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Eyepiece brightness question

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#1 BPoletti

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Posted 13 July 2020 - 08:48 PM

Greetings,

 

I am a relative noob with a Celestron Nexstar 8SE for about 6 weeks.  I take it back on the deck in suburban St. Louis 15 miles or so east of downtown.  Still plenty of glorious light pollution.  But I'm having a lot of fun and learning the scope.  I'm quite happy with the scope.  

 

My compliment of eyepieces include Celestron Plossls in 32mm and 40mm.  But I have become particularly partial to wide field eyepieces of medium focal lengths.  I also am using a 2" diagonal and visual back.

 

With the 8SE field stop of around 39mm, does it make sense to get an eyepiece, say a decent 62 degree or wider 32mm or 40mm thgat has a field stop around the same or larger than the 8SE to get a brighter wide field image of the sky even with a little vignetting?  By using a larger section of the light coming into the eyepiece, does that at all enhance the image?  Or all all the individual objects the same brightness, just larger with shorter focal length lenses?  (Did I just answer my own question?)

 

If I head to dark skies (REALLY dark skies) do the long focal length lenses make for better viewing?   

 

Some things are just not completely intuitive.  At least to me.  

 

Thanks for your help,

 

Bill



#2 Jon Isaacs

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Posted 13 July 2020 - 09:03 PM

Bill:

 

I'm glad you're enjoying your scope.

 

Your question is a good one. This is how I see it:

 

The largest possible field stop for a 2 inch eyepiece is 46 mm. Even though the rear port/rear baffle is 38 mm, it's far enough away and far enough out of focus that you won't see it.  A camera would see it, your eye won't.

 

You'll want more eyepiece's to cover higher magnifications but a 40 mm SWA or even a 55 mm Plossl would be a worthwhile addition, being much wider than your current eyepiece's.

 

Jon


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#3 KBHornblower

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Posted 13 July 2020 - 09:18 PM

It takes a lot of vignetting to make a noticeable loss of light near the edge of the field.  It is sort of analogous to a shallow penumbral lunar eclipse.  When I put my widest eyepiece in my big Dob, I can tell it is cutting off about 1/3 of the out-of-focus disk of a star at the edge, but when it is focused I hardly notice it.



#4 M11Mike

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Posted 13 July 2020 - 09:40 PM

Hi Bill - you will see more in a dark observing location with any eyepiece or scope for that matter. 

 

BTW - I use a 50mm Axiom in my 8SE - and I don't notice any vignetting.  My 50mm's actual observable field of view IS larger than that of my 42mm, etc.

 

Being just 15 miles from downtown St. Louis means you have a LOT of light pollution.  With that much light pollution - your sky background will be quite bright with long FL eyepieces.  Pumping up the power will darken your sky - but that can't compare with observing from a dark location.  

 

M11Mike    


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#5 vtornado

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Posted 13 July 2020 - 09:48 PM

I have found in light pollution smaller exit pupils can help pull out faint items.

You have a nice slow f/10 scope.  I think a 2 inch 30mm eyepiece with a 40+ mm field stop would help you.

Some of the less expensive 2 inch eyepieces work well in an f/10 scope, like the GSO superview 2in 30mm 68 degree.


Edited by vtornado, 13 July 2020 - 10:03 PM.


#6 gnowellsct

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Posted 14 July 2020 - 12:26 AM

Greetings,

 

I am a relative noob with a Celestron Nexstar 8SE for about 6 weeks.  I take it back on the deck in suburban St. Louis 15 miles or so east of downtown.  Still plenty of glorious light pollution.  But I'm having a lot of fun and learning the scope.  I'm quite happy with the scope.  

 

My compliment of eyepieces include Celestron Plossls in 32mm and 40mm.  But I have become particularly partial to wide field eyepieces of medium focal lengths.  I also am using a 2" diagonal and visual back.

 

With the 8SE field stop of around 39mm, does it make sense to get an eyepiece, say a decent 62 degree or wider 32mm or 40mm thgat has a field stop around the same or larger than the 8SE to get a brighter wide field image of the sky even with a little vignetting?  By using a larger section of the light coming into the eyepiece, does that at all enhance the image?  Or all all the individual objects the same brightness, just larger with shorter focal length lenses?  (Did I just answer my own question?)

 

If I head to dark skies (REALLY dark skies) do the long focal length lenses make for better viewing?   

 

Some things are just not completely intuitive.  At least to me.  

 

Thanks for your help,

 

Bill

I answer this two ways.  First, I own a C8 (and other scopes) and use it often with 30 and 40mm 70 degree oculars.  I wouldn't have it any other way!

