Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

exit pupil

  • Please log in to reply
7 replies to this topic

#1 Dazz_4130

Dazz_4130

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 03 Jun 2020

Posted 05 June 2020 - 04:07 PM

Hi,

 

Can someone explain the ranges of what is best to observe at certain exit pupil values



#2 Bataleon

Bataleon

    Ranger 4

  • *****
  • Posts: 368
  • Joined: 29 Aug 2019

Posted 05 June 2020 - 04:42 PM

Hi,

Can someone explain the ranges of what is best to observe at certain exit pupil values

First off, exit pupil is determined by the objective diameter of a scope or binocular divided by the magnification given by a specific eyepiece. So, let's say you have an 8" SCT with a f/l of 2032 and you're using a 15mm eyepiece, that's 2032 f/l ÷ 15 = magnification ~135.46x, then objective 203 ÷ 135.46 = ~1.50, so the exit pupil in this case is 1.5mm

The diameter of the dark adapted pupil in your eye is largely due to age and genetics. Some people are at a disadvantage by heredity, and then as we age, our eyes become less capable of dilating to the adequate exit pupils necessary for low light situations. The average dark adapted pupil is between 5-9mm, 9 being the maximum possible for most people. As you might suspect, the more exit pupil afforded by a particular scope and eyepiece will lend itself to seeing fainter objects in the sky, but only to the extent your eye can adapt itself in the first place. But I digress...

To answer your question, generally somewhere in the 1.5-2mm area is the best exit pupil for viewing fainter objects such as DSO. 2mm is the "sweet spot" for most people because it matches the average resolving power of the human eye, which is around 60 arc seconds. As you get into brighter objects like planets, large stars, doubles etc, you only need about 1mm or less exit pupil and since you'll be using higher power eyepieces for these type of targets, you should be well within that range. So, there isn't really an advantage to chasing too much exit pupil for a given object because at that point, you're letting too much light into your eye and your brain's ability to resolve is reduced.

Sent from my SM-N960F using Tapatalk

Edited by Bataleon, 05 June 2020 - 04:57 PM.

  • Asbytec, naramsin, ponybird and 1 other like this

#3 russell23

russell23

    Voyager 1

  • *****
  • Posts: 10,846
  • Joined: 31 May 2009
  • Loc: Upstate NY

Posted 05 June 2020 - 05:07 PM

Another way to calculate exit pupil is to divide the focal length of the eyepiece by the focal ratio of the telescope.
  • Starman1, ponybird and Dazz_4130 like this

#4 N3p

N3p

    Apollo

  • -----
  • Posts: 1,290
  • Joined: 25 Jun 2018

Posted 05 June 2020 - 05:36 PM

Wide fields low power is nice too..  To look at star clouds, large nebulae. In my opinion, a wider exit pupil is nice too. My ES 24mm 68d gives me a very nice 4.8mm exit pupil, great low power value.

 

Just saying.


  • Dazz_4130 likes this

#5 stevenrjanssens

stevenrjanssens

    Vostok 1

  • -----
  • Posts: 181
  • Joined: 24 May 2018
  • Loc: Vancouver, BC

Posted 05 June 2020 - 06:31 PM

This used to be stickied in the best of eyepieces thread. Looks like that thread has been removed?

 

https://www.cloudyni...ation/?p=645399



#6 Ernest_SPB

Ernest_SPB

    Apollo

  • *****
  • Posts: 1,429
  • Joined: 13 Nov 2010
  • Loc: St.-Petersburg, Russia

Posted 06 June 2020 - 07:33 AM

Hi,

 

Can someone explain the ranges of what is best to observe at certain exit pupil values

In a good scope with moderate aperture: 

<0.7 mm (in diameter) - planets, fragments of moon, close double stars

0.7-1 mm - Moon and Solar disc overview, core of globular clusters, compact planetary nebulae

1-1.5 mm - compact open clusters, bright nebulae, globular clusters, Jupiter and Saturn satellites

around 2 mm - tiny and dim galaxies

around 3 mm - bright DSO (like most Messier)

>4 mm - large extended DSO


  • DHEB and kauschovar like this

#7 russell23

russell23

    Voyager 1

  • *****
  • Posts: 10,846
  • Joined: 31 May 2009
  • Loc: Upstate NY

Posted 06 June 2020 - 08:25 AM

Hi,

 

Can someone explain the ranges of what is best to observe at certain exit pupil values

For a given scope, the following exit pupil relations occur:

 

1.  As magnification increases exit pupil gets smaller.

2.  As exit pupil gets smaller the image gets dimmer.

 

So the shorter the focal length of the eyepiece the smaller the exit pupil and therefore the dimmer the image. 

