----------------------------

#1397 Feb 16 4:25 AM

Hello friends, I need some advice about this question...

It has any sense to buy an ep with a field stop bigger than the size

of the secondary, let.s say a mak or mak newt... probably high

vigneting will appear, am I right?... if this thoughts are correct a

practical limit for wide field ep, plus the exit pupil factor, will

be the size of the secodary mirror of the compound system. I.m

considering a mak newt but they use to have secondaries about 30mm

and for instance the new panoptics have 48mm field stops.

Thanks for help and good luck.

Alejandro.

----------------------------

#1399 Feb 16 8:00 AM

It has any sense to buy an ep with a field stop bigger than the

size > of the secondary, let.s say a mak or mak newt... probably high

> vigneting will appear, am I right?... if this thoughts are correct

a > practical limit for wide field ep, plus the exit pupil factor, will

> be the size of the secodary mirror of the compound system. I.m

> considering a mak newt but they use to have secondaries about 30mm

> and for instance the new panoptics have 48mm field stops.

A quick and dirty method is to remove the eyepiece, position your eye

at the eyepiece location, first try on optical axis, can you see the

enire mirror(primary)? now move your eye 24mm to one side (48/2=24mm)

can you still see the entire primary? if you can not, then you will

have vignetting with that particular eyepiece.

Many MN's that are optimized for planetary performance come with very

small secondaries, only an eyepiece with a very small field stop can

capture the entire light from its primary.

Thanks,

Vahe

----------------------------

#1401 Feb 16 12:53 PM

- Thanks friend, but there must be numeric way to know this things...

Alejandro.

-- In tec-scopes@yahoogroups

wrote: > > It has any sense to buy an ep with a field stop bigger than the

> size

> > of the secondary, let.s say a mak or mak newt... probably high

> > vigneting will appear, am I right?... if this thoughts are

correct > a

> > practical limit for wide field ep, plus the exit pupil factor,

will > > be the size of the secodary mirror of the compound system. I.m

> > considering a mak newt but they use to have secondaries about

30mm > > and for instance the new panoptics have 48mm field stops.

>

> A quick and dirty method is to remove the eyepiece, position your

eye > at the eyepiece location, first try on optical axis, can you see

the > enire mirror(primary)? now move your eye 24mm to one side

(48/2=24mm) > can you still see the entire primary? if you can not, then you will

> have vignetting with that particular eyepiece.

> Many MN's that are optimized for planetary performance come with

very > small secondaries, only an eyepiece with a very small field stop

can > capture the entire light from its primary.

>

> Thanks,

> Vahe

----------------------------

#1404 Feb 16 3:47 PM

--- In tec-scopes@yahoogroups

sptakfun@y...> wrote: > Hello friends, I need some advice about this question...

> It has any sense to buy an ep with a field stop bigger than the size

> of the secondary, let.s say a mak or mak newt... probably high

> vigneting will appear, am I right?... if this thoughts are correct a

> practical limit for wide field ep, plus the exit pupil factor, will

> be the size of the secodary mirror of the compound system. I.m

> considering a mak newt but they use to have secondaries about 30mm

> and for instance the new panoptics have 48mm field stops.

> Thanks for help and good luck.

> Alejandro.

Hi Alejandro,

the exit pupil of telescope working at low power should match the width of=

our

eye pupil openings in the dark and is appr. 6mm for an average person (coul=

d

be slightly less or more). To find out what is the focal length of the low=

est

power eyepiece is needed just multiply your scope f/ratio by 6.

For example, for APO140 f/7.0 the focal length will be 42mm .

Checking is simple: with F=980mm , this eyepicece gives power 23.3X, and

140mm / 23.3 gives 6mm exit pupil.

For MCs it is harder to find the low power eyepiece since we are talking ab=

out

longer f/ratios and for f/12 we would need 72mm FL eyepiece!.

About vignetting.

The vignetting in MCs (and in MNs with small diagonals) starts immidiatelly=

near the center of FOV, since most MCs designed with smalles possible CO. =

The central obstruction affects the image quility much more than small ligh=

t

loss causing by vignetting. How small is it? - we calculated the light loss=

in

MC, and for f/12, f15ratio it is with in 0.2-0.3 magnitude! - not easy d=

etectable

for our eye (but CCD will catch shuch vignetting).

On practice even bigger light loss is acceptable; we were using the 6" MAK =

with 31mm Nagler type 5 or Leitz 30mm (+ AP Maxbright diagonal) with no

problem.

Yuri

----------------------------

#1410 Feb 17 6:13 AM

- Thanks Yuri, so with a bigger field stop than the secondary you can

see a field of view limited by the secondary not limited by the field

stop, and the vigneting depends on the design of the telescope, not

depends on the field stop... the true field of view depends on the

field stop so the maximum true field of view depends on the diameter

of the secondary (this determines the max field stop "usefull"), in

the case of a 25mm secondary it has no sense to get the max true

field of view with a 30mm field stop ep...you use a 25mm field stop

ep to get what you want. Other thing is the magnification you get, in

my opinion, wide magnified views are very nice, nagler type wiews.

What I mean with this question is if it.s well worth to buy a 2" ep

with a MN when you can get the same field of view with a 1.25",

taking into account that you focus your attention in the field stop

to select the ep? The magnification is a secondary factor...

Thanks again.

Alejandro.

-- In tec-scopes@yahoogroups

wrote: > --- In tec-scopes@yahoogroups

> sptakfun@y...> wrote:

> > Hello friends, I need some advice about this question...

> > It has any sense to buy an ep with a field stop bigger than the

size > > of the secondary, let.s say a mak or mak newt... probably high

> > vigneting will appear, am I right?... if this thoughts are

correct a > > practical limit for wide field ep, plus the exit pupil factor,

will > > be the size of the secodary mirror of the compound system. I.m

> > considering a mak newt but they use to have secondaries about

30mm > > and for instance the new panoptics have 48mm field stops.

> > Thanks for help and good luck.

> > Alejandro.

>

> Hi Alejandro,

> the exit pupil of telescope working at low power should match the

width of= > our

> eye pupil openings in the dark and is appr. 6mm for an average

person (coul= > d

> be slightly less or more). To find out what is the focal length of

the low= > est

> power eyepiece is needed just multiply your scope f/ratio by 6.

> For example, for APO140 f/7.0 the focal length will be 42mm .

> Checking is simple: with F=980mm , this eyepicece gives power

23.3X, and > 140mm / 23.3 gives 6mm exit pupil.

> For MCs it is harder to find the low power eyepiece since we are

talking ab= > out

> longer f/ratios and for f/12 we would need 72mm FL eyepiece!.

> About vignetting.

> The vignetting in MCs (and in MNs with small diagonals) starts

immidiatelly= >

> near the center of FOV, since most MCs designed with smalles

possible CO. = >

> The central obstruction affects the image quility much more than

small ligh= > t

> loss causing by vignetting. How small is it? - we calculated the

light loss= > in

> MC, and for f/12, f15ratio it is with in 0.2-0.3 magnitude! -

not easy d= > etectable

> for our eye (but CCD will catch shuch vignetting).

> On practice even bigger light loss is acceptable; we were using the

6" MAK = >

> with 31mm Nagler type 5 or Leitz 30mm (+ AP Maxbright diagonal)

with no > problem.

> Yuri