Go Back   IceInSpace > Equipment > Eyepieces, Barlows and Filters

Reply
 
Thread Tools Rate Thread
  #1  
Old 04-03-2021, 05:31 PM
Hemi
Registered User

Hemi is offline
 
Join Date: Dec 2010
Location: Darwin
Posts: 447
An EP Role Call

Team IIS,

All of these EP threads have been awesome. Its humbling to know that I share this site with such learned and skilled astronomers.

I wonder if I might be able to use you guys in suggesting an EP line up for me.
Ive attached a spreadsheet that ive listed my visual use scopes at various focal lengths, vs EP focal lengths for various exit pupils and magnifications.

Im interested in deep sky (big and small), moon and planets. I observe mainly from Bortle 5, but have reasonably easy access to Bortle 2/3. Its hot and humid for half the year! I am shortsighted, have significant, astigmatism, aged 50 (just!) and lots of floaters. I do however prefer observing without glasses. (but that might be that ive just not got great EP's)

Assume we are starting from scratch, and 6 EPs max for the entire range of scopes and Focal lengths.

Focal lengths is the minimum info I'm after, but specifics are welcome. No budgetary constraints. Premium EP's only. Ive spent years in the basement, and feel my skills are sufficient now for a penthouse view! I currently only have 3 Ep's that I would consider "premiumish" an ES30 82, ES4.7 82 and TV PO 24. But delete these from your mind and assume nothing.

I hope you can take up the challenge!

....oh and Barlows/Powermates can be included in addition to the 6.
Attached Thumbnails
Click for full-size image (Screen Shot 2021-03-04 at 15.33.51.png)
143.6 KB40 views
Reply With Quote
  #2  
Old 04-03-2021, 07:35 PM
astro744
Registered User

astro744 is offline
 
Join Date: Apr 2007
Posts: 983
See first para at https://www.televue.com/engine/TV3b_...n=Advice&id=78

More advice at https://www.televue.com/engine/TV3b_page.asp?id=154

I have not yet found a telescope that does not work well with Tele Vue eyepieces. Tele Vue eyepieces simply work well in short and long and fast and slow refractors and Newtonians, SCTs, MCTs etc.

Tele Vue make a number of designs and your choice is then based on your apparent field of view preference as well as any preference for eye relief. I would recommend longer eye relief option should you ever choose to observe with glasses in whatever AFOV you prefer that is available for that option.

See https://www.televue.com/engine/TV3b_page.asp?id=214

Note anything with the same field stop diameter will give the same true field of view regardless of eyepiece focal length. (Magnification will vary).

Yes you can spend a lot of time and money trying different eyepieces that have their focal plane curvature matching your telescope curvature or are an affordable match for a longer f ratio telescope but not for a fast telescope. You mention no budgetary constraints so I point you to what I believe is a product well proven in the field and one that is well designed, well tested, well supported and performs well in any telescope you are ever likely to own. You cannot go wrong with Tele Vue eyepieces (or their telescopes).

Bintel is the Australian dealer for Tele Vue and offer full warranty and excellent after sales support.

If you call Tele Vue you may even get Al Nagler on the phone. Have a chat and see what he says.

I have not listed anything specific but perhaps consider the Delos in combination with a 31mm Nagler and/or maybe 22mm Nagler too then Delos. This keeps your eye relief around 20mm all round. DeLite is another option for shorter focal lengths and narrower AFOV than Delos.

If you can look through as many as possible at any covid safe star parties all the better because each type has its own feel, even amongst the Naglers; Type 4 feel different to Type 5 & 6.
Reply With Quote
  #3  
Old 04-03-2021, 08:28 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,886
My experience as someone who is short sighted and with astigmatism is that I need to keep my exit pupil below 2.5mm, preferably 2mm. Unless I’m binoviewing in which case it can go as high as 4mm.

I’ve personally preferred Orthos and planning a full set of Tak Abbe Orthos at some point in the future. I do prefer binoviewing everything though! The tiny FOV doesn’t feel so small through both eyes.
Reply With Quote
  #4  
Old 04-03-2021, 10:02 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 16,809
I came back to doing some visual only recently and have spent a fair bit of time researching eyepieces and collecting quite a good set.

In the past I would consider mainly only Televue. But I found out since I last was doing visual they have quite a lot of competition. I agree with the first poster though that getting TV eyepieces is a safe bet. But they are considerably ore expensive than some of their fierce competitors.

