Eyepiece selection based on exit pupil

[amaury]

Hi all,

I have been planning my eyepiece selection based on exit pupil.

For context I have an 8" f/6 Dob.

I am following a minimalistic/simplistic approach. Basically I am planning to have three magnifications.

Low power: 30mm focal length eyepiece / 40X Magnification / 5 mm Exit Pupil

Mid power: 12.5mm focal length eyepiece / 96X Magnification / Approx. 2 mm Exit Pupil   (Apparently around 2mm exit pupil is the optimal exit pupil for visual acuity)

High power: 2x Barlow the 12.5mm eyepiece or a dedicated 6mm eyepiece/ 200X Magnification / Approx. 1 mm Exit Pupil (to match my scope focal ratio)

So basically two eyepieces and a 2x Barlow, or three eyepieces. Is it too simplistic?

is there any point with going beyond 200x with my telescope in the UK? (I guess not)

I am missing out something without a 1.5mm exit pupil or a 3-4 mm exit pupil?

Edited April 7, 2021 by amaury
Formatting and wording

[kev100]

I’d go for a dedicated 6mm ep, and avoid a Barlow. It’s a personal thing, but I’ve never got along with them (I know lots do, though). I have a 5mm BST and a William Optics 6mm SPL, and the latter gets loads of use.

[KP82]

I agree with kev100 and would recommend you get a dedicated eyepiece for 5 - 6mm rather than barlowing the 12.5mm.

Depending on what you like to observe, you may or may not need an eyepiece between the 12.5 and 30mm. Personally I'd get something around 18 - 24mm.

Above 200x are usually for Mars opposition, Lunar close-ups and splitting doubles. Other than that 200x is usually the limit under UK skies. 

[Rob_UK_SE]

You can certainly push beyond 200x magnification with an 8” scope, but you sometimes have to be patient as the moments of stable views are less frequent. If you are aiming for a wide spacing between three eyepieces, I would suggest going for three that you can use almost every night. 

5mm exit exit pupils are great for darker skies, but if you have any light pollution where you typically observe you would, in my opinion, be better off with a low power eyepiece that has a 4mm exit pupil (24mm / 50x). Ideally this would be an eyepiece that also has a wider field of view. 4mm would help to improve contrast and still works well with filters such as OIII. Depending on the type of targets you wish to observe, 100x magnification from a 12mm eyepiece is a good workhorse range for a wide variety of DSOs. The 2mm ‘sweet spot’ concept happens to work well for your scope’s focal length (1200mm?), but is not an exact science, so to speak. 

I second Kev’s point about a dedicated third eyepiece and would suggest a 7mm eyepiece which would result in 171x. This would be usable on most evenings and is ideal for globs, planetary nebula, Jupiter, Saturn, etc in an 8” f6 scope.

...You would end up with 50x, 100x and 170x.

Edited April 7, 2021 by Rob_UK_SE

[Jiggy 67]

You can certainly go way beyond x200 in UK skies depending on the seeing at the time. With good to excellent seeing x300 with an 8’ scope is achievable 

Edited April 7, 2021 by Jiggy 67

[amaury]

1 hour ago, Rob_UK_SE said:

5mm exit exit pupils are great for darker skies, but if you have any light pollution where you typically observe you would

I normally observe from a bortle 4 area in South Downs. Is that Dark enough to justify a 5mm exit pupil? 

 

1 hour ago, Rob_UK_SE said:

The 2mm ‘sweet spot’ concept happens to work well for your scope’s focal length (1200mm?), but is not an exact science, so to speak.

Yes, 200mm aperture 1200mm focal length f/6 Skywatcher Dobsonian. I've read in many places that the 2mm is related to the average size of the retina we are used to during the day. Apparently that is the size of the most sensitive part of the retina and the one that gives the best visual acuity.  So there is some science behind that "optimal" exit pupil size.

