Eyepiece selection based on exit pupil

[Deadlake]

19 minutes ago, jetstream said:

Contrast is fixed and is the difference between sky brightness and the surface brightness of the object.

What do you mean by surface brightness for an object, what happens when that object is extended, that is difficult to define.

@jetstream still waiting for Tele Vue filters and APM EP’s.

Edited April 12, 2021 by Deadlake

[jetstream]

An object that is not a point source is an extended object- they dim equally (extended object and sky) with magnification unlike stars. I'm not the best to answer all this eventhough I do understand it.

From Clark, Blackwell Bartels Carlson etc

" A long held concept in amateur astronomy observing is to increase the magnification of a telescope to "increase the contrast of the object being viewed." While the effect is real, the explanation is incorrect. As one changes magnification, all objects change size (e.g. object and sky background), so the contrast stays constant. But the eye's sensitivity to contrast changes as the object size changes, with lower contrast objects easier to detect when they appear larger, meaning at higher power. "

" its surface brightness (e.g. apparent magnitude per square arc-second)"

"

A low-contrast object is more easily detected if it is larger. For an extended object such as a galaxy viewed in a telescope, magnification does not change the contrast with the background, because both the sky's and the object's surface brightnesses are affected equally. Some visual observers have stated that a dim object's contrast with the sky background increases with higher magnification, but this is clearly wrong. The contrast merely looks greater because of the increased detection capabilities of the eye. Clark (1990) coined a name for the maximum magnification that will help detection: the "optimum magnified visual angle" (OMVA). This angle is shown in Figure 2.6 and also Figure 2.7b."

https://www.bbastrodesigns.com/visual.html

https://www.bbastrodesigns.com/blackwel.html

 

 

[wookie1965]

2 hours ago, faulksy said:

just ordered 2 from https://r-sky.org/en/payment-and-shipping

also ordered 2 monk hoods of ebay last night, dont know if they will be any good though 🤣

Can you send me a link to the ones on ebay please 

[bomberbaz]

2 hours ago, faulksy said:

just ordered 2 from https://r-sky.org/en/payment-and-shipping

I have ordered one of these too, prefer something properly made than getting all heath robinson. 

Why buying so many @faulksy, you grown a spare head in lockdown 😂

[faulksy]

16 minutes ago, bomberbaz said:

I have ordered one of these too, prefer something properly made than getting all heath robinson. 

Why buying so many @faulksy, you grown a spare head in lockdown 😂

you could say that steve

what it was, i ordered the ebay ones last night, then today i thought i would get the proper ones 🤣

[faulksy]

30 minutes ago, wookie1965 said:

Can you send me a link to the ones on ebay please 

here you go paul

https://www.ebay.co.uk/itm/Newest-Medieval-Hooded-Cowl-Hat-Monk-Clergy-Witch-Wizard-Cape-Halloween-Costume/154088170119?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3Da1ae6034b5a14490b4319016a5596b9c%26pid%3D100675%26rk%3D1%26rkt%3D15%26mehot%3Dnone%26sd%3D154088170119%26itm%3D154088170119%26pmt%3D0%26noa%3D1%26pg%3D2380057%26brand%3DUnbranded&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3A591aff00-9bd6-11eb-acc7-46f19c705b38|parentrq%3Ac7fe81161780aaecf68c5892ffed3994|iid%3A1

[John]

59 minutes ago, jetstream said:

....The contrast merely looks greater because of the increased detection capabilities of the eye....

 

 

As a visual observer, if the contrast looks greater, the target object stands out a little better. As my only "tools" apart from the scope and eyepiece are my eye and brain, if I've optimized their operation through selecting a certain eyepiece and got a better view, or maybe seen something rather than nothing, then that's a win as far as I'm concerned

 

 

 

[Stu]

2 hours ago, amaury said:

Thanks @Stu, as @johninderby said, using exit pupil alone or any variable alone is an over simplification of a very complex subject such as the visibility of a given astronomical object. 