Out of the quite a few eyepieces I have gotten recently these to me are the outstanding ones:

1. Baader Morpheus. My favourite eyepiece is the 17.5mm. I got the 6.5, 9, 12.5 and 2 x 17,5 (for a binoviewer).

2. APM XWA 9 and 20mm 100 degree eyepieces, 12.5 Hi FW and 30mm UFF. These are my 2nd favourite. I am using a 103/6 refractor. The 9mm XWA is sharp to the edges, the 20mm is good for most of it but not right to the edge. That cleans up with a barlow. Very immersive and a fraction of the TV price. I did have a 13mm Ethos when it first came out. Lovely eyepiece but don't remember it wowing me anymore than the APM does.

Bintel 2 inch Barlow - seems very high quality. This is a Long Pern Taiwan product and has ED glass. It slightly outperforms the APM 2 inch Barlow.
I also have a 1.25 inch Baader Q Barlow but have yet to test it.

3. Televue, 3.5T6, 9T1, 16T5 Naglers, 32mm Plossl, 15mm Plossl. All very nice. The 16mm is just a little bit less appealing than the Morpheus 17.5 because it has better eye relief and is not very sensitive to eye placement so you get a comfortable view. Delite 18.3mm still fairly new to me.

4. Pentax XW 7 and 10mm. 70 degree, superb, a touch sensitive to eye placement. I haven't looked through the 10mm as its new.

5. Masuyama 10mm 85 degree - I really like this eyepiece. A bit more contrast and detail in the centre and some aberrations show up in the last 30% which don't bother me. I plan on getting one or two more.

6. I have quite a few ortho type eyepieces. University Optics Edmunds RKE, Fujiyama.

The top 5 for me are:

Morpheus 17.5
APM 12.5 Hi FW
APM XWA 20mm
APM 9mm XWA
APM30 UFF.

I find out I prefer wider view eyepieces a lot more than narrow Ortho types. The RKE 28mm though is a standout good value eyepiece. Edmunds Optics. About $163.

A sampling of some of the above would be good rather than all Televue.

A memorable eyepiece from the past was the Nagler 22 T4. It often comes up on peoples all time favourite eyepiece list.

Greg.
Reply With Quote
  #5  
Old 05-03-2021, 12:03 AM
Hemi
Registered User

Hemi is offline
 
Join Date: Dec 2010
Location: Darwin
Posts: 447
Thanks everyone for taking the time to reply. The Brands I have in mind are TV, Baader (Morpheus) Stellarview (optimus) and APM XWA....but if we were brand and aFOV agnostic, could I press you for a selection of 6 focal lengths to work with all those scopes/focal lengths/exit pupils?

Hemi
Reply With Quote
  #6  
Old 05-03-2021, 03:58 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 16,809
I can only comment for 2000mm (8 inch SCT scope),1200mm (Takahashi FS152 and TEC180FL), 819mm (AP130GT), 630mm (CFF105 F6).

I have also viewed through an AP140 F7.7 so 1078mm.

Ethos 13mm worked really well on the TEC180 at 1260mm
AP130GT 819mm, 3.5mm Nagler is pushing it a bit so I would start at 5mm (floaters appear) even 9 or 10mm. Morpheus 6.5mm seems just about right for max magnification. 9 and 10mm works well for usual high mag, 17.5 is a nice wide field but not too wide, 20mm wide angle 100degree is very wow.
30mm is good for nice widefields and for locating objects - cruising.

FS152 1260mm focal length, worked well with Panoptic 19mm and 22mm T4 Nagler was just wow.

So really for focal lengths 630 to 1260mm its 9/10mm, 12.5, 14, 17.5, 20,22, 30.

So 9-30mm with 17.5 -20 the sweet spot.

2000mm best view was 19mm Panoptic and 26mm plossl.

Greg.
Reply With Quote
  #7  
Old 06-03-2021, 08:07 AM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 448
Quote:
Originally Posted by Hemi View Post
Team IIS,

All of these EP threads have been awesome. Its humbling to know that I share this site with such learned and skilled astronomers.

I wonder if I might be able to use you guys in suggesting an EP line up for me.
Ive attached a spreadsheet that ive listed my visual use scopes at various focal lengths, vs EP focal lengths for various exit pupils and magnifications.