Second, I've been around the block on some of these issues (by no means all).   But I mount apos on top of my SCTs and so I know what a fully illuminated non-vignetted view looks like.

 

So I would put it this way.  Using the 30 and 40 mm two inch format 70 degree eyepieces is immensely satisfying.  But if you know what vignetting looks like and know how to look for it, yeah, it can be seen.  But even if you KNOW how to look for it you don't really pay attention.  And your guests at the eyepiece (remember when we had guests at the eyepiece?) won't see it unless you coach them.  

 

Greg N


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#7 gnowellsct

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Posted 14 July 2020 - 12:29 AM

 but that can't compare with observing from a dark location.  

 

M11Mike    

Yeah we need to head out to St. Louis and chase him around till he cries uncle, piles the gear into a car, and heads out for somewhere dark.


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#8 BPoletti

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Posted 14 July 2020 - 08:25 AM

Yeah we need to head out to St. Louis and chase him around till he cries uncle, piles the gear into a car, and heads out for somewhere dark.

 

UINCLE!!!  I'M READY.  I keep trying to get out of town to blow the dust off of 5 months of stay-at-home.  A little road trip to NM, AZ, NV, UT, MT, WY....  Someplace dark and SAFE.  and SAFE, where I can set up with an extra battery pack and spend the evening exploring what is obscured at home.   

 

My biggest concern is finding a SAFE place.  I'm an old guy and would prefer being somewhere in a group observing than  alone at 2:00am in a dark location. 


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#9 gnowellsct

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Posted 14 July 2020 - 12:54 PM

  

 

My biggest concern is finding a SAFE place.  I'm an old guy and would prefer being somewhere in a group observing than  alone at 2:00am in a dark location. 

Well, I never cheat at gamblin' / cuz it makes some people sore. / But just in case I got an extra ace and a big long 44. / A red hot 44!   (Rovin' Gambler, New Christy Minstrels)

 

Pack some heat with that c8?  No better not to.   You'll end up shooting at raccoons.   My mom loved hiking out doors and I used to tell her that the best way to go would be to get eaten by a bear.  She perished the hard way, six years of Alzheimer's.     I think she would have preferred the bear. 

 

More seriously:  I have a kind of anxiety that builds up before I go out to do solo observing in a remote spot.  But if I push through it, some of the most beautiful nights I've had have been out alone.  It is serene.  

 

And frankly, people who are looking to commit crimes don't go out into places where there is no one to be found.

 

Greg N


Edited by gnowellsct, 14 July 2020 - 01:00 PM.

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#10 M11Mike

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Posted 14 July 2020 - 05:28 PM

Greg's right.  Very serene when you're alone.  

 

When I observe alone - I bring along a small portable radio and put on some soft relaxing music to keep me company.  That helps me.  One CORONA (BEER that is) helps too!!! 

 

I must admit - I do get a "little" scared now and then when observing alone.  I prefer going out with a "buddy". 

 

I'm an older fellow - and you never know, you could fall or have a health issue - nice to have a friend there "just in case".

 

When I go to my remote site in Esperance, NY - it's most always with my astro buddy - Mike M.   If I go out near home (I have 2 dark observing spots within 5 miles) and both I believe to be quite safe - then going it alone is less of a concern.

 

Mike P aka M11Mike      



#11 SkippyMcSkipperson

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Posted 16 July 2020 - 10:29 AM

 

 

My biggest concern is finding a SAFE place.  I'm an old guy and would prefer being somewhere in a group observing than  alone at 2:00am in a dark location. 

 I am also about 15-20 miles East (as the crow flys) from St. Louis and that city is getting a bit crazy right now.  I will not be crossing the river until I am required to do so, at least for a while.

 

I live out in the rural areas East of the city and it isn't terrible, Bortle 5 at my house, but the views to the West definitely suffer a bit with the city glowing over there.  The other problem is the refinery near Woodriver, that thing has it's own glow as well.  



#12 gnowellsct

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Posted 16 July 2020 - 10:46 AM

Greg's right.  Very serene when you're alone.  

 

When I observe alone - I bring along a small portable radio and put on some soft relaxing music to keep me company.  That helps me.  One CORONA (BEER that is) helps too!!! 

 

I must admit - I do get a "little" scared now and then when observing alone.  I prefer going out with a "buddy". 

 

I'm an older fellow - and you never know, you could fall or have a health issue - nice to have a friend there "just in case".

 

When I go to my remote site in Esperance, NY - it's most always with my astro buddy - Mike M.   If I go out near home (I have 2 dark observing spots within 5 miles) and both I believe to be quite safe - then going it alone is less of a concern.

 

Mike P aka M11Mike      

Your astrobuddy is Mike W!




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