To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 


I seem to recall his focal length was fairly short, around 40 inches, for
which 100 yards would be adequate on eyepiece placement.  The problem with
using the sky is you are doing things in the dark.  Plus I found it far
easier to set this up with the tube horizontal, to make it easier to move
the main mirror around.  In the end, however, I think I checked my setup
against a star on the horizon before I drilled.

=Matt

>Delivered-To: mlbrown{at}0
>From: "Scott Ewart" 
>To: 
>Subject: Re: Re: ATM focuser placement calculations and tube lenght
>Date: Thu, 13 Mar 2003 01:55:25 -0500
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>
>----- Original Message -----
>From: "Matthew L. Brown" 
>To: 
>Sent: Wednesday, March 12, 2003 1:08 AM
>Subject: Fwd: Re: ATM focuser placement calculations and tube lenght
>
>
>|
>| Use your eye looking thru the eyepiece to judge.  Rack the focusser in and
>| put in a low power eyepiece.
>|
>| If you can, move the main cell along the length of the tube until things
>| focus.  That is, move the cell a half inch or so and wedge it in
>| place.  Then run around and look in the eyepiece.  This is easiest done in
>| the daytime, looking at something a 100 yards away or more.
>|
>
>Depending on focal length, 100 yds. may be way too short.  The longer the
>focal length, the more you have to move the focuser to focus shorter than
>infinity.  My 4.5" f/5.5 (24.5" f.l.) can focus inside a
large room, but the
>36" f/5 Yard Scope can't focus on a phone pole a mile away without holding
>the eyepiece a few inches outside the focuser.  Just use something in the
>sky.  It's the only way to be sure where it's going to focus.
>
>
>| If the focus is getting better, you're going in the right direction.  If
>| not, move it the other direction.  If it isn't changing much, you may be
>| really far from focus (apparent focus sharpness changes quickest right
>| around focus).
>|
>| =Matt
>|
>|
>| >scott
>| >
>| >I am assembling the tube tonight, followed at least some of your advice
>| >
>| >I installed the focuser about 10" from the front of the
tube, installed
>the
>| >mirror cell at the measured distance from the focuser (as specified by
>| >newt),  installed the secondary and aligned it so my eye thru the focuser
>| >was centered in the secondary, and the primary reflection
>| >
>| >I took the scope outside and tried to look at the moon, and yes I can see
>| >it, but no detail what so ever, unable to achieve focus, so I concluded
>the
>| >mirror cell is in the wrong place, is this a valid assumption?
>| >
>
>Do you reach a point where the moon gets to a minimum size, just not sharp,
>then gets bigger as you rack the focuser?  Or is best focus in one
>direction, but you just run out of room?  If it's the former, then you may
>have an alignment problem or something.
>
>
>| >data that has changed from below, the focuser is 3", tube is
8", newt say
>| >everything is OK, but I have installed wrong, or the stated focal length
>of
>| >the purchased mirror is not correct
>| >
>| >you recommended moving the mirror cell until the focal plane is the
>desired
>| >distance above the fully racked in focuser
>| >how do I do this, ie how do I know when the focal plane is the desired
>| >distance above the focuser, and how do I know what the desired distance
>is?
>| >
>| >thanks for your help
>| >kevin
>| >

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