That’s basically what I was saying; you implied that contrast was independent of exit pupil, which it is, but the object visibility is what is explained and calculated in the link I posted. Effectively it is related to exit pupil as per the graphs produced.

[Stu]

2 hours ago, Deadlake said:

Thanks @Stu, i’ll remember that contrast is on the hot list of subjects like the glass question etc..😃😀😃

My apologies, was just attempting to contribute to the conversation.

[jetstream]

30 minutes ago, Stu said:

That’s basically what I was saying; you implied that contrast was independent of exit pupil, which it is, but the object visibility is what is explained and calculated in the link I posted. Effectively it is related to exit pupil as per the graphs produced.

100% Agree Stu- when we get right down to it exit pupil is a huge factor. An example is my 24" f4.1- it gives big magnification at nice bright exit pupils. The ability to observe with the parameters mentioned gives views that are a sight to behold on brighter galaxies and DSO and with this big fast scope I dont usually "up the mag for contrast" as the 2500mm fl gives the big image scale and the f4.1 gives great eye illumination.

IMHO the real goal is to observe targets with as much eye illumination as possible and with a scope that provides the needed mag.

Since eye illumination is provided by the exit pupil it is a major factor .

There is contrast then there is "perceived contrast" which @John is referring to.

[Deadlake]

8 hours ago, Stu said:

My apologies, was just attempting to contribute to the conversation.

Sorry @Stu if I've caused offence, just surprised at the number of comments when noting contrast and exit pupil.

[amaury]

8 hours ago, Stu said:

My apologies, was just attempting to contribute to the conversation.

That is appreciated Stu. I am learning a lot from you guys. 

8 hours ago, jetstream said:

24" f4.1- it gives big magnification at nice bright exit pupils.

Wow!! that is a beast of a telescope! I'd be happier with darker skies with my 8" f6. 

8 hours ago, jetstream said:

"perceived contrast"

It's that "perceived contrast" that have made mid/hig power with ultra mega wide field of views (20mm/100afov for example) very appealing. 

 

[Stu]

1 hour ago, Deadlake said:

Sorry @Stu if I've caused offence, just surprised at the number of comments when noting contrast and exit pupil.

No offence taken, just being tired and grumpy 🤣.

I think the problem is we use the word contrast to mean a number of different things, both actual and perceived I guess. I think the term object visibility is probably useful, as described by Bartels and his calculator as I believe this is explaining the comment from John below:

10 hours ago, John said:

As a visual observer, if the contrast looks greater, the target object stands out a little better.

The contrast remains the same, but the larger image scale helps the eye perceive it more clearly. Larger scopes are able maintain larger exit pupils at higher powers, one reason why they are the weapon of choice for hunting small faint galaxies.

Very complex subject as said, and I continue to learn through conversations like this so it’s all good! 👍👍

[jetstream]

5 hours ago, amaury said:

Wow!! that is a beast of a telescope! I'd be happier with darker skies with my 8" f6. 

Dark skies are everything with respect to DSO IMHO.

5 hours ago, amaury said:

It's that "perceived contrast" that have made mid/hig power with ultra mega wide field of views (20mm/100afov for example) very appealing. 

After a while this stuff gets very predictable...seasoned members can help out and cut the learning curve down greatly and with the effect of more money left in the wallet.

[Second Time Around]

On 12/04/2021 at 10:38, amaury said:

I'll do my homework with this. 

It helps yes. The thing is that with my scope (8" 1200mm f/6 Dob), the APM 20 gives me 1.67deg TFOV, that makes the APM 30 UFF redundant as it almost overlaps in terms of TFOV (1.75 TFOV for the UFF).

The one thing I would be giving up with the 20/100 is eye relief (15mm). I only have very small corrections (-0.5 Sph and -0.5 Cyl) so I think I can do visual observing without glasses. Maybe I am wrong here and I am being too optimistic. 

