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Old 24-09-2008, 12:40 PM
ausastronomer (John Bambury)
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ausastronomer is offline
Join Date: Mar 2005
Location: Shoalhaven Heads, NSW
Posts: 2,513
Hi Rob,

I just wanted to add a few tips to Gary's very detailed comments based on my practical experience of having used several Argo Navis units on dobs ranging from 8" to 25" over the past 5 years or so.

On "any" hand made dob there will always be "mount fabrication errors". If you are a fine wood craftsmen these will be minimal and likely go undetected in normal use of Argo Navis, however in most cases they can lead to detectable pointing inaccuracies caused by the mount. On a couple of scopes I have seen they are significant. As Gary alluded to these will manifest as larger pointing errors when pointing to parts of the sky farthest away from your original alignment stars. This is where TPAS is your very very best friend. Whilst its use and implementaion may appear a little daunting at first it is very easy to use and truly, can make a poorly made telescope look good. That's not casting any aspersions on your craftmanship, just a generalisation that it will offer a significant improvement in pointing accuracy in just about every case. Gary can be very proud of the achievement he has made in putting something as complex, revolutionary and beneficial as TPAS, in the hands of amateur astronomers at a bargain price. It is important that you understand the "basic" operations and setup of Argo Navis as Gary has explained, before you start on TPAS, but my further advice to Gary's advice is to start using TPAS as soon as you think you can handle it.

I would be turning atmospheric refraction "off". If you are doing "normal" Deep Sky and Planetary observing it is not necessary to use it. Whilst it's a nice feature to have in some cases for certain specific purposes, it is not necessary in most normal observing situations. With refraction turned off and a good TPAS model established both my scopes point with an accuarcy better than 10' even very low down. The pointing errors caused by mount fabrication errors of hand made telescopes, tube and truss flexure pointing low down and mirror flop, will greatly outweigh the compensation necessary due to atmospheric refraction in 99% of situations involving hand made dobsonians. When refraction modelling is "on", as Gary explained earlier, it is important to have the date, time and observing location accurately set. If you change location and don't change the settings, with refraction "on" you can find your pointing out by a good bit. I find it far easier to turn refraction off and then I don't need to worry about changing observing locations. With refraction off, I have observed as far apart as Coonabarabran and Albury with my location remaining set at my home address on the Central Coast and had excellent pointing accuracy at all times to all parts of the sky. If I had refraction on I would have undoubtedly had to adjust the observing locations to a fairly accurate degree.

John B
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