NavList:

   

Reply

Richard Reed points towards a telephoto lens , but this is NOT the way to
go for measuring altitudes above the horizon. The aim is to measure a large
angle with great accuracy. Both the sky-body and the horizon have to appear
in the frame. If the two are so close together as to fit into the view of a
telephoto lens, then the altitude is in the range where refraction becomes
unpredictable. But Greg's camera will not work with altitudes much greater
than 20 degrees, which is a severe limitation for celestial navigation.

On the other hand, there may be something to be said in favour of a special
device which consists of two such narrow-field cameras coupled together,
with a precise hinge and notched stops at exact angular intervals, say 2
degrees apart. One camera could carry a very black filter (when necessary,
for the Sun) and they could work with appropriately different apertures.
The two images could be superimposed on a common LCD display, but the final
angle readout could be done, not via that display, but after correction and
analysis of the two pictures.

The mechanical demands for precise repeatable angular alignment would be
considerable, to comparable standards as apply to a sextant.

Is there anything in such a notion? Give me a sextant, in preference...

George

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Richard Reed" 
To: 
Sent: Tuesday, June 22, 2010 11:23 AM
Subject: [NavList] Re: On The Water Trial of Digital Camera CN


I've been lurking on this topic and seen the discussions about image
distortion and pixel size.

Greg said:  "0.4 moa of a single pixel width for the 50mm / 10 mp".  My
brother, who's become a DSLR fanatic, leads me to believe that 20 mp would
be really expensive.  Wouldn't it be possible to calibrate the image
distortion of a prime telephoto (are these available/expensive?) in order
to get better pixel precision?  Maybe a bit like using a 7x scope.  I
suppose sensor speed is an issue for magnification, just like it's hard to
hold a sextant still with the 7x scope.  Can we even dream about accurate
star sights this way?

I recently surfed up some articles (now lost) on photometry, and this kind
of thing is at least well-studied academically.

Richard

----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------

   

Reply