I totally understand that usefulness now. I just never considered  ~20mm / 100 degree eyepieces because of the massive price premium of those specs. I was aiming more towards 24mm/82 deg or 30mm/70 because they have an equivalent TFOV in my scope, better eye relief, and better prices, at the expense of more exit pupil (which wouldn't be an issue in truly dark skies, but it doesn't get better than bortle 4 for me).   

At night, like many others, I find I need about 0.25 dioptres extra cylinder correction and about 0.50 extra spherical correction (the latter is called night myopia).  I have special astronomy glasses as a result.  Even with 0.5 dioptres of astigmatism you're likely to notice it at exit pupils above about 4mm.   With an extra 0.25 correction you'll notice it at exit pupils above about 3mm.  See https://www.televue.com/engine/TV3b_page.asp?id=54&Tab=_Choose

This link is to the Televue Dioptrx, that is a way to correct astigmatism without wearing glasses.  A Dioptrx fits onto the top of many Televue eyepieces.  However, like with glasses, you need more eyelief to be able to see all the field of view.  How much depends on the shape of your eye sockets (and if wearing glasses their style makes a big difference). 

For many of us that means eye relief of 20mm+.  So this precludes all of the 100 degree FOV eyepieces on the market - the Ethos for example has only 15mm.  My view is that if I can't see all of the FOV anyway the extra cost isn't justified.

Some other non-Televue eyepieces also accept a Dioptrx, although sometimes an O ring is needed.  They need a top diameter of about 41-44mm. Those that do of 22-30mm focal length include:

Pentax XW: all I'm told

Explore Scientific 68 degree: 24mm and 28mm

APM Ultra Flat Field (and copies): 24mm (but not the 30mm)

Omegon Redline SWA 70 degree: 22mm.  This is an inexpensive hidden gem that many rate as very nearly as good as the 22mm Nagler and with more eye relief.  I sold mine only because it's a 2 inch eyepiece rather than a 1.25.

By the way, I prefer a Dioptrx because they have better coatings than even my expensive glasses.  Additionally, the angle of my astigmatism varies between prescriptions and a Dioptrx enables me to compensate for that.

 

Edited April 13, 2021 by Second Time Around
Cylinder and spherical the wrong way round in the first sentence.

[Second Time Around]

I see the Omegon 22mm Redline is out of stock until mid-May.  The TS 22mm Expanse is the same eyepiece and is in stock but is slightly more expensive when VAT is added.

https://www.omegon.eu/eyepieces/omegon-eyepiece-redline-sw-22mm-okular-2-/p,33239

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4922_TS-Optics-Eyepiece-Expanse-22-mm-Wide-Angle---2-inch-telescope-connection.html

[amaury]

1 hour ago, Second Time Around said:

At night, like many others, I find I need about 0.25 dioptres extra spherical correction and about 0.50 extra cylinder correction (the latter is called night myopia).  I have special astronomy glasses as a result.  Even with 0.5 dioptres of astigmatism you're likely to notice it at exit pupils above about 4mm.   With an extra 0.25 correction you'll notice it at exit pupils above about 3mm.  See https://www.televue.com/engine/TV3b_page.asp?id=54&Tab=_Choose

This link is to the Televue Dioptrx, that is a way to correct astigmatism without wearing glasses.  A Dioptrx fits onto the top of many Televue eyepieces.  However, like with glasses, you need more eyelief to be able to see all the field of view.  How much depends on the shape of your eye sockets (and if wearing glasses their style makes a big difference). 

For many of us that means eye relief of 20mm+.  So this precludes all of the 100 degree FOV eyepieces on the market - the Ethos for example has only 15mm.  My view is that if I can't see all of the FOV anyway the extra cost isn't justified.

Some other non-Televue eyepieces also accept a Dioptrx, although sometimes an O ring is needed.  They need a top diameter of about 41-44mm. Those that do of 22-30mm focal length include:

Pentax XW: all I'm told

Explore Scientific 68 degree: 24mm and 28mm

APM Ultra Flat Field (and copies): 24mm (but not the 30mm)

Omegon Redline SWA 70 degree: 22mm.  This is an inexpensive hidden gem that many rate as very nearly as good as the 22mm Nagler and with more eye relief.  I sold mine only because it's a 2 inch eyepiece rather than a 1.25.

