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40mm Explore Scientific In f/5 Newtonian

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#1 AngrySaladman

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Posted 30 June 2020 - 10:06 PM

Hi all - I just received a 40mm explore scientific 52 degree eyepiece. I have a 6", 750mm focal length, f/5 newtonian. I had to adjust the focuser to maximum length and still pull the eyepiece out past the holding screws in order to have any focus on the sky. 

 

Did I purchase an eyepiece with too long of a focal length? Thanks in advance.



#2 petert913

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Posted 30 June 2020 - 10:12 PM

No, you just need an extension tube.  Newtonians have limited focus travel.

 

https://agenaastro.c...-extension.html


Edited by petert913, 30 June 2020 - 10:12 PM.


#3 AngrySaladman

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Posted 30 June 2020 - 10:15 PM

Ah - thank you very much Skylab. Such a simple answer. I will look for a 2" extension tube.



#4 Redbetter

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Posted 01 July 2020 - 12:29 AM

Isn't that a 2" eyepiece?  If so you will need a 2" diameter extension tube rather than the 1.25" in the link.

 

While I have [not] seen this eyepiece and realize it is set up for 2" format, it's specs suggest to me that its field stop and eye relief are similar to standard 40 Plossls.  If so, the field stop and focal plane are near the bottom of the barrel.  That is why it requires so much focuser travel.  

 

As for whether the focal length of the eyepiece is too much for the scope, it is literally in the eye of the beholder.  At f/5 you will end up with 8mm exit pupil.  This is more than most eyes can fully accommodate.  The positive is that it means that you know that the field is essentially as bright as it can be (mostly useful with filters for large diffuse nebulae.)   The down side is that any extra pupil effectively reduces the aperture and leaves you operating at a smaller image scale than you would otherwise be available.  Generally a larger image is an easier one to pick detail from.

 

My pupil is probably somewhere in the 6.5mm range (or perhaps less) anymore based on some tests I have done.  However, I still use a 41 Pan with nebula filters at times with an f/5 because it provides the largest true field of view (field stop diameter) that I have available in 2".


Edited by Redbetter, 01 July 2020 - 10:16 AM.

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#5 ButterFly

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Posted 01 July 2020 - 01:09 AM

The worst part about an exit pupil that big in a dob is the central obstruction.  At 30%, the hole in the middle of the exit pupil is 2.4mm.  At 20%, it's 1.6mm.  Essentially everything on axis is peripheral vision.  That kills a lot of detail.

 

I do still use a 41 Pan in a scope that's f/4.7 with a Paracorr and a 17% obstruction.  It gives bright views and lots of stars.  But it also kills the dust lanes in Andromeda.  The Rosette nebula is very nice with it.  You have to choose your targets carefully.  There is no easy way to predict beforehand, so just try it out.



#6 Jon Isaacs

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Posted 01 July 2020 - 03:31 AM

The worst part about an exit pupil that big in a dob is the central obstruction.  At 30%, the hole in the middle of the exit pupil is 2.4mm.  At 20%, it's 1.6mm.  Essentially everything on axis is peripheral vision.  That kills a lot of detail.

 

I do still use a 41 Pan in a scope that's f/4.7 with a Paracorr and a 17% obstruction.  It gives bright views and lots of stars.  But it also kills the dust lanes in Andromeda.  The Rosette nebula is very nice with it.  You have to choose your targets carefully.  There is no easy way to predict beforehand, so just try it out.

I doubt that it is the central obstruction that is killing the dust lanes.  The diffraction effects of the central obstruction are on a very small scale, planetary.  

 

When I measured my dark adapted pupil a few years ago, I measure it photographically at 7.7mm.  This what the end of a story that began when I bought the 41mm Panoptic to increase the field of view of the 25 inch F/5 I had. I figured I had aperture to burn and I could use the field of view.  What happened was that the field of view was not much help but I noticed that extremely faint objects were brighter with the 8.2 mm exit pupil than they were with a 7mm exit pupil.  It really didn't make sense for someone in their late 60s but the measurement explained it.

 

Still, I would not recommend a 40mm eyepiece in an F/5 scope even for someone with a dark adapted pupil large enough to take advantage of it.  There are very few situations where a 40mm eyepiece with a 8mm exit pupil is a better choice than a 30mm with a 6mm exit pupil and 33% greater magnification.

 

Jon


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#7 Tony Flanders

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Posted 01 July 2020 - 04:16 AM

I would not recommend a 40mm eyepiece in an F/5 scope even for someone with a dark adapted pupil large enough to take advantage of it.  There are very few situations where a 40mm eyepiece with a 8mm exit pupil is a better choice than a 30mm with a 6mm exit pupil and 33% greater magnification.


I use a 40-mm wide-field eyepiece reasonably often in my f/5 Dob to achieve a wider true field of view -- an irreproachable reason. I have no problems whatsoever with the shadow of the secondary mirror unless I try to use it during the daytime or while viewing the Moon, both of which are bright enough to stop my own pupil down from its normal 5.5 mm.
 
Of course I lose a lot in terms of detail and ability to see faint objects. But that's not the point. If I wanted to see more detail I would be using higher magnification. When I really need the oversize true field of view, I am willing to sacrifice the detail.
 
Having said that, the OP is not achieving a particularly wide field of view, due to using a Plossl. An inexpensive 30-mm wide-field eyepiece would show nearly as wide a true field of view with a much richer and more detailed image.


Edited by Tony Flanders, 01 July 2020 - 04:18 AM.

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#8 ButterFly

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Posted 01 July 2020 - 06:02 AM

The fovea is about 1.5mm.  That's where all the detail is.  Our brains fill than area in just as they do with the blind spot at our optic nerves.

 

I prefer the 41 Pan for aimless panning around over the 31 Nagler.  It's just easier for me to keep my eye at the eye relief point to avoid kidney beaning  while moving around.  The 31mm typically does much better for targets.  There is lots of detail in the North America nebula at large scales.  The 31 does better on it - the blind spot is about 1mm.  Giant dark nebulas are great 41 Pan targets.  You just can't magnify those without losing the surrounding 30% more brightness.  The 41 Pan was certainly a refractor purchase, but it has its place in a dob too.



#9 AngrySaladman

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Posted 01 July 2020 - 08:30 AM

Thank you for all the replies. My main purpose in using this eyepiece is to achieve a wider field of view and more stars. I have other pieces for higher magnification. Hubble - the Explore Scientific is a Plossl? What would you suggest for a maximum field of view in an f/5 newtonian without breaking the bank?



#10 Jon Isaacs

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Posted 01 July 2020 - 08:50 AM

I use a 40-mm wide-field eyepiece reasonably often in my f/5 Dob to achieve a wider true field of view -- an irreproachable reason. I have no problems whatsoever with the shadow of the secondary mirror unless I try to use it during the daytime or while viewing the Moon, both of which are bright enough to stop my own pupil down from its normal 5.5 mm.
 
Of course I lose a lot in terms of detail and ability to see faint objects. But that's not the point. If I wanted to see more detail I would be using higher magnification. When I really need the oversize true field of view, I am willing to sacrifice the detail.
 
Having said that, the OP is not achieving a particularly wide field of view, due to using a Plossl. An inexpensive 30-mm wide-field eyepiece would show nearly as wide a true field of view with a much richer and more detailed image.

 

As they say, it all depends..  My two choices are a 31mm is 82° eyepiece and a 41 mm 68° eyepiece, this provides the widest possible field of view in a 2 inch eyepiece.

 

The 31mm provides 32% greater magnification with 9% narrower field of view.  There are situations where the loss of 9% is significant but it's rare. Most often, the 32% greater magnification provides the preferred view.

