#21  
Old 12-02-2013, 05:30 PM
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I can only find a link which says: Strehl 0.951

http://www.astro-foren.de/showthread...5020#post55020



Auch der Strehlwert ist für eines dieser Foto-Objektive sehr hoch, was für fotografische Zwecke gar nicht erforderlich ist:
10 inch GSO RC Wieviel Strehl braucht ein Astro-Objektiv ? ATIK4000-techn.Daten



here:
http://rohr.aiax.de/PF_GSO10RC_06.jpg



Where does this Strehl of 0.27 come from for a GSO RC 10"?
I can't find the link.


cheers
Allan
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  #22  
Old 12-02-2013, 07:23 PM
Poita (Peter)
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Luke at Andrews said he is getting two 16"ers in this April, and wouldn't know anything about them until they arrive.
So I guess we wait and see!
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Old 12-02-2013, 08:50 PM
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This was the first mention of Strehl ratio I saw in the review...

Remember that an instrument with a 50% obstruction will lose some 40% of its Strehl ratio due to the obstruction.
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  #24  
Old 12-02-2013, 09:33 PM
Poita (Peter)
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Does the 16" have a 50% obstruction? If it is f9 I would think the obstruction would be less than 40% (I'm known for getting calcs wrong though).

For the non Deutsch public, on that image t he heading on the left says 'before optimisation' for the 0.278 figure and the heading on the right says 'after optimisation' for the 0.904 figure.

Last edited by Poita; 12-02-2013 at 10:01 PM.
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Old 13-02-2013, 10:36 AM
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Quote:
Originally Posted by Satchmo View Post
This was the first mention of Strehl ratio I saw in the review...

Remember that an instrument with a 50% obstruction will lose some 40% of its Strehl ratio due to the obstruction.
Hi Mark,
then why do people pay megabucks for obstructed RC telescopes with published high Strehl ratios?
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Old 13-02-2013, 10:51 AM
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Hi Mark,
then why do people pay megabucks for obstructed RC telescopes with published high Strehl ratios?
To guarantee optical and mechanical quality. 0.5 wave spherical aberration plus a 50% obstruction will probably have an Encircled Energy Ratio ( the Strehl value taking into account obstructive effects ) of less than 0.5... A low Strehl value does not render a telescope useless for imaging. Most of the worlds large RC's probably have 50% obstructions...their resolutions even with large obstruction are way below the seeing limit anyway.
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Old 13-02-2013, 11:25 AM
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So going back to the 10" GSO test you saw being 'very poor optical quality', I cannot find any reviews that give that conclusion. The review that you are pointing to (as far as I can tell) actually verified quite high quality optics for imaging, unless I am looking at the wrong one?

I'm interested in any poor reports as a mate of mine is seriously considering the 16" model.
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  #28  
Old 13-02-2013, 11:57 AM
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Originally Posted by Poita View Post
So going back to the 10" GSO test you saw being 'very poor optical quality', I cannot find any reviews that give that conclusion. The review that you are pointing to (as far as I can tell) actually verified quite high quality optics for imaging, unless I am looking at the wrong one?

I'm interested in any poor reports as a mate of mine is seriously considering the 16" model.

From what I hear the problem with GSO is that their quality assurance is not consistent.
You could buy a good one or a bad one & it's just luck.
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Old 13-02-2013, 12:39 PM
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That isn't much of a problem if you live in the same city as the vendor, you just make the required quality level a condition of sale.

What I am interested in though is any review that states that the GSO RC scopes are of a 'very poor optical quality'. That would be a concern if true.
I can't find any yet.
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Old 13-02-2013, 12:40 PM
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I'm interested in any poor reports as a mate of mine is seriously considering the 16" model.
The whole point is that on cheaper mass produced instruments there is much greater spread of quality. You would have to be thinking wishfully to that one report ( good or bad ) had any statistical meaning. Ten might start to give you a good idea. These optical systems do not come with a any guarantee certificate nor are the optical pieces even serialised.