Im interested in deep sky (big and small), moon and planets. I observe mainly from Bortle 5, but have reasonably easy access to Bortle 2/3. Its hot and humid for half the year! I am shortsighted, have significant, astigmatism, aged 50 (just!) and lots of floaters. I do however prefer observing without glasses. (but that might be that ive just not got great EP's)

Assume we are starting from scratch, and 6 EPs max for the entire range of scopes and Focal lengths.

Focal lengths is the minimum info I'm after, but specifics are welcome. No budgetary constraints. Premium EP's only. Ive spent years in the basement, and feel my skills are sufficient now for a penthouse view! I currently only have 3 Ep's that I would consider "premiumish" an ES30 82, ES4.7 82 and TV PO 24. But delete these from your mind and assume nothing.

I hope you can take up the challenge!

....oh and Barlows/Powermates can be included in addition to the 6.
With those disparate telescope focal lengths, one set will not do all scopes equally well.
But I can suggest a set that could do OK.
For the SCTs: 40mm, 20mm, 13-14mm, 10mm, 8mm
For the 102mm: 24mm, 12mm, 8mm, 6mm, 4.7-4.8mm, 3-4mm
For a set for all the scopes:
41mm TeleVue Panoptic
22mm TeleVue Nagler or 21mm Ethos.
13mm, 10mm, 8mm TeleVue Ethos
TeleVue 3-6mm Zoom
6 eyepieces, covers all scopes.
Reply With Quote
  #8  
Old 06-03-2021, 08:18 PM
Hemi
Registered User

Hemi is offline
 
Join Date: Dec 2010
Location: Darwin
Posts: 447
Thanks again Don, Greg.

I will ponder those focal lengths.
Was thinking to start off by getting eye pieces for 1, 1.4 and 2 mm exit pupils? From my spread sheet I could initially get away with...

6, 10,14, 20??
Reply With Quote
  #9  
Old 07-03-2021, 01:29 AM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 448
Quote:
Originally Posted by Hemi View Post
Thanks again Don, Greg.

I will ponder those focal lengths.
Was thinking to start off by getting eye pieces for 1, 1.4 and 2 mm exit pupils? From my spread sheet I could initially get away with...

6, 10,14, 20??
Given the other eyepieces you have, yes, that would be fine.
Reply With Quote
  #10  
Old 09-03-2021, 09:18 PM
Hemi
Registered User

Hemi is offline
 
Join Date: Dec 2010
Location: Darwin
Posts: 447
First Cab off the rank!

OK so the 13/14mm will be the first one Iím going to buy. This i think is my favourite focal length.

The candiates....

Televue Ethos 13mm
Stellarview Optimus 13.5mm
Baader Morpheus 12.5mm or 14mm

Greg, you have all of these donít you? Iím sure someone does.
Alas, live in Darwin so trying first wont be an option. But ive never been guided in the wrong direction from advice on here (and cloudy).

Hemi
Reply With Quote
  #11  
Old 10-03-2021, 01:17 AM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 448
Of the eyepieces you picked, I think you have them in the right sequence of preference, though I would also look at the APM XWA HDC 13mm, the same eyepiece as the Optimus, but in a different, lighter, housing.
Reply With Quote
  #12  
Old 10-03-2021, 07:27 AM
Hemi
Registered User

Hemi is offline
 
Join Date: Dec 2010
Location: Darwin
Posts: 447
Quote:
Originally Posted by Don Pensack View Post
Of the eyepieces you picked, I think you have them in the right sequence of preference, though I would also look at the APM XWA HDC 13mm, the same eyepiece as the Optimus, but in a different, lighter, housing.
Thanks Don,

I thought they were the same EP from your previous advice, but my impression was that Stellarview have a very well respected (superior?) quality control process, hence chose that. Lighter would definitely be better, as it will often be used in the Fs60q.

Another reason is that no one seems to have the apm.

Does the 20mm field stop in the APM vs the 23mm Field stop in the TV and SV make a significant difference?

Hemi

Last edited by Hemi; 10-03-2021 at 09:10 AM.
Reply With Quote
  #13  
Old 10-03-2021, 07:44 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 16,809
Quote:
Originally Posted by Hemi View Post
OK so the 13/14mm will be the first one Iím going to buy. This i think is my favourite focal length.

The candiates....

Televue Ethos 13mm
Stellarview Optimus 13.5mm
Baader Morpheus 12.5mm or 14mm

Greg, you have all of these donít you? Iím sure someone does.
Alas, live in Darwin so trying first wont be an option. But ive never been guided in the wrong direction from advice on here (and cloudy).

