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Re: Instrument Error
From: Bill B
Date: 2005 Apr 29, 15:48 -0500
From: Bill B
Date: 2005 Apr 29, 15:48 -0500
> Fred wrote re:sun/moon lunars: > Were you always looking at the sun through the clear part of the > horizon mirror or at it's reflected image? I generally aim the > telescope at the lower body, as it's easier to hold the sextant. If > you're reversing the body at which the telescope is aimed between early > and late afternoon, that might account for the difference. Fred Yes, I was reversing. Early pm shots with moon in glass and Sun in mirror. Late pm shots with Sun in glass and moon in mirror. > If reversing, part or all of it might be shades. That thought did cross my mind, but I perceive little if any difference between IE checks using the Sun's limbs (3 shades on index and horizon mirror) or a star (no shades). In my case blaming the shades at this juncture would be the same as a poor craftsman blaming his tools. I imagine I should do a sanity check when both bodies are at approx. the same elevation by doing a few observations and then reversing. Given my lack of prowess with instrument that test might well be garbage in, garbage out. > > And > > Whether reversing or not, the gap in late afternoon might be caused by > not aiming straight and not centering the horizon body on the optical > axis. Those sorts of errors always cause the distance to be too great. Hard to know, it is tough because of brightness difference to kiss the discs. Irradiation *should* cause me to undershoot. I did notice in tripod-mounted IE checks using the Sun if I got the discs in the center and kissing, and left the settings alone for several minutes the tangency did drift by several tenths of an arc minute as the images moved right and down. (Sun past meredian passage of course.) > The data also suggest more practice is needed, as you know. You think? :-) Probably need to stop the marathon sessions (up to 19 observations in one sitting) as well and relax. As I noted earlier, when I popped out and took one cut standing, went back to work, and repeated in 10-15 minutes, 5 out of 6 observations were within 0.2'. Thanks for your insights. Bill