2mm Exit Pupil for DSO = Perfection?

[Stardaze]

I'm debating my next EP purchase and thought I'd glean some info surrounding the above. Currently I have: APM XWA 20 & 13, Ethos 8, ES 6.7 82, XW 5 all for a 10" f5 dob. I have been thinking of moving the ES 6.7 on and tidying the high power element up, maybe an E6 or Delos 6 and eventually a 3.5 too. The XW 5 is a big jump from the 8mm. 

The other point that I have been deliberating is the mention of a 2mm exit pupil as being perfection for DSO hunting, which I really enjoy. I have thought of adding a Baader Classic 10mm ortho, maybe the 6mm too if I like it, as a cost-effective intro for me here. So, how does a 10mm stack up amongst this lot in your opinion?

 

13mm - x98

10mm - x127

8mm - x159

6mm - x211

5mm - x254

[Ricochet]

I've read somewhere that the optimum is actually 2.4mm but so long as you have something in the ball park you'll do fine. What you need to remember is that this is for observing unfiltered extended objects only. For anything using a filter you need to increase the exit pupil and for objects that resolve as point sources (open and globular star clusters) you can go down to about 1mm. 

Personally, I think it is much more important to choose your eyepieces based on exit pupil rather than magnification for DSO observation. Choosing steps based on doubling or halving the image brightness works very well. Choosing eyepiece focal lengths based on this is easily done by starting with an existing eyepiece and either multiplying or diving the focal length by 1.4. 

Using this method both your 13-8 and 8-5 jumps are too big. If I were in your position my choice would be to sell the 8 and 6.7 and replace them with 10 and 7mm XWs, if you are happy with the narrower field of view. 

[Littleguy80]

I would say the 2mm exit pupil is an excellent guide but don't take it as a hard and fast rule. With my 10" dob, I have an APM HDC 20mm, Noblex 12.5mm (2.65mm exit pupil) and Delos 10mm (2.12mm exit pupil. After that I barlow the Noblex to get an approximate 7mm eyepiece before the XW 5mm. I recently compared a 7mm DeLite to the barlowed Noblex. Different experiences but the dedicated DeLite showed no more than the Noblex. Lovely eyepiece though. I also have a 9mm Baader Genuine Ortho (1.91mm exit pupil). This comes out when I'm looking for really faint stuff. It goes a bit deeper than the 10mm Delos. 

I was once told that experience is the best guide on eyepiece selection. Based on current usage, what are you feelings? If you work up through your eyepieces on a DSO, does it feel like a good progression? The 10mm BCO would be a good option. Inexpensive and will go deep when you need it. I find I use my orthos as more specialist eyepieces, used on trickier targets though. Most of the time I'm mainly using the 20mm APM and 12.5mm Noblex.

One further note, the Fujiyama Orthos are very similar to the Baader Genuine Orthos are offer a few more focal length/exit pupil options than the BCOs. The 6mm BCO is supposed to be the weakest of that range.

Edited May 7, 2021 by Littleguy80

[Stardaze]

56 minutes ago, Ricochet said:

I've read somewhere that the optimum is actually 2.4mm but so long as you have something in the ball park you'll do fine. What you need to remember is that this is for observing unfiltered extended objects only. For anything using a filter you need to increase the exit pupil and for objects that resolve as point sources (open and globular star clusters) you can go down to about 1mm. 

Personally, I think it is much more important to choose your eyepieces based on exit pupil rather than magnification for DSO observation. Choosing steps based on doubling or halving the image brightness works very well. Choosing eyepiece focal lengths based on this is easily done by starting with an existing eyepiece and either multiplying or diving the focal length by 1.4. 

Using this method both your 13-8 and 8-5 jumps are too big. If I were in your position my choice would be to sell the 8 and 6.7 and replace them with 10 and 7mm XWs, if you are happy with the narrower field of view. 

So my 13mm provides 2.6mm so is in the ball-park, but there's quite a gap to the 8mm (1.6mm) on that basis. I think that from 10mm and below, I am happy to sacrifice FOV for a better presentation. I'm not totally convinced with the eye positioning of my XW but then it's been a while since I used it (mostly for Mars last year) and it could just be down to a difference to my other EP's, regular use will no doubt solve it. Thanks for your input. 

