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40mm eyepieces for 3.5

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#1 RMay

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Posted 03 December 2019 - 12:07 PM

I own Q 16 and 24mm Brandons, plus an assortment of TV Plossls, Wide Fields, and Naglers, including a 32mm Plossl and 24mm WF.

I’m now considering buying a used TV 40mm Plossl for the brightness it may offer while scanning. In a review I read that you might use a 32mm for scopes faster than F8 and that a 40mm is good for others (slower), including the Q.

Any thoughts on that, or is it just overkill?

Thanks,

Ron

#2 Gregory Gross

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Posted 03 December 2019 - 12:47 PM

My reasoning if I were in your shoes, Ron, would be this: Conventional wisdom (which you probably already know) states that a 32mm Plossl will give you the widest field of view for a 1.25" eyepiece barrel as far as Plossls are concerned. I'm thinking that a 40mm Plossl does give you lower magnification and would visually brighten objects. But its field of view is narrower, which might make it less useful for scanning (or maybe the difference between the 32 and 40mm EPs would be a wash).
 
In all my scopes, I find that it's most useful and economical to have a wider gap between eyepiece focal lengths at the lower end of the magnification range. In a scope with a 1300mm focal length, a 32mm eyepiece yields 41x and a 40mm eyepiece yields 33x, which is only an 8x magnification drop. To my eye, I'd have a hard time telling the difference between 41x and 33x.
 
Just to offer my own speculative thoughts (indeed speculative because those thoughts are untested by real-world experience using either a 32 or 40mm EP in a Questar), I'd opine that adding a 40mm TV Plossl would be overkill if you already have a 32mm Plossl.
 
Tele Vue doesn't include their 32mm Plossl on their table of eyepieces that come to focus in Questar finder mode. What's your experience using the 32mm Plossl in your Questar? Do you find you can reach focus in finder mode with that eyepiece?


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#3 RMay

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Posted 03 December 2019 - 07:06 PM

Hi Gregory, thank you for your thoughtful input .

I don’t recall ever having a problem with the 32mm TV eyepiece achieving focus at the finder, although I have to admit I usually only ever used it for low scans after using various other eyepieces, and I’m not sure if I ever really used it in finder mode.

I agree that any added brightness may not be much of a factor, since skies up here in Northern California are not dark enough for me to be able to see any discernible difference. It’s not a major investment... maybe a coin toss will help me decide...

🤷‍♂️

Thanks again for your insight and input.

Ron

#4 RobertPettengill

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Posted 03 December 2019 - 07:39 PM

Your field of view will be severely restricted. 32 mm is about the maximum usable eyepiece focal length before the view becomes severely vignetted
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#5 Gregory Gross

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Posted 03 December 2019 - 07:57 PM

Your field of view will be severely restricted. 32 mm is about the maximum usable eyepiece focal length before the view becomes severely vignetted

Rob, does that vignetting appear when a 40mm Plossl is used in a Questar or regardless of what a 40mm Plossl is used in?

 

This is somewhat off topic, but I've been wanting to get a 28mm RKE for some time. I'm hoping Santa will leave one in my stocking this year. I've only ever looked through one of these in my 10" Dob, and that famous effect where the eyepiece lens housing disappears is really neat. I wonder how well this eyepiece would work in a Questar. This may be a better choice for low-power scanning, though a 28mm EP in a scope with 1300mm of focal length (46x) is a touch on the high side in terms of magnification for low-power views.



#6 BillHarris

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Posted 04 December 2019 - 12:11 AM

Remember, the brightest visual image of an extended object (a nebula, etc) is with an eyepiece that gives an exit pupil of 7mm. This also called the "richest field magnification". For my 8" (200mm) f/4 newtonian, this works out to be a 28mm eyepiece. One exit pupil formula is to divide the eyepiece fl by the f/ratio-- 28mm/4=7mm with my 8".

With the Questar, the "RFT" eyepiece would be 98mm in fl (98/14=7). A 100mm 1-1/4" with that fl would be like looking into a pipe (apparent field of view of some 7.5*).

Just scribbling on a napkin...
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#7 davidmcgo

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Posted 04 December 2019 - 11:37 AM

The 40mm TeleVue Plossl does not focus with the finder, it has a different field stop position compared to the 32mm on down.

 

The 32 is nice and wrings about as much field as a 1.25” eyepiece allows.  In  my experience the 40 doesn’t make things noticeably brighter and the field isn’t any bigger.

 

Dave


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#8 RMay

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Posted 04 December 2019 - 12:17 PM

All: your comments are noted with much appreciation and thanks; I’ll stick with my 32...

Ron
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#9 RichA

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Posted 06 December 2019 - 06:48 PM

I own Q 16 and 24mm Brandons, plus an assortment of TV Plossls, Wide Fields, and Naglers, including a 32mm Plossl and 24mm WF.

I’m now considering buying a used TV 40mm Plossl for the brightness it may offer while scanning. In a review I read that you might use a 32mm for scopes faster than F8 and that a 40mm is good for others (slower), including the Q.

Any thoughts on that, or is it just overkill?

Thanks,

Ron

Whichever eyepiece offers the highest power with the widest field size is the one to get.  The widest field I believe I've seen in a 1-1/4" eyepiece might have been with a Celestron Ultima (modified 5-element "Plossl") 35mm.  I had a 45mm 1-1/4" Celestron Plossl but lost about a magnitude in it compared with a 32mm Plossl.




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