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Is there ANY POINT in using ORTHO's with a larger CENTRAL OBSTRUCTION scopes?

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#1 dfoster356

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Posted 10 April 2021 - 06:54 PM

I've had mixed results doing this.  

 

What have you experienced?


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#2 junomike

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Posted 10 April 2021 - 07:14 PM

IME it's all about the eyepiece F/L and OTA F/L and not the eyepiece design.


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#3 KBHornblower

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Posted 11 April 2021 - 09:59 PM

The presence or absence of a central obstruction should not be a factor in choosing the eyepiece.  A good eyepiece of any type "gets out of the way" and lets the telescope do its stuff without introducing additional aberrations or scattered light.


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#4 luxo II

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Posted 12 April 2021 - 07:34 AM

With orthos - like many older eyepiece designs - it all depends on focal ratio.

 

In f/10 or longer scopes orthos are fine.

But in less than f/10 expect to be disappointed.

At f/6 or less you WILL be disappointed for sure.

 

NB recently tested quite a wide range of old and new eyepieces to see how they perform in a variety of scopes.

 

IMHO the race to ever-shorter focal ratios has two downsides:

 

a) the fast scopes - f/6 and under - are fussy when it comes to eyepieces. You may end up spending thousands on a boxful of Televues to match an f/5 newtonian or f/7 refractor, when instead if you had opted for f/10 or more you could use just about any eyepiece and get fine results.

 

b) many of the fast scopes - and I mean newtonians f/6 or less, or APO refractors f/7 and less - are somewhat marginal optically. If the manufacturer had opted for a slower f/ratio its a lot easier to produce better quality optics. 


Edited by luxo II, 12 April 2021 - 08:07 AM.

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#5 Jon Isaacs

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Posted 12 April 2021 - 09:35 AM

a) the fast scopes - f/6 and under - are fussy when it comes to eyepieces. You may end up spending thousands on a boxful of Televues to match an f/5 newtonian or f/7 refractor, when instead if you had opted for f/10 or more you could use just about any eyepiece and get fine results.

 

 

One does not need to spend thousands of dollars on TeleVue eyepieces to use in faster scopes. If one is satisfied with "just about any old eyepiece at F/10", then invest in a decent 2X Barlow or Telecentric and that same eyepiece will be working at F/10..

 

Many invest in quality wide fields for F/10 scopes.. 

 

Jon


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#6 dfoster356

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Posted 12 April 2021 - 10:33 AM

With orthos - like many older eyepiece designs - it all depends on focal ratio.

 

In f/10 or longer scopes orthos are fine.

But in less than f/10 expect to be disappointed.

At f/6 or less you WILL be disappointed for sure.

 

NB recently tested quite a wide range of old and new eyepieces to see how they perform in a variety of scopes.

 

IMHO the race to ever-shorter focal ratios has two downsides:

 

a) the fast scopes - f/6 and under - are fussy when it comes to eyepieces. You may end up spending thousands on a boxful of Televues to match an f/5 newtonian or f/7 refractor, when instead if you had opted for f/10 or more you could use just about any eyepiece and get fine results.

 

b) many of the fast scopes - and I mean newtonians f/6 or less, or APO refractors f/7 and less - are somewhat marginal optically. If the manufacturer had opted for a slower f/ratio its a lot easier to produce better quality optics. 

Looking back over my mixed results, this would explain a lot.  Thanks for the succinct analysis.  


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#7 Mitrovarr

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Posted 12 April 2021 - 10:46 AM

That's overly harsh. Orthos aren't ideal at F/6 or whatever but they're not terrible. Same with plossls. The edge of the field starts to become less sharp but it isn't a train wreck, not even at F/4.
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#8 Jon Isaacs

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Posted 12 April 2021 - 08:16 PM

That's overly harsh. Orthos aren't ideal at F/6 or whatever but they're not terrible. Same with plossls. The edge of the field starts to become less sharp but it isn't a train wreck, not even at F/4.

 

Indeed. 

