Edmund 32mm RKE Wide-Angle Erfle 2" eyepiece
Larry Sayre
in a Newtonian in the F/4.5 to F/5 range? Is this eyepiece threaded for
2" filters? With its 42.7mm field stop, this might be a great candidate
for a 2" superwide in the 30-32mm category. As mentioned by someone
else in a different (current) thread, with its Edmund certified 42.7mm
field stop this eyepiece (advertised at only 68 degrees AFOV) would
actually show a bit wider TFOV of the sky than the 31mm Nagler with its
advertised 42mm field stop. By my calculations, the 32mm Edmund (31.8mm
actual) must actually weigh in at 76.94 degrees AFOV. Using the same
calculation method, I get 77.63 degrees AFOV for the 31mm Nagler.
Bottom line: Is it worth the asking price of $225 plus shipping?
AFOV method used: AFOV = FS/FL_eyepiece * 57.296
Lawrence Sayre
Starstuffed
from Orion performs excellently for me. I figured it does something like
72.5 degrees AFOV in my F4.5 system. It was only $129 when I bought it but
has gone up to $139. If you aren't satisfied, they will refund your money.
I am,
Starstuffed
Jon Isaacs
>calculation method, I get 77.63 degrees AFOV for the 31mm Nagler.
>Bottom line: Is it worth the asking price of $225 plus shipping?
>AFOV method used: AFOV = FS/FL_eyepiece * 57.296
Try the formula
AFOV=2*Atan(FS/(2*FLep)*57.3
which should be correct and you will get 67.75 degrees, close enough to the
advertised 68 degrees.
Not sure what the field stop on the Nagler might be but I calculate that the
maximum possible AFOV for a 31 mm 2 inch eyepiece is about 75 or so degrees,
based on a FS that is 1/10th of an inch less than 2 inches.
Jon Isaacs
David Knisely
> Try the formula
>
> AFOV=2*Atan(FS/(2*FLep)*57.3
Well, I'm afraid that this formula doesn't exactly work (for one thing,
I think you dropped a right-hand parenthesis somewhere). I have a 30mm
Ultrascopic eyepiece with a *measured* apparent field of about 52
degrees and a field stop diameter of 26.1mm, which yields a true
*measured* field in my 1410mm focal length telescope of 63.78 arc
minutes on the sky. I can't seem to get your formula to yield anything
close to reality, no matter where I put in the parenthesis. To get the
true field, the field stop method is by far the best way to get an
accurate idea of the true field an eyepiece may provide with a given
telescope. A person might "equate" the old AFOV/mag and Field Stop
formulae for true field of view and then solve for the apparent field.
However, the result would still be a little off of reality, as the old
TFOV = AFOV/Mag is only an approximation at best (although better than
some other formulae which are posted in various places on the
Internet). Even the formula mentioned earlier comes up a bit more than
two degrees under the measured true field of my 30mm real eyepiece. The
best method to determine the apparent field of view of an eyepiece is to
actually measure it, but barring that, the figures given by some
eyepiece manufacturers should be taken as approximate only. Clear skies
to you.
--
David W. Knisely KA0...@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
Jon Isaacs
>Well, I'm afraid that this formula doesn't exactly work (for one thing,
>I think you dropped a right-hand parenthesis somewhere).
I worked this formula in Mathcad, I think the confusion does come from the
parenthesis.
The basis for the formula is a bit of geometry I have seen posted here before
which uses the field stop and the focal length.
It does not seem to agree with your eyepieces, it does seem to agree with the
RKE in question.
AFOV=2*atan(Field stop/(2*focal length))
Answer is in radians.
Jon Isaacs
Brian Tung
> I worked this formula in Mathcad, I think the confusion does come from the
> parenthesis.
>
> The basis for the formula is a bit of geometry I have seen posted here before
> which uses the field stop and the focal length.
>
> It does not seem to agree with your eyepieces, it does seem to agree with the
> RKE in question.
>
> AFOV=2*atan(Field stop/(2*focal length))
This formula does not take into account any possible distortion. Two
eyepieces with the same focal length and field stop, but one with barrel
distortion and the other with pincushion distortion, will yield different
AFOVs, but this formula will give the same number for both.