3 hours ago, kev100 said:

avoid a Barlow

 

1 hour ago, KP82 said:

would recommend you get a dedicated eyepiece for 5 - 6mm rather than barlowing the 12.5mm

 

1 hour ago, Rob_UK_SE said:

dedicated third eyepiece

It's pretty clear that dedicated eyepieces are preferred instead of Barlow. I'll go for a dedicated third EP then. Thanks.

51 minutes ago, Jiggy 67 said:

With good to excellent seeing x300 with an 8’ scope is achievable

With excellent conditions sure it is achievable. 

[mikeDnight]

If you observe using a blackout hood, you'll maximise your potential dark adaption and you could even exceed a 5mm exit pupil. Much of the trouble occurs as stray light hits the eye from the side. Just to illustrate how effective a hood can be, I've observed IC434 and the tiny black notch of the Horse Head using a 100mm refractor from the suburbs of Burnley in Lancashire using this method. As far as high powers go, I really think a barlow is a great idea. Modern barlow's, even the less expensive Skywatcher Delux are excellent, and because it won't break the bank it should be part of every observers kit. Most modern multi element eyepieces use an inbuilt barlow in their design, so Barlow's are not a bad thing. Again, if you ever get the urge to observe double stars, you'll really appreciate a high power eyepiece or barlow. Don't underestimate your telescope or your skies. My 100mm on an equatorial holds 500X with ease on binary stars and they are a joy to observe, so 300X on an 8" Dob is certainly doable.

Edited April 7, 2021 by mikeDnight

[Jiggy 67]

39 minutes ago, amaury said:

With excellent conditions sure it is achievable. 

I wouldn’t rule out a Barlow, just get a good one, they double your eyepiece collection. I wouldn’t be without one 

[Rob_UK_SE]

I also observe in Bortle 4 skies and near to the South Downs (East Sussex / Birling Gap end). When going from a 4mm to 5mm exit pupil I don’t, personally, perceive any real benefit towards additional nebulosity in objects such as the Veil. However, this will be very dependent on how wide each individual’s eyes can dilate when dark adapted.

Mike’s point about wearing a hood is really good advice. 

Some modern barlows, such as the Baader one, are really great. My preference not to use them is simply the convenience and comfort associated with native focal lengths. 

[amaury]

1 hour ago, mikeDnight said:

Just to illustrate how effective a hood can be, I've observed IC434 and the tiny black notch of the Horse Head using a 100mm refractor from the suburbs of Burnley in Lancashire using this method

I was just watching a video from an astronomy club where they mention this is the best virtually-free hack you can do in astronomy. I'll give it a go. 

38 minutes ago, Rob_UK_SE said:

When going from a 4mm to 5mm exit pupil I don’t, personally, perceive any real benefit towards additional nebulosity in objects such as the Veil

That is very interesting. I wonder if the hood would help you out to open more your pupils. I certainly see your point tho'

1 hour ago, Jiggy 67 said:

I wouldn’t rule out a Barlow, just get a good one

I'll do my research eventually with a Barlow. I don't want it to be the weak link in my optics. 

[Second Time Around]

I too also like a minimalist approach.  

However, I'd very much recommend a zoom eyepiece as your workhorse for medium and high powers.  One zoom eyepiece will cover multiple focal lengths and so is really excellent value for money.  I have both the Svbony 7-21mm zoom at around £45 on eBay plus the Baader 8-24mm at £189. The Baader has a wider field of view and is sharper and more contrasty, although it's not 4x better.  

Despite having high quality fixed focal length eyepieces, I use my zooms a lot more often.  The zoom plus a Barlow lens and a low power, wide field eyepiece is often all I use the whole evening.  

Fixed focal length eyepieces may be slightly better corrected when compared with a zoom at the same magnification.   But that's not always a fair comparison as that magnification may not be the optimum for a given object.  This is because one of the many advantages of a zoom is to be able to dial in precisely the best focal length.  For instance, this may be 13mm or even 13.1mm, which may actually show more detail than shorter or longer fixed focal length eyepieces - even better quality ones.  

