the Graphing function in MaxIm DL, and drawing a narrow horizontal box to capture the 1st order spectrum. Most instructions (including those from Tom Field at Rspec) will tell you to use Vega, but in a 17" or larger scope, it's far too bright to be useful. Thus, you need to use a dimmer A0V star (or almost any type A star) with very distinct hydrogen Balmer lines. I have used Gamma U Maj, and also Rho Hydra and a number of other stars for this -- it all depends on how bright you can tolerate, or how dim you can go. Every now and then when I run across another A0V star in TheSkyX, I capture a few images just for checking the focus. The zero order star image looks terrible -- a soft little donut -- but the details in the spectrum are clear. The actual focus position changes across the spectrum because the red end is dispersed further (longer ray path than the blue end), so I try to find the sharpest notch in the spectrum for H-beta, which is roughly in the middle of the spectrum.

    I was also advised to use a non-linear calibration with the SA-200, so I've done that and saved it so I can recall it whenever I want. Note that the spectral resolution in Angstroms or Nm per pixel will change with your binning. I get 11.3 A/pixel unbinned, but about 22.7 A/pixel when shooting 2x2, and I've had to save non-linear calibrations for both settings, since we commonly shoot photos binned 2x2.

    In my system, using the Luminance as a reference, the SA-200 focuses at -950 microns. I have entered that focal offset in the filter information box in Maxim, so I can use an auto-focus routine to set the position for Luminance (the one from PWI is by far the best), and the focus is automatically changed by 950 microns whenever the SA-200 is selected. I'll try to attach images of Gam U Maj and/or Rho Hya in case it helps.
    Gam U Maj Non-Linear.jpg

    Rho Hydra Raw Spectrum.jpg

    I hope that is helpful, though by now I'm guessing you already know all this stuff. still fair game to record it here in case anyone else needs to know this.

    Clear skies,

    Brad Vietje
    Newbury, VT



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