It's conceivable that it's true for orthoscopic eyepieces (by definition?)
but I haven't given it any serious thought.
Brian Tung <br...@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
Larry Sayre
> Try the formula
>
> AFOV=2*Atan(FS/(2*FLep)*57.3
>
> which should be correct and you will get 67.75 degrees, close enough to the
> advertised 68 degrees.
>
> Not sure what the field stop on the Nagler might be but I calculate that the
> maximum possible AFOV for a 31 mm 2 inch eyepiece is about 75 or so degrees,
> based on a FS that is 1/10th of an inch less than 2 inches.
>
> Jon Isaacs
>
The field stop of the 31mm Nagler is defined by Tele Vue (on their web
site) to be exactly 42mm. Either the Edmund 32mm eyepiece is 77 degree
AFOV while being advertised as 68 degree, or the Nagler 31mm is about 69
degree AFOV while being advertised as 82 degree. I believe my method
(derived from a simple substitution into the Al Nagler provided TFOF =
FS/FL_scope * 57.296) is the better of the two.
Lawrence Sayre
Jon Isaacs
>site) to be exactly 42mm. Either the Edmund 32mm eyepiece is 77 degree
>AFOV while being advertised as 68 degree, or the Nagler 31mm is about 69
>degree AFOV while being advertised as 82 degree. I believe my method
>(derived from a simple substitution into the Al Nagler provided TFOF =
>FS/FL_scope * 57.296) is the better of the two.
>
>Lawrence Sayre
I am not prepared to say which is the better of the two. I certainly offer none
at this time. There doesn't seem to be any supporting analysis for either
formula. And one thing to consider is Al Nagler's formulas are likely to apply
to his eyepieces, he may even be presenting the fieldstop diameter as an
"apparent field stop", ie the field stop that when used with his formula gives
the correct AFOV.
I think that Brian Tung has some insight into this matter and he said that
different types of distortion in the eyepiece need to be accounted for. So I
think it is possible that two eyepieces with the same focal length and field
stop diameter can have different AFOV's.
Looking in the Amateur Astronomer's Handbook, I found no formula but rather
several ways to measure the AFOV/TOFV.
My "guess" is that there is no simple method that accurately computes the AFOV
from the focal length and the field stop diameter, considering the complexity
of the optical path through the glass, and the issue of where the field stop is
placed (at the focal plane??).
So, I have come to the conclusion that David Knisely is right when he says that
this is something that needs to be directly measured and if that is not
possible then the manufacturer's specification is the best guideline.
But I ask this question, is the really called an RKE .... Erfle? I was under
the impression that RKE stood for Reverse Kellner Eyepiece, so it seems a bit
odd to have Reverse Kellner Eyepiece that is an Erfle...
jon isaacs
Barry Simon
> How well does the 2" Edmund 32mm RKEŽ Wide-Angle Erfle Eyepiece perform
> in a Newtonian in the F/4.5 to F/5 range?
This is a nice eyepiece. Two of my friends have them. They are old
however, dating from the late 70's/early 80's. It has cemented
elements and both eyepieces that I am familiar with have started to
spiderweb as the cement has started breaking down. One of the guys
asked about a replacement and found that it is no longer available. I
assume then that your only source for the eyepiece will be used. I
would inspect carefully for element separation and I would suspect
that as it, at least in my experience, is a common occurence, may be
an issue in the future with whatever you find.
Barry Simon
TMBack
>Erfle? I was under the impression that RKE stood for
>Reverse Kellner Eyepiece, so it seems a bit odd to have
>Reverse Kellner Eyepiece that is an Erfle...
>
>jon isaacs
RKE is a trade name for all the Edmund eyepieces,
and Barlow that Dr. David Rank designed.
Thomas Back
Peter R Hobson
This eyepiece (or at least one with the same name) is clearly still
available according to Edmund Optics Catalogue 2002. It has part code
NT41-347 and listed as costing (in USA $) $225.00.
