Re: ATM Diagonal

["Asaf S.-T." <asafs@ibm.net>]

>Hi Asaf
> First, you have to deside what is the longest focal
>length and the widest field of view. Although there are
>a lot of SWA's out there, the loss of light with these
>might be too much for a 6 incher. I'd recommend considering
>a 25mm with 50 degrees as about the largest you'd use.
>This gives a magnification of about 49x with your mirror.
>In order to figure the mirror size, we need to know the
>distance from the center of the tube to the focal plane.
>This will usually be about 2/3 of the focus tubes outwards
>travel. Maybe more or less depending on if you have a
>low profile and what extras you'd like to tack on. For
>prime Newton camara work, the focuser needs to move inwards
>about 60-70mm to allow for body and adapters. Some short
>Barlows will require some inwards travel as well.
>Now, if we figure this for 100% illumination over 100%
>of the field of view, we should have enough info. It
>should be noted that 100% over 70% of the FOV is normally
>considered adequate but you can work this back if you like.
>To get 100%, the light from the edge of the primary must
>be able to hit the field stop of the eyepiece on that
>side of the system. This is usually at the focus point.
>Now, figuring your 50 degrees and 49x, this means that
>you'll be looking at about 1 degree through the primary.
>That's half a degree to either side. Now we need to now
>how much further the light from the 1 degree off
>axis light cone would travel before hitting
>the primaries center axis, so we can do some triangles.
>The original half angle of the primary comes out to 3.58 degrees
>on each side of the primaries axis, using arctan of 3/48.
>To this we subtract our 0.5 degrees to get 3.08 degrees,
>The angle of the new light cone. Using half the primaries
>diameter of 3 inches, the crossing point is now 55.8 inches
>back. This is the cone of unobstructed light for 100%.
>I'll guess at the values of your tube and focuser of
>8 inch tube with 1-1/4 focuser hieght. This gives a distance
>of 5.25 inches to the eyepieces field stop. The cone
>is 55.8-48= 7.8 inches further back or the cross of the
>100% cone is 13.05 inches from the secondary. Using
>ratios of secondary/primary = secondary_cone/primary_cone,
>We get a secondary diameter of 1.4 inches for the minor
>axis. This is a 23% obstruction. If we take it down to the
>70% of FOV for our 100%, we get a 1.0 inch with an obstruction
>of 17%. So, it looks like anything between 1 and 1.4 would
>be good.

Now, THAT was a great lecture. Thanks for your help.

----------------------------------------
Asaf Shtull-Trauring, Amateur Astronomer

http://www.maxnm.com/asaf/