1.25" v's 2" accessories

The short answer is that 1.25" is just fine except for cases where the
combination of long focal length and wide AFOV requires the use of 2".
Also for certain types of cameras. 1.25" eyepieces start limiting
their AFOV around 20-25mm, depending on design. So, you'll definitely
want a 2" wide field eyepiece of around 35-40mm. You might want a
second 2" of about 20-25mm, though I wouldn't make that a priority.

Nothing wrong with 1.25" filters and barlow, either. You just might
want to add a 2" light polution filter.

Greg

Hopefully, I will find some astronomers in my area that I can get
together with. Would be nice for many reasons, one being the mutual
benefit of swapping scope accessories to gauge performance without
having to buy everything "blindly" (sort of speak).

Thanks,

Errol
NOLA

On Mon, 02 Jan 2006 23:45:03 -0600, David Knisely
wrote:

+james posted:
+
+ amateur derived formulae????? Where do you get off making that
+ statement?
+
+Well, I misunderstood what you were writing. You weren't as clear as
+you might have been as to what you were calculating or why. In the
+first paragraph you didn't say exactly what field of view you meant. I
+was only referring to the formulae that give the true field of view *on
+the sky*: TFOV = (180/Pi)*(EFSD/Fl) (or the older approximation: TFOV =
+AFOV/Mag). Thus, I assumed you were referring to this as well. You
+also didn't write a full equation, as the equal sign wasn't there, so
+again, there was a little confusion. The inverse of the sine function
+in equations is most often designated as "arcsin" or "sin-1", (there is
+usually no "e" on the end of the sine or arcsine functions when placed
+inside equations) so that raised a bit of a question mark as to what the
+equation really was ("e" to me is 2.71828..). The numbers I got when
+putting Fl equal to the telescope's focal length and not the eyepiece
+focal length were close enough to reality that I thought you were going
+for that with some of the 'cobbled together' formulae I see some people
+try to make fit the true field numbers.
******

In this case 'e' is just a variable. I do believe that I did explain
what 'e' was. As for the trig functions, whether I spelled them out
instead of using familiar abbreviations is really nit picking. To be
honest with this limited tool it is rather difficult to actually
publish the equation in a form that would appear in a mathematics
paper. Instead I used some liberties to make the equation a bit more
readable.

james

I am well aware of the old saying about opinions. Trust me it need not
be repeated here.

One thing to note is that eyepiece apparent field of view is not
entirely dictated by the barrel diameter. A 10 mm Abbe Orthoscopic
will have about 45 degrees apparent field of view whether it is in a 2
inch or 1.25 inch or even a 0.965 inch barrel. This is due to the
eyepiece focal length and the field stop diameter.

When the eyepiece field stop and the eyepiece focal length are equal
the apparent field of view is 57.3 degrees or one radian. To gain this
at eyepiece focal lengths of greater than about 27 to 30mm, the lens
will have to sit in a 2 inch barrel or the 1.25 inch barrel will act
as a stop and thus limit the view to where it will be somewhat less
than actually obtainable. 2 inch eyepieces are needed for longer focal
length eypieces.

As for what is the best field of view for usage, that really depends
on what you intend to observe and the telescope that you are using.

james

On Tue, 03 Jan 2006 09:32:17 -0700, Sketcher
wrote:

+On Tue, 03 Jan 2006 02:05:40 GMT, james wrote:
+
+My arguement is that apparent field of view is not, in my opinion, the
+primary specification for choosing an eyepiece. Nor is barrel
+diameter.
+
+Hi James,
+
+Some folks will share your opinions. Others will not. Such is the
+nature of opinions.
+
+When I was into comet hunting the true and apparent fields of view
+were *very* important. For this purpose I found a 65 to 70 degree
+apparent field of view to be optimal.
+
+In order to get the largest practical true field at the magnifications
+I found to be most suitable for my task, barrel diameter wasn't
+something to be ignored. For some scopes a 2" OD eyepiece would be
+best. For others a 1.25" would be better. The determining factor
+(for me) would be the f-ratio of the telescope.
+
+Different people have different needs and preferences when it comes to
+eyepieces. It is in recognition of this fact that such a wide variety
+of eyepieces exist (and get purchased) in the market place.
+
+Sketcher
+To sketch is to see.