Here is its spec:
Total Length (glass only) 54.34mm, Total Length (housing) 65mm
Barrel OD 32mm
EFL 31.8mm, BFL 8.85mm
Eye Relief 18mm, Field Stop 42.7mm
Apparent Field 68°, Distortion (0.7 field) 7%
Spot Size (on axis) (0.7 Field)
0.0006mm 0.005mm
Lateral Color (0.7 field) = 0.05mm
P Hobson
>
> Larry Sayre <lsa...@apk.net> wrote in message news:<ap22tv$cs0$1...@plonk.apk.net>...
> > How well does the 2" Edmund 32mm RKE® Wide-Angle Erfle Eyepiece perform
> > in a Newtonian in the F/4.5 to F/5 range?
>
>SNIP ---
> spiderweb as the cement has started breaking down. One of the guys
> asked about a replacement and found that it is no longer available. I
> assume then that your only source for the eyepiece will be used. I
>SNIP ---
> Barry Simon
>
>
********************************************************
Dr Peter R Hobson
Reader, Department of Electronic and Computer Engineering
Brunel University, Uxbridge UB8 3PH G.B.
Al M
I have had the Edmunds 32mm Erfle for years. I preferred it over the
U.O. 40mm MK70 Koenig. It functions well in my MN56 (f/6). It is not
threaded to accept filters, so I place it into an extension tube and
place the filter there. It is a much higher quality than the other
Erfles out there and also more expensive.
Al M
joni...@aol.com (Jon Isaacs) wrote in message news:<20021021200504...@mb-mv.aol.com>...
Starstuffed
the "K" in "RKE" stood for kellner.
Starstuffed
Jon Isaacs
>RKE is a trade name for all the Edmund eyepieces,
>and Barlow that Dr. David Rank designed.
Thomas:
Maybe you could comment on the more significant issue here, the relationship
between the field stop diameter, the focal length and the apparent field of
view.
Jon Isaacs
Mike McJimsey
caveats: it's not threaded for filters (already mentioned by someone else) and
it needs quite a bit of out-travel on the focuser because the focal plane is
just beyond the bottom of its barrel. I usually have to pull it out of the
focuser a bit to make it easier to focus. Another person's suggestion of adding
an extension to allow for filters would also alleviate the problem with the
location of the focal plane. Unfortunately, I haven't had a chance to try it
with any scopes faster than f/6. However...
A while ago I downloaded the optical prescription of this eyepiece from the
Edmund Optics web site (along with the prescriptions for all of their other RKE
eyepieces) so I could ray trace it. It is remarkably well corrected
(particularly on axis) and should be able to handle f/4.5 just fine. The
on-axis spot size at f/4.5 is less than 1 arcminute, which is generally regarded
as the angular resolution limit of the human eye under good conditions. The
spot sizes at 10, 20, and 30 degrees off axis are 2, 7, and 22 arcminutes.
These spot sizes are only about 35% larger than at f/6 and less than twice those
at f/8. To compare, for a well-designed Plossl at f/4.5 (like Tele-Vue's), the
spot sizes at 10, 20 and 25 degrees off axis are about 1, 5 and 8 arcminutes.
The main off-axis aberration is astigmatism, as it is with most well-corrected
non-Nagler type eyepieces.
The story about field of view is kind of interesting. For the specified 68
degree AFOV, the true field is actually 37.6mm in the focal plane. With the
42.7mm field stop (which is actually the retaining ring for the last lens
element), the AFOV is closer to 77 degrees. This might explain some of the
discrepancies with the formulas used by others. By the way, the 7% distortion
spec shown below applies to 70% of a 37.6mm field (i.e., a 26.3mm field). It
increases to 9% at 70% of a 42.7mm field, and to 18% at the full 42.7mm field.
All distortion is pincushion, which makes the AFOV larger than what most
equations would predict.
Kindest regards,
Mike McJimsey
Peter R Hobson wrote:
>
> Dear Barry
>
> This eyepiece (or at least one with the same name) is clearly still
> available according to Edmund Optics Catalogue 2002. It has part code
> NT41-347 and listed as costing (in USA $) $225.00.