43 minutes ago, Littleguy80 said:

I would say the 2mm exit pupil is an excellent guide but don't take it as a hard and fast rule. With my 10" dob, I have an APM HDC 20mm, Noblex 12.5mm (2.65mm exit pupil) and Delos 10mm (2.12mm exit pupil. After that I barlow the Noblex to get an approximate 7mm eyepiece before the XW 5mm. I recently compared a 7mm DeLite to the barlowed Noblex. Different experiences but the dedicated DeLite showed no more than the Noblex. Lovely eyepiece though. I also have a 9mm Baader Genuine Ortho (1.91mm exit pupil). This comes out when I'm looking for really faint stuff. It goes a bit deeper than the 10mm Delos. 

I was once told that experience is the best guide on eyepiece selection. Based on current usage, what are you feelings? If you work up through your eyepieces on a DSO, does it feel like a good progression? The 10mm BCO would be a good option. Inexpensive and will go deep when you need it. I find I use my orthos as more specialist eyepieces, used on trickier targets though. Most of the time I'm mainly using the 20mm APM and 12.5mm Noblex.

One further note, the Fujiyama Orthos are very similar to the Baader Genuine Orthos are offer a few more focal length/exit pupil options than the BCOs. The 6mm BCO is supposed to be the weakest of that range.

I'm glad you've responded Neil as your collection and reasoning behind what you use has struck a chord with me for a while, I think you may have also mentioned the 2mm when you bought your Delos. I do use the APM 13 and 20 the most for sure but there is a very noticeable drop in brightness from the 13 to 8, it can sometimes take me a while to acclimate to that. The Delos 10 has looked quite interesting to me too but following @Ricochet advice, I'd need a 6.5/7 to plug the gap to the XW5 (Delos offering 8mm/6mm of course). Thanks for the info, plenty to mull over and in the end, I guess there's no substitute for trying something. I'll look in to the Fuji Ortho's too.

Edited May 7, 2021 by Stardaze

[Littleguy80]

A 10mm will sit well between your 13mm and 8mm. Beyond that, I'm not sure you need to change too much. When you get to the shorter focal lengths it's nice to have a larger selection so you can adjust the view to fit the seeing. 

[Stardaze]

2 minutes ago, Littleguy80 said:

A 10mm will sit well between your 13mm and 8mm. Beyond that, I'm not sure you need to change too much. When you get to the shorter focal lengths it's nice to have a larger selection so you can adjust the view to fit the seeing. 

Looking at the 1mm drop does now highlight a gap here. I'll maybe put my funds here instead. That said, the impending few months might be better invested into the shorter end to enjoy some lunar and planets. Maybe the XW 3.5/5/7 combo with a 10mm Delos works better too. 

[Littleguy80]

4 minutes ago, Stardaze said:

Looking at the 1mm drop does now highlight a gap here. I'll maybe put my funds here instead. That said, the impending few months might be better invested into the shorter end to enjoy some lunar and planets. Maybe the XW 3.5/5/7 combo with a 10mm Delos works better too. 

That would make a nice setup. I have a Vixen HR 3.4mm at the top end for those nights when the seeing is at it's best. It's more of an ortho like eyepiece with a narrower FOV but very very good. That focal length will be used quite infrequently with a 10" dob and generally only when the seeing is excellent. That's why I went a bit more specialised with that. The XW 5mm is the normally where I stop for planetary Lunar. 

[Stardaze]

3 minutes ago, Littleguy80 said:

That would make a nice setup. I have a Vixen HR 3.4mm at the top end for those nights when the seeing is at it's best. It's more of an ortho like eyepiece with a narrower FOV but very very good. That focal length will be used quite infrequently with a 10" dob and generally only when the seeing is excellent. That's why I went a bit more specialised with that. The XW 5mm is the normally where I stop for planetary Lunar. 

Yes I realise that shorter than 5mm would be for very infrequent use so wasn't on the priority list just now. The HR does get excellent reviews and is a little more specialist I've noted, have they been discontinued now though? I want to get the 5mm-13mm options plugged for now. A long way off but I'd also like to dabble with a APM 30 too. Trying to get it right and buy once. 🙂

[Littleguy80]

43 minutes ago, Stardaze said:

Yes I realise that shorter than 5mm would be for very infrequent use so wasn't on the priority list just now. The HR does get excellent reviews and is a little more specialist I've noted, have they been discontinued now though? I want to get the 5mm-13mm options plugged for now. A long way off but I'd also like to dabble with a APM 30 too. Trying to get it right and buy once. 🙂

Yeah, HR’s are discontinued but seem to come up secondhand quite frequently. 

