2 inch focuser for 6 inch f 5

i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?
the tube diameter would be 8 inch.
secondary size is 1.52 inch.
i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?
thank you for your replies.

"zaph" wrote

i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?
the tube diameter would be 8 inch.
secondary size is 1.52 inch.
i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?
thank you for your replies.

You can't get a 100% illuminated field with a 1.52" secondary with a 6" f/5.
The secondary has to be much bigger than that. If you are questioning as to
whether or not a 2" focuser on this size scope would be a problem, it sounds
like you have a lot more learning to do about Newtonian design. The short
answer is no, there's not a problem using one at all.

Here's the site for installing an excellent, easy to use design program:

http://www.dalekeller.net/atm/newton...t/newtsoft.htm

Howard Lester wrote:
"zaph" wrote

i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?
the tube diameter would be 8 inch.
secondary size is 1.52 inch.
i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?
thank you for your replies.

You can't get a 100% illuminated field with a 1.52" secondary with a 6" f/5.
The secondary has to be much bigger than that. If you are questioning as to
whether or not a 2" focuser on this size scope would be a problem, it sounds
like you have a lot more learning to do about Newtonian design. The short
answer is no, there's not a problem using one at all.

Here's the site for installing an excellent, easy to use design program:

http://www.dalekeller.net/atm/newton...t/newtsoft.htm


thanks for the link.
i'm going to do more research before i make up my mind exactly what i'm
going to build.
i'm just exploring possibilities now.
royce is now offering a 6 inch f 5 conical cross section mirror that
got me thinking.
all 6 inch f 5 scopes i've seen have 1.25 inch focusers.
maybe if i want a 2 inch focuser i should build an 8 inch f 5?

"zaph" wrote

thanks for the link.
i'm going to do more research before i make up my mind exactly what i'm
going to build.
i'm just exploring possibilities now.
royce is now offering a 6 inch f 5 conical cross section mirror that
got me thinking.
all 6 inch f 5 scopes i've seen have 1.25 inch focusers.
maybe if i want a 2 inch focuser i should build an 8 inch f 5?

NO! Mirror size has NOTHING to do with it! (f/ratio has something to do
with it.) You can put a 2" focuser on a 3" telescope, and you can put a
0.965" focuser on a 20" (as stupid as that would be).

You seem pretty hung up on a 2" focuser. Nothing wrong with that. Just get
the mirror you want for the telescope size you want, determine what f/ratio
suits your NEEDS, and go from there. Until you know what you want your
telescope needs are, you shouldn't be even thinking of getting anything.
What's with the 100% illumination? The only reason for needing that is for
serious "imaging," not for visual.

zaph wrote:
i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?
the tube diameter would be 8 inch.
secondary size is 1.52 inch.

This 1.52" secondary would prevent illumination of the wide fields of
view possible in 2" eyepieces. For an F/5, to fully illumnate a 47mm
FoV would require about 2.4" secondary, which is way to large for a 6"
scope (central obstruction). The 1.52" secondary can only fully
illuminate about a 1.2" FoV (which is just about perfect for a 1.25"
focuser.)

i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?

Yes, it would allow the use of 2" eyepieces, but it will not fully
illuminate the 47mm FoV.

wrote:
zaph wrote:
i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?
the tube diameter would be 8 inch.
secondary size is 1.52 inch.

This 1.52" secondary would prevent illumination of the wide fields of
view possible in 2" eyepieces. For an F/5, to fully illumnate a 47mm
FoV would require about 2.4" secondary, which is way to large for a 6"
scope (central obstruction). The 1.52" secondary can only fully
illuminate about a 1.2" FoV (which is just about perfect for a 1.25"
focuser.)

i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?

Yes, it would allow the use of 2" eyepieces, but it will not fully
illuminate the 47mm FoV.

thanks mitch.
that's just what i wanted to know.
i suspected something like that.

On 2 Jan 2007 07:26:49 -0800, "zaph" wrote:

i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?

Yes.

the tube diameter would be 8 inch.
secondary size is 1.52 inch.
i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?

Even without bothering to ray-trace the system, it's safe to say that
you'll have some vignetting with the above arrangement. OTOH, IMO
it's unrealistic, for most visual purposes, to design a Newtonian that
will not vignette at all with a wide-field, 2" eyepiece. You would end
up needing a relatively *large* diagonal (secondary) mirror with any
such f/5 Newtonian. A *serious* variable star observer *might* want
such a telescope; but the rest of us would be better off with some
minor vignetting.

Seriously, it would take quite a bit of vignetting before most visual
observers would notice it. IIRC (it's been a while since I've
designed a Newtonian!) a fully illuminated field of 1/4 inch diameter
is more than adequate for most visual purposes.

I suggest you ray-trace the system to see what you're really
designing!
--
Bill
Celestial Journeys
http://cejour.blogspot.com

I built a 6 inch f/5 Dob and then later modified it for astrophotography, I
cannot rember my caluations but if your tube is 8 inch (mine was) a 1.52
secondary moonlite low profile Crayford focuser will be okay visually, That
what I had on mine with no vignetting.

When I moved to photography I changed to a 1.83 secondary with a 1.5inch
extension to the focuser to get enough back focus and a fully illumated
field, I did have some vignetting.

Phil
"zaph" wrote in message
ups.com...
i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?
the tube diameter would be 8 inch.
secondary size is 1.52 inch.
i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?
thank you for your replies.

Phil wrote:
I built a 6 inch f/5 Dob and then later modified it for astrophotography, I
cannot rember my caluations but if your tube is 8 inch (mine was) a 1.52
secondary moonlite low profile Crayford focuser will be okay visually, That
what I had on mine with no vignetting.

When I moved to photography I changed to a 1.83 secondary with a 1.5inch
extension to the focuser to get enough back focus and a fully illumated
field, I did have some vignetting.

Phil
"zaph" wrote in message
ups.com...
i'm thinking about building a 6 inch f 5 reflector.
could i use a 2 inch focuser with no problems?
the tube diameter would be 8 inch.
secondary size is 1.52 inch.
i would like to use a moonlite low profile crayford focuser.
i want a fully illuminated field and no vignetting.
this is for visual use, and not imaging.
would this work?
thank you for your replies.


thanks for the reply phil.
it was informative and helpful.