>
> Here is its spec:
>
> Total Length (glass only) 54.34mm, Total Length (housing) 65mm
> Barrel OD 32mm
> EFL 31.8mm, BFL 8.85mm
> Eye Relief 18mm, Field Stop 42.7mm
> Apparent Field 68°, Distortion (0.7 field) 7%
> Spot Size (on axis) (0.7 Field)
> 0.0006mm 0.005mm
>
> Lateral Color (0.7 field) = 0.05mm
>
> P Hobson
>
TMBack
Mike McJimsey answered that question as well as I
could.
Thomas Back
David Knisely
> I worked this formula in Mathcad, I think the confusion does come from the
> parenthesis.
>
> The basis for the formula is a bit of geometry I have seen posted here before
> which uses the field stop and the focal length.
>
> It does not seem to agree with your eyepieces, it does seem to agree with the
> RKE in question.
>
> AFOV=2*atan(Field stop/(2*focal length))
>
> Answer is in radians.
No, the answer is in degrees unless you have your calculator set up for
output in radians, which most probably aren't. Even then, it doesn't
agree precisely with *any* of my eyepieces, and that indicates that
something is seriously wrong. The actual measured data on my eyepieces
is:
____Eyepiece___________________AFOV_______EFSD____Your-AFOV___% dev.__
30mm Ultrascopic (Orion) 52.3 deg. 26.08mm 47.0 deg. -10.1%
27mm Kellner (Jaegers) 52.5 deg. 25.38mm 50.3 deg. -4.2%
24mm Koenig (University Opt.) 59.6 deg. 23.98mm 53.1 deg. -10.9%
20mm Celestron Plossl 51.7 deg. 17.25mm 46.7 deg. -9.7%
15mm Ultrascopic (Orion) 58.3 deg. 14.40mm 51.3 deg. -12.0%
14mm Meade Ultrawide 83.1 deg 20.2mm* 71.6 deg. -13.8%
10mm Ultrascopic (Orion) 48.7 deg. 8.32mm 45.2 deg. -7.2%
10mm Celestron Plossl 48.6 deg. 8.00mm 43.6 deg. -10.3%
6.4mm Super Plossl (Meade) 48.7 deg. 5.31mm 45.1 deg. -7.4%
6mm Orthoscopic (Brandon) 44.2 deg. 4.62mm 42.1 deg. -4.8%
Your formula consistently *underestimates* the apparent field of view by
an average of about 9 percent (greatest deviation was nearly 14% under
the true figure). The fact that it is underestimating the values for
apparent field (sometimes by significant amounts) indicates that there
is still something wrong with your formula. Even the original formula
given earlier by the gentleman (AFOV = 57.3*EFSD/EFL) may be somewhat
closer to reality than the one you cite. Again, geometric arguments are
approximate only, so the best way to determine apparent field of view is
to actually measure it. In any case, true field of view is the factor
most observers are interested in, and for getting a good idea of what to
expect, the field stop method is the one of choice here. Clear skies to
halfro
Larry Sayre
> One of the guys asked about a replacement and found that it is no longer available.
>
> Barry Simon
It is most definately still available from Edmund Optics (though it is
no longer available from Edmund Scientific). See:
http://www.edmundoptics.com/IOD/DisplayProduct.cfm?productid=2074
PS: The regular 1.25" RKE's are still available at Edmund Optics also.
Eyepieces are a bit hard to find from the http://www.edmundoptics.com
home page, but just do a search for "eyepiece" and you are on your way.
I believe Edmund Optics is now a separate entity from Edmund Scientific.
Lawrence Sayre
Unknown
>>Using the same
>>calculation method, I get 77.63 degrees AFOV for the 31mm Nagler.
>
>>Bottom line: Is it worth the asking price of $225 plus shipping?
>
>>AFOV method used: AFOV = FS/FL_eyepiece * 57.296
>
>Try the formula
>
>AFOV=2*Atan(FS/(2*FLep)*57.3
>*****************************************
Actually it is 6/PI* ARCSIN((FS/FLep/2) in degrees.
