1.25" v's 2" accessories

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, and
hence, also clueless into which size future investments should be made.

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... I did this because it seemed money well spent; 5 filters and
barlow w/ shipping for $90. In my mind, I was maximizing my abilities
with what accessories where coming stock with the Dob.

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. Of course I could send back the
1.25" accessories and exchange them for 2", but then I'd have to
purchase 2" eye pieces as well; spending more money. Is it worth it?

Please make up my mind for me.....

Errol NOLA

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, and
hence, also clueless into which size future investments should be made.


A very good question.

You might want to look around Cloudy Nights a bit:

Here is an article on how to set up a Dob (scope is similar to Orion XT-8).

Don't know if they have any 1.25 vs. 2" articles or reviews, but worth
looks.

FWIW, at this point in time, all my EPs, diagonals, etc. are 1.25" ..
though at least two of my scopes will support 2" accessories.

Phil

Phil Wheeler 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, and
hence, also clueless into which size future investments should be made.


A very good question.

You might want to look around Cloudy Nights a bit:

Here is an article on how to set up a Dob (scope is similar to Orion XT-8).


Oops! He


http://www.cloudynights.com/item.php...d=1058&pr=3x74

Seems that site was designed for me. Thankx!

Errol
NOLA

For starts I will not make your mind up.

Fact #1 all eyepeices are not best used in all scopes.

Fact #2 is that what eyepiece you used depends on what you want to
look at and what scope you are using at that time.

Some basic generalities:

1) 1.25 inch eypieces are generally limited to about 55 degree field
of view max. 2 inch eyepieces can achieve higher fields of view.
Upwards to 80 degrees.

2) Generally speaking 2 inch eyepieces are heavier and cost more due
to larger diameter and thickness of the glass used in 2 inch
eyepieces.

3) Longer focal lengths are better off in 2 inch eyepieces. Generally
greater than 30 mm focal length eyepieces.

Also limiting your choice to just eyepiece diameter is limiting visual
performance. A good selection of eyepieces will have both for
different viewing conditions and objects.

The only recommendation that I can make it that you have a selection
that comprises of both. Most of you 2 inch eyepieces will be used for
low pwer and wide field of views. Most of your 1.25 inch eyepieces
will be short focal length for high magnification and specific
viewing. Even some of your eyepieces can be narrow field of view for
double stars and planetary work.

james



On 31 Dec 2005 12:16:01 -0800, "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, and
+hence, also clueless into which size future investments should be made.
+
+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... I did this because it seemed money well spent; 5 filters and
+barlow w/ shipping for $90. In my mind, I was maximizing my abilities
+with what accessories where coming stock with the Dob.
+
+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. Of course I could send back the
+1.25" accessories and exchange them for 2", but then I'd have to
+purchase 2" eye pieces as well; spending more money. Is it worth it?
+
+Please make up my mind for me.....
+
+Errol NOLA

Please make up my mind for me.....

For starts I will not make your mind up.

James,

I suppose the choice phrase was, at risk of sounding somewhat gay,
"cutesy." (no offence to any gay astro's, or "gastro's" per' se.......
oh there I go again). No, you're right, my mind will be made up through
the composite of all the ideas expressed through this group; and the
great google search engine.

I do thank you for your time and being so generous with your knowledge.
Please keep it coming..

Errol
NOLA

james posted:

1) 1.25 inch eypieces are generally limited to about 55 degree field
of view max. 2 inch eyepieces can achieve higher fields of view.
Upwards to 80 degrees.


Well, I'm afraid that this is not exactly true. Many 1.25" eyepieces
are capable of *apparent fields* of view of from 65 degrees to as much
as 83 degrees (this is what your eyeball sees when looking into the
eyepiece whether it is in a telescope or not). Examples of wider field
1.25" eyepieces include the shorter focal lengths Naglers, the 1.25"
Panoptics, the older Meade 14mm Ultrawide, the 1.25" Swans, the
Speers-Walers, and a few others. However, the 1.25" barrel eyepiece
field stop diameter is limited to no more than about 27mm, which limits
the *true* field of view on the sky. The Field Stop formula for true
field demonstrates this (and is the most accurate predictor of true
field of view):

TFOV = 57.3*(EFSD/Fl) where EFSD is the eyepiece field stop diameter and
Fl is the telescope's focal length.

