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what is the possible lowest power for 14 inch F4.6

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#1 jtrevino

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Posted 30 January 2013 - 07:34 PM

Not sure If I should post this here or in the EP forum. I soon will be getting an Orion XX14G and would like to know if there is a limitation on how low of a power I can have. The scope has a FL of 1650mm. The Orion website claims the lowest power is 47X with the supplied 35mm EP. I'm just wondering if it is possible to go lower. Is there a point where you can't go any lower? I'm just thinking that I'd like to get an EP with the lowest possible magnification for wide views. :question:

thanks in advance!

#2 JIMZ7

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Posted 30 January 2013 - 07:55 PM

That would be the very lowest eyepiece you would use 35mm. Actually a 32mm would be the best choice. You have a f/4.6 scope. The largest eye pupil in millimeters that would not waste any light would be 7mm. Take 4.6 x 7mm would equal a 32mm eyepiece. Of course a 2" eyepiece has a large FOV than a 1.25" eyepiece. But I believe the 35mm will be fine on your scope.

Jim :dob:

#3 Mirzam

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Posted 30 January 2013 - 08:03 PM

The lowest magnification possible for any telescope has been traditionally determined by the human eye, with its maximum dilated pupil opening of about 7mm. The exit pupil of the telescope must be no bigger than this or some of the light (effective aperture) is lost.

The formula for exit pupil of a telescope is eyepiece focal length/primary f-ratio. So for your 35 mm eyepiece the exit pupil is 7.6 mm--a little too big. Note also that as you get older the eye dilates less so your ability to use low powers degrades.

All this being said, there is no real harm in using lower powers--you will get a wider fov and this may be great for viewing certain objects, such as the Pleiades. Just be aware that you are throwing some light away.

A final consideration is that very low powers used in conjunction with an obstructed telescope can cause the appearance of a shadow in the middle of the fov when looking at the Moon. It's a mystery to me why this only happens at very low powers but it does.

JimC

#4 gpelf

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Posted 30 January 2013 - 08:05 PM

It depends on your pupil size during dark adaption.
Younger folks typically have around a 7mm size, As we age (Like me) around 5mm, This will dictate your answer.
Go to Stellafanes website and in the mirror making area they have a telescope calculator where you can enter your scope parameters, including pupil size, and it will give you your answer.

Clear Skies,
Greg

#5 Mirzam

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Posted 30 January 2013 - 08:20 PM

Of course the other way to get wide field views without throwing any light away is to invest in a wide field eyepiece, such as a Nagler or Ethos. To make an appropriate choice figure out the eyepiece focal length that gives your telescope an exit pupil of 7 mm or less. As mentioned above this would be any eyepiece of 32 mm focal length or less.

Then divide the apparent fov of your candidate eyepiece by the magnification that your scope will yeild when using the eyepieces. This gives the "true" field of view.

For example, a 31 mm Nagler (which like all Naglers has an 82 degree apparent fov) would give a magnification of 53x. The true field of view would be 82/53 or 1.54 degrees--pretty nice!

Another example, a 21 mm Ethos (which like all Ethoi has a 100 degree apparent fov) would yeild 78.6x and a 1.27 degree true fov. The higher power may be advantageous despite the slightly narrower fov because the sky background will be reduced a bit.

In my own experience I rarely use very low powers anymore, even with non-goto scopes because of light pollution and the effectively brighter sky background.

JimC

#6 davidpitre

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Posted 30 January 2013 - 09:57 PM

It depends on your pupil size during dark adaption.

True. Your lowest useful power is mostly dictated by your pupil size at maximum dilation. You can use less magnification but the light from you eyepiece will be cut off by your eye so that you are not using what your 14" is designed to deliver.
Be careful about assumptions you hear about dilated pupil size. They can very greatly. I am 48 and my pupil size is less than 5.5mm.

#7 DJCalma

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Posted 30 January 2013 - 10:28 PM

Somewhere down the line try someone elses 35mm. or even 40 mm. eyepiece. See if you like it, then decide. It will not harm you or your scope to use an eyepiece with an exit pupil greater than 7mm.