On the Newts GSO website states better than +/- 1/12 wave surface RMS which is about 1/3 wave wavefront if talking basic spherical correction- I have tested ones much better than this and some worse. But I can guarantee the only thing that stick in the mind of readers is the 1/12 wave bit !

If you buy a high end system like RC Optical you will get an interferogram and an optical certificate of guaranteed performance. Thats what I would call piece of mind.

If you can star test a telescope to weed out a lemon and up to making mechanical mods needed then the cheap RC's would seem worth a look.
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Old 13-02-2013, 01:48 PM
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I've seen multiple good reports, but I like to see the bad ones to find out where the issues are.
You mentioned a review that pointed to the GSO having very poor optical quality, that one I would like to read to see where the problem was.

People have said various things across the forums about quality problems with the GSO RC units, but no one so far who actually *has* one.

I'm just trying to sort the FUD from the facts.

Well aware that any consumer product can have issues, look at the hassles Paul had with Lunt recently, but it does seem that a lot of the GSO RC bashing tends to come not from people who have actually used one.

C'mon, someone out there must have a dud one to report on
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  #32  
Old 13-02-2013, 05:16 PM
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My 10" RC has been brilliant and I can't fault it, but then I use it for astrophotography not optical testing.
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Old 13-02-2013, 07:28 PM
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Quote:
Originally Posted by Satchmo View Post
To guarantee optical and mechanical quality. 0.5 wave spherical aberration plus a 50% obstruction will probably have an Encircled Energy Ratio ( the Strehl value taking into account obstructive effects ) of less than 0.5... A low Strehl value does not render a telescope useless for imaging. Most of the worlds large RC's probably have 50% obstructions...their resolutions even with large obstruction are way below the seeing limit anyway.
I'd suggest you are mixing your metaphors here.

Strehl is calculated via looking at the RMS values of a * particular system* .

Optical designs vary (eg obstructed vs non-obstructed) hence it makes no sense to compare Airy disk functions between the two.

You can derive (through RMS data ) a Strehl number of say 0.97 for an obstructed system.

What this is saying is: 97% of the flux captured by this system goes where it is expected (ie focus) compared to a theoretically perfect system of the same design.

A system of the *same design* with a Strehl of 0.5 is scattering light all over the shop... clearly rubbish and should be avoided.
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Old 14-02-2013, 07:52 AM
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You can derive (through RMS data ) a Strehl number of say 0.97 for an obstructed system.
I follow your point - its rteally a semantical one as you understand the pint I am trying to make.

I did talk about the Encircled Energy Ratio- this is the performance of the system taking into account the central obstruction (Essentially a modified Strehl ratio of the system including negative effects of central obstruction.). The owner of an 0.999 Strehl ratio scope with 50% obstruction would find his airy pattern and contrast transfer function similar to the owner of an unobstructed scope with 1/2 a wave or more of spherical aberration ( equivilent to an Strehl of around 0.6 )

So simply comparing Strehl ratio between different kinds of scopes, is a bit of furfy for the unwary. Good interferometry software is perfectly capable of putting out an EER number as well as Strehl but it is not the kind of thing instrument manufacturers want to advertise.

I propose manufacturers who supply optical data on finished instruments include a figure for the Encircled Energy Ratio- lets rename it the OSR or obstructed strehl ratio then the waters will be much clearer for those that want to compare relative performance of optical systems. After all its the ability of the instrument to focus light from an available aperture into the Airy Disc that we are interested in - and I know that pixel size in imaging further complicates these considerations.

Last edited by Satchmo; 14-02-2013 at 09:20 AM.
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Old 14-02-2013, 09:19 AM
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Quote:
Originally Posted by Satchmo View Post
To guarantee optical and mechanical quality. 0.5 wave spherical aberration plus a 50% obstruction will probably have an Encircled Energy Ratio ( the Strehl value taking into account obstructive effects ) of less than 0.5... A low Strehl value does not render a telescope useless for imaging. Most of the worlds large RC's probably have 50% obstructions...their resolutions even with large obstruction are way below the seeing limit anyway.
Hi Mark,
Thanks for that information.
Is there an online calculator which can provide an Encircled Energy Ratio
& a Strehl ratio for a given system - in theory assuming that it was perfect?

