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OCS and barlow impact on SCT's focal length

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#1 YKSE

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Posted 30 May 2016 - 07:15 AM

As I understand it, OCS is recommended for binoviewing for spherochromatism correction, especially for SCT since it shortens SCT focal length too. Then there comes questions:

1. Placing OCS after and before the diagonal, does it give the same shortening of focal length of a SCT? If gives different magnifcation by my measurement.

2. How about using a barlow instead of an OCS? Does it shorten the focal length of a SCT too?

3. If yes to the second question, how about using a barlow in SCT in Cyclops?



#2 Eddgie

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Posted 30 May 2016 - 08:01 AM

The Barlow placement will change the amount of amplification and hence the mirror spacing so if you move the Barlow to the front of the diagonal, the mirrors will have to be moved further apart.

 

Both the OCS and the Barlow are amplifiers and work the same way.  Both extend the light cone. 

 

The difference in using a Barlow or OCS in mono-vision mode and binoviewer mode is the spacing.    If you use a single eyepiece and a conventional diagonal for a given OCS or Barlow lens, the amplification will not be as great as with the binoviewer because the amount of back focus needed to reach focus is not nearly as much.

What determines the actual power of a the Barlow or OCS is the distance between the amplifier and the field stop.  The further forward you move the amplifier from the field stop of the eyepiece, the more powerful it will become.

 

To cancel the effects of a binoviewer's light path length on a standard moving mirror SCT, you need about 2.6x of magnification.   

 

I have a theory that this is the reason that Baader sells the 2.6x GPC.   When used with the T2 mirror (which is the way Astro-Physics sells it and there are many other good reasons why the equip the Mark V with a mirror rather than a prism) the 2.6 GPC will just about exactly cancel out the back focus of the standard SCT.

 

I don't think that is a coincidence.  I think it was designed exactly for that purpose.



#3 YKSE

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Posted 30 May 2016 - 08:44 AM

Thanks for the explanations Eddgie.

Yes, placing the OCS before and after the diagonal will give different magnifications, how about the focal length of the SCT, has it been shortened to the same(or almost the same) amount?



#4 Eddgie

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Posted 30 May 2016 - 09:05 AM

It is hard to know exactly what the focal length of the system itself would be with the amplifier in place because the focal length of the primary optics varies with mirror spacing.

 

To answer your question though, when using a binoviewer, adding about 2.3x to 2.6x of amplification will result in a mirror spacing that is about the same you would get if you were to use the factory 1.25" diagonal and 1.25" visual back.  

For the C5, C8, C9.25, and C11, these components (1.25" visual back and diagonal) gave about the stated focal length and about f/10 focal ratio for all of these scopes.    If the system was well made, this mirror spacing would provide the best possible performance.

 

Adding back focus would increase the spherical aberration of the system at the rate of 1/23d wave for every 25mm of back focus added.   For small amounts this can be completely ignored, but for binoviewer type amounts of back focus, it can have enough effect to lower contrast by a meaningful amount.    Pretty much any Barlow giving 2x to 3x of amplification will bring the mirror spacing back close enough to optimal that the small residual differences can be ignored.

 

For general observing, the OCS or Barlow is not at all critical, but for the most demanding high resolution work in and SCT, I would recommend some kind of Barlow or OCS.



#5 YKSE

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Posted 01 June 2016 - 04:27 AM

Chris Lord has this paper about calculating SCT or Mak's focal length, measuring the initial data seems difficult to me.

http://www.brayebroo.../EFLMAKCASS.pdf



#6 mikey cee

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Posted 01 June 2016 - 07:36 PM

YKSE, on 01 Jun 2016 - 04:27 AM, said:

Chris Lord has this paper about calculating SCT or Mak's focal length, measuring the initial data seems difficult to me.

http://www.brayebroo.../EFLMAKCASS.pdf

The minute I saw that web site my eyes immediately glazed over!!  :belushi:  :jawdrop:



#7 YKSE

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Posted 02 June 2016 - 02:37 AM

My questions are raised because I did some measurement the other day, with a yardstick on a table some 20 meters away, I measured FOV with OCS ahead and behind the diagonal, assuming same effect on focal length, which I'm not sure, the measurement and calculation looks like this:

GPC_T2_front_back_measurement.jpg

 

1.25x OCS works 1.17x  after diagonal, and 1.23x ahead of the diagonal,

for 1.7x, it's 1.37x and 1.51x, and finally 2.6x is 2.48x and 3.07x.



#8 Eddgie

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Posted 02 June 2016 - 07:49 AM

These figures are pretty consistent with what other people have measured.

 

As you can see now, the actual magnification factor can change just by changing eyepieces and refocusing.

 

The widest possible low power field I was able to achieve was using a Baader 10mm SCT to T2 thread adapter, the T2 Prism, and a Mark V attached to the top via the Quick Connector.    The light path length for this setup is 171mm.

 

This will yield a focal length increase of about 220mm. 

 

The Maxbright will result in a shorter focal length increase (as will other similar binoviewers when directly connected) but of course the smaller apertures will restrict you to something like 20mm wide fields, so a 24mm wide field in the Mark V will  work at the full apparent field. (IF you put a 24mm Panoptic in a Maxbright, the apparent field is reduced from 68 degrees to something less than 60 degrees.   The rear aperture of the BV simply acts like a smaller field stop). 

 

As an aside, if you have a binoviewer that supports very large field stop, I found the 35mm Ultima to be the best compromise between apparent field and exit pupil.   The apparent field of a 40mm Plossl will be kind of unrewarding to use.  The 35mm Ultima/Orthoscopic/Parks is much more like a standard Plossl.

 

The 35mm Ultrascopic with the above configuration gave the very widest possible field I got from my SCT. It is only a tiny bit wider than a 24mm Pan, but if you need the extra exit pupil for a dim nebula, the additional reduction in magnification could be worth it.

 

I don't own this scope anymore but here was my own optimal configuration:

 

 

 

Attached Thumbnails

  • Clearence with 10mm.jpg


#9 YKSE

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Posted 02 June 2016 - 09:42 AM

Yes, Ed, different EPs' focusing position should change the FL of SCT too, I seem to recall seeing somewhere Celestron mentions very small amount of primary mirror movement with a full turn of focus knob. Can't find the exact number mentioned.

The 10mm SCT to T2 is surely the shortest one, along with 2" prism diagonal screws on the SC thread directly (using the Baader Lock ring to lock the diagonal).




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