The APM UFF 30mm is very good. The FOV is about the same as the APM HDC 20mm. The larger exit pupil makes it good for filters though. Looking forward to testing it on the Veil soon. 

[Stardaze]

8 minutes ago, Littleguy80 said:

Yeah, HR’s are discontinued but seem to come up secondhand quite frequently. 

The APM UFF 30mm is very good. The FOV is about the same as the APM HDC 20mm. The larger exit pupil makes it good for filters though. Looking forward to testing it on the Veil soon. 

The FOV is slightly more but the exit pupil difference was more the idea given filter use. It should provide a welcome boost of brightness with the 0III. I didn't realise you had one.

[Littleguy80]

1 minute ago, Stardaze said:

The FOV is slightly more but the exit pupil difference was more the idea given filter use. It should provide a welcome boost of brightness with the 0III. I didn't realise you had one.

Yep, sold the ES82 30mm and got that as a replacement. Similar idea of improving optical quality with a drop in FOV. The whole FOV feels more accessible with the 30mm UFF vs the 20mm HDC/XWA. The APM is a lot lighter and smaller than the ES so it works better with the frac for widefield. The whole Veil complex in a single FOV is a special thing!

[Piero]

An exit pupil of 2.0mm is a good all-around, a trade-off between brightness and magnification. In my opinion, it is good to consider it as a reference for a medium power when building up an eyepiece set, not as a rigid constraint.

[Stardaze]

17 minutes ago, Littleguy80 said:

Yep, sold the ES82 30mm and got that as a replacement. Similar idea of improving optical quality with a drop in FOV. The whole FOV feels more accessible with the 30mm UFF vs the 20mm HDC/XWA. The APM is a lot lighter and smaller than the ES so it works better with the frac for widefield. The whole Veil complex in a single FOV is a special thing!

Once I have my EP case where I want it to be (couple of filters to add too mind) I would like a frat to complement the dob. I love the idea of being able to get the whole of the  Veil into view. 

[Littleguy80]

53 minutes ago, Stardaze said:

Once I have my EP case where I want it to be (couple of filters to add too mind) I would like a frat to complement the dob. I love the idea of being able to get the whole of the  Veil into view. 

I have a short 80mm frac (500mm focal length). It's nice for widefield and white light solar. I got it as a travel scope that was airline portable. Seems funny to think of that as a requirement at the moment. While I enjoy the frac, the dob is always my first choice. I guess that speaks to what I enjoy as much as the quality of the two scopes.

[Stardaze]

9 minutes ago, Littleguy80 said:

I have a short 80mm frac (500mm focal length). It's nice for widefield and white light solar. I got it as a travel scope that was airline portable. Seems funny to think of that as a requirement at the moment. While I enjoy the frac, the dob is always my first choice. I guess that speaks to what I enjoy as much as the quality of the two scopes.

I think I recall you buying it now you mention for travelling. I'm not in any rush just yet, I think anything else will compromise my views a little anyway, but would sure be great for those quick, last minute sessions. Be handy for lunar and planets in the main too. 

[Littleguy80]

7 minutes ago, Stardaze said:

I think I recall you buying it now you mention for travelling. I'm not in any rush just yet, I think anything else will compromise my views a little anyway, but would sure be great for those quick, last minute sessions. Be handy for lunar and planets in the main too. 

It's a nice middle ground between binoculars and the dob. When the seeing is good my dob will still outperform on Lunar and Planetary. 

[jetstream]

I had a very good 10mm Ethos that I stupidly sold, very high contrast- the current 10mm Delos goes a little bit deeper but I miss the big view of the E. The 10mm Delos is a mainstay of use for me across the dobs.

@Stardaze if it were me I'd sell the (very nice ) 6.7 ES 82- go for the low scatter XW 7mm, get a 10mm Delos and thats about it. If a 3.5mm XW came wandering by I would grab it but it would be last on my list.

You have a very good EP collection happening.

Re: 2mm exit pupil

Its a very good place to be DSO (galaxies) wise at your focal length and with a widefield like the 10mm Delos. Just my thoughts.