Hemi
I have Morpheus 12.5, APM Hi FW 12.5, APM 9 and 20mm XWA.
They are all spectacular eyepieces. The 13mm TV Ethos is a legendary eyepiece.
I had one many years ago. Expensive though and the APM version is not that far behind it at a fraction of the cost. APM sells them direct as well as through some dealers. Don may have one. As I focused on imaging I sold off the nice eyepieces I had except for a couple. There are CloudyNight comparison reviews between APM XWA and TV Ethos. Spolier alert - the conclusions are usually the TV is best but not by a lot and its up to you if you want to pay 2-3X more for a small gain. I am not. I am really happy with my APM eyepieces, those with the Baader Morpheus and the Pentax 7 and 10mm XW are really very very good. Stellarvue's review well also but also getting a bit pricey. It places the APM's as an all round good choice as we all have to operate in a budget.

You can't go wrong with Televue but you can get similar or better performance for cheaper generally speaking.

Greg.
Reply With Quote
  #14  
Old 12-03-2021, 06:10 AM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 448
Quote:
Originally Posted by Hemi View Post
Thanks Don,

I thought they were the same EP from your previous advice, but my impression was that Stellarview have a very well respected (superior?) quality control process, hence chose that. Lighter would definitely be better, as it will often be used in the Fs60q.

Another reason is that no one seems to have the apm.

Does the 20mm field stop in the APM vs the 23mm Field stop in the TV and SV make a significant difference?

Hemi
We don't know the actual field stop dimension in the 13mm APM.
The 13.5mm Stellarvue is 22.6mm, which is probably what the APM is.
The TeleVue Ethos is 22.3mm, and I would bet the KUO 100į eyepieces are equal to each other.
I know APM has 20mm for the field stop on their website, but I distrust the accuracy of that figure. That would make it 10% narrower than the Ethos, which it isn't.

APM is getting more 13s at the end of March.
Weight is
13 APM--461g
13.5mm Optimus--581g
13mm Ethos--590g
Both of the latter have stainless steel bottom barrels.
Reply With Quote
  #15  
Old 12-03-2021, 04:18 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 16,809
Exit pupil explanation

Hey Don,

Would you mind explaining what exit pupil is and how it relates to choosing an ideal eyepiece for a particular focal length?

I understand it is calculated as eyepiece focal length/ telescope Fratio.

What is an ideal exit pupil and what are the liabilities of violating it?

I see folks talking about exit pupil all the time in the CN eyepiece forum and I haven't really studied what it is and how important it is.

Greg.
Reply With Quote
  #16  
Old 12-03-2021, 04:27 PM
Hemi
Registered User

Hemi is offline
 
Join Date: Dec 2010
Location: Darwin
Posts: 447
Quote:
Originally Posted by gregbradley View Post
Hey Don,

Would you mind explaining what exit pupil is and how it relates to choosing an ideal eyepiece for a particular focal length?

I understand it is calculated as eyepiece focal length/ telescope Fratio.

What is an ideal exit pupil and what are the liabilities of violating it?

I see folks talking about exit pupil all the time in the CN eyepiece forum and I haven't really studied what it is and how important it is.

Greg.
Hey Greg, Don will no doubt give an experienced explanation, but ive been reading a lot about this for a while, and ended up with my selection that way, and the spreadsheet at the top of the thread has Exit pupil at the Centre.

Maybe these will help?

https://skyandtelescope.org/astronom...-pupil-primer/

https://stargazerslounge.com/topic/2...he-exit-pupil/
Reply With Quote
  #17  
Old 12-03-2021, 05:19 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 16,809
Quote:
Originally Posted by Hemi View Post
Hey Greg, Don will no doubt give an experienced explanation, but ive been reading a lot about this for a while, and ended up with my selection that way, and the spreadsheet at the top of the thread has Exit pupil at the Centre.

Maybe these will help?

https://skyandtelescope.org/astronom...-pupil-primer/

https://stargazerslounge.com/topic/2...he-exit-pupil/
That's a great help- thank you. Although the second article had some clearly false data in it. In the first sentence under Introduction the author says exit pupil is independent of the telescope and the magnification. Yet all the maths for exit pupil are totally dependent on telescope focal length, aperture and F ratio. Unless he meant something else and expressed himself poorly. It sounds like English is not his first language, but apart from that it was helpful.

