Re: ATM Newtonian design help needed

[Andrew.Martyn@dao.defence.gov.au]

Paul wrote:

>I've already built the tube (7.4" ID, 0.45" thick) and have a 1.43"
>diagonal mirror.  I have a Parkes 1.25" rack and pinion focuser - its
>height is either 4" if mounted on the outside of the tube or 2.8" mounted
>from the inside of the tude.  NEWT says the diagonal is too small for 100%
>illumination for the 4" height and gives 0.2913" 100% illiminated diameter
>at 2.8" height.  I think my diagonal may be too small but this was what
was
>recommended when I purchased the mirror (I was on holiday in the US and
>made a on the spot decision to finally build a scope that I'd been putting
>off for years).

>Should I mount the focuser on the inside on the tube?
>What real effect will I see if I mount it on the outside?
>What is the minimum 100% illuminated field diameter I would want (visual
>use only)?

Hi Paul,

I realise that some of this advice is a bit late, but bear it in mind when
you redesign or build a new scope. Since we are both Australians I'll talk
in mm instead of inches, Newt does accept measurements in mm.

In working out diagonal sizes etc the first thing you have to determine is
what is the lowest power you intend to work with and what its field size
is. For your 150mm f6 scope a common low power eyepiece would be a 25mm
with a 50 deg fov. This would give you a power of 36 and an effective fov
of 1.39 degrees. 1.39 degrees equates to approx 22mm at the focal plane.
Now as general rule for visual observing we do not want the illumination at
the edge of the field drop to less than 75%. So therefore you need a 75%
illumination fov of greater than 1.39 deg or 22mm.

Another thing you have watch out for is the size of the 100% of the
illuminated fov. There are cases where you can get 75% illumination at the
edge of the fov, but only get a very small 100% illumination fov. I like to
try and obtain a 100% illuminated fov of 60% of my actual fov. This ensures
that when I have something reasonable centred, I'm getting the maximum
light possible. In your case this equates to 60% of 1.39 deg or 22mm which
gives 0.84 deg and 13.2mm.

Now moving on to your actual case, if I use your figures of a 152mm f6
mirror with a 36mm diag, distance to front of tube of 200mm, tube diam of
188mm, thickness 11mm, spare in travel of 6mm, and focuser diam of 32mm,
using newt I get for:

the 102mm focuser height: 100% fov 0.04 deg or 0.65mm, 75% fov 1.35 deg or
21.4mm

the 71mm focuser height: 100% fov 0.45 deg or 7.1mm, 75% fov 1.49 deg or
23.7mm

Looking at these I would say that the taller focuser height is unacceptable
due to its small 100% fov. Mounting the focuser inside gives pretty
acceptable results. The only thing to watch out for is that by mounting it
on the inside of the tube, you do not obstruct the light path unduly.

If you rebuild the scope, try using a lower profile focuser and a thinner
tube. By the way, what sort of tube is 0.45" thick, it must be heavy. The
trick to getting a larger illuminated fov, is to try and get the focal
plane as close to the diagonal as possible. I would not recommend a larger
diagonal.

ps. I'm moving to Brisbane at the end of the year, any good astro societies
to join? Good luck with the exams!

Andy