 

If you stay above a one millimeter exit pupil you generally can avoid seeing any floaters in your eyes.  However, there are a number of targets such as the Moon, double stars, planetary nebula, globular star clusters, planets up to a point, that can handle exit pupils below 1mm. 

 

There is generally a "rule" that exit pupils smaller than 0.5mm are not beneficial because of the image will degrade below that exit pupil.  However, that is often debated on this forum so ... experience is sometimes the way to go and see what works for you. 

 

You could approximate a good range of exit pupils using your telescope's focal ratio as a basis.

 

Let's say you have an f/6 scope.  So then you could cover a range of exit pupils and the eyepiece FL needed: 

 

1mm -->  6mm FL

2mm --> 12mm FL

3mm --> 18mm FL

4mm --> 24mm FL

 

Below 1mm perhaps eventually look to have a 0.75mm and 0.5mm exit pupil:

 

0.75mm --> 4.5mm FL

0.5mm --> 3mm FL

 

Now one thing is do not obsess about being exact with these.  Finding eyepieces you like is the top priority.  For example, in my 102mm f/7 my most used eyepieces are:

 

18.2mm --> 2.6mm exit pupil

13mm FL --> 1.9mm exit pupil

8mm FL --> 1.1mm exit pupil

4.7mm FL --> 0.7mm exit pupil

 

I would say if you are looking to add eyepieces try to get something that would give an exit pupil between 1.6mm and 2.6mm and then something that would give somewhere between 0.8mm and 1.2mm and then anything else you want to try.

 

Generally, if you are using a UHC filter to enhance nebula you will want an exit pupil of 2mm or larger to really maximize the filter's usefulness.  So maybe you would want a 3.5mm exit pupil as well.

 

As far as object classes, I've never found that above 1mm the class of object matters too much.  You have to keep in mind that not only is image brightness changing with different exit pupils, but image scale is changing as well as you change magnification (and therefore exit pupil). 

 

So if it was all about image brightness you would always want the largest possible exit pupil.  However, you also want to better resolve the image with a larger magnification. So the key, for each object is to find the lowest magnification (brightest image) that provides the resolution you are looking for.  And that varies with different objects within a class.  Not all open clusters are at their best at the same exit pupil. 


  • csphere.d likes this

#8 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 46,143
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 06 June 2020 - 12:17 PM

Hi,

 

Can someone explain the ranges of what is best to observe at certain exit pupil values

I don't know what size or type of scope you own, but Ernest's recommendations are appropriate.

 

Personally, I use the following exit pupils, magnifications rounded off(also listed in times per inch of aperture)for:

 

M     ExP   M/in of aperture
 
60    5.2    5    large objects like star clusters, or nebulae with a filter, or dark nebulae
80    3.8    7    normal low power for large fields, large objects, carbon stars in rich fields.
100  3.0    8    normal low power for fainter objects and somewhat smaller objects
130  2.4    10  low medium power for fainter, smaller targets like globulars, large planetaries, smaller open star clusters.
150  2.2    12  low medium power for fainter, smaller targets like globulars, large planetaries, smaller nebulae, small open star clusters.
165  1.9    13  medium power for most DSOs like galaxies
185  1.7    15  medium power for most DSOs like galaxies that need a little more power.
230  1.4    18  high medium power for planetaries, galaxies, smaller globulars, details in galaxies.
300  1.0    24  low high power for planets, Moon, planetary nebulae, small globulars, 
390  0.8    31  high power for outer planets, moons of planets, planetary nebulae
490  0.6    40  very high power for outer planets, moons of planets, planetary nebulae

 

As you can see, I have a lot of medium and low power eyepieces--too many--but the winnowing process is still on going.

I need to lose about 3 eyepieces eventually.


  • DHEB likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics