Finder EP size recommendation needed for 90mm MAK

Hi All

I am getting back into astronomy after many years with some imaging with my
digital camera as my main interest. Moon, Sun and planets I believe are
doable.

I have a 90mm f1200 Mak with a 25mm Sirius Plossl with 2X and 3X barlows on
order (hopefully to use one EP with Scopetronics adapter ring for 3 powers),
Digi-T adapter and I use a red dot finder.

On my old 6"f8 scope I sold years ago I used a Rini 40mm as my locating EP
and the views thru it was like looking thru a porthole.

I have tried to sift thru many google threads on the 32mm vs. 40mm EP and
aspect of the FOV being the same. Don't quite understand this though.

With the 25mm I have a mag of 48X. A 40mm would give me 30X, and the 32mm
about 38X.

Would the 32mm (budget $$ recommendations accepted) give me a significantly
larger FOV than my 25mm or would the 40mm (budget $$ recommendations
accepted) at a lower power prove more useful as a locating EP?

Thanks
Brian

Would the 32mm (budget $$ recommendations accepted) give me a significantly
larger FOV than my 25mm or would the 40mm (budget $$ recommendations
accepted) at a lower power prove more useful as a locating EP?

Thanks
Brian

The field of view of an eyepiece is set by the field stop, which is a ring at
the focal plane of the eyepiece. The 1.25 inch format limits the size of the
field stop to essentially the inside diameter of the barrel, something less
than 1.25 inch, normally 1.10 inches, the Inner diameter of the standard filter
thread.

With a 32mm eyepiece, this works out to about 50 degrees, the standard field of
view for a Plossl.

Using a longer focal length eyepiece cannot make the field stop any larger so
the true field of view is not going to be any wider.

If you don't like that looking down a tunnel view of the 40mm eyepiece, then a
32 mm is your best choice. Generally I think the 32mm is your best choice
anyway.

I would suggest getting some individual eyepieces as well. A nice eyepiece at
a great price is the Synta 9mm Widefield, same as the Orion Expanse, 66 degree
FOV good eyerelief, best deal I know of is www.scopestuff.com for $39 shipped
to your door.

jon

Amyotte wrote:
Hi All

I am getting back into astronomy after many years with some imaging with my
digital camera as my main interest. Moon, Sun and planets I believe are
doable.

I have a 90mm f1200 Mak with a 25mm Sirius Plossl with 2X and 3X barlows on
order (hopefully to use one EP with Scopetronics adapter ring for 3 powers),
Digi-T adapter and I use a red dot finder.

On my old 6"f8 scope I sold years ago I used a Rini 40mm as my locating EP
and the views thru it was like looking thru a porthole.

I have tried to sift thru many google threads on the 32mm vs. 40mm EP and
aspect of the FOV being the same. Don't quite understand this though.

With the 25mm I have a mag of 48X. A 40mm would give me 30X, and the 32mm
about 38X.

Would the 32mm (budget $$ recommendations accepted) give me a significantly
larger FOV than my 25mm or would the 40mm (budget $$ recommendations
accepted) at a lower power prove more useful as a locating EP?

Thanks
Brian



to compare the true field of view of eyepieces, multiply the eyepiece's
apparent field of view times it's focal length. the bigger number
yields larger field of view.

using Celestron Ultima eyepieces for example,
30mm X 50* afov = 1500
42mm X 36* afov = 1512 - not much difference from the 30mm.
35mm X 49* afov = 1715 which would be the max available in 1.25" eyepieces.

to approximately determine the degrees of true field of view that an
eyepiece would yield in your scope, divide the above numbers by your
scope's focal length. the 35mm would yield a 0.9* true field of view.
[1715 / 1900 (your scopes focal length)]

peace,
jon II

35mm X 49* afov = 1715 which would be the max available in 1.25" eyepieces.

One also has to be careful about distinguishing between the AaFOV and AmFOV.

AaFOV= Apparent Advertised FOV
AmFOV= Apparnet Measured FOV

The 1.10in filter thread restriction would imply the maximum FOV possible for
the 35mm would be 46 degrees...