 

[Stardaze]

21 minutes ago, jetstream said:

I had a very good 10mm Ethos that I stupidly sold, very high contrast- the current 10mm Delos goes a little bit deeper but I miss the big view of the E. The 10mm Delos is a mainstay of use for me across the dobs.

@Stardaze if it were me I'd sell the (very nice ) 6.7 ES 82- go for the low scatter XW 7mm, get a 10mm Delos and thats about it. If a 3.5mm XW came wandering by I would grab it but it would be last on my list.

You have a very good EP collection happening.

Re: 2mm exit pupil

Its a very good place to be DSO (galaxies) wise at your focal length and with a widefield like the 10mm Delos. Just my thoughts.

 

Thanks Jerry, I'm going to do exactly this. Just taken some pics of the EP's that will need new homes to fund this direction, thanks for your input, appreciated. Eventually I can see me swapping the APM 13 for an Ethos for those big views, that will future-proof me for sure. 

[Louis D]

2 hours ago, jetstream said:

go for the low scatter XW 7mm, get a 10mm Delos and thats about it. If a 3.5mm XW came wandering by I would grab it but it would be last on my list.

I have the 10mm Delos and love it.  Absolutely no flaws edge to edge even without a coma corrector at f/6 (yes, I've swapped repeatedly and found no difference).  By comparison, the 12mm ES-92 really needs a CC.

I also have the 9mm Morpheus, and it appears to be very close to Delos quality.

The 7mm XW shows a bit of chromatic aberration off axis on bright stars, so a bit of a disappointment for me.  Otherwise, it is quite sharp and contrasty.

My venerable 5.2mm XL is still an excellent performer at that focal length.  Sharp and contrasty edge to edge with no faults that I can detect.

The 3.5mm XW is very sharp and lacks any faults that I can see.  However, that is one tiny exit pupil it produces, so I rarely use it.  I tend to binoview at those exit pupils to keep floaters at bay.  I picked it up for $215 during an Amazon flash sale, otherwise I wouldn't own it.

All that said, I've thought about picking either or both 6mm and 8mm Delos eyepieces.  However, I rarely feel a need to split the difference between the 7mm and 9mm and 7mm and 5.2mm eyepieces.  I do have the 5-8mm Speers-Waler zoom if I really feel the need for those focal lengths.  However, it's eye relief is lacking for eyeglass wearers despite the LER of Waler meaning long eye relief.

I highly recommend the 30mm APM UFF.  It's basically flawless edge to edge with high sharpness and contrast.

[Don Pensack]

Choosing eyepieces based on exit pupil has some problems.

1) It results in having the low power magnifications be too close together.

Take the 8" f/6 scope.  A 24mm eyepiece yields 50x and a 4mm exit pupil.

A 1.414 increase (50% difference in brightness) is an exit pupil of 2.8mm, but is only 70x.  That's too close to 50x to see any appreciable difference in magnification.

From experience, I can tell you it takes 35-40x spacing to even notice the difference in magnification.

2) it results in high power eyepieces being too far apart.

Take the same 8" f/6 scope. A 1mm exit pupil is a 6mm eyepiece at 200x.

A 1.414 increase in exit pupil is an exit pupil of 0.7mm, with is an eyepiece of 4.2mm.  

But 4.2mm is a magnification of 285x.  That is too much of a jump at the high end where you might be bumping into the ceiling allowed by your Seeing conditions.  Smaller jumps,

like 40-50x are more practical as you edge up on the limit, specially if you are doing lunar/planetary observing.

 

So I advocate ignoring exit pupil and picking eyepieces based on a rational pattern of magnification increases.

In that 8" scope (I owned one that size for 11 years), I found jumps of 50x just about ideal to have enough change from one to the other but not make the high power jumps too large.

And a 50x/100x/150x/200x/250x sequence is exit pupils of 4mm, 2mm, 1.3mm, 1.0mm, 0.8mm

Edited May 7, 2021 by Don Pensack

[Louis D]

1 hour ago, Don Pensack said:

And a 50x/100x/150x/200x/250x sequence is exit pupils of 4mm, 2mm, 1.3mm, 1.0mm, 0.8mm

And I'll once again advocate for widest field at about 30x and exit pupil of 6.7mm with something like a 40mm SWA.  It's a noticeable jump up in true field of view when trying to frame large clusters like the Pleiades or Collinder 70, that is unless you're using a 25mm ES-100 to get a 4.2mm exit pupil, in which case the difference is only 0.25 degrees, so not a big improvement then.