6/PI is equal to about 1.9099
This holds for eyepiece design that meets orthoscopy, that is
rectilenear distortion and angular magnification distortion are fully
corrected.
Thus when the field stop equals the focal length the apparent field of
view is one radian. Sound familiar?
Mathcad gives the answers in radians, then you multiple radians by
57.3 to get degrees.
********************************************************
>which should be correct and you will get 67.75 degrees, close enough to the
>advertised 68 degrees.
>
>Not sure what the field stop on the Nagler might be but I calculate that the
>maximum possible AFOV for a 31 mm 2 inch eyepiece is about 75 or so degrees,
>based on a FS that is 1/10th of an inch less than 2 inches.
>
*******************************
The 31 Type 5 Nagler has a 42 mm Eye Stop. This yields a calculation
of about 81.4 degrees. Published specs is 82 degrees. Therror in the
formula is due to the fact that the Nagler is not fully corrected for
rectilinear distortion and angualr magnification distortion. It is
almost corrected for both. The 49.5mm field stop and 31mm focal
length of your example yields an apparent field of view of 101.1
degrees. While this is not impossible, I seriously doubt that
orthoscopy can be maintained with such a wide field stop and/or focal
length as you describe. This eyepiece is going to have some major
problems. Bring the field stop down to 33 to 35 mm range and it should
preform well.
In eyepiece design you cannot get something for nothing. If you want
wide field view you give up distortion. If you want fully corrected
eyepieces then you give up apparent field of view. It is a matter of
trade offs and what one can tolerate visually.
james
>Jon Isaacs
>
>
David Knisely
> Actually it is 6/PI* ARCSIN((FS/FLep/2) in degrees.
>
> 6/PI is equal to about 1.9099
>
> This holds for eyepiece design that meets orthoscopy, that is
> rectilenear distortion and angular magnification distortion are fully
> corrected
Again, you need to put parentheses in the correct places. Is the first
operation to divide the focal length of the eyepiece by two and then
take the field stop an divide it by that number, or to divide the
quotient FS/FLep by two? If it is the former (ie: (FS/(FLep/2)), then
the results for my 30mm Ultrascopic make no sense, as the inverse sine
of a number greater than one is undefined. If it is the latter
((FS/Flep)/2), then the results are real, although still only
approximate for Your AF Formula: AFOV = (6/PI)*ARCSIN((FS/FLep)/2). The
original "Old" formula was, I believe, AFOV = (180/Pi)*FS/Flep.
____Eyepiece_______________Apparent Field___FS___ Your AF Form.__Old
Form._
30mm Ultrascopic (Orion) 52.3 deg. 26.08mm 49.2 deg. 49.8
deg.
27mm Kellner (Jaegers) 52.5 deg. 25.38mm 53.5 deg. 53.9
deg.
24mm Koenig (University Opt.) 59.6 deg. 24.04mm 57.4 deg. 57.4
deg.
20mm Celestron Plossl 51.7 deg. 17.23mm 48.7 deg. 49.4
deg.
15mm Ultrascopic (Orion) 58.3 deg. 14.40mm 54.8 deg. 55.0
deg.
14mm Meade Ultrawide 83.1 deg 20.2mm* 88.2 deg. 82.7
deg.
10mm Ultrascopic (Orion) 48.7 deg. 8.32mm 46.9 deg. 47.7
deg.
6.4mm Super Plossl (Meade) 48.7 deg. 5.31mm 46.8 deg. 47.5
deg.
6mm Orthoscopic (Brandon) 44.2 deg. 4.62mm 43.2 deg. 44.1
deg.
Looking at the deviations from the measured apparent fields of my
eyepiece set:
____Eyepiece____________Measured A.F.____Your AF__% Dev.__Old Form._%
Dev.
30mm Ultrascopic (Orion) 52.3 deg. 49.2 deg. -5.9% 49.8 deg.
-4.8%
27mm Kellner (Jaegers) 52.5 deg. 53.5 deg. +1.9% 53.9 deg.
+2.7%
24mm Koenig (University Opt.) 59.6 deg. 57.4 deg. -3.7% 57.4 deg.
