lowest 2 inches eyepiece?

[N3ptune]

Hello everyone,

I am questioning myself about how to choose the lowest power 2 inches EP for my telescope. Is it ruled by the exit pupil factor only ?

Knowing 7mm is the maximum pupil aperture then a 38mm would be too much for my telescope, because .6mm of aperture would be wasted.

38mm = 26.31x (38/5 = 7.6 Exit pupil)
32mm = 31.25x (32/5 = 6.4 exit pupil)
28mm = 35.7x (28/5 = 5.6 exit pupil)

Is there another factor in order to choose?

Thanks
 

 

[cloudsweeper]

The other issue is the long eye relief, which makes positioning of the eye critical: slight movement causes blackout/kidney beaning.

Doug.

[N3ptune]

The eye relief on 2 inches EP look less like a problem then with a 1.25 inches EP, I guess something like 20 mm or maybe more would be fine.

So a 35 mm would give me 7mm exit pupil. If it has a good eye relief, then it should be the lowers power possible to use on my instrument? A Newtonian 230x1000 ?

[cloudsweeper]

From those figures, the focal ratio is 4.35, so with a 35mm EP, the exit pupil is 8.0mm.

I sometimes use a 50mm EP in my ST120 when the exit pupil is 10mm.  There will be issues with loss of light and criticality of eye positioning, but it still gives a low mag/wide angle view.

Doug.

 

[Charic]

As a guide you should multiply your telescopes focal ratio by the size of your pupil to  figure out where you should be, with regards to  your low powered eyepiece ( there's no lower limit for a refractor scope! ).

If your  (eye/s) entry pupil matches the (eyepiece) exit pupil, you should achieve the full amount of light/image provided from the eyepiece.   It really does not matter (to me) if you lose or gain a little  exit pupil, as our own eyes are  ever controlling, constantly adjusting to the light levels you see in the eyepiece, despite the fixed aperture exit pupil size?  so from a dim Star one minute to a bright Star the next, maybe a Planet,  your pupils will auto adjust,  protecting your eyes by default,  all without you having to think about it, so a 7mm exit pupil  may be wasted due to your age ( pupils smaller as you age) and brightness level, if the pupil can only provide you with 6mm or not enough if you have a smaller exit pupil and your own pupils can enlarge to that of a baby barn owl?
Testing has proven that an exit pupil of between 1-2mm is optimal for Planetary detailing, but less than 0.5 and more than 7mm can be pushing the extremes for our telescopes, but all down to the users taste and ability to see the images at those extremes.

So when choosing eyepieces,  an eyepiece that equally matches the focal ratio of the scope  (in your case f/5 - F-1000 A-200) will give you a 1mm exit pupil at a power that the scope is capable of achieving (the size of the Aperture) under the right conditions, so 200X power 1mm Exit Pupil from a 5mm Eyepiece, then if you take a 10mm, you have half the power 100x but twice the exit at 2mm.
For your wide angle, Telescope focal ratio x Entry pupil (eye).
Eye relief is at the designers discretion!

HTH

[N3ptune]

6 minutes ago, cloudsweeper said:

From those figures, the focal ratio is 4.35, so with a 35mm EP, the exit pupil is 8.0mm.

What is 4.35 F ratio ?

[Stu]

12 minutes ago, N3ptune said:

What is 4.35 F ratio ?

Focal ratio of the scope is focal length divided by aperture, in your case

1000/230 = 4.35

[Charic]

Stu.......I know his print is small ( BKP 200 DS reflector 203mm x 1000mm on EQ5 ) but its a 203mm (8") scope 1000/203=4.926 ( 5) f/5, which is why our sums differ?

[N3ptune]

1 hour ago, Charic said:

 Telescope focal ratio x Entry pupil (eye).
 

Ok then in that case I would try to remain into this range:

38mm = 26.31x (38/5 = 7.6 Exit pupil)
37mm = 7.4mm exit pupil
36mm = 7.1mm exit pupil
35mm = 7mm exist pupil
34mm = 6.8mm exit pupil
33mm = 6.6mm exit pupil
32mm = 31.25x (32/5 = 6.4 exit pupil)
31mm = 6.2mm exit pupil
30mm = 6mm exit pupil
28mm = 35.7x (28/5 = 5.6 exit pupil)

My telescope is F5 so to stay in an optimal exit pupil, let's say 6mm, ( and considering I may not be at 7 right now and it's not going to improve) I would say quickly that 30 to 35 looks like the best choices for my telescope.

But if a little excess means nothing, then a 38mm or more could work also.