By the way, I prefer a Dioptrx because they have better coatings than even my expensive glasses.  Additionally, the angle of my astigmatism varies between prescriptions and a Dioptrx enables me to compensate for that.

Thanks Steve. Very useful information as usual.

That chart from televue is handy. Another reason for me to maybe stay around the 4mm exit pupil or maybe observe with my right eye which doesn't have any cylinder correction (I tend to observe with me left eye for some reason, the one with -0.5 Cyl)

I've read good things about that Omegon (and re-brands), I'll keep it in mind.

The APM Ultra Flat Field 30mm have been reported to be eyeglasses-friendly when the eyecup is folded down. Still not anywhere near the 22mm of eye relief they state in the specs. 

 

[Second Time Around]

I noticed that I had spherical and cylinder the wrong way round in my earlier post.  Now corrected.

[Louis D]

1 hour ago, amaury said:

The APM Ultra Flat Field 30mm have been reported to be eyeglasses-friendly when the eyecup is folded down. Still not anywhere near the 22mm of eye relief they state in the specs. 

It's still quite comfortable to use with eyeglasses.  In use, it feels more like the 18mm of eye relief that my Pentax XWs and 14mm Morpheus have.

Edited April 13, 2021 by Louis D

[amaury]

On 13/04/2021 at 22:31, Louis D said:

It's still quite comfortable to use with eyeglasses.  In use, it feels more like the 18mm of eye relief that my Pentax XWs and 14mm Morpheus have.

@Louis D, Do you think the same can be achieved with an ES82 24mm? the specs say it has 17.5mm of eye relief, in practice Don mentioned it has more like 14mm of effective eye relief. Haven't read any comments about folding the eyecups in the ES82 eyepieces. 

[Louis D]

9 hours ago, amaury said:

@Louis D, Do you think the same can be achieved with an ES82 24mm? the specs say it has 17.5mm of eye relief, in practice Don mentioned it has more like 14mm of effective eye relief. Haven't read any comments about folding the eyecups in the ES82 eyepieces. 

Since the current generation of ES-82s all have recessed eye lenses, I can understand where it would lose about 3mm of eye relief right there.

I know I can just take in the whole field of my first-gen mushroom top 30mm ES-82 with eyeglasses on.  I've measured it to have 16mm of usable eye relief while the ES spec says 22mm, and that's about how it feels, or possibly 17mm.  You have to press in to take in the entire field of view, but it is doable without scratching your glasses.

Since the 24mm version has a smaller eye lens, I would expect less usable eye relief in the original gen-1 version, and even less with the current gen's recessed eye lens.  I'd guess probably 13mm of usable eye relief, which is great if you're not wearing glasses.

[faulksy]

On 12/04/2021 at 22:11, bomberbaz said:

I have ordered one of these too, prefer something properly made than getting all heath robinson. 

Why buying so many @faulksy, you grown a spare head in lockdown 😂

Has your head cover come Steve?

[bomberbaz]

13 hours ago, faulksy said:

Has your head cover come Steve?

came the other day mike, not had chance to use it but seems like a good bit of kit.

Tried it outside on sunny day and it was a total blackout.

I take it yours has also arrived?

[faulksy]

1 hour ago, bomberbaz said:

came the other day mike, not had chance to use it but seems like a good bit of kit.

Tried it outside on sunny day and it was a total blackout.

I take it yours has also arrived?

hi steve. no mate it hasnt. got my order number. but no confirmation or anything

Edited May 10, 2021 by faulksy

[bomberbaz]

16 hours ago, faulksy said:

hi steve. no mate it hasnt. got my order number. but no confirmation or anything

I got plenty of email off him, guys in russia apparently. 

Just send him an email and ask for an update mike, seemed like a decent sort.