The first article was very clear and very well written.

In a round about way exit pupil is much the same as magnification on telescopes so I see the same concerns expressed in terms of ideal magnifications.

I did like the explanation for lower powered eyepieces showing eye induced optical imperfections. I noticed that with my APM 30mm UFF. After viewing through 12.5mm and 17.5mm eyepieces where the stars are beautiful and stable pinpoints the 30mm showed some "flare" which moved when I moved my eye. That was what Don pointed out to me on another forum.
Good to know.

It also explains why there seems to be a sweet spot just like say the sweet spot in a tennis racket. For my AP130GT F6 819mm focal length that sweet spot is around 17.5mm and 7-12.5mm for higher mag.

A 3.5mm Nagler is good when the seeing is great but a bit fuzzy otherwise
(not the eyepiece or scope's fault).

So in the end it boils down to you need 3 types of focal lengths - the high, medium and low power eyepieces.

Too short a focal length is not going to be very nice just as too long a focal length is not going to be very nice.

Greg.
Reply With Quote
  #18  
Old 12-03-2021, 05:40 PM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 448
Exit pupil is the size of the image formed behind the eyepiece that enters the eye.
It ranges from the size of your dark adapted pupil on the low power end to about 0.5mm on the extremely high power end. Exit pupils larger than the size of your pupil get vignetted by the iris in the eye (you are stopping down the diameter of the scope) and exit pupils on the small end can be intercepted by floaters in the eye (you can see them on the Moon at high powers).
So there is a range.
The size of the exit pupil determines the brightness of the overall field seen--larger is brighter.
The diameter of your pupil determines the maximum brightness of the field seen.

Astigmatism in the eye affects all observers with maximum pupil. We see the stars with points on them. Without astigmatism, we'd all see the stars as tiny points.
As the exit pupil gets smaller, astigmatism bothers the image less and less.
At some point most people notice the image no longer has any astigmatism in the star images as the exit pupil gets smaller. From that point and smaller, glasses or optical correction are not needed. Hyperopia or myopia can simply be focused away, so only astigmatism requires glasses to see the stars as points.
You mention a 30mm APM eyepiece. I see horrible star images without glasses, but tiny pinpoints with them. That eyepiece yields too large an exit pupil for me to see an unastigmatic image. But when I reach an 8mm eyepiece, I see no astigmatism at all, without glasses.
My astigmatism is minor, though, and so I can dispense with glasses at an exit pupil of about 1.7mm. People with worse astigmatism may need to wear glasses at all powers.

2-3mm of exit pupil is often the point of best acuity for the observer (corrected vision, that is), so the image quality in that range is usually great. It's small enough to avoid a lot of large pupil astigmatism, and large enough that floaters in the eye don't interfere with the image.

Once you get to about a 1mm exit pupil, the Airy disc resolves. From that point down (higher powers) the star images get larger but no further improvement in resolution occurs.
However, the image gets larger so smaller details become easier to see, and the background sky darkens, so fainter stars become visible.

Exit pupils below 0.5mm get exceedingly dark (think how little light is entering the eye), so are usually not valuable to use. Also, seeing conditions usually prohibit such tiny exit pupils to be useful because of high magnifications. In fact, many people max out their magnifications with about a 1mm exit pupil (the focal length of the eyepiece matches the f/ratio of the scope).

I usually describe low power as 4 to 10x/inch of aperture, medium power as 10-20x/inch, and high power as 20-30x/inch.. Above that is the realm of ultra high power and it can be occasionally used, but not that often.
In exit pupil terms, that's 6mm to 2.5mm, 2.5mm to 1.3mm, and 1.3mm to 0.8mm. The 0.8mm to 0.5mm is the realm of extreme high powers.

I don't know what more to say except that exit pupil choice is only one of many ways to pick a set of eyepieces. Personally, I prefer to do it by picking the magnifications that work well in my scope.
Reply With Quote
  #19  
Old 12-03-2021, 07:13 PM
astro744
Registered User

astro744 is offline
 
Join Date: Apr 2007
Posts: 983
Do not dismiss low power viewing just because larger exit pupils bring out the worst in ones eyes. Either use long eye relief eyepieces and wear glasses when observing or get the Tele Vue Dioptrx (available in various strengths).