-3.7%
20mm Celestron Plossl 51.7 deg. 48.7 deg. -5.8% 49.4 deg.
-4.4%
15mm Ultrascopic (Orion) 58.3 deg. 54.8 deg. -6.0% 55.0 deg.
-5.7%
14mm Meade Ultrawide 83.1 deg 88.2 deg. +6.1% 82.7 deg.
-0.5%
10mm Ultrascopic (Orion) 48.7 deg. 46.9 deg. -3.7% 47.7 deg.
-2.1%
6.4mm Super Plossl (Meade) 48.7 deg. 46.8 deg. -3.9% 47.5 deg.
-2.5%
6mm Orthoscopic (Brandon) 44.2 deg. 43.2 deg. -2.3% 44.1 deg.
-0.2%
-------------------------------------------------------------------------
Mean Deviations: (Your formula) -2.6% (Original formula)
-2.4%
Maximum Individual Deviations: (Your Formula) +6.1% (Original form.)
-5.7%
It appears that the original formula, in addition to being somewhat
simpler, is, on average, slightly more accurate than yours, although not
enormously so. For these reasons, the original AFOV~= 57.3*(FS/FLep) is
probably good enough for general purposes (probably within 10 percent or
so), although again, the true field of view is probably a more important
for the amateur. Clear skies to you.
Unknown
(x/y)/2 = x/y*1/2 = x/(2*y) = x/2 * 1/y = (x/2)/y
(x/y)/2 does not equal x/(y/2) and the rest is false.
I did forget a parenthesis in the arcsine portion. The formula should
have read
AFOV = (6/PI) * ARCSIN(( FS/FLep)/2) also the following is true
AFOV = (6/PI) * ARCSIN((FS/2)/FLep)
This works for fields stops greater than or less than the focal length
of the eyepiece. This is derived from the other.
proof
30mm Ulstrascopic with 26.08 FS
26.08/30 = 0.869333 0.869333/2= 0.434777
26.08/2 = 13.04 13.04/30 = 0.434777
ARCSIN(0.434777) = ARCSIN(0.434777) = 25.77
25.77*1.9099 = 49.22 degrees.
QED
Now this is the AFOV for orthoscopy. It may or maynot be the total
field of view of the eyepiece. This is a prediction tool only. This
formula accepts that there is no distortion in the total FOV.
Sometimes refered to as the geometric field of view. If distortion is
acceptable then the above formula is modified considerably.
the term FS/FLep is multiplied by the quantity of 1 plus the error
times the tangent cubed of theta. Where error is the positive error
distortion in the design and theta is the geometric field of view.
AFOV = (6/PI) * ARCSIN((FS/FLep)*(1 + e*tan3(theta)/2)
this will always lead to a wider field of view when distortion is
built into the design. Distortion = 0 then the formula reverts to the
more simpler version.
Opening the FS will open the field of view but invite distortion.
Manufacturers in an attempt to make their product appear better to the
consumer will sacrifice some distortion to open the field of view. The
question becomes how much distortion is acceptable? That depends on
the viewer.
Yes in truth one can calculate the field of view. The final test is in
the viewing. How much field of view does a particular eyepiece present
and how much is truly usable? Only using it will tell.
I prefer the geometric field of view equation as it will predict the
useful field of view of the eyepiece. It also lets me know about how
much distortion the manufacturer is allowing in the design to gain
wider field of view. To me wider field of view is no good if it is not
usable.
regards
james
On Tue, 29 Oct 2002 06:42:07 GMT, David Knisely <KA0...@navix.net>
wrote:
Larry Sayre
Putting this all together, and applying it to the subject eyepiece, I'm
still waiting for someone to tell me if the latest edition of the Edmund
Optics 32mm REK Wide-Angle Erfle 2" eyepiece with its Edmund declaired
42.7mm field stop will show more sky (TFOV) than the 31mm Nagler Type 5
with its Tele Vue declaired 42mm field stop. Has anyone done a head to
head? $225 vs $680 means that for many the Edmund may be just the
ticket for wide field viewing.
