34 replies to this topic

[Sebastian_Sajaroff]

Svbony eyepieces behave OK on a slow telescope (F/8 or higher), they’re so so on an F/6 and poor on an F/4.
F/4 is the realm of expensive eyepieces.

[svenkill]

why is it that faster scopes struggle more with eye piece performance?  Does it have to do with the focusing point being shorter so it's bending the light more, making the tolerances less for bad lenses?

[davidgmd]

why is it that faster scopes struggle more with eye piece performance?  Does it have to do with the focusing point being shorter so it's bending the light more, making the tolerances less for bad lenses?

  
Exactly. Often referred to here as a “steeper light cone.”

[Jon Isaacs]

why is it that faster scopes struggle more with eye piece performance?  Does it have to do with the focusing point being shorter so it's bending the light more, making the tolerances less for bad lenses?

 

It's not the scope that struggles, it's the eyepiece that struggles with the fast focal ratio.

 

It's not a question of the quality of the lenses, it's a question of the design of the eyepiece. Eyepieces that are sharp across the field in a fast scope generally have more elements.

 

Jon

[svenkill]

It's not the scope that struggles, it's the eyepiece that struggles with the fast focal ratio.

 

It's not a question of the quality of the lenses, it's a question of the design of the eyepiece. Eyepieces that are sharp across the field in a fast scope generally have more elements.

 

Jon

That makes sense! I’m still learning the difference between f ratios and what it all means. I have an f/5 scope and didn’t know it made a difference on eye piece selection until reading on here. Thank you!

[Olimad]

Another good result i get by adding the 0,5x to the 6mm redline is an amélioration of the edge of field aberrations i get on my f4 newtonian. Far less astigmatism and the stars on the center and on the edge are focused similarly.

Way less aberrations than the 6 or 9mm alone or the UFF 10mm on my f4. 

Made the test on the pléiades and beehive. 

 

Happy to have found that. 

[star acres]

I have the 6, 9, and 20. The telescope? 114/500 = F. 4.38. The 6 is most prone to kidney beaning, so hunker down and get behind it. It's not a Cadillac, but it's not a .965, either. The 9 MM has less kidney beaning. Both show a bit of out of focus up to the edges and the edges are themselves colored and a bit fuzzy. It's my favorite of the three. The 20 gives a lot of center shadows in daylight and the edges get pretty far out of focus, but they still help you put your object in the center. The Red Bands are good materials and mine are quite blacked out. The 6 and 20 are 5 lenses and the 9 is 6 lenses. They have lots of eye relief spacing. The 6 and 9 share a bunch of parts.

[Wimpy.mcpoat]

I bet it all on red!

[hlhr]

Good afternoon all,

 

First of all, I apologize for disturbing this long dormant thread, but I have an honest question regarding the 6mm redline eyepiece. I have the 6mm, 9mm and 15mm ones. Compared to the other two eyepices, the 6mm shows a considerably darker view. Very ,much akin to what happens when you put a 2x barlow on any eyepiece. An usually bright image, becomes darker.

At first I thought that perhaps the 6mm was a 12mm with an integrated barlow. Google says otherwise. 

Why then does the image through the 6mm is a lot darker than through the other two eyepieces of the same line? 

 

ps: If I barlow the 6mm, I can barely see anything but the brightest of objects.

[The Cloud Gazer]

Good afternoon all,

 

First of all, I apologize for disturbing this long dormant thread, but I have an honest question regarding the 6mm redline eyepiece. I have the 6mm, 9mm and 15mm ones. Compared to the other two eyepices, the 6mm shows a considerably darker view. Very ,much akin to what happens when you put a 2x barlow on any eyepiece. An usually bright image, becomes darker.

At first I thought that perhaps the 6mm was a 12mm with an integrated barlow. Google says otherwise. 

Why then does the image through the 6mm is a lot darker than through the other two eyepieces of the same line? 

 

ps: If I barlow the 6mm, I can barely see anything but the brightest of objects.

Hello!

 

This is because the exit pupil gets smaller with a shorter eyepiece focal length. It has nothing to do with the eyepiece; a longer focal length is going to be brighter, and a shorter focal length will show a dimmer view. You should see the same effect going from the 15 to the 9mm. 

 

Your 2x barlow divides the focal length of an eyepiece in half***, this explains why you get a dimmer view. When used in your 15mm, you get a 7.5mm eyepiece; with your 9mm, you get a 4.5mm eyepiece. 

 

The exit pupil is the size of the disk of light exiting the eyepiece (that your eye acquires). It is given by:

 eyepiece focal length/telescope focal ratio

 

Let's say, for the sake of example, that you have an f/6 scope, then for each of your eyepieces:

- 15mm: 15/6 = 2.5mm exit pupil

- 9mm: 1.5mm exit pupil

- 6mm: 1mm exit pupil

 

Depending on the type of objects, certain exit pupils are preferred; there are some guidelines, but in the end, it is up to you to see what works best through experimentation (depends on your sky, your seeing conditions, your eyes, your telescope etc.). 

 

The largest exit pupil that still brings a brighter image corresponds to the size of your dilated pupils. Depends on your age. Up to ~7mm, let's say but it really depends on your eyes.

 

The smallest exit pupil... well. People usually say 0.5mm. You can always go to 0.2mm if you want, to see if you get anything out of the additional power. I know that when I go below 1mm exit pupil, I have "floaters" in the eye that can be annoying, depending on the target. For some other target, I go to crazy high powers for the aperture and still like it.

 

Experiment! See what works!

 

***Actually, it is more correct to see it as multiplying the focal length of your scope by 2. But just so you can more easily compare what exit pupil you will end up with when comparing with non-barlowed eyepieces, you can think of it as halving the eyepiece focal length.

Edited by The Cloud Gazer, 19 May 2025 - 02:20 PM.