On the other hand, a 6.7mm exit pupil is not very useful for hunting faint fuzzies due to the washed out background, but it's hardly noticeable when taking in bright clusters.  That TFOV is also handy for centering bright objects when your finders aren't exactly aligned with the main scope.

[Stardaze]

2 hours ago, Don Pensack said:

Choosing eyepieces based on exit pupil has some problems.

1) It results in having the low power magnifications be too close together.

Take the 8" f/6 scope.  A 24mm eyepiece yields 50x and a 4mm exit pupil.

A 1.414 increase (50% difference in brightness) is an exit pupil of 2.8mm, but is only 70x.  That's too close to 50x to see any appreciable difference in magnification.

From experience, I can tell you it takes 35-40x spacing to even notice the difference in magnification.

2) it results in high power eyepieces being too far apart.

Take the same 8" f/6 scope. A 1mm exit pupil is a 6mm eyepiece at 200x.

A 1.414 increase in exit pupil is an exit pupil of 0.7mm, with is an eyepiece of 4.2mm.  

But 4.2mm is a magnification of 285x.  That is too much of a jump at the high end where you might be bumping into the ceiling allowed by your Seeing conditions.  Smaller jumps,

like 40-50x are more practical as you edge up on the limit, specially if you are doing lunar/planetary observing.

 

So I advocate ignoring exit pupil and picking eyepieces based on a rational pattern of magnification increases.

In that 8" scope (I owned one that size for 11 years), I found jumps of 50x just about ideal to have enough change from one to the other but not make the high power jumps too large.

And a 50x/100x/150x/200x/250x sequence is exit pupils of 4mm, 2mm, 1.3mm, 1.0mm, 0.8mm

 

35 minutes ago, Louis D said:

And I'll once again advocate for widest field at about 30x and exit pupil of 6.7mm with something like a 40mm SWA.  It's a noticeable jump up in true field of view when trying to frame large clusters like the Pleiades or Collinder 70, that is unless you're using a 25mm ES-100 to get a 4.2mm exit pupil, in which case the difference is only 0.25 degrees, so not a big improvement then.

On the other hand, a 6.7mm exit pupil is not very useful for hunting faint fuzzies due to the washed out background, but it's hardly noticeable when taking in bright clusters.  That TFOV is also handy for centering bright objects when your finders aren't exactly aligned with the main scope.

Well, I'm pretty committed now as the 8mm and 6.7mm have sold..

So my idea of a lineup would be:

30        41x.        6mm ep

20        64x.        4mm 

13         98x.       2.6mm

10        127x.       2mm

7          181x.       1.4mm

5.         254x.      1mm

3.5.      363x.      0.7mm

Looks great from an exit pupil perspective but there are some large jumps from 7mm down. I agree that a 3.5 wouldn't be used much and so bottom of the shopping list for sure. It's a massive jump as Don highlights from the 5mm but it does provide a 32x magnification? Even 5mm to 7mm is a large jump.

@Louis DI've never thought of a 40mm as being of use really as it gives an 8mm exit pupil but magnification of 32x? Obviously some of that light will be wasted but is the FOV with a filter possibly still worth it? 

 

Edited May 7, 2021 by Stardaze

[jetstream]

21 minutes ago, Stardaze said:

but is the FOV with a filter possibly still worth it? 

No.

[Littleguy80]

26 minutes ago, Stardaze said:

Obviously some of that light will be wasted but is the FOV with a filter possibly still worth it? 

I got a 40mm Aero ED for use with my frac. I’ve used it with the dob and filters and had some good results. I’m not sure I would buy a 40mm just for the dob. It’s behind everything else on your list for sure. 

[Stardaze]

44 minutes ago, jetstream said:

No.

 

37 minutes ago, Littleguy80 said:

I got a 40mm Aero ED for use with my frac. I’ve used it with the dob and filters and had some good results. I’m not sure I would buy a 40mm just for the dob. It’s behind everything else on your list for sure. 

I didn't think so, but thought you guys knew something I didn't for a moment there 😀

Edited May 7, 2021 by Stardaze