What would be the best choice? Everything seems to work somewhat, what would you people pick for my telescope Newtonian 203 x 1000 F5 ? something optimal for the DSO's like the Veil. I would think an EP with 20mm relief, 60 to 82 degrees fov, and which has a price in between 300CAD to 399CAD range maximum. (175GBP to 233GBP)?

Or less but the eyepiece has to do some coma correction, F5 is fast.

 

 

[Charic]

My eyes don't seem to compute with this age thing? their smaller than average, so the specialist tells me? at 4.9mm

Of the three SWA's that Sky-watcher provide, I went with  the 32mm variant, using information found here http://www.swindonstargazers.com/beginners/eyepieces.htm , but this was prior to having my pupils measured 11 Months ago. @rwilky is also a member here at SGL.

Not seeing your age, I would plump for the 32mm, if your wanting to stay with that eyepiece, and closer to 6mm Exit pupil.

As you  can see, the math involved can be a little different to what  is achievable/desirable.

From the specifications you state, The Sky-watcher SWA Panaview 32mm  with its 70° afov  6.4mm Exit and  24mm eye-relief might be the one to choose. If you choose  the 38mm @ 7.6mm Exit or a 28mm(not Panaview) @ 5.6mm Exit, their average is 6.6mm Exit, closer to that provided from the 32mm SWA Panaview 

TeleView Panoptic EP's in 27mm & 35mm are dearer options, but are reputedly good optics,  and I think, similar to my Delos, are personally tested to f/4, which may help  with any coma your observing, or a cheaper EP with an additional coma corrector.
For visual use, coma is not a major issue for myself, as I keep my targets centralized, but for photographic images where sharpness across the field is a concern, an additional corrector may be required?

[N3ptune]

Sky-watcher SWA Panaview 32mm looks nice, visually also, but It seems like a UK exclusive product. The televue is probably easier to get here, but too expensive.

 

 

 

[jetstream]

You might want to stay within the 5mm-5.5mm exit pupil range, just a thought.

[Alien 13]

I have never quite understood this some of the best instruments I have looked through have had exit pupils well into double figures (rifle sights tank sights) I know that the background gets brighter but thats what I prefer.

Alan

[Charic]

Alien13........for my scope a 6mm eyepiece produces a 1mm exit pupil, and provides a power of x200 from the Skyliner There is also the sweet-spot? for me this is the 12mm eyepiece, providing 2mm exit pupil. You will find more information in this detailed billing http://bpastro.org/astronomy/amateur-astronomy/selecting-eyepieces

Larger exit pupils provide  for a brighter image overall, though you  may lose the contrast between the target and the background sky. In order to have more contrast, in order to see the fainter targets,  you may need to magnify the image, but  have a smaller exit pupil.

[Piero]

The only factor that would come into my mind is your sky darkness.

Unless you live in a dark location, I wouldn't go beyond 4-5mm exit pupil. If you do 5-6mm exit pupil could be an option though!

[N3ptune]

This is a great resume from Charic's website:

http://bpastro.org/astronomy/amateur-astronomy/selecting-eyepieces

Quote

This is not the case with reflecting telescope because the central obstruction (the secondary mirror) would start to become visible once the exit pupil reached a diameter of about 10mm. To simplify: The first few eyepieces that you buy should have an eyepiece/telescope combination that produces an exit pupil 7mm or less in diameter. If you are middle-aged the exit pupil should be 6mm or less, and by age 60 you should shoot for 5mm.

The first part was the missing information I was searching for at the beginning of this thread. A seller told me almost the same thing one time, he said to me:  After 35 mm on my telescope, I am going to see the central obstruction, implying 35mm was the maximum possible to use.

The website says 10 mm exit pupil.

Two new conflicting ideas!

 

 

[YKSE]

Seeeing central obstruction (CO) happens when the dilated pupil is much smaller than exit pupil, in my limited experience.

32mm plossl was my finder and low power eyepiece in 130P (f5), Under night sky, I've never seeing CO in 6.4mm exit pupil , neither have I seen CO with 40mm or 50mm eyepiece in C8(f10).

The only time I saw CO was 50mm in C8 during daytime measurement where my dilated pupil was no more than 2-3 mm, even then, it's not difficult to dodge CO to get a proper view.

[Dave In Vermont]

My methodology was honed by a rather simple principle: I believe what I read as being true (sort of). But I don't necessarily act accordingly - I try to see it for myself. Instead I'll check it out if I have the equipment on hand to try it. I learned many years ago (decades, really - <gasp!>) that there can be a gulf of difference in human-eyes and vision. And I've seen things work that the going thought swore up & down wouldn't. I won't spend my money on a 'proverbial' hunch. But if I have the means to 'look' <koff> at such myself - I will take a look.