 

Some experienced observers with large fast Dobs use Orthos for contrast.

 

Also, I'm not sure what the large central obstructions has to do with orthos. Most scopes with large COs are relatively slow Cassegrains and CATs.  Even the Orion 4.5 inch F/4 Starblast has a 30% CO, smaller than nearly all SCTs, smaller than many Maks.

 

My 22 inch F/4.4 came with a 3.5 inch = 89mm secondary, 16% For comparison, a Celestron C-11 has a 95mm secondary, half the aperture but a larger secondary. A C-9.25 has an 85mm secondary.

 

Jon



#9 dfoster356

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Posted 13 April 2021 - 11:25 AM

Indeed. 

 

Some experienced observers with large fast Dobs use Orthos for contrast.

 

Also, I'm not sure what the large central obstructions has to do with orthos. Most scopes with large COs are relatively slow Cassegrains and CATs.  Even the Orion 4.5 inch F/4 Starblast has a 30% CO, smaller than nearly all SCTs, smaller than many Maks.

 

My 22 inch F/4.4 came with a 3.5 inch = 89mm secondary, 16% For comparison, a Celestron C-11 has a 95mm secondary, half the aperture but a larger secondary. A C-9.25 has an 85mm secondary.

 

Jon

Large CO's here means by percentage of objective.  I wasn't fully clear about this, so I understand the confusion. 

 

Orthos are known for contrast.  Small (% of D) CO  scopes are as well.  

 

This is what led me to inquire about the experiences of others when combining Orthos with largish CO's.  


Edited by dfoster356, 13 April 2021 - 11:26 AM.

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#10 Mitrovarr

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Posted 13 April 2021 - 11:28 AM

I would say orthos are the wrong direction to go for good DSO contrast. Go large AFOV eyepiece. Higher magnification = darker sky = better contrast.

#11 KBHornblower

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Posted 13 April 2021 - 01:16 PM

I've had mixed results doing this.  

 

What have you experienced?

Can you describe your mixed results in technical detail?

 

I have done tests with large obstructions, about 33%, alongside unobstructed apertures the same size.  I could see the brightening of the diffraction rings in star images, and that is something that subtly reduces the arcsecond-scale detail contrast on the face of a planet.  It is not the catastrophic bogeyman that some of us think it is out of misunderstandings, and it does not reduce the contrast between a nebula or galaxy and the surrounding dark sky.  I does not mask the ill effects of scattered light from inferior eyepieces or other sources, so the superior characteristics of a good ortho are not wasted on objects that will fit into its field of view.


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#12 dfoster356

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Posted 13 April 2021 - 01:17 PM

I would say orthos are the wrong direction to go for good DSO contrast. Go large AFOV eyepiece. Higher magnification = darker sky = better contrast.

I tend to agree with you there.  I do prefer orthos on planets and occasionally the moon.  



#13 dfoster356

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Posted 13 April 2021 - 01:33 PM

Can you describe your mixed results in technical detail?

 

I have done tests with large obstructions, about 33%, alongside unobstructed apertures the same size.  I could see the brightening of the diffraction rings in star images, and that is something that subtly reduces the arcsecond-scale detail contrast on the face of a planet.  It is not the catastrophic bogeyman that some of us think it is out of misunderstandings, and it does not reduce the contrast between a nebula or galaxy and the surrounding dark sky.  I does not mask the ill effects of scattered light from inferior eyepieces or other sources, so the superior characteristics of a good ortho are not wasted on objects that will fit into its field of view.

Yes.  Post #4 and my response in #6 sums it up pretty well.  



#14 Jon Isaacs

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Posted 13 April 2021 - 03:18 PM

Large CO's here means by percentage of objective.  I wasn't fully clear about this, so I understand the confusion. 

 

Orthos are known for contrast.  Small (% of D) CO  scopes are as well.  

 

This is what led me to inquire about the experiences of others when combining Orthos with largish CO's.  

 

In my thinking, the diffraction effects of a larger CO on planetary contrast is visible but only really meaningful when comparing scopes of similar apertures.  The diffraction effects of the aperture on planetary contrast are far greater.