I particularly like the ability to increase the magnification to make use of brief moments of good seeing (a steady atmosphere).  It takes more time to swap out an eyepiece, and the opportunity may then be missed.  You can't see anything if you haven't got an eyepiece in the focusser!

Zooms also enable the field of view to be varied to frame an object to get the prettiest view.  For this reason I particularly like them for clusters.

They're also handy when you're using filters.  You don't have to unscrew and then replace the filter when you change magnifications.

Many of those who post here and advocate fixed focal lengths are experienced observers.  It's so easy to forget what it was like as a beginner!  A zoom eyepiece enables beginners to easily learn what difference a change of magnification makes on all the various classes of object.  It also shows them what focal lengths would be most useful to their eyes, their telescope, and their observing conditions.  They then have the option of buying/not buying the most appropriate fixed focal length eyepieces for them.  For these reasons I'd always recommend that beginners buy a zoom as their first eyepiece.

I'd supplement the zoom with a Barlow lens - I wouldn't be without mine (in fact I own 3 of varying powers). 

One of the downsides of Barlows is that they transmit a little less light.  That used to be important years ago, but multicoating of lenses has made the difference only marginal.  In any event, many modern medium and high power eyepieces actually contain a Barlow to increase the eye relief anyway!  This is because Barlows preserve (and even slightly increase) the eye relief of the eyepiece they're used with.  Small eye relief means that you have to get very close to the eyepiece, and even then you may not be able to see the full field of view.  This is especially so if you wear glasses.

Additionally, Barlows often improve the performance of an eyepiece, especially towards the edge of the field. 

The multiplication factor varies but 2x is most common.  Some of these 2x Barlows can also be used at 1.5x, although it's not always mentioned in the blurb, and it's one of these I'd recommend.  These dual 1.5x/2x Barlows allow the black lens cell to be unscrewed from the body of the Barlow and then screwed into the filter thread at the bottom of an eyepiece to give approx 1.5x.   First Light Optics do one in their Astro Essentials range that even has a standard T thread at the top for attaching a camera.  It's just £25.  Go to https://www.firstlightoptics.com/barlows/astro-essentials-125-2x-barlow-with-t-thread.html

The exact amplification varies from eyepiece to eyepiece depending on where the field stop is located.  At 2x amplification with an 8-24mm zoom this will give you magnifications of approx 100-300x.  Most nights in the UK the seeing (atmospheric turbulence) won't be good enough to go as high as 300x but you'll really appreciate having the option when it does.  However, you'd get more use from the approx 75-225x that 1.5x amplification will give you.  Additionally, at a given magnification the field of view will be bigger with 1.5x amplification.  This is because the vast majority of zooms have a wider field of view at the high power end.
  

[amaury]

@Second Time Around thanks for the input. I actually bought the Svbony 8-24 zoom based on your feedback some weeks ago. It's outstanding value for the money. 

The Baader is around 3X more expensive than my zoom, I agree is sharper and more contrasty but not even close to 3x better. It has a wide field of view only in the last two shortest focal lengths. Low power is subpar and the 16mm is plossl-like at best, which is not bad, I just have a hard time trying to justify a 3x price.

 

 

  

[Spile]

I am also a fan of a good zoom and the figures for my 200P and Baader HPZ IV (ignore the last row it is for my only other EP) are not a million miles away from your exit pupil requirements.

[amaury]

@Spile The 12mm of the BHZ is pretty much bang on with a 2mm exit pupil. You x2.25 barlow it to 0.9mm exit pupil. That pretty much covers what I have in mind plus some extra high power alternatives (I mean x150 and x338). Do you use your x225 and x338 powers often? 

[Spile]

23 hours ago, amaury said:

@Spile The 12mm of the BHZ is pretty much bang on with a 2mm exit pupil. You x2.25 barlow it to 0.9mm exit pupil. That pretty much covers what I have in mind plus some extra high power alternatives (I mean x150 and x338). Do you use your x225 and x338 powers often? 

