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Needing some input on longer FLs

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#1 jmillsbss

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Posted 21 March 2019 - 09:31 AM

CN  -  I'm still kinda new to this hobby/obsession and I'm still piecing together a decent eyepiece collection.  I have a 10" reflector and as such can see quite a good many faint fuzzy spots.  It's not a Go-To, so I'm having to learn the sky and star-hop quite a bit. I have to get close with the wider fov and then start stepping in.  Honestly, I love the hunt!  BUT, the longest eyepiece FL I have is a cheap, but decent, 32mm Celestron plossl that was in the EP kit.  I'm building a collection of Explore Scientific EPs one piece at a time.  I've got several of their 82 degree shorter focal lengths and a 62 degree 26mm. I also have their 2" focal extender.  I'd love to have their 30/82 or 40/68 but that's stretching my self-imposed budget constraint.  I have been seriously considering their 62 degree 40mm but now I've begun to consider the Tele Vue 55mm plossl or 40mm plossl.  I already know, when I mention the TV piece that this board perks up!  My question:  For the money, for the search for the faint fuzzies, which eyepiece gets the job done.  If anybody's got anything to sale, let me know.  Thanks in advance.....  J



#2 csa/montana

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Posted 21 March 2019 - 09:40 AM

Welcome to Cloudy Nights!  Glad to have you join us!

 

I'm moving your post to Eyepieces, in order to get better assistance.  Enjoy the night skies!



#3 DavidWasch

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Posted 21 March 2019 - 10:11 AM

I believe you have the Orion 10" dob, which is f/5. Even with young eyes, you probably have a maximum pupil size around 7mm.

 

The eyepiece's exit pupil is going to be its focal length/5-- or 8mm. So, you can probably safely take the 40mm off you list because a good amount of the light it puts out would never make it into your eye.

 

If you're closer in age to 50, your pupil may be a maximum of 5mm or less. At that size, the 30mm would also be a sub-optimal eyepiece. There are threads on measuring your pupil size on this site, and here's a pretty good article that goes into the topic and provides ways for measuring your pupil size: https://www.skyandte...a-pupil-primer/



#4 sg6

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Posted 21 March 2019 - 10:15 AM

Realistically you will have to look at 2" eyepieces.

The 32mm plossl delivers much the same as the 40mm plossl, so no real gain. The 62 degree 40mm you will have to work out.

 

In a 1200mm scope the 62/40 gives 2 degree view.

The 55mm ploss with 50 degree gives 2.29 so the TV 55mm ploss would deliver 15% more in your scope, whatever its focal length is. Didn't expect that much difference.

 

The plossl has the problem of eye relief, there is a lot of it, 38mm. You may find that much eye relief a troublesome.

 

Exit pupil size is oddly something I never consider, suppose that is field is paramount then something has to be compromised. As ever you cannot have everything.



#5 BillP

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Posted 21 March 2019 - 10:19 AM

Hi.  I'm assuming your 10" reflector is a fast focal ratio, f/4.7 like the Orion XT10?  If so you need to be cognizant of two things when searching for an eyepiece to get you a large TFOV: Exit Pupil and Coma.  With fast parabolic mirrors, they show coma in the off-axis.  When working the mid to high magnifications the coma is not bothering.  But when at low magnifications and largest TFOVs then you will see it a lot.  So at some point you may want to pick up a TV Paracorr on the used market.  The older Type 1 Paracorr works just fine and costs less.

 

The other thing is the exit pupil that results from the eyepiece+scope combination.  Exit Pupil = Focal Length of Eyepiece / Focal Ratio of Telescope.  So for you assuming f/4.7, a 40mm eyepiece will yield 40/4.7=8.5mm Exit Pupil.  Since your eye dilates to a max of 7mm (the normal), the larger 8.5mm exit pupil using the 40mm eyepiece means that your eye at 7mm will be masking part of the larger 8.5mm diameter light cone from the scope, so the view will be dimmer than what the 10" can give you and also the background sky will look brighter making it hard to find fainter items.  In my XT10 using a 40mm eyepiece even M42 looks lackluster!  So would recommend that you get a focal length that is between 5-6mm so the background sky will be darker making DSO pop more when you search for them.  So staying on the 25-30mm range would be best.  Your 32 Plossl gets you about 1.4 degree TFOV and a 6.8mm exit pupil.  The 28mm ES 68 gets you a little bigger 1.6 degree TFOV at a nice 6mm exit pupil.  The 24mm ES82 gets about the same TFOV as the 28ES68 but at a nicer 5.1mm exit pupil.  The 30ES82 gets you a larger 2 degree TFOV but a bit brighter background at a 6.4mm exit pupil so similar to how the view looks in the 32 Plossl only larger TFOV.  No inexpensive answer to do this well unfortunately.


