Confirm my understanding of EP's!? Exit pupil, FL, FOV.......

[parallaxerr]

I have been thinking about a new scope for some time but feel I haven’t done enough with my current scope, a Sky-Watcher ED80. So, I though it may be time to move on from my Baader Hyperion Zoom and get some fixed FL EP’s, specific to what I like to view.

I have read a lot about exit pupil diameter, how it affects viewing and what size exit pupils are good for various objects and conditions and have started calculating my EP requirements based on what’s realistic with my scope and my favourites objects to date, which consist of the following:

M42 – Orion Nebula, M45 – Pleiades, M31 – Andromeda & Double Cluster. Moon, Jupiter, Saturn & Mars (albeit very very small!)

From this list I have arrived at EP’s at either end of the FL range and figure I’ll split the difference with a few others in order to help me search out other objects.

The limiting factor at the lower FL end of the range appears to be exit pupil, I have read that 1mm is about the best for planetary. When adding the barlow to my Hyperion Zoom I do suffer from floaters as I crank up the mag and exit pupil gets down to around 0.7mm (although this may be made worse by the poor eye-relief & FOV in this configuration), so maybe I should aim for 0.8mm to 1mm. Realistically this means my shortest EP would be a 6mm giving x100 mag. Some people claim they have successfully pushed the ED80 to x240 but I’ve never had seeing good enough and even when it is good, anything above x100 gets “soft”, maybe due to the fact I’m using a zoom and a barlow, that’s a lot of lenses!

Interestingly, I have a cheap Celestron 12mm Plossl and barlow and they appear to match the performance of the Baader at the same FL which goes some way to confirming that cheaper EP designs with fewer lenses win on planets.

For wider field stuff the exit pupil can grow and although up to 7mm is possible, several articles state that 3-4mm is more realistic and better suited for light polluted skies. A 25mm at 50 degree FOV gives a 3.33mm exit pupil and frames the larger objects (M42, M45 etc.) quite nicely.

I had a thought that a shorter FL EP with a wider FOV would be beneficial in as much as I get a larger image, but then I realised the exit pupil goes down in size which will reduce the brightness.

I have been eyeing up the Williams Optics EP’s from FLO, specifically the 6mm SPL Planetary for high power and the SWAN’s for wider field, they are at a nice price point and get good reviews. I have also been considering the Vixen SLV’s as they have a broad range of FL’s and I’d prefer to have a matching set!

Every time I understand one element of EP choice, another factor creeps in, but I hope I have most if not all bases covered now. So the question is, am I barking up the right tree with my summation?

 

Cheers all, Jon

[cloudsweeper]

Just been working on all this.  Exit pupil = EP focal length divided by focal ratio.  7mm is good for brightness, esp. in dark skies.  4 or 5mm is better with poor skies, as it dims the background, so increasing contrast.  Smaller values are used for higher mag/detail.  Just work out what you need for a particular 'scope.

Perhaps the most valuable lesson concerning exit pupil is that it determines surface brightness of extended (not point or star-like) sources, which has nothing to do with aperture!  Where aperture comes in is that for a larger aperture you get higher mag for a given brightness.

Exit pupil really is a very important parameter in observation.

Doug.

[John]

Being in my mid-50's I reckon my max dilated pupil is around 6mm so I tend to stick to this as a max exit pupil size.

Using ultra and hyper wide eyepieces (which I also rather like for their views ) also means that I can get as much sky as possible in the field AND maintain a sensible exit pupil with my scopes that range from F/5.3 to F/7.5.

So my max field / min magnification eyepieces are 31mm 82 degrees AFoV and 21mm 100 degrees AFoV respectively.

[parallaxerr]

3 hours ago, cloudsweeper said:

Perhaps the most valuable lesson concerning exit pupil is that it determines surface brightness of extended (not point or star-like) sources, which has nothing to do with aperture!  Where aperture comes in is that for a larger aperture you get higher mag for a given brightness.

Exit pupil really is a very important parameter in observation.

Hi Doug,

That sounds very much like the summary I read on the Astronomics website which made me think I need to maintain a decent exit-pupil for faint objects. Thanks for confirming that exit pupil is as important as I thought!

[parallaxerr]

51 minutes ago, John said:

Being in my mid-50's I reckon my max dilated pupil is around 6mm so I tend to stick to this as a max exit pupil size.

Using ultra and hyper wide eyepieces (which I also rather like for their views ) also means that I can get as much sky as possible in the field AND maintain a sensible exit pupil with my scopes that range from F/5.3 to F/7.5.

So my max field / min magnification eyepieces are 31mm 82 degrees AFoV and 21mm 100 degrees AFoV respectively.

Makes sense John, thanks. Due to the ratio of my scope I don't think I'll get anywhere near my max exit pupil, I'd need a 52m EP but at least I now know to keep it as large as possible for faint objects.

I am swinging towards the Vixen SLV's, so did you return or pay for the one's FLO loaned you?

[Alien 13]

I have looked through instruments with exit pupils between 10 mm and 20  mm and the view was marvelous needs a dark sky though.

Alan

[John]

3 hours ago, parallaxerr said:

...I am swinging towards the Vixen SLV's, so did you return or pay for the one's FLO loaned you?

