ST-i Guiding Kit application range

Discussion in 'General' started by steven_usa, Jul 14, 2016.

  1. steven_usa

    steven_usa Standard User

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    What is the focal length range that the lens for the ST-i Guiding Kit is intended to support?

    Here is an M42 reference image my guider camera (any C-mount) and this ST-i lens resulted in (5 sec exposure):
    http://www.astrobin.com/240959/0/

    The point was just to show the resulting FOV: 2.266 deg w/ 20.561 arcsec/pixel

    I'm told the ST-i lens is f/2.8, 100mm FL?

    Could this be used to guide large FL, like 1200-1800? PHD inputs the guider camera FL, and I don't think 100 is the correct value in this case (but maybe that only influences the calibration time, not the actual guiding? although good calibration influences the guiding, right?)

    It's a nice solid kit, but is it suitable for large FL setups? (for instance Orion claims their 50mm guide scope is suitable up to 1500mm FL -- but that depends on what sensors they assumed, right?)

    Regards,
    Steve
     
  2. Doug

    Doug Staff Member

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    The lens is 100 mm focal length.

    The centroid calculation used in guiding is very robust. It can measure the star position to a tiny fraction of a pixel.

    What you need to look at is the image scale - the ratio of resolution on the sky for the guider and main chip. A 10:1 ratio is conservative. As a rule of thumb I'd recommend a maximum ratio of 20:1. If you go beyond 20:1... well, you'll just have to try it and see if it works for your setup.

    To determine the image scale, there's a simple formula:

    Image scale (arc seconds per pixel) = 205 * pixel size (microns) / focal length (mm).

    So the ST-I and guider kit has a scale of 205 * 7.4 / 100 = 15 arc-seconds. That means it should be able to handle 0.75 arc seconds.

    Let's say we have an 8" SCT at 2000 mm focal length, and a 9 micron pixel camera. That is 205 * 9 / 2000 = 0.92 arc-seconds. That should work!

    Of course mileage may vary. If you have a crappy mount it may not work. If you have good tracking then you can dial down the guider aggressiveness to effectively get some averaging in the feedback loop, and that will be more robust.

    Doug
     
  3. steven_usa

    steven_usa Standard User

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    Alright, I think I follow that. My C-mounted guider chip is 8.4 micron, and using a DSLR with 4.3 micron pixels... I plug the numbers like this:

    (main native) image_scale = 205 * 4.3 / 1800 = 0.4897222222222222
    (main reduced) image_scale = 205 * 4.3 / 1278 = 0.6897496087636933 (.71X reducer)
    (guider native) image_scale = 205 * 8.4 / 100 = 17.22
    (guider 2x bar) image_scale = 205 * 8.4 / 200 = 8.61

    (guider native) / (main reduced) = 17.22 / 0.6897496087636933 = 24.96558139534884 > 20:1 = FAIL

    BUT, if I were somehow able to use a 2X Barlow on the ST-i (correctly spaced)...

    (guider 2x bar) / (main reduced) = 8.61 / 0.6897496087636933 = 12.48279069767442 (close to 10:1, MIGHT WORK)


    I found a C-mount 2X Barlow for $25, so I'll give it a try.

    Alternatively, if I could find like a 0.5X reducer (instead of .71X)? I'm using a VC200L scope, I'm not sure if such a reducer could be found. But dropping my reduced FL from 1800 to 900 would get that ratio with the native 100mm FL guider down to 18:1.

    Could I "emulate" such a reducer by increasing the distance between the DSLR lens and the reducer lens with spacers? (or is it the "other way", that I'd need to decrease the distance -- which it is already at the minimum)

    Thanks Doug for the explanation!
     
  4. Doug

    Doug Staff Member

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    Reducers are generally designed to work at a specific spacing; you will probably get inferior results if you alter that. (Barlows are more forgiving, it mostly just changes the magnification.)

    Given your resolution I'd recommend going with the reducer on the DSLR regardless of the guiding. I'd suggest just giving the setup a try with the 0.71 reducer and see if it works. If not I'd try to find a 0.5X reducer.
     

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