Lawrence Sayre
Unknown
While the view in the 31 Nagler is beautiful, I can not see the
justification for a $620 price tag. I have looked through one and it
is wide. The 32 erfle from Edmund I have not. So I can not give you a
side by side comparison.
james
On Tue, 29 Oct 2002 19:55:45 -0500, Larry Sayre <lsa...@apk.net>
wrote:
Dave Mitsky
>
> But I ask this question, is the really called an RKE .... Erfle? I was under
> the impression that RKE stood for Reverse Kellner Eyepiece, so it seems a bit
> odd to have Reverse Kellner Eyepiece that is an Erfle...
>
> jon isaacs
What I've heard is that RKE stands for Rank Kellner for Edmund or Rank
Kellner Eyepiece.
Dave Mitsky
Larry Sayre
>
> What I've heard is that RKE stands for Rank Kellner for Edmund or Rank
> Kellner Eyepiece.
>
> Dave Mitsky
Dave,
I believe this is correct. Why Edmund chose to call their 2" 32mm 6
element Erfle (3 cemented 2 element achromats in 3 groups) an RKE is a
mystery. Feeding off of the success of the 1.25" RKE series (real
RKE's) is always a possibility.
Lawrence Sayre
ralphjunius
> Putting this all together, and applying it to the subject eyepiece, I'm
> still waiting for someone to tell me if the latest edition of the Edmund
> Optics 32mm REK Wide-Angle Erfle 2" eyepiece with its Edmund declaired
> 42.7mm field stop will show more sky (TFOV) than the 31mm Nagler Type 5
> with its Tele Vue declaired 42mm field stop. Has anyone done a head to
> head? $225 vs $680 means that for many the Edmund may be just the
> ticket for wide field viewing.
>
> Lawrence Sayre
Lawrence, I have both eyepeices. They have about the same TFOV. The
31mm Terminagler is a much better corrected eyepiece at the edge of
the field in fast scopes. The big Edmund 32mm does a very good job in
my SCT's at f/10. It does what I consider ok in my 10" f/5.6 dob, but
nowhere near the edge correction of the Terminagler at f/5.6. At
f/4.5 and below the Edmund has a lot of coma/astigmatism.
The big Edmund has more eye relief and is more comfortable and easy to
use than the Terminagler; its also a lot lighter. To make the Edmund
more parfocal with your other eyepieces it really needs a 2" tube
extension because the field lens is at the end of the 2" chrome
barrel. Clear skies!
Matt Kriebel
wrote:
At this point RKE is just a name for Edmund-designed 1.25" eyepieces. I
doubt very many in Edmund Insutrial know what RKE stands for either. It
sorta lost its meaning.
--
Matt Kriebel * Oooops!
mkr...@cruzio.com *
*********************************************************************
Now 90% closer to 80% of the world's kooks!
Unknown
the eyepiece itself? Erfle design should be workable down to scopes
with speeds of f/4. The design of the Nagler type 5 may help in
correcting coma in fast scopes. Wonder how well the 31 Nagler works in
very slow scopes (f/15 & >)?
james
On 30 Oct 2002 04:08:19 -0800, ralph...@cox.net (ralphjunius)
wrote:
Unknown
The RKE eyepiece is a modification of the Kellner eyepiece. Instead of
the achromat being the eye lens it is the field lens. The eye lens is
a double convex. This is a reversal of the original Kellner design.
Edmund had Dr. David Rank design the eyepiece for them thus the name
RKE. He also designed the Erfle eyepiece which is a modificationof the
origianl Eerfle design. The Erfle design was a 2-1-2 where the double
lens was the achromat and the single lens was a double convex lens. It
was later modified for a 2-2-2 system of three achromats. This allowed
a wider field of view. This is the design of the RKE Erfle sold by
Edmund Optics (2-2-2).
mark d. doiron
> > >
> > > What I've heard is that RKE stands for Rank
> > > Kellner for Edmund or Rank Kellner Eyepiece.
> > >
"Matt Kriebel" <mkriebNOS...@cruzio.com> wrote in message
news:mkriebNOSPAMDAMMIT-3...@cnews.newsguy.com...