This is where 2" barrel eyepieces have an advantage, as their field stop
sizes can be as much as 46mm across. Clear skies to you.
--
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 *
**********************************************

On Sat, 31 Dec 2005 16:51:00 -0600, David Knisely
wrote:

+james posted:
+
+ 1) 1.25 inch eypieces are generally limited to about 55 degree field
+ of view max. 2 inch eyepieces can achieve higher fields of view.
+ Upwards to 80 degrees.
+
+
+Well, I'm afraid that this is not exactly true. Many 1.25" eyepieces
+are capable of *apparent fields* of view of from 65 degrees to as much
+as 83 degrees (this is what your eyeball sees when looking into the
+eyepiece whether it is in a telescope or not). Examples of wider field
+1.25" eyepieces include the shorter focal lengths Naglers, the 1.25"
+Panoptics, the older Meade 14mm Ultrawide, the 1.25" Swans, the
+Speers-Walers, and a few others. However, the 1.25" barrel eyepiece
+field stop diameter is limited to no more than about 27mm, which limits
+the *true* field of view on the sky. The Field Stop formula for true
+field demonstrates this (and is the most accurate predictor of true
+field of view):
+
+TFOV = 57.3*(EFSD/Fl) where EFSD is the eyepiece field stop diameter and
+Fl is the telescope's focal length.
+
+This is where 2" barrel eyepieces have an advantage, as their field stop
+sizes can be as much as 46mm across. Clear skies to you.
*****

I always have used 1.9099 * arcsine(e/2) where "e" is field stop/focal
length. This reduces down to 1.9099 * arcsine( FS/(2*FL)), where FS is
the field stop diameter and FL is the eyepiece focal length. This
gives the maximum field of view over which orthoscopy is maintained.
IF you want to add in field of view where either rectilinear
distortion and/or angular magnification distortion is involved then
"e" is modified. This then will yield a wider field of view.

Yes in theory as well as practicallity, a 1.25 inch eyepiece can be as
wide as 90 degrees. The statement I made will hold true for most
designs. Adding a divergent lens will deffinitely widen field of view
at the expense of some distortion. As long as later elements
reasonable correct for this is fine. Even the wide angle eyepeices
that you have mentioned have some form of distortion and less than
advertized field of view for orthoscopy.

Yes the field stop of a 1.25 inch eyepiece is limited to a max of
30mm. More realistically it is about 27 to 28 mm. This limits field of
view to eyepieces with focal lengths of 30mm unless the field lens is
a divergent lens. As above a divergent lens as the field lens is not
without its issues.


Maybe my error is not fully stating all the conditions.

james

james posted:

I always have used 1.9099 * arcsine(e/2) where "e" is field stop/focal
length. This reduces down to 1.9099 * arcsine( FS/(2*FL)), where FS is
the field stop diameter and FL is the eyepiece focal length. This
gives the maximum field of view over which orthoscopy is maintained.
IF you want to add in field of view where either rectilinear
distortion and/or angular magnification distortion is involved then
"e" is modified. This then will yield a wider field of view.


Unfortunately, this formula (like so many other amateur-derived
formulae) does not yield results which are quite as close to reality as
the somewhat simpler (and more commonly used) Eyepiece Field Stop formula:

TFOV = (180/Pi)*(EFSD/Fl)

where Pi is the usual 3.1415926... EFSD is the eyepiece field stop
diameter, Fl is the telescope focal length, and * is short hand for the
multiplication operation (the division operations in parethesis are to
be done first). In most cases, the 180/Pi (the number of degrees in one
Radian) can be just rounded to 57.3 (although for a few review tests, I
have had to use 57.296 when doing accurate measurement comparisons).

Here are the real measured characteristics of my own set of eyepieces
and the true field of view they yield in my 10 inch f/5.6 Newtonian
(1410mm measured focal length, fields of view measured with averages of
multiple star-drift timings for a star on the celestial equator).
Only the 40mm Mk-70 Konig and the 30mm WideScan III are 2" barrel
eyepieces (although the 14mm Meade Ultrawide has an outer barrel sleeve
to allow it to be used in a 2" focuser):