The only real way to be sure is to measure it.

jon

(Jon Isaacs) wrote in
:

35mm X 49* afov = 1715 which would be the max available in 1.25"
eyepieces.

One also has to be careful about distinguishing between the AaFOV and
AmFOV.

AaFOV= Apparent Advertised FOV
AmFOV= Apparnet Measured FOV

The 1.10in filter thread restriction would imply the maximum FOV
possible for the 35mm would be 46 degrees...

Hi Jon,

If that's true, then how do the KK Widescan Type III eyepieces achieve an
84 deg. AFOV in a 1.25 inch eyepiece?

The 1.10in filter thread restriction would imply the maximum FOV
possible for the 35mm would be 46 degrees...

If that's true, then how do the KK Widescan Type III eyepieces achieve an
84 deg. AFOV in a 1.25 inch eyepiece?

What is the focal length of the eyepiece? The eyepiece I was referring to had
a 35mm focal length. One is certainly not going to achieve an 84 degree FOV
with a 35mm 1.25 inch eyepiece and probably not with a 2 inch eyepiece.

My calculations show that it would be possible with a 19mm eyepiece.

The first order approximation is:

Fieldstop/FLeyepiece*57.3 = AFOV in degrees.

jon

Brian posted:

Would the 32mm (budget $$ recommendations accepted) give me a significantly
larger FOV than my 25mm or would the 40mm (budget $$ recommendations
accepted) at a lower power prove more useful as a locating EP?

Not really. The problem is that the hole in the back of the telescope is not
large enough to permit light transmitted by the telescope optics from the
edges of a field wider than about a one degree true "sky" field from getting
to the eyepiece (that and the fact that the barrel of an 1.25 inch eyepiece
won't let the light in either). Hence, the maximum field for the telescope is
just about a degree. I use a 30mm Ultrascopic (26.1mm field stop), and in my
little StarMax 90mm EQ Maksutov, it starts to show just a hint of vignetting
at the outer edges of the field of view, so a longer focal length eyepiece
would give you lower power but not any more field of view. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

(Jon Isaacs) wrote in
:


The 1.10in filter thread restriction would imply the maximum FOV
possible for the 35mm would be 46 degrees...

If that's true, then how do the KK Widescan Type III eyepieces achieve
an 84 deg. AFOV in a 1.25 inch eyepiece?

What is the focal length of the eyepiece? The eyepiece I was
referring to had a 35mm focal length. One is certainly not going to
achieve an 84 degree FOV with a 35mm 1.25 inch eyepiece and probably
not with a 2 inch eyepiece.

My calculations show that it would be possible with a 19mm eyepiece.

The first order approximation is:

Fieldstop/FLeyepiece*57.3 = AFOV in degrees.

They have a 20mm a 16mm and a 13mm in the 1.25" format.

Thanks for the replies. I will go the 32mm or so route.

Brian


"Amyotte" wrote in message
...
Hi All

I am getting back into astronomy after many years with some imaging with
my
digital camera as my main interest. Moon, Sun and planets I believe are
doable.

I have a 90mm f1200 Mak with a 25mm Sirius Plossl with 2X and 3X barlows
on
order (hopefully to use one EP with Scopetronics adapter ring for 3
powers),
Digi-T adapter and I use a red dot finder.

On my old 6"f8 scope I sold years ago I used a Rini 40mm as my locating EP
and the views thru it was like looking thru a porthole.

I have tried to sift thru many google threads on the 32mm vs. 40mm EP and
aspect of the FOV being the same. Don't quite understand this though.

With the 25mm I have a mag of 48X. A 40mm would give me 30X, and the 32mm
about 38X.

Would the 32mm (budget $$ recommendations accepted) give me a
significantly
larger FOV than my 25mm or would the 40mm (budget $$ recommendations
accepted) at a lower power prove more useful as a locating EP?

Thanks
Brian

They have a 20mm a 16mm and a 13mm in the 1.25" format.

As was originally stated the comments about the field stop size concerned a
35mm 1.25 inch eyepiece......

The 20 mm is pushing the limit, the estimation based of field stop size
indicates 80 degrees max. AFOV.

jon isaacs