That is the thing, I kind of forget about focal lengths now I am using the zoom and just "twist" until everything looks just right. Of course it depends on conditions and if they are poor I won't even consider the Barlow. But when there is no turbulence etc and I have a close double then I'll rack up to 200x and beyond.  

I would say my observing time is split like this...

42mm WA - 30% 

Zoom - 60%

Zoom and Barlow - 10%

 

[amaury]

On 07/04/2021 at 20:19, Rob_UK_SE said:

I also observe in Bortle 4 skies and near to the South Downs (East Sussex / Birling Gap end). When going from a 4mm to 5mm exit pupil I don’t, personally, perceive any real benefit towards additional nebulosity in objects such as the Veil.

@Rob_UK_SE I was thinking about this comment. I understand you don't observe benefits when going from 4mm to 5mm exit pupil in terms of observing more faint details but do you observe contrast detriment when going from 4mm to 5mm?

I believe the other real perceivable advantage of going from 4mm to 5mm EP is the extra True Field of View, I just wonder how much the trade off  is in terms of contrast from a Bortle 4 location. 

[Zermelo]

On 07/04/2021 at 13:57, amaury said:

I have been planning my eyepiece selection based on exit pupil

 

Had you seen this thread:

 

 

[amaury]

1 hour ago, Zermelo said:

Had you seen this thread

Yes I have, very handy indeed.

The only uncertainty I have is with this statement:

"Most of us live under moderately light polluted skies. A large exit pupil could increase the sky background brightness so much that it appears light grey. Therefore, under moderately light polluted sky, it would be better not to have eyepieces with more than 4mm-4.5mm exit pupil"

That seems sensible if you are observing from a bortle 6 backyard.

I observe from a Bortle 4 area, I believe I can stretch that recommendation to 5mm exit pupil for my low power EP in that context.

[Ricochet]

36 minutes ago, amaury said:

That seems sensible if you are observing from a bortle 6 backyard.

I observe from a Bortle 4 area, I believe I can stretch that recommendation to 5mm exit pupil for my low power EP in that context.

In practice you probably wouldn't really notice the difference between exit pupils of 4.5mm and 5mm and either is fine. I think that really you want an eyepiece that will maximise the field of view so it is a case of choosing between: 

• 20mm 100°
• 30mm 82°
• 40mm 68°

In my 8" f6 dob my mono view set is 28mm, 14mm, 10mm and then a 2x telextender to give me 7 and 5. The "missing" 20mm (or 28mm) is the one you can probably skip, the rest I get regular use from. You might find a zoom or more closely spaced fix eyepieces are useful for the higher magnifications where you are hitting atmospheric limits. 

[Zermelo]

33 minutes ago, amaury said:

I observe from a Bortle 4 area, I believe I can stretch that recommendation to 5mm exit pupil for my low power EP in that context

Others on here will have much more experience than I do, but I too live in an area that's supposed to be Bortle 4 so I'll offer an opinion for what it's worth.
Until recently, the longest EP I possessed was a 26mm Plossl that gave me a 1.6 degree TFOV with an exit pupil of 5.2mm. Like you, I was wondering how far I could push it, and settled on a 30mm Vixen giving 2 degrees but at the cost of a 6mm pupil. The first outing was a bit disappointing, the background sky was noticeably lighter and the contrast not that great. But there was a half moon on the other side of the sky; when I tried it on a moonless night, the view was noticeably better. Based on that, I'd say 5mm wouldn't be too much for you.

[wookie1965]

On 07/04/2021 at 19:37, mikeDnight said:

If you observe using a blackout hood, you'll maximise your potential dark adaption and you could even exceed a 5mm exit pupil. Much of the trouble occurs as stray light hits the eye from the side. Just to illustrate how effective a hood can be, I've observed IC434 and the tiny black notch of the Horse Head using a 100mm refractor from the suburbs of Burnley in Lancashire using this method. As far as high powers go, I really think a barlow is a great idea. Modern barlow's, even the less expensive Skywatcher Delux are excellent, and because it won't break the bank it should be part of every observers kit. Most modern multi element eyepieces use an inbuilt barlow in their design, so Barlow's are not a bad thing. Again, if you ever get the urge to observe double stars, you'll really appreciate a high power eyepiece or barlow. Don't underestimate your telescope or your skies. My 100mm on an equatorial holds 500X with ease on binary stars and they are a joy to observe, so 300X on an 8" Dob is certainly doable.