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#6 CollinofAlabama

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Posted 21 March 2019 - 11:37 AM

What Bill Paolini said!  He's exactly right.

 

Getting any eyepiece for an F/4.7 scope over about 30mm is just counterproductive, unless a person has abnormally large dilation of his pupil.  So don't do it.  Bill's right.  The ES 28mm 68º, which isn't cheap, but is relatively light weight for two inch wide field eyepieces, is your best choice, until you get to the ES 30mm 82º, which costs a lot more and is much, much heavier.  Even the ES 34mm 68º is not optimal, since it will produce a 7.2mm exit pupil, and is bound to waste some light.  You could use it to minimize the waste, while maximizing the view, but it's really an F/6 and above telescopic eyepiece.  And don't bother with the Hyperion 31mm from Baader.  The edge correction was unacceptible for me at F/10.4, and at F/4.7, I can only imagine how bad it would be.  You either bite the bullet for the 30mm ES 82º, bite less of a bullet (and less resulting field) and get the 28mm 68º, or live with smaller fields.  I will forewarn you that going the cheap route will only cost you more in the end, as someone who's walked that road many a time.  Sorry for the tough astronomy love, but them's the facts


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#7 vdog

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Posted 21 March 2019 - 03:43 PM

A couple of things to consider:

 

  • You're probably not going to enjoy your 2" EPs as much without a coma corrector, so add that expense to the cost of the EPs.
  • You're probably not going to get your money's worth out of the corrector unless you get multiple 2" EPs. 
  • Because of the exit pupil issue, you really don't want any EPs longer than 28mm (depending on your eyes, even that may be too wide an exit pupil)

 

My solution was to avoid getting any 2" EPs for now. I figure that if I ever have the money to buy a fleet of 100 degree 2" EPs, I'll also have the money for a corrector, and vice-versa.

 

So, I went with a 1.25" barrel ES 68 degree 24mm as my low-mag option.  Minimal coma, no having to mess with the adapter.  Unless money is no object, it's something to think about.  


Edited by vdog, 21 March 2019 - 03:45 PM.


#8 jmillsbss

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Posted 22 March 2019 - 09:02 AM

Okay, thanks for all the input.  I knew I could count on you!!!  Here's another one, since everybody talked about exit pupil.....I got out yesterday before sunset and inserted several eyepieces in the scope and projected the bright sky on a piece of paper and noticed that different EPs focused a crisp white disk at similar distances from the EP (eye relief?) but the disks were different diameters.  Is that what we're talking about?  That disk is exit pupil?  So what happens to all the light that exists in the part of the image disc that exceeds the diameter of your eyeball's pupil?  Say a fella's fully dark-adapted pupil is 6mm and his eyepiece projects 9.5mm.  Can you just not jam the extra 3.5mm into your eyeball, can you have to look around for it in the EP, or is that only to do with FOV?  Thanks again!


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#9 Starman1

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Posted 22 March 2019 - 02:29 PM

OK, the telescope projects an image of the sky on its focal plane.

The eyepiece, a simple magnifier, looks at a portion of that focal plane, and that is the field of view of the eyepiece.

But, note that the entire mirror produces the star image on axis, and this is true at all powers.

 

A low power eyepiece may yield an exit pupil (the small disc of light behind the eyepiece where the image of the primary mirror forms) larger than the eye's pupil.

 

If the exit pupil is larger than the pupil of the eye, a lot of the light is blocked by your iris.

The image isn't any dimmer than it is when the exit pupil matches the eye because the magnification is lowered, which yields a brighter image per square millimeter

in the image.  The eye is no longer seeing all the light from the primary mirror, though, so you are using a smaller telescope at that lower power.

So, there is no reason to go lower in power than the exit pupil that matches the eye's pupil diameter.

And Exit pupil of an eyepiece is the focal length of the eyepiece divided by the f/ratio of the telescope.

 

Another disadvantage to using a larger than the eye exit pupil is that the shadow of the secondary mirror becomes a larger percentage of the image seen.

That shadow can be ignored at higher powers, but it becomes obnoxiously noticeable if you try to use too low a power (i.e. too large an exit pupil).

 

Normally, I don't advise going over a 6mm exit pupil (30mm eyepiece on an f/5 scope) unless you know your pupil is larger than 6mm.

Maximum pupil diameter normally gets smaller with age (mine is 4.5mm), but there is a lot of variation in that.  It's usually smaller in diabetics or people

who take blood pressure medicines, and larger in those with dark eyes and no health problems.

 

Here is an explanation of exit pupil you might find informative:

https://www.skyandte...a-pupil-primer/

and

https://en.wikipedia...wiki/Exit_pupil

and alittle more in depth:

https://www.handprin....html#exitpupil

This whole website is amazing.


Edited by Starman1, 22 March 2019 - 02:31 PM.

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#10 Jon Isaacs

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Posted 22 March 2019 - 02:45 PM

So what happens to all the light that exists in the part of the image disc that exceeds the diameter of your eyeball's pupil?  Say a fella's fully dark-adapted pupil is 6mm and his eyepiece projects 9.5mm.  Can you just not jam the extra 3.5mm into your eyeball, can you have to look around for it in the EP, or is that only to do with FOV?  Thanks again!

 

 

Don explained this nicely.   The simple story is that 6mm enters your eye, rest of the light falls on the eye itself, never entering the eye itself.  Since the amount of light is proportional to the area of the exit pupil, in this situation, only 40% of the light collected by the telescope will enter the eye and be seen.

 

In terms of exit pupils, even if one's eye does dilate to close to 8mm, generally smaller exit pupils will show more because a smaller exit pupil means the image is larger, the magnification is greater.  It is only for large, dim objects that a large exit pupil is an advantage and generally then it's only an advantage for nebulae that benefit from filters.  

 

Jon



#11 25585

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Posted 22 March 2019 - 06:20 PM

The same optics as the ES68 28mm, are used in their earlier Maxvision. Lots cheaper https://www.rotherva...-eyepieces.html


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#12 spaceoddity

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Posted 22 March 2019 - 07:56 PM

The same optics as the ES68 28mm, are used in their earlier Maxvision. Lots cheaper https://www.rotherva...-eyepieces.html

Looks like they bought out the meade 5k SWA's and removed the meade badge. I have the 28 in that series, good eyepiece but I've since replaced it with the APM UFF 30 for a little more FOV. I like the 28 SWA too much to sell though, at least for now. 


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#13 jmillsbss

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Posted 23 March 2019 - 12:52 PM

OK, the telescope projects an image of the sky on its focal plane.

The eyepiece, a simple magnifier, looks at a portion of that focal plane, and that is the field of view of the eyepiece.

But, note that the entire mirror produces the star image on axis, and this is true at all powers.

 

A low power eyepiece may yield an exit pupil (the small disc of light behind the eyepiece where the image of the primary mirror forms) larger than the eye's pupil.

 

If the exit pupil is larger than the pupil of the eye, a lot of the light is blocked by your iris.

The image isn't any dimmer than it is when the exit pupil matches the eye because the magnification is lowered, which yields a brighter image per square millimeter

in the image.  The eye is no longer seeing all the light from the primary mirror, though, so you are using a smaller telescope at that lower power.

So, there is no reason to go lower in power than the exit pupil that matches the eye's pupil diameter.

And Exit pupil of an eyepiece is the focal length of the eyepiece divided by the f/ratio of the telescope.

 

Another disadvantage to using a larger than the eye exit pupil is that the shadow of the secondary mirror becomes a larger percentage of the image seen.

That shadow can be ignored at higher powers, but it becomes obnoxiously noticeable if you try to use too low a power (i.e. too large an exit pupil).

 

Normally, I don't advise going over a 6mm exit pupil (30mm eyepiece on an f/5 scope) unless you know your pupil is larger than 6mm.

Maximum pupil diameter normally gets smaller with age (mine is 4.5mm), but there is a lot of variation in that.  It's usually smaller in diabetics or people

who take blood pressure medicines, and larger in those with dark eyes and no health problems.

 

Here is an explanation of exit pupil you might find informative:

https://www.skyandte...a-pupil-primer/

and

https://en.wikipedia...wiki/Exit_pupil

and alittle more in depth:

https://www.handprin....html#exitpupil

This whole website is amazing.

I didn't know that green eyes and high blood pressure were factors as well!  Just when I get my mind wrapped around something, y'all throw something else into the equation!!!

 

I settled on the Explore Scientific 82*/24mm eyepiece.  It has a bit bigger TFOV over the 28ES68 and at a slightly smaller 5.1mm exit pupil.  I already have the 26mm ES62 but the TFOV is smaller and at a lower mag.  All things considered, it looks like the best choice, but as we all know, we can't be sure what it's gonna do until the sun goes down!

 

We were out last night as the sun was setting and the first stars were coming into view.  Talk about old eyes, I'm amazed that the 4 year old can see the belt of Orion and I'm just seeing Betelgeuse and RIgel!  But in that twilight, I located M42 with the 26mm ES62 and just to get a feel for darker images at higher magnification, switched out and put in the 8.8/ES82 and it was unbelievably darker in the twilight background.  The responses on this board have explained that effect and now I know what you guys were talking about.

 

So, thank you all again for the insight and counsel... I can barely stand the wait til Monday night!!!


Edited by jmillsbss, 23 March 2019 - 02:53 PM.



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