 

In terms of Orthos and faster scopes, the eye relief is a serious problem.  I view the planet's on a good night in my 10 inch F/5 at 300x-400x, that's a 5mm or 3.5 mm eyepiece. Orthos just work.

 

Jon



#15 dfoster356

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Posted 13 April 2021 - 03:30 PM

In my thinking, the diffraction effects of a larger CO on planetary contrast is visible but only really meaningful when comparing scopes of similar apertures.  . Orthos just work.

 

Jon

I agree 100%.

 

I would only add - "orthos just work" as they are designed - I too have struggled with their eye relief, but have been rewarded for doing so.  

 

Thanks for your insight.


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#16 Mitrovarr

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Posted 13 April 2021 - 04:30 PM

I'm kind of wondering what eyepiece it is that doesn't "just work"

#17 teashea

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Posted 13 April 2021 - 08:45 PM

It does seem that orthos are often mentioned in the context of planetary observation, rather than with DSO's.  



#18 dfoster356

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Posted 14 April 2021 - 07:53 PM

It does seem that orthos are often mentioned in the context of planetary observation, rather than with DSO's.  

I agree.  The focal lengths I own are better suited to brighter objects.  I've often wondered how a longer FL ortho EP would perform on clusters etc.  

 

Conditions in my region are infrequently ideal.  It's hard to justify redundancy in my EP collection.  

 

I'd be interested to hear reports of the "longer" FL orthos - say above 12mm or so.  



#19 Mitrovarr

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Posted 14 April 2021 - 07:59 PM

Orthos aren't widely used for DSO observation because the narrow AFOV is too significant a disadvantage there.

#20 blakestree

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Posted 14 April 2021 - 08:40 PM

I agree.  The focal lengths I own are better suited to brighter objects.  I've often wondered how a longer FL ortho EP would perform on clusters etc.  

 

Conditions in my region are infrequently ideal.  It's hard to justify redundancy in my EP collection.  

 

I'd be interested to hear reports of the "longer" FL orthos - say above 12mm or so.  

My 32mm TAO is near the top among my favorites. It's one of those "magical" eyepieces. The public has been taught that they need wide-field oculars.



#21 The Ardent

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Posted 14 April 2021 - 09:09 PM

They are useful in city and suburban skies where the narrow FOV blocks out sky glow and allows better observation of any single target. 
 

Consider two eyepieces of same focal length: an ortho of 45 deg apparent field vs a wide field 100 deg fov. The wider ep will show 4x more area of skyglow. Pardon my inexact math. 

Orthos aren't widely used for DSO observation because the narrow AFOV is too significant a disadvantage there.

I observed from the city for 10 years. 


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#22 Mitrovarr

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Posted 14 April 2021 - 09:15 PM

The trick is to increase the magnification so you frame the same area of the sky. That way, you get better contrast and a darker sky because of the much higher magnification.



#23 Voyager 3

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Posted 15 April 2021 - 02:13 AM

The trick is to increase the magnification so you frame the same area of the sky. That way, you get better contrast and a darker sky because of the much higher magnification.

This . This is the main reason for using wider field EPs . You don't have to go to a larger exit pupil to get a larger field .



#24 Old Speckled Hen

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Posted 15 April 2021 - 03:10 AM

I've had mixed results doing this.  

 

What have you experienced?

What are mixed results?



#25 dfoster356

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Posted 15 April 2021 - 06:41 PM

They are useful in city and suburban skies where the narrow FOV blocks out sky glow and allows better observation of any single target. 
 

Consider two eyepieces of same focal length: an ortho of 45 deg apparent field vs a wide field 100 deg fov. The wider ep will show 4x more area of skyglow. Pardon my inexact math. 

I observed from the city for 10 years. 

 

 

The trick is to increase the magnification so you frame the same area of the sky. That way, you get better contrast and a darker sky because of the much higher magnification.

Interesting and insightful perspectives!




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