#8 johnnyha

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Posted 30 January 2013 - 11:43 PM

The readily available 2" 55mm Televue Plossl will give you 28X.

#9 Jon Isaacs

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Posted 31 January 2013 - 06:08 AM

The readily available 2" 55mm Televue Plossl will give you 28X.


28x, a 12mm exit pupil and the same true field of view as 41mm Panoptic or a 38mm Orion Q-70. If one is fortunate enough to have pupils that dilate to 6 mm, the effective aperture is reduced from 14 inches to 7 inches, the central obstruction grows from 23% to 46% representing a dark spot of 2.75mm, it maybe start to become visible.

I would not recommend a 55mm eyepiece in a F/4.6 telescope, there is no upside over a ~40mm SWA and very little over a ~31mm UWA but the downsides are major.

Jon

#10 johnnyha

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Posted 31 January 2013 - 06:12 AM

Yeh it would be crazy to use in that scope, but it does give the lowest mag. :grin:

#11 Jon Isaacs

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Posted 31 January 2013 - 07:27 AM

Yeh it would be crazy to use in that scope, but it does give the lowest mag. :grin:


There even longer focal lengths available. Gary Russel sells an 85mm focal length 2 inch "Super-Plossl(Bottom of Page)".

That would provide 19x with a 18.5mm exit pupil and a 4.25 mm shadow, the effective aperture would be under 5 inches.

Not a good choice either.

Jon

#12 Cotts

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Posted 31 January 2013 - 07:32 AM

I would add that using Exit Pupils very close to that of your eye will enhance any irregularities in your eye lens such as astigmatism which lurk in the outer edges of your lens just like they do in mediocre telescope mirrors and lenses.

Stars may appear irregular, spiky etc. and you will wonder about the quality of your mirror.

There is a solution - the Dioptrix - from Televue which acts to remove your astigmatism. It works miracles when I use it on my 31 Nagler... My vote for the best $100 item in the hobby!!!

Dave

#13 Jarad

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Posted 31 January 2013 - 07:42 AM

I wouldn't think of it in terms of lowest mag, I would think in terms of widest TFOV.

The widest possible TFOV in a 2" focuser is with a 41mm Panoptic or 40mm Pentax or similar. A 55mm plossl is lower mag, but the same TFOV.

A 31 nagler or 30mm ES are a bit narrower, but come close.

If you don't use a paracorr, you could get:
41 Panoptic: 39.9x, 1.70 degree TFOV, 8.9mm exit pupil
35 Panoptic: 46.7x, 1.45 degree TFOV, 7.6mm exit pupil
31 Nagler: 52.8x, 1.55 degree TFOV, and 6.7mm exit pupil
21 Ethos: 77.9x, 1.28 degree TFOV, 4.6mm exit pupil

So without a paracorr, I would pick the 31 Nagler. It actually gives a wider TFOV than the 35 pan with a better exit pupil. The 21 Ethos gives up a bit of TFOV, but will also give you a darker sky background.

With a paracorr, that becomes:
41 Panoptic: 45.9x, 1.48 degree TFOV, 7.8mm exit pupil
35 Panoptic: 53.7x, 1.27 degree TFOV, 6.6mm exit pupil
31 Nagler: 60.7x, 1.35 degree TFOV, and 5.9mm exit pupil
21 Ethos: 89.6x, 1.12 degree TFOV, 4.0mm exit pupil

So the 35 Pan is useable with the paracorr as a cheaper option to the 31 Nagler (although it gives up a bit of field).

If your pupils can get to 7mm, I would get the 31 nagler without a paracorr or either the nagler or 35 Pan with the paracorr.

If your pupils only get to 5mm or less, I would look at the 21 Ethos, or similar focal length 100-degree design.

Jarad

#14 Jon Isaacs

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Posted 31 January 2013 - 08:38 AM

If your pupils can get to 7mm, I would get the 31 nagler without a paracorr



:waytogo:

I use the 31mm Nagler with a Paracorr and if I want a wider field of view, I temporarily remove the Paracorr.

