8.75" Dobsonian

Hi,
I am thinking about building an 8.75" Dobsonian scope, mainly for deep space
fuzzies. What should I expect in performance and what is the best Focal
length to go for? I was thinking about a F4.5, but there are other options
available (7.3 and a 5.8).
Any help would be apreciated,
Thanks,
Paul

On Tue, 2 Sep 2003 21:31:13 +0100, "Paul"
wrote:

Hi,
I am thinking about building an 8.75" Dobsonian scope, mainly for deep space
fuzzies. What should I expect in performance and what is the best Focal
length to go for? I was thinking about a F4.5, but there are other options
available (7.3 and a 5.8).
Any help would be apreciated,
Thanks,
Paul



Performance depends on the quality of the optics and how well the tube
is baffled. If you're after a mini-light bucket then maybe the f/4.5
would be a good choice, though f/5.8 would be perfectly acceptable. At
f/7.3 you're moving into the realms of a planetary scope - optimised
for high power use but not as well suited for a Dobsonian mount (the
tube would be around 5ft long).

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

"Paul" wrote in message
...
Hi,
I am thinking about building an 8.75" Dobsonian scope, mainly for deep
space
fuzzies. What should I expect in performance and what is the best Focal
length to go for? I was thinking about a F4.5, but there are other options
available (7.3 and a 5.8).
Any help would be apreciated,
Thanks,
Paul
The 'downsides' of the shorter focal length designs, are that collimation
will become extremely difficult, and coma will be very severe.
It's 'plus', is the very wide FOV, and on larger mirrors, the shorter
overall length. On some mounts this becomes vital.
For a Dobsonian, the 4.5, will be quite a short scope, and will need a
reasonably tall base, (assuming that you are not very short). Personally,
I'd probably look at the f/5.8, and still budget to buy a Paracorr to help
correct the coma.

Best Wishes

On Tue, 2 Sep 2003 22:08:35 +0100, "Roger Hamlett"
wrote:

The 'downsides' of the shorter focal length designs, are that collimation
will become extremely difficult, and coma will be very severe.
It's 'plus', is the very wide FOV, and on larger mirrors, the shorter
overall length. On some mounts this becomes vital.
For a Dobsonian, the 4.5, will be quite a short scope, and will need a
reasonably tall base, (assuming that you are not very short). Personally,
I'd probably look at the f/5.8, and still budget to buy a Paracorr to help
correct the coma.

Best Wishes


I think you overstate the problems - "Extremely difficult collimation"
and "Very severe coma" are misleading terms. Collimation on my f/5.3
is simple and the work of moments, and on a freind's f/4 hardly more
difficult. Coma is barely evident on the f/4 scope (although it is
there it is not objectionable). A Paracorr would not be required for
f/5.8 unless you were extremely critical of the performance at very
edge of field.

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

In article , ChrisH
wrote:

On Tue, 2 Sep 2003 22:08:35 +0100, "Roger Hamlett"
wrote:

The 'downsides' of the shorter focal length designs, are that
collimation
will become extremely difficult, and coma will be very severe.
It's 'plus', is the very wide FOV, and on larger mirrors, the shorter
overall length. On some mounts this becomes vital.
For a Dobsonian, the 4.5, will be quite a short scope, and will need a
reasonably tall base, (assuming that you are not very short).
Personally,
I'd probably look at the f/5.8, and still budget to buy a Paracorr to
help
correct the coma.

Best Wishes


I think you overstate the problems - "Extremely difficult collimation"
and "Very severe coma" are misleading terms. Collimation on my f/5.3
is simple and the work of moments, and on a freind's f/4 hardly more
difficult. Coma is barely evident on the f/4 scope (although it is
there it is not objectionable). A Paracorr would not be required for
f/5.8 unless you were extremely critical of the performance at very
edge of field.

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

Torc uttered:

Having built a 8.75inch at f7.3 - I can honestly say that I had fun and
enjoy tracking down the fuzzies - the fov on a 25mm plossl is around a
degree (2xmoon diameters). The clincher for me was the width of the
Orion nebula and the height of the scope was eyeball height for me at
zenith (around 5ft 6inches).

To get round any balance issues simply build bigger distances between
the bearings...

Clear skies

Torc

"ChrisH" wrote in message
...
On Tue, 2 Sep 2003 22:08:35 +0100, "Roger Hamlett"
wrote:

The 'downsides' of the shorter focal length designs, are that collimation
will become extremely difficult, and coma will be very severe.
It's 'plus', is the very wide FOV, and on larger mirrors, the shorter
overall length. On some mounts this becomes vital.
For a Dobsonian, the 4.5, will be quite a short scope, and will need a
reasonably tall base, (assuming that you are not very short). Personally,
I'd probably look at the f/5.8, and still budget to buy a Paracorr to
help
correct the coma.

Best Wishes


I think you overstate the problems - "Extremely difficult collimation"
and "Very severe coma" are misleading terms. Collimation on my f/5.3
is simple and the work of moments, and on a freind's f/4 hardly more
difficult. Coma is barely evident on the f/4 scope (although it is
there it is not objectionable). A Paracorr would not be required for
f/5.8 unless you were extremely critical of the performance at very
edge of field.

ChrisH
It does depend of course on what eyepieces you are using. The assumption I
made, was that if buying a 'short focal length' scope, the intention would
be to use it for relatively wide-field work. However if you stick to using
relatively high magnifications, it is not such a problem.
Collimation, becomes increasingly critical with faster focal ratios, with
the allowable tolerance being less than half the distance on a f/4.5 scope,
that is acceptable on a f/5.8. The error allowed is related to a cube power
on the focal length, with a f/5 scope, having an allowable tolerance, just
1/8th that of a f/10 example. At the lower focal ratios, very good design of
every part becomes increasingly necessary, to make the movements easy, and
fine. It is an increasing complexity, that is unnecessary, unless the short
focal length is needed.
The balance for me, might well shift to the f/4.5 scope, with a 12" mirror,
since otherwise the tube length becomes unweildy/unuseable, but why look for
extra problems on the smaller scope. A unit around the mid f/5's, is a damm
useable all round scope, won't be too long, and coma will be acceptable most
of the time. I just felt that f/4.5, was getting uneccesarily 'short',
unless there is some special reason to go this far. :-)

Best Wishes