I returned them to FLO because they were on loan. FLO subsequently sold them at a good discount in their clearance section. They are very nice eyepieces - I was impressed with them

[Louis D]

If you're willing to put up with a slightly dimmer view (no one seems to know why) and a 45 degree field instead of a 50 degree field below 9mm, the discontinued, Japanese made Vixen LVs are a bargain on the used market for around $60 to $75 apiece.  They were around $100 when new 18 years ago when I bought mine.  They run rings around the BST Explorer, XCel LX, HD-60, etc. due to their very high polish, excellent internal blackening, superb multicoatings, lack of kidney-beaning, and true 20mm of eye relief.

[Stu]

23 hours ago, parallaxerr said:

I have been thinking about a new scope for some time but feel I haven’t done enough with my current scope, a Sky-Watcher ED80. So, I though it may be time to move on from my Baader Hyperion Zoom and get some fixed FL EP’s, specific to what I like to view.

 

I have read a lot about exit pupil diameter, how it affects viewing and what size exit pupils are good for various objects and conditions and have started calculating my EP requirements based on what’s realistic with my scope and my favourites objects to date, which consist of the following:

 

M42 – Orion Nebula, M45 – Pleiades, M31 – Andromeda & Double Cluster. Moon, Jupiter, Saturn & Mars (albeit very very small!)

 

From this list I have arrived at EP’s at either end of the FL range and figure I’ll split the difference with a few others in order to help me search out other objects.

 

The limiting factor at the lower FL end of the range appears to be exit pupil, I have read that 1mm is about the best for planetary. When adding the barlow to my Hyperion Zoom I do suffer from floaters as I crank up the mag and exit pupil gets down to around 0.7mm (although this may be made worse by the poor eye-relief & FOV in this configuration), so maybe I should aim for 0.8mm to 1mm. Realistically this means my shortest EP would be a 6mm giving x100 mag. Some people claim they have successfully pushed the ED80 to x240 but I’ve never had seeing good enough and even when it is good, anything above x100 gets “soft”, maybe due to the fact I’m using a zoom and a barlow, that’s a lot of lenses!

 

Interestingly, I have a cheap Celestron 12mm Plossl and barlow and they appear to match the performance of the Baader at the same FL which goes some way to confirming that cheaper EP designs with fewer lenses win on planets.

 

For wider field stuff the exit pupil can grow and although up to 7mm is possible, several articles state that 3-4mm is more realistic and better suited for light polluted skies. A 25mm at 50 degree FOV gives a 3.33mm exit pupil and frames the larger objects (M42, M45 etc.) quite nicely.

 

I had a thought that a shorter FL EP with a wider FOV would be beneficial in as much as I get a larger image, but then I realised the exit pupil goes down in size which will reduce the brightness.

 

I have been eyeing up the Williams Optics EP’s from FLO, specifically the 6mm SPL Planetary for high power and the SWAN’s for wider field, they are at a nice price point and get good reviews. I have also been considering the Vixen SLV’s as they have a broad range of FL’s and I’d prefer to have a matching set!

 

Every time I understand one element of EP choice, another factor creeps in, but I hope I have most if not all bases covered now. So the question is, am I barking up the right tree with my summation?

 

 

 

Cheers all, Jon

 

Jon, I think in general you have chosen the right tree to bark up ?

I have used numerous small Apo refractors, all doublets similar to the ED80 ranging from a Tak 60mm and William Optics SD 66mm, through to a TV85. I have generally found that they are capable of taking high magnification down to an exit pupil of around 0.5mm. At this sort of level I do suffer from floaters but am willing to look around them to get the view. This means around x140 to x160 on a good night with the ED80 should be attainable for planetary observing. One or two good eyepieces in this area would be very useful I think.

You probably then have maybe three other exit pupil sizes which are useful. Around 2mm supposedly gives optimum contrast for small, fainter objects such as galaxies and planetary nebulae. 3 to 4mm is good for larger objects such as open clusters and large nebulae such as the Veil. Under a dark sky you can open things up much more - stick a widefield 31mm nag in or an ES 30mm 82 degree and you get lovely wide fields with a large exit pupil which will take a UHC or OIII filer very nicely.

 

[parallaxerr]

Hi Stu,

Thanks for the informative post. You have confirmed my thinking with regards to choosing my EP's based on exit pupil. Given that observing is limited due to daylight hours at the moment, I have started with planetary EP's.

I nearly pulled the trigger on the WO SPL's as I'm not in the position to commit to Naglers etc. at the moment and the WO's get reasonable reviews. But, then I came across a post suggesting that they're available under several different brands...

RVO have them at the best price I could find here...

http://www.rothervalleyoptics.co.uk/altair-lightwave-ler-125-planetary-eyepieces.html

Ideally I wanted a 4, 5 & 6mm but there's no 4mm in the range, however, I remembered I have the Baader Hyperion 1.25" x2.25 barlow which will give 4mm when fitted to the 9mm. The 9mm may also be useful on some other objects so I ordered 5, 6 & 9mm.

Below are the options I have with the Baader barlow, a nice range of mags and exit pupils for planetary observing. The 2.2 & 2.7mm will likely be too much for my ED80, having said that, the moon is good down to 3.5mm with the Hyperion Zoom barlowed, so you never know.

When the nights start drawing in I'll start thinking about wider EP's for nebulae/DSO's but I'm interested to see what these can do in the mean time.

Cheers, Jon.

[Stu]

Good stuff Jon. I assume you mean 5, 6 and 9? That should give you a good spread so you can experiment with what works best for you. The 5 native and 9 barlowed should give good high mags for you.

Let us know how you get on.

Stu

[parallaxerr]

Oops, quite correct Stu, 5, 6 & 9mm....corrected