Starboard wrote:

As a astro-newb and soon-to-be owner of an 8" Dob (which for those not
familiar, can accommodate either 1.25 or 2" eyepieces & accessories), I
am clueless as to the advantages and disadvantages between the two ...
I guess one could say I have put the cart before the horse in that I
have already made purchases for 1.25" accessories, e.g. filters and
barlow...
However, if 2" is "noticeably superior", then I am "destined" to resort
to 2" (and all necessary, associated, accessories), ergo making all
current 1.25" purchases - wasteful.

I just read your post carefully for the first time, and realized that
none
of the respondents has answered your question. In fact, the answer is
very simple indeed.

No, you have *not* wasted anything by purchasing 1.25-inch accessories,
because you -- like essentially all other amateur astronomers -- *will*
end
up using 1.25-inch eyepieces. The choices are really betwen using
*only*
1.25-inch eyepieces or between using a combination of 1.25-inch and
2-inch
eyepieces. With rare exceptions, using 2-inch eyepieces alone is *not*
an option.

Why? Because with fairly rare exceptions, eyepieces are always put
into the smallest barrel that will physically accomodate the necessary
lenses, field stop, and so on. For long-focal-length widefield
eyepieces,
like a 40mm eyepiece with a 70-degrees apparent field of view, it's
physically impossible to use a 1.25-inch barrel, so this kind of EP
is offered only in 2-inch barrels. But assuming the same design,
every element of an 8mm eyepiece is 1/5th the size of the
corresponding element in a 40mm eyepiece -- making it easy
to put the stuff into a 1.25-inch barrel. In fact, it could be put into
a 1/2-inch barrel, but there wouldn't be any point to that, since
no telescope has a 1/2-inch focuser.

In general, high-power eyepieces (i.e. eyepieces with a short focal
length) are offered *only* in 1.25-inch barrels. So even if you end up
purchasing 2-inch filters and a 2-inch Barlow in the long run, you'll
still end up using your 1.25-inch filters and 1.25-inch Barlows for
your 1.25-inch eyepieces.

- Tony Flanders

With my first 2 scopes(8" F6 and 16" F4.5 dob), used all 1.25"
eyepieces mainly to avoid hassle of changing out adapters.Used 80mm F5
finder /2nd scope on 16" dob for wide field views( 1*).BTW seeing all
4 segments of Veil in one FOV with 80mm and OIII filter was one of my
most memorable ever.Now have 20" F4.5 dob and have invested in 2"
eyepieces and Paracorr. Added 31mm and 17mm 2" eyepieces to my 12mm
1.25/2" Nagler and 8.8mm 84* Meade 1.25/2" for a decent range of mags
without having to change adapters.Still need 1.25" occulars for higher
powers.

Paul


wrote:
Starboard wrote:

As a astro-newb and soon-to-be owner of an 8" Dob (which for those not
familiar, can accommodate either 1.25 or 2" eyepieces & accessories), I
am clueless as to the advantages and disadvantages between the two ...
I guess one could say I have put the cart before the horse in that I
have already made purchases for 1.25" accessories, e.g. filters and
barlow...
However, if 2" is "noticeably superior", then I am "destined" to resort
to 2" (and all necessary, associated, accessories), ergo making all
current 1.25" purchases - wasteful.

I just read your post carefully for the first time, and realized that
none
of the respondents has answered your question. In fact, the answer is
very simple indeed.

No, you have *not* wasted anything by purchasing 1.25-inch accessories,
because you -- like essentially all other amateur astronomers -- *will*
end
up using 1.25-inch eyepieces. The choices are really betwen using
*only*
1.25-inch eyepieces or between using a combination of 1.25-inch and
2-inch
eyepieces. With rare exceptions, using 2-inch eyepieces alone is *not*
an option.