____Eyepiece___________________AFOV________Mag.___ ___TFOV_________EFSD__
40mm Mk-70 Konig (Univ. Op.) 68.0 deg. 35.3x 113.0'arc 46.00mm
30mm Widescan III 84.0 deg. 47.0x 107.9'arc 44.00mm
30mm Ultrascopic (Orion) 52.3 deg. 47.0x 63.78'arc 26.08mm
27mm Kellner (Jaegers) 52.5 deg. 52.2x 62.51'arc 25.38mm
24mm Panoptic (Tele Vue) 69.5 deg. 58.5x 66.79'arc 27.00mm
24mm Konig (Univ. Op.) 59.6 deg. 58.8x 58.86'arc 24.04mm
20mm Celestron Plossl 51.7 deg. 70.5x 42.31'arc 17.23mm
15mm Ultrascopic (Orion) 58.3 deg. 94.0x 35.38'arc 14.40mm
14mm Meade Ultrawide 83.1 deg 100.7x 49.17'arc 20.3mm*
10mm Ultrascopic (Orion) 48.7 deg. 141.0x 20.35'arc 8.32mm
10mm Celestron Plossl 48.6 deg. 141.0x 19.67'arc 8.00mm
6.4mm Super Plossl (Meade) 48.7 deg. 220.3x 13.01'arc 5.31mm
6mm Orthoscopic (Brandon) 44.2 deg. 235.0x 11.29'arc 4.62mm

To take some examples, the 30mm Ultrascopic yields a *measured* true
field of view on the sky of 63.78 arc minutes (1.063 degrees). The
formula you cite predicts a 1.012 degrees, or about 4.8 percent off of
reality. The field stop formula predicts a 1.060 degrees (only 0.3
percent off of reality), so it is working somewhat better. The 15mm
Ultrascopic yields a measured true field of view on the sky of 35.38 arc
minutes or about 0.600 degrees, yet the formula you cite would calculate
out a field of 0.559 degrees, or about 5.2 percent in error. By
contrast, the usual field stop formula predicts a true field of 0.585
degrees, which is only 0.8 percent off. With accurate eyepiece field
stop diameter and telescope focal length figures, the field stop formula
can generally yield results which are often within two percent of the
true field of view (as opposed to the old AFOV/Mag formula which is
lucky to get within 6 percent of reality). Thus, it is probably the
better of the two formulae to use when calculating true field of view.

Yes in theory as well as practicallity, a 1.25 inch eyepiece can be as
wide as 90 degrees. The statement I made will hold true for most
designs. Adding a divergent lens will deffinitely widen field of view
at the expense of some distortion.


Well, "most designs" covers quite a lot of ground. It (a 55 degree AFOV
"limit") might hold for things like the Abbe Orthos, Kellners, Plossls,
and the simpler designs, but won't generally hold for the others like
Erfles, Konigs, Radians, Panoptics, Naglers, Speers-Walers, and other
wider field units. Also, the negative lens used in most well-corrected
wider-field designs is there not to "widen the field" but to flatten it
(the negative field lens is known as a "Smyth" lens). With these
designs, the core elements correct for astigmatism fairly well, but at
the expense of introducing strong field curvature. Thus, the Smyth lens
is added out in front as a field flattener to compensate for this. The
distortion will depend on the exact design of the eyepiece, but most
eyepieces have at least some distortion.

Yes the field stop of a 1.25 inch eyepiece is limited to a max of
30mm.

Well, it had better be noticably smaller than 30mm I.D., as otherwise,
the walls of the barrel are going to be *awfully* thin (only 0.875mm
thick). 2mm is probably close to the minimum barrel thickness in 1.25"
O.D. barrel eyepieces to get standard filter threads to work, so the
maximum field stop diameter for an eyepiece filter threads would be
about 27.75mm assuming the field stop is the eyepiece barrel itself.
Clear skies to you.

--
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 *
**********************************************

On Mon, 02 Jan 2006 18:42:00 -0600, David Knisely
wrote:

+Unfortunately, this formula (like so many other amateur-derived
+formulae) does not yield results which are quite as close to reality as
+the somewhat simpler (and more commonly used) Eyepiece Field Stop formula:
+
*******

amateur derived formulae????? Where do you get off making that
statement?


+TFOV = (180/Pi)*(EFSD/Fl)
+

What are you calculating? The system or the apparent field of view of
the eyepiece. To me it looks as if you are deriving some equation for
the system. This is deduced from the FL as teh focal length of the
telescope. The formula that I am using calcualtes the apparent field
of view of the eyepiece. Not eyepiece and telelscope.

+where Pi is the usual 3.1415926... EFSD is the eyepiece field stop
+diameter, Fl is the telescope focal length, and * is short hand for the
+multiplication operation (the division operations in parethesis are to
+be done first). In most cases, the 180/Pi (the number of degrees in one
+Radian) can be just rounded to 57.3 (although for a few review tests, I
+have had to use 57.296 when doing accurate measurement comparisons).
********

Please. I do not need a lecture on mathematics.

james