> At this point RKE is just a name for Edmund-designed 1.25"
> eyepieces. I doubt very many in Edmund Insutrial know what
> RKE stands for either. It sorta lost its meaning.
Matt et al--
rke stands for Rank-modified Kellner eyepiece. Dr David Rank designed
this eyepiece type.
clear, dark skies--
mark d.
David Knisely
> Would not the coma in an f/4.5 be more due to the primary mirror than
> the eyepiece itself? Erfle design should be workable down to scopes
> with speeds of f/4. The design of the Nagler type 5 may help in
> correcting coma in fast scopes. Wonder how well the 31 Nagler works in
> very slow scopes (f/15 & >)?
The coma is from the primary mirror, although there may be a few
eyepieces that produce a little coma as well. Most of what appears to
disort star images at the edge of the field with short focus telescopes
is astigmatism induced by the eyepiece and not coma. Most eyepieces
simply cannot easily handle the f/4 or f/4.5 light cones. The Naglers
correct fairly well for astigmatism, but not for coma caused by the
primary mirror. To deal with primary mirror coma, a "coma corrector" is
needed, such as Tele Vue's Paracorr. I have used the 31 Nalger, and it
works pretty well, although it does have some pin-cushion distortion. I
used it on a number of short-focus larger aperture Newtonians, and the
star images were pretty sharp (at least with the coma corrector
installed). At very slow f/ratios, you probably wouldn't need the 31
Nagler, as less expensive moderate to wide field eyepiece would do
fairly well. Clear skies to you.
Larry Sayre
> Lawrence, I have both eyepeices. They have about the same TFOV. The
> 31mm Terminagler is a much better corrected eyepiece at the edge of
> the field in fast scopes. The big Edmund 32mm does a very good job in
> my SCT's at f/10. It does what I consider ok in my 10" f/5.6 dob, but
> nowhere near the edge correction of the Terminagler at f/5.6. At
> f/4.5 and below the Edmund has a lot of coma/astigmatism.
>
> The big Edmund has more eye relief and is more comfortable and easy to
> use than the Terminagler; its also a lot lighter. To make the Edmund
> more parfocal with your other eyepieces it really needs a 2" tube
> extension because the field lens is at the end of the 2" chrome
> barrel. Clear skies!
Thanks for this report!!! It looks like my F/5.23 Dob will have to wait
for a Terminagler. :-( Good to hear that the TFOV's are essentially
the same, and that for F/10 type scopes many would save a bunch by
buying the Edmund eyepiece over the Terminagler.
Lawrence Sayre
Matt Kriebel
Uh, Mark, I know that. Please re-read my post and se what I was getting
at.
mark d. doiron
> > rke stands for Rank-modified Kellner eyepiece.
> > Dr David Rank designed this eyepiece type.
"Matt Kriebel" <mkriebNOS...@cruzio.com> wrote in message
news:mkriebNOSPAMDAMMIT-6...@cnews.newsguy.com...
> Uh, Mark, I know that. Please re-read my post
> and se what I was getting at.
Matt--
sorry. when i saw that no one had put the correct meaning of rke, i
assumed that all in the thread didn't know. as for whether folks at
Edmund Scientifics know that the rke ep is named for one of their
optical engineers, i couldn't say.
Dave Mitsky
> Would not the coma in an f/4.5 be more due to the primary mirror than
> the eyepiece itself? Erfle design should be workable down to scopes
> with speeds of f/4. The design of the Nagler type 5 may help in
> correcting coma in fast scopes. Wonder how well the 31 Nagler works in
> very slow scopes (f/15 & >)?
>
>
> james
You're correct about the coma in an f/4.5 scope resulting from the
mirror. In fact, astigmatism, not coma, is the primary field edge
aberration in most eyepieces, especially ones with wide-fields.
The Erfle design is not well-corrected for astigmatism and will not
function well in fast telescopes (see
http://www.roboticobservatory.com/jeff/optics/optical_definitions.htm).
Nagler eyepieces have no coma correcting properties although a few
such designs, the University Optics Pretoria for example, have been
marketed in the past.
Dave Mitsky