Why? Because with fairly rare exceptions, eyepieces are always put
into the smallest barrel that will physically accomodate the necessary
lenses, field stop, and so on. For long-focal-length widefield
eyepieces,
like a 40mm eyepiece with a 70-degrees apparent field of view, it's
physically impossible to use a 1.25-inch barrel, so this kind of EP
is offered only in 2-inch barrels. But assuming the same design,
every element of an 8mm eyepiece is 1/5th the size of the
corresponding element in a 40mm eyepiece -- making it easy
to put the stuff into a 1.25-inch barrel. In fact, it could be put into
a 1/2-inch barrel, but there wouldn't be any point to that, since
no telescope has a 1/2-inch focuser.

In general, high-power eyepieces (i.e. eyepieces with a short focal
length) are offered *only* in 1.25-inch barrels. So even if you end up
purchasing 2-inch filters and a 2-inch Barlow in the long run, you'll
still end up using your 1.25-inch filters and 1.25-inch Barlows for
your 1.25-inch eyepieces.

- Tony Flanders

Tony Flanders wrote:

I just read your post carefully for the first time, and realized that
none
of the respondents has answered your question. In fact, the answer is
very simple indeed.

Uh, Tony, I answered his question (didn't you see that response?).

2" barrel eyepieces are not necessarly "noticably superior" to any others. Basically, all 2" barrels will allow you to do is access the *potential" of wider true fields of view on the sky than you might be able to get with 1.25" eyepieces. If you don't need a huge field, then you won't need a 2" eyepiece or the hardware that goes with them. With an 8 inch Newtonian, you may be also limited on the maximum field size by the size of your secondary mirror, which if not big enough won't fully illuminate the outer portions of wider fields of view. Thus, going to 2" eyepieces might not gain you all that much. You should be able to get at least a full degree of true field on the sky with some of the longer focal length 1.25" eyepieces in an 8 inch f/6 telescope, which is generally plenty except for maybe a few of the really large Deep-sky objects. *SOME* 2" eyepieces are superior, but there are also some that are just downright dogs with horrid performance, so the barrel size
doesn't tell the whole story. For an example of a "noticably superior" 1.25" eyepiece, the Tele Vue 24mm Panoptic is probably one which fits that description. You may want to eventually go 2", but for right now, 1.25" should serve you fairly well.


--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

wrote in message
oups.com...
Starboard wrote:

As a astro-newb and soon-to-be owner of an 8" Dob (which for those not
familiar, can accommodate either 1.25 or 2" eyepieces & accessories),
I
am clueless as to the advantages and disadvantages between the two ...
I guess one could say I have put the cart before the horse in that I
have already made purchases for 1.25" accessories, e.g. filters and
barlow...
However, if 2" is "noticeably superior", then I am "destined" to
resort
to 2" (and all necessary, associated, accessories), ergo making all
current 1.25" purchases - wasteful.

I just read your post carefully for the first time, and realized that
none
of the respondents has answered your question. In fact, the answer is
very simple indeed.

No, you have *not* wasted anything by purchasing 1.25-inch accessories,
because you -- like essentially all other amateur astronomers -- *will*
end
up using 1.25-inch eyepieces. The choices are really betwen using
*only*
1.25-inch eyepieces or between using a combination of 1.25-inch and
2-inch
eyepieces. With rare exceptions, using 2-inch eyepieces alone is *not*
an option.

Why? Because with fairly rare exceptions, eyepieces are always put
into the smallest barrel that will physically accomodate the necessary
lenses, field stop, and so on. For long-focal-length widefield
eyepieces,
like a 40mm eyepiece with a 70-degrees apparent field of view, it's
physically impossible to use a 1.25-inch barrel, so this kind of EP
is offered only in 2-inch barrels. But assuming the same design,
every element of an 8mm eyepiece is 1/5th the size of the
corresponding element in a 40mm eyepiece -- making it easy
to put the stuff into a 1.25-inch barrel. In fact, it could be put into
a 1/2-inch barrel, but there wouldn't be any point to that, since
no telescope has a 1/2-inch focuser.