Generally pick the magnification that best frames the target but for general sweeping of the night sky nothing beats low power on a 4” flat field Nagler-Petzval (except maybe a 5” Nagler-Petzval). Either of these telescopes are wonderful for low powers with either a 41mm Panoptic to maximise true field or a 31 mm Nagler for a bit more apparent field but a little less true field. The Tele Vue NP telescope have a fiat field so you don’t have to worry about curvature of field provided you use highly corrected eyepiece that Tele Vue has to offer. There may be other eyepieces that work well with such telescopes.

Also any short refractor can be used for low power sweeping and depending on focal length and eyepiece used you will see some curvature of field and most of the time it’s not too detrimental but certainty telescope/eyepiece combinations will work well and others not so well. Dark skies are recommended for low power viewing to maintain contrast.

See the article (extract) on the joys of low power viewing at:
https://www.televue.com/engine/TV3b_...=Advice&id=164

If you can source “The Best of Amateur Telescope Making Journal, Volume 1, Willmann-Bell publisher, it has the full article as well as many others. Volume 2 is highly recommended too.

Last edited by astro744; 14-03-2021 at 02:24 PM. Reason: Corrected typo; Petzval not Petzal
Reply With Quote
  #20  
Old 12-03-2021, 09:27 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 16,809
Fantastic writeup Don. Very well explained and answered all my questions.

Thanks very much for taking the time to do that.

Greg.



Quote:
Originally Posted by Don Pensack View Post
Exit pupil is the size of the image formed behind the eyepiece that enters the eye.
It ranges from the size of your dark adapted pupil on the low power end to about 0.5mm on the extremely high power end. Exit pupils larger than the size of your pupil get vignetted by the iris in the eye (you are stopping down the diameter of the scope) and exit pupils on the small end can be intercepted by floaters in the eye (you can see them on the Moon at high powers).
So there is a range.
The size of the exit pupil determines the brightness of the overall field seen--larger is brighter.
The diameter of your pupil determines the maximum brightness of the field seen.

Astigmatism in the eye affects all observers with maximum pupil. We see the stars with points on them. Without astigmatism, we'd all see the stars as tiny points.
As the exit pupil gets smaller, astigmatism bothers the image less and less.
At some point most people notice the image no longer has any astigmatism in the star images as the exit pupil gets smaller. From that point and smaller, glasses or optical correction are not needed. Hyperopia or myopia can simply be focused away, so only astigmatism requires glasses to see the stars as points.
You mention a 30mm APM eyepiece. I see horrible star images without glasses, but tiny pinpoints with them. That eyepiece yields too large an exit pupil for me to see an unastigmatic image. But when I reach an 8mm eyepiece, I see no astigmatism at all, without glasses.
My astigmatism is minor, though, and so I can dispense with glasses at an exit pupil of about 1.7mm. People with worse astigmatism may need to wear glasses at all powers.

2-3mm of exit pupil is often the point of best acuity for the observer (corrected vision, that is), so the image quality in that range is usually great. It's small enough to avoid a lot of large pupil astigmatism, and large enough that floaters in the eye don't interfere with the image.

Once you get to about a 1mm exit pupil, the Airy disc resolves. From that point down (higher powers) the star images get larger but no further improvement in resolution occurs.
However, the image gets larger so smaller details become easier to see, and the background sky darkens, so fainter stars become visible.

Exit pupils below 0.5mm get exceedingly dark (think how little light is entering the eye), so are usually not valuable to use. Also, seeing conditions usually prohibit such tiny exit pupils to be useful because of high magnifications. In fact, many people max out their magnifications with about a 1mm exit pupil (the focal length of the eyepiece matches the f/ratio of the scope).

I usually describe low power as 4 to 10x/inch of aperture, medium power as 10-20x/inch, and high power as 20-30x/inch.. Above that is the realm of ultra high power and it can be occasionally used, but not that often.
In exit pupil terms, that's 6mm to 2.5mm, 2.5mm to 1.3mm, and 1.3mm to 0.8mm. The 0.8mm to 0.5mm is the realm of extreme high powers.

I don't know what more to say except that exit pupil choice is only one of many ways to pick a set of eyepieces. Personally, I prefer to do it by picking the magnifications that work well in my scope.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 08:11 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
Advertisement
Limpet Controller
Advertisement
Testar
Advertisement
Bintel
Advertisement
NexDome Observatories
Advertisement
Astronomy and Electronics Centre
Advertisement