 

In a 12.5 inch F/5, the difference would be 1.66° at 39x versus 1.52° at 51x. Even with a dark adapted pupil that could almost accept the 8 mm exit pupil off the 41mm, it's rarely used in my F/5 scopes.

 

Of course, these are two very expensive eyepieces that represent the best of what is possible in a ~30  mm and ~40 mm wide field eyepieces.

 

Jon

Jon



#11 AngrySaladman

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Posted 01 July 2020 - 10:24 AM

Am I wrong in my calculation that a 40mm, 52 degree in a 750mm focal length tube will obtain a 2.77 degree field of view? Obviously that doesn't account for lost light for an exit pupil bigger than my own.



#12 JOEinCO

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Posted 01 July 2020 - 10:44 AM

Am I wrong in my calculation that a 40mm, 52 degree in a 750mm focal length tube will obtain a 2.77 degree field of view? Obviously that doesn't account for lost light for an exit pupil bigger than my own.

That is correct (using a field stop 36.3mm for that eyepiece as published by ES).



#13 Jon Isaacs

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Posted 02 July 2020 - 02:47 AM

Am I wrong in my calculation that a 40mm, 52 degree in a 750mm focal length tube will obtain a 2.77 degree field of view? Obviously that doesn't account for lost light for an exit pupil bigger than my own.

That is correct.  :waytogo:

 

One thing:  When the exit pupil is larger than your dark adapted pupil, you do not lose field of view, stars are dimmer, the objects are not as bright as they could be.  But the field of view will be 2.77 degree. 

 

Jon



#14 Tony Flanders

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Posted 02 July 2020 - 04:52 AM

Thank you for all the replies. My main purpose in using this eyepiece is to achieve a wider field of view and more stars. I have other pieces for higher magnification. Hubble - the Explore Scientific is a Plossl? What would you suggest for a maximum field of view in an f/5 newtonian without breaking the bank?

The number of stars that you can see in a single field of view depends on the true field of view (TFOV) and on the magnitude of the faintest star that you can see. For any given aperture, the faintest star that you can see is a function of magnification, especially at wide exit pupils. Regardless of how wide your own pupils open, you will be able to see much fainter stars with a 6-mm exit pupil than with an 8-mm exit pupil, and much fainter still at a 4-mm exit pupil.

 

Achieving wide TFOV is always a compromise. In general, eyepieces with wider apparent fields of view (AFOV) allow you to achieve larger true fields of view without compromising (or with less compromise) in terms of magnification.

 

In other threads, Jon has suggested the GSO 30-mm SuperView as an inexpensive wide-field eyepiece that doesn't compromise too much in terms of image quality. Based purely on the specs, your 40-mm eyepiece ought to deliver a TFOV around 2.8 degrees wide at 18.75X, compared to 2.7 degrees at 25X for the GSO 30-mm SuperView. For me, the extra magnification would be well worth the slight loss in true field of view.

 

Be aware that the arithmetic for TFOV in wide-field eyepieces isn't entirely reliable even if the stated AFOV and focal length are precisely correct -- which they often aren't, especially apparent field of view. That's because all eyepiece necessarily distort the image somewhat. To put that in a different way, the focal length at the edge of the field of view is never exactly the same as in the center of the field of view.

 

Another factor for me is that I get diminishing returns from larger apparent fields of view. I can take in almost the full field of view of a Plossl entirely, at a glance. With a 68-degree eyepiece I need to look around a bit, and the edge of the field is often a little hard to see (depending on the design and the focal length). With a 82-degree AFOV or wider, I find the edge of the field really difficult to see, even if I try hard. So for me, although an 82-degree eyepiece does indeed have a wider effective field of view than a 68-degree eyepiece, it's not as wide as the numbers suggest.

 

Despite the benefits of a wide apparent field of view, I still find something very solid and comforting about eyepieces with AFOV of 50 degrees or less, where what you see is really what you get. Most people feel just the opposite. Just saying ...
 


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