I've been pleasantly surprised on occasion. In this I agree with the gentleman from Sweden in his "limited experience" <LOL>. Science is the result of making sure of things by experiments. Some 'hard & set rules' are made to verify by further experimenting. Just try not to blow-up your neighborhood!

All the best -

Dave

 

[Charic]

12 hours ago, N3ptune said:

...... conflicting ideas!

Another good read here,  "Affect of Eye Pupil on Binocular Aperture"  but the same  principles apply to telescope users. http://www.cloudynights.com/documents/binoexit.pdf
The .pdf is secured but still  readable!

[BGazing]

there are a lot of people here who are MUCH more knowledgeable than i am, but just a couple of thoughts and others are feel free to correct me:

- do you regularly observe under light polluted skies or from relatively dark skies? if the former, your low power eyepiece should probably not exceed 4-5 mm. otherwise, your low power views would be too bright (exception below)

- do you plan to use agressive filters (OIII, H-beta)? if yes, extra light is something to work with, so extra focal length would not hurt and would actually be beneficial, as the filters (especially agressive ones) tend to dim the image.

 

[N3ptune]

I read most of the last document pasted by Charic and all makes sense.

--> I will choose an EP that will have a smaller exit pupil then my maximum pupil diameter. (I have to measure my own pupil and see about that)

But a good estimate would be around 6mm exit pupil (I still think) to get strong results on faint DSO's. (I already have EPs in the range of 4 - 5 mm so I think it's secure to get closer to 6 mm right now. )

These are the following choices possible to order in Canada. (Televue are not listed, because of the price. )

-Explore Scientific BRESSER 70° Series 30mm - 158$

-William Optics 33 mm Super Wide Angle 2" Eyepiece - 221$

-Explore Scientific 82° Series 30mm Eyepiece (2") [EPWP8230-01] - 376$

-Explore Scientific 68° Series 34mm Eyepiece (2") [EPWP6834-01] - 376$

-Celestron - Luminos™ 31mm 2" Eyepiece [93435] - 300$

-Baader 31mm Hyperion - 260$

-ANTARES OMMER30 74 degrees- 150$

-ANTARES OMER32W 68 degrees-100$

-UNIVERSITY OPTICS UOWS32 70 degrees -116

-UNIVERSITY OPTICS UOWS30 -116

Which one should I pick?

============

I already have a LET 28mm and has discussed before I don't like it because it's blurry all around the edge, only the center is clear. (So almost 2/3 of the FOV blurry) while my Xcel LX 25mm has a great field of view, nearly perfect.

Thanks

 

[BGazing]

Again, others will surely give a wiser advice than me, but I would stay away from erfles and the likes in a fast scope and spend some more. Not sure how Aspheric 31mm performs in a very fast scope, if I remember the discussions here, not a stellar pairing.

Es 34 68 deg would be my pick. If you do not like it, Es holds the value well on a resell market...

[N3ptune]

Bgazing,

You are right, I forgot to consider the scope is fast , thanks also for the suggestion the ES. but... I don't feel like buying anything finally, the store is collaborating a minimum with me and everything is complicated, they have nothing in stock and they don't care. I am unhappy about this 2 inches eyepiece project.

--> If live doesn’t want me to have the perfect 2 inches EP, their must be a reason for that, maybe I don't deserve it or it would be bad for me.

--> I struggle painfully with natural elements: It's 30 degrees Celsius and more for 3 weeks now, extreme humidity, extreme mosquitos, tiredness, the moon is there, foot pain.

 

The 2 inches project is on the ice.

[John]

If you don't like blurry edges you might find that none of the eyepieces you have listed escape this in an F/5 newtonian. Some will show less of this than others though but for "sharp right across" at F/5 either the budget needs to increase or the requirement for a wide field to diminish somewhat.

The ES 68 might be the exception to this, of the eyepieces mentioned above.

 

 

[N3ptune]

4 hours ago, John said:

If you don't like blurry edges you might find that none of the eyepieces you have listed escape this in an F/5 newtonian.

Well has discussed before, my Celestron Xcel-LX 25mm 1.25inches is doing an great job for the blur, the FOV is more then acceptable, my actual 28mm LET 2 inches is far worst. This is my example from 1 eyepiece to another one, they are not equal.

I received an e-mail from the store today, they tell me they have a ES 28mm 68degrees. it's not the size I hoped for, but it could be a good EP also compare to LET.