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    Whole horizon mirror comments
    From: Lu Abel
    Date: 2011 Apr 25, 13:43 -0700
    As I mentioned in a previous post, I took my US Power Squadrons celestial class out for a round of sight-taking earlier this month.  One of my students is a former US Navy deck officer who had handled sextants during his stint in the Navy.   He's now a computer chip designer, so pretty technically savvy.  He decided to go out an purchase a full-horizon Astra in anticipation of reviving his celestial skills.

    The day we took sights was clear but somewhat humid, which resulted in a murky horizon, even for sun shots during the afternoon.

    Here are some comments he just emailed me about his experience with the whole-horizon mirror on his Astra:

    "In haze conditions it was very hard to have a high confidence of the horizon for index error. The southward shots we took have a margin of error just due to haze.  While this will affect both mirror types for altitude shots the whole horizon mirror made it very hard to determine IE.  I ended up using a combination of the southern land edge & bridge [San Mateo bridge across San Francisco Bay] to determine IE (I needed a very sharp edge to determine IE). I thought the half mirrors would be more robust in determining IE."

    I have to agree with him on the difficulty of aligning the two views of the horizon to determine Index Error with a whole-horizon mirror.   I tried both his Astra and a Davis Mark 25 that day.   With a split-horizon mirror, it's really easy to see the direct and reflected images of the horizon and bring them into alignment to determine IE.   With the whole-horizon mirror, my experience was that it was really, really difficult to see the two distinct images of the horizon, especially as the images got close to each other. 

    There are certainly strong arguments for both types of mirrors.   I'm not against whole-horizon mirrors, but I do feel their commercial advocates tend to promote their benefits without mentioning their drawbacks.  Pay your money, take your choice....

    BTW, I really enjoyed our sight-taking session because I got to handle a variety of sextants myself -- Davis Mark 15, 20, and 25's, the Astra, two different WW II navy/merchant marine sextants and a Cassens & Plath that one of my students bought on eBay for $300.   The latter had an index error of almost a full degree (!) (time for a trip to Robert White Instruments!) but otherwise was rock solid.  After subtracting that ridiculous Index Error, the shots were all great, many within a mile or so of our location, which is great for a newbie, especially with our fuzzy horizon.

    Lu
       
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