This is a link for an online calculator
http://www.wilmslowastro.com/software/formulae.htm

but it doesn't show the above calculations.


cheers
Allan
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Old 14-02-2013, 09:23 AM
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I follow your point -

I did talk about the Encircled Energy Ratio- this is the performance of the system taking into account the central obstruction (Essentially a modified Strehl ratio of the system including negative effects of central obstruction.). The owner of an 0.999 Strehl ratio scope with 50% obstruction would find his airy pattern and contrast transfer function similar to the owner of an unobstructed scope with 1/2 a wave or more of spherical aberration ( a Strehl of around 0.6 )
......
Indeed but the elephant in the room is MTF gives a handle on a resolution limit due diffraction and should not be confused with an optical error.

A larger aperture obstructed system can produce higher resolution than an smaller unobstructed one.

A 1/2 wave spherical error does more than just lower contrast. A telescope with this sort of error simply doesn't focus, and produces caustic images full of halos. The most famous example being Hubble pre-servicing mission.

Last edited by Peter Ward; 14-02-2013 at 10:01 AM.
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Old 14-02-2013, 10:23 AM
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Indeed but the elephant in the room is MTF gives a handle on a resolution limit due diffraction and should not be confused with an optical error.

Indeed a larger aperture obstructed system can produce higher resolution than an smaller unobstructed one.

A 1/2 wave spherical error does more than just lower contrast. A telescope with this sort of error simply doesn't focus, and produces caustic images full of halos. The most famous example being Hubble pre-servicing mission.
From memory that resolution increase with obstruction is only for sources fairly similar in brightness..its a small anomaly rather than a "feature"

I did specify effect on MTF in my earlier post ( Contrast Transfer Function ) . The effect on it by obstruction or spherical aberration is similar, so optical quality and obstructiuon are both important influences on the MTF.

I think supplying MTF curves would be the ultimate piece of documentation but how many punters would understand it? People seem to have grasped the Strehl Ratio, so I think furthering our understanding of what an instrument is capable of through an EER or Obstructed Strehl Ratio would be a positive step.

I don't think its accurate to say that an instrument with 1/2 wave spherical aberration does not focus. Anyone owning a high quality optic with a 50% obstruction is seeing an in focus Airy Pattern similar to an unobstructed telescope with 1/2 a wave of spherical aberration.

The Hubble Space Telescope had a 1/2 wave RMS figure ( around 2 waves P-V) . Official report on the whole debarcle here...

Last edited by Satchmo; 14-02-2013 at 10:38 AM.
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Old 14-02-2013, 12:00 PM
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I don't think its accurate to say that an instrument with 1/2 wave spherical aberration does not focus. Anyone owning a high quality optic with a 50% obstruction is seeing an in focus Airy Pattern similar to an unobstructed telescope with 1/2 a wave of spherical aberration.....
Well, OK, it does focus, just not very well.

Sure, with the "best focus" airy disks are similar with 50% obstructed systems and say unobstructed 1/2 wave error system.

But what I've found time and time again with imaging, that's not the whole story...the system with the error has a more divergent "off focus" spread, scattering or both which is significantly larger than the airy disk and seeing nearly always turns into halos....something you don't get with a well corrected system.

Hope that makes sense
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Old 15-02-2013, 06:35 PM
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Here was me thinking I was going to read something about a 16" RC from GSO, have they actually been sighted in the wild yet?
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Old 15-02-2013, 06:59 PM
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Here was me thinking I was going to read something about a 16" RC from GSO, have they actually been sighted in the wild yet?
I spoke to Luke at Andrews, they haven't even seen a photo of one yet. They have two coming in in April.
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