Mike did you make your own or buy the hood I could do with one from home. 

[John]

My skies are around bortle 5.

With my 12 inch F/5.3 dobsonian I have a couple of low power / wide field options:

- 31mm / 82 degrees which gives 51x, a true field of 1.6 degrees and an exit pupil of 5.84mm

- 21mm / 100 degrees which gives 76x, a true field of 1.32 degrees and an exit pupil of 3.96mm

Most of the time I find the 21mm is the most effective eyepiece for picking out faint deep sky objects.

I've had a number of galaxy hunting sessions where the 21mm is the only eyepiece I've used in fact.

 

[Rob_UK_SE]

Hello amaury,

I most frequently observe from our garden which benefits from Bortle 4 skies. For low power views of large objects (m81 and m82 together, the Pleiades, m31, the Veil, Double Cluster etc.)  I can only echo John’s comments that I too prefer using a 21mm eyepiece. This results in an exit pupil of 4.2mm. I also have a 30mm, but find the contrast is less effective from home when compared to the 21mm. By contrast (excluding the terrible pun), when I travel to darker skies I find the larger exit pupil useful, but it is subtle to my eyes at least. The darker skies have a much more dramatic effect (comparing the 21mm at home vs away). I have recently ordered a new scope so it will be interesting to see the impact of the new focal ratio on these eyepieces as I am going from f5 to f4.6.

I think if I was observing in a Bortle 6 area I would probably be aiming for approximately a 3mm exit pupil as my low power option (16mm or 17mm).

Just to complicate matters further, it’s also worth considering the impact of an eyepiece’s light transmission, scatter and the quality of the coatings as they all play a part in the quality of views and contrast achieved too.

Rob

 

Edited April 11, 2021 by Rob_UK_SE

[amaury]

2 hours ago, Ricochet said:

In practice you probably wouldn't really notice the difference between exit pupils of 4.5mm and 5mm and either is fine. I think that really you want an eyepiece that will maximise the field of view so it is a case of choosing between: 

• 20mm 100°
• 30mm 82°
• 40mm 68°

In my 8" f6 dob my mono view set is 28mm, 14mm, 10mm and then a 2x telextender to give me 7 and 5. The "missing" 20mm (or 28mm) is the one you can probably skip, the rest I get regular use from. You might find a zoom or more closely spaced fix eyepieces are useful for the higher magnifications where you are hitting atmospheric limits. 

your three EP set plus a barlow is what I am planning more or less. I'm about to buy the APM Ultraflat field 30mm 70 AFOV . Price scales up rather quick with those premium 80 AFOV eyepieces. I believe that eyepiece is a good balance between optical quality, field of view, exit pupil, eye relief and value.

[amaury]

2 hours ago, Zermelo said:

Until recently, the longest EP I possessed was a 26mm Plossl that gave me a 1.6 degree TFOV with an exit pupil of 5.2mm. Like you, I was wondering how far I could push it, and settled on a 30mm Vixen giving 2 degrees but at the cost of a 6mm pupil.

I have a 25mm Plossl, with my 8" f/6 dob, it gives me a very narrow 1.04 TFOV at x 48 and 4.16mm exit pupil. Very narrow but sharp edge to edge.  I'm about to buy the APM UFF 30mm, that gives me 1.75 TFOV (that's a much wider field of view relative to what I have) at x40 with 5mm exit pupil.

2 hours ago, Zermelo said:

Based on that, I'd say 5mm wouldn't be too much for you.

Thanks, that is encouraging