Short Focal Ratio Dobs

Hi, what's the shortest focal length Dob that's practical? As the aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...

"Chuck" wrote in message
. ..
Hi, what's the shortest focal length Dob that's practical? As the
aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...

Chuck,

Increasing coma, and a very small diffraction limited field.

Diffraction Limited Field of a Newtonian based on Roger Sinnott's
Astronomical Computing (Sky & Tel, May, 1991), for "faster" Dob....

f/4.5 0.064 inches
f/4 0.045 inches
f/3.5 0.03 inches
f/3 0.019 inches

But if you are looking for something with a lot of light grasp, and seeing
limited anyway, perhaps it isn't a big deal.

Clear skies, Alan

Royce thinks it makes a big difference:

http://www.rfroyce.com/mirror_performance.htm

S. Sherrod

"Alan French" wrote in message
...
"Chuck" wrote in message
. ..
Hi, what's the shortest focal length Dob that's practical? As the
aperture
gets larger (greater than 20") what is the downside of making an F3
dob
rather than an F4.5 (more common). Thanks ...

Chuck,

Increasing coma, and a very small diffraction limited field.

Diffraction Limited Field of a Newtonian based on Roger Sinnott's
Astronomical Computing (Sky & Tel, May, 1991), for "faster" Dob....

f/4.5 0.064 inches
f/4 0.045 inches
f/3.5 0.03 inches
f/3 0.019 inches

But if you are looking for something with a lot of light grasp, and
seeing
limited anyway, perhaps it isn't a big deal.

Clear skies, Alan

Thanks to you both. At F5, for the blank that I'm looking at the length
will be 135" I was hoping to get away with an F3 primary using a 2x barlow
to get up to F6, oh well ....
"Alan French" wrote in message
...
"Chuck" wrote in message
. ..
Hi, what's the shortest focal length Dob that's practical? As the
aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...

Chuck,

Increasing coma, and a very small diffraction limited field.

Diffraction Limited Field of a Newtonian based on Roger Sinnott's
Astronomical Computing (Sky & Tel, May, 1991), for "faster" Dob....

f/4.5 0.064 inches
f/4 0.045 inches
f/3.5 0.03 inches
f/3 0.019 inches

But if you are looking for something with a lot of light grasp, and seeing
limited anyway, perhaps it isn't a big deal.

Clear skies, Alan

I can't advise you as to what focal ratio to choose, but I suspect that at
F5 or faster, you might want to look into using a Tele Vue Paracorr. Why not
check out the TV web site and see what you think.
I notice that Royce doesn't mention the Paracorr in his analysis. I don't
know what to make of that omission.
Ciao,
Bill Meyers

Chuck wrote:

Thanks to you both. At F5, for the blank that I'm looking at the length
will be 135" I was hoping to get away with an F3 primary using a 2x barlow
to get up to F6, oh well ....
"Alan French" wrote in message
...
"Chuck" wrote in message
. ..
Hi, what's the shortest focal length Dob that's practical? As the
aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...

Chuck,

Increasing coma, and a very small diffraction limited field.

Diffraction Limited Field of a Newtonian based on Roger Sinnott's
Astronomical Computing (Sky & Tel, May, 1991), for "faster" Dob....

f/4.5 0.064 inches
f/4 0.045 inches
f/3.5 0.03 inches
f/3 0.019 inches

But if you are looking for something with a lot of light grasp, and seeing
limited anyway, perhaps it isn't a big deal.

Clear skies, Alan

On Fri, 14 Nov 2003 02:39:21 GMT, "Chuck" wrote:

Thanks to you both. At F5, for the blank that I'm looking at the length
will be 135" I was hoping to get away with an F3 primary using a 2x barlow
to get up to F6, oh well ....

Such a fast focal ratio is best if not only suited for astrophotography,
an activity for which Dobsonian-mounted Newtonians are ill-advised, and
due to its overt simplicity.

snip

Alan

Hi, what's the shortest focal length Dob that's practical? As the aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...

I own a 12.5 inch F4.06 DOB. It does work and it works quite nicely with a
Paracorr, it is also very touchy about collimation and it is hard on eyepieces.


If you look around, you will see essentially no amateur scopes that are less
than F4, in my view that is with good reason.

In my view, if F3 were workable, all large DOBs would be F3 because it really
would make the scope more manageable.

If you really want to go ahead with this project, I would not process until you
have found someone who owns such a scope and take the time to not only talk to
them about it but also look through it.

jon

"Chuck" wrote in message ...

Hi, what's the shortest focal length Dob that's practical?

Steve Mock at my astro club has a 25" F/3.5 Dob. It is a little cranky,
takes only expensive eyepieces, and certainly isn't optimized for high
power, but the images are really quite acceptable. And at 25", the
difference between F/3.5 and F/4.5 is the difference between a short
stepladder and a rather scary drop down to Earth.

A propos of the diffraction-limited circle, it is worth remembering
that in most locations, the atmosphere won't allow a 25" scope to
operate anywhere close to diffraction-limited anyway.

Also worth remembering that most pro scopes these days are F/2
or thereabouts.

- Tony Flanders

"Alan W. Craft" wrote in message
...
Such a fast focal ratio is best if not only suited for
astrophotography,
an activity for which Dobsonian-mounted Newtonians are ill-advised

Using an EQ Platform drive, a Dob is suitable for astrophotography.

http://astronomy-mall.com/regular/pr...ms/images.html

See:
http://www.astrosurf.com/altaz/600_e.htm
Photos and description of a 40" f/3...
Clear skies,
Shneor Sherman

"Chuck" wrote in message ...
Hi, what's the shortest focal length Dob that's practical? As the aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...