The 31mm Nagler is just about perfect for a Newtonian in the F/4-F/5 range. It comes within 10% of the maximum possible true field of view and provides a reasonable exit pupil with excellent off-axis correction. The Paracorr takes it to another level.

Jon

#15 Thomas Karpf

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Posted 31 January 2013 - 08:49 AM

A final consideration is that very low powers used in conjunction with an obstructed telescope can cause the appearance of a shadow in the middle of the fov when looking at the Moon. It's a mystery to me why this only happens at very low powers but it does. JimC


The black dot is the secondary mirror (seen from the back).

It's because it's only at low powers that it's big enough to be noticed. If your scope has a 20% obstruction, than that black dot is 20% of the exit pupil size; not so visible when you have a high powered eyepiece (0.5mm exit pupil, 0.1mm black dot), but much more with a lowe powered eyepiece (7mm exit pupil, 1.4mm black dot). With compound scopes (SCT, MCT, RC, etc.), the obstruction percentage is greater and so the black dot is bigger.

#16 FirstSight

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Posted 31 January 2013 - 09:02 AM

The lowest magnification possible for any telescope has been traditionally determined by the human eye, with its maximum dilated pupil opening of about 7mm.


Actually, the loss of light (or inefficient use of aperture) from a low-power eyepiece whose exit pupil in a given scope exceeds the human eye pupil diameter is a relatively minor, usually inconsequntial effect, and not of itself a sufficient reason to avoid focal-lengths going below a given power in the scope. Typically, at low powers the viewing purpose is far more toward panoramic view of a wide true field, rather than for the sort of faint object detection or detailed resolution for which the more aperture, the more light, the better. There are, however a couple of more potentially consequential issues with going "too low" in power (i.e. using excessively long focal-length eyepieces) for a given scope:
1) What Cotts said above: exit pupils at or beyond the human eye pupil diameter will employ the often more imperfect outer portions of the eye's pupil, degrading the image with astigmatism, etc. Of course, YMMV tremendously on this factor; for some people, big problem, for others, no problem at all.
2) for REFLECTORS too large an exit pupil risks making the shadow of the secondary mirror visible in the center of the field of view, which will seriously degrade the image. This is also potentially true for any other telescope design involving any central obstructions, i.e. cadiotropic scopes. For REFRACTORS, this is a complete non-issue, since there is no central obstruction, it's all "clear" aperture.

NEVERTHELESS, as an informal rule-of-thumb for reflectors, it's still useful to use the 5mm to 7mm human eye pupil diameter as a guideline for determining the longest focal length (i.e. lowest power) which will probably still be a "safe" choice (remembering, however that if your eye pupils have lots of astigmatism toward the edges, you might be more comfortable sticking with a somewhat shorter focal-length/not quite as low-power limit than the maximum eye pupil diameter heuristic would otherwise indicate). FWIW, around 30mm, e.g. a 31T5 Nagler is a "safe" low-power focal-length for the overwhelming majority of scopes of all types in existence.

#17 Jon Isaacs

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Posted 31 January 2013 - 09:04 AM

A final consideration is that very low powers used in conjunction with an obstructed telescope can cause the appearance of a shadow in the middle of the fov when looking at the Moon. It's a mystery to me why this only happens at very low powers but it does. JimC


The black dot is the secondary mirror (seen from the back).

It's because it's only at low powers that it's big enough to be noticed. If your scope has a 20% obstruction, than that black dot is 20% of the exit pupil size; not so visible when you have a high powered eyepiece (0.5mm exit pupil, 0.1mm black dot), but much more with a lowe powered eyepiece (7mm exit pupil, 1.4mm black dot). With compound scopes (SCT, MCT, RC, etc.), the obstruction percentage is greater and so the black dot is bigger.


There is a second factor that comes into play. As Thomas says, the size of the shadow is proportional to the central obstruction and the exit pupil. With compound scopes, it's generally not such an issue because their slow focal ratios limit the size of the exit pupil and there for the size of the shadow.