In general, high-power eyepieces (i.e. eyepieces with a short focal
length) are offered *only* in 1.25-inch barrels. So even if you end up
purchasing 2-inch filters and a 2-inch Barlow in the long run, you'll
still end up using your 1.25-inch filters and 1.25-inch Barlows for
your 1.25-inch eyepieces.

- Tony Flanders


Well, Tony, I'll stick with what *I* said, just as David did with what he
said.

--
Jan Owen

To reach me directly, remove the Z, if one appears in my e-mail address...
Latitude: 33.6
Longitude: -112.3

David Knisely wrote:

Uh, Tony, I answered his question (didn't you see that response?).

Sorry -- no insult intended. Both your response and Jan Owen's were
extremely thoughtful and comprehensive, and I'm sure the OP learned
a lot from them. It's just that you answered a slightly different
question
from the one the OP asked.

The questions that both of you answered were "Are 2-inch EPs
inherently superior, and should I start investing in them?" That's
what people usually ask, and in some sense, it's the *right*
question to ask.

The questions the OP actually asked were "Are 2-inch EPs
inherently superior, and should I expect to *replace* my
entire 1.25-inch collection with 2-inch EPs?"

Obviously, this was based on the assumption that
it's *possible* to replace all 1.25-inch EPs with
2-inch equivalents. You could theoretically do so --
achieving high magnifications with 2-inch eyepieces
combined with 4X 2-inch Powermate, but this would be
a very unusual way to build an eyepiece collection.
(Yes, I know that if you owned a *really* slow scope,
say f/30 or thereabouts, you might not even need the
Powermate -- but that's even more unlikely.) For most
people, there is no practical alternative to owning at
least a few 1.25-inch eyepieces.

Very likely the OP understood that all of this by the
time he had finished reading your posts; I just wanted
to state it explicitly.

- Tony Flanders

On Thu, 05 Jan 2006 06:16:10 -0800, tony_flanders wrote:

David Knisely wrote:

Uh, Tony, I answered his question (didn't you see that response?).

Sorry -- no insult intended. Both your response and Jan Owen's were
extremely thoughtful and comprehensive, and I'm sure the OP learned
a lot from them. It's just that you answered a slightly different
question
from the one the OP asked.

The questions that both of you answered were "Are 2-inch EPs
inherently superior, and should I start investing in them?" That's
what people usually ask, and in some sense, it's the *right*
question to ask.

The questions the OP actually asked were "Are 2-inch EPs
inherently superior, and should I expect to *replace* my
entire 1.25-inch collection with 2-inch EPs?"

Obviously, this was based on the assumption that
it's *possible* to replace all 1.25-inch EPs with
2-inch equivalents. You could theoretically do so --
achieving high magnifications with 2-inch eyepieces
combined with 4X 2-inch Powermate, but this would be
a very unusual way to build an eyepiece collection.
(Yes, I know that if you owned a *really* slow scope,
say f/30 or thereabouts, you might not even need the
Powermate -- but that's even more unlikely.) For most
people, there is no practical alternative to owning at
least a few 1.25-inch eyepieces.

Very likely the OP understood that all of this by the
time he had finished reading your posts; I just wanted
to state it explicitly.



And thank you very much for doing so. There are *many*
subtleties that go unspoken in so many threads. The behaviour of
eyepieces with different focal ratios is a big one.

As I read this group longer and longer, certain voices (and you know who
you are) are starting to become recognizable as the experts. Any time you
can say *why* you have the opinion you do is very helpful.

I still don't understand why my intuitive feeling that 1.25" eyepieces
would benefit from a 2" diagonal is wrong, but I'm grateful that someone
bothered to tell me that it is.

Tom R