Back to second factor: The moon is bright enough that it causes your pupil to contract to daylight levels. If you pupil is open to 7mm and the exit pupil is 7mm with a 1.75mm secondary shadow, you are not going to see the shadow.

If you eye is only open to 2.5 mm, then that 1.75mm shadow might represent a 70% CO and can easily seen.

Jon

#18 Dick Jacobson

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Posted 31 January 2013 - 09:11 AM

I think it's a very good idea to have two low-power eyepieces, one that has an exit pupil that exactly matches your eyes, the other with the widest possible field of view. I have a scope almost identical to yours (14" f/4.5) and my pupils are 5 mm, typical of a middle-aged or older person. I use a 22mm Nagler for bright wide-field views (pupil = 22/4.5 = 4.9 mm) and a 34mm for ultra-wide views. The 34mm loses some light because the exit pupil is too large (effectively reducing the scope to a 10") but is still very useful for finding objects and viewing very large nebulae or star clusters. For the ultra-wide eyepiece the main thing you want is a large Field Stop (40mm or larger for a 2" focuser).

#19 Mirzam

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Posted 31 January 2013 - 09:21 AM

:waytogo:

Thanks for the explanation!

JimC

#20 jtrevino

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Posted 31 January 2013 - 11:03 AM

All this great info!! You guys are truly AWESOME!! :bow: :bow: :bow:

So here is what I was thinking, the xx14g will come with a 35mm 2in EP with an Apparent FOV of 56 degrees.

I don't think at this time I have the cash to purchase an expensive EP.

I was leaning towards the 38 or 32mm Orion Q70 (more towards the 38mm).

I have a 26mm Q70 and I like the views in my 8inch f4.9

#21 panhard

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Posted 31 January 2013 - 11:18 AM

First try the 26 mm eyepiece in your new scope to see how the views are. You may not like the view as much as in your 8" scope.

#22 cloudmagnet

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Posted 31 January 2013 - 11:35 AM

jtrevino- My DOB is almost identical to your's- 14", f/4.8

My favorite ep is the Nagler 31mm- this gives me 57 power. The fov is wide enough that even with a Paracor in place, the entire Pliedies is in view. Since I'm strickly visual and love DSo's, this combo practically lives in the focuser.

#23 Starman1

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Posted 31 January 2013 - 11:49 AM

My pupil opens only to 4.5mm these days.
So, on my f/5.75 scope (the f/ratio with a Paracorr), I shouldn't use an eyepiece longer than 25.9mm.

Yet, I find the 31 Nagler a great eyepiece to use in my scope. I don't see the shadow of the secondary; nor do I mind losing a little light from the scope because the magnification is so low and the field is quite wide.

So take calculations of the appropriate lowest power with a small grain of salt--you can go a little longer in focal length and not be losing much of anything. Lowest power is about widest field, not highest resolution.

#24 Jon Isaacs

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Posted 31 January 2013 - 12:17 PM

jtrevino- My DOB is almost identical to your's- 14", f/4.8

My favorite ep is the Nagler 31mm- this gives me 57 power. The fov is wide enough that even with a Paracor in place, the entire Pliedies is in view. Since I'm strickly visual and love DSo's, this combo practically lives in the focuser.


Something doesn't quite compute here for me. Your scope has 1700mm focal length, the 31mm Nagler has a 42mm field stop. Without the Paracorr, this translates to a 1.42 Degree TFoV, with the Paracorr this translates to a 1.23 degree TFoV. The Pleiades as a cluster are about 2 degrees in diameter.

My 12.5 inch f/4.06 (1288mm) with the 31mm Nagler and a Paracorr (=1482mm) has a TFoV of 1.6+ degrees, I am not seeing the entire Pleiades in field of view.

Jon

#25 nevy

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Posted 31 January 2013 - 12:23 PM

I can just fit the Pleiades in my 12" , F5 , focal length 1500 mm, scope with a Meade 40 mm super wide , and it fits very nicely in my 55 & 56 mm plossls , I can also look at the moon without seeing the secondary shadow.






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