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SCTs and binoviewers and magnification, oh my!

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#1 swsantos

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Posted 02 August 2020 - 09:25 AM

"This new learning amazes me!" - King Arthur to Sir Bedevere from Monty Python and the Holy Grail

 

Got myself a pair of 12.5mm Morphei to go with the Maxbright II and the GPC 1.25 hoping for 200X magnification to duplicate the normal magnification that I use for planetary observation in cyclops mode with the 10mm Ethos. First time out with that I spent a lot of time looking at Saturn and Jupiter thinking to myself that wow things do look bigger in a binoviewer, the illusion of those objects appearing bigger than what I was used to in cyclops mode no doubt created by the binocular summation. Little disappointed that the view was slightly muddy relative to cyclops mode but that's OK because in total the view was easier to take in and study using both eyes. The view reminded be of when I try and go past 200X in cyclops mode with my skies that rarely allow much more that that.

 

Then it all came into "focus" and I remembered my research into SCTs and binoviewing the concept of operating focal length. Somehow I thought that that only happened to other people why would I have to worry about such things? Certainly my SCT wouldn't annoy me with such considerations because we have known each other for so long and have I have always treated it so well. Once the reality of me not having considered the operating focal length of my SCT with the binoviewer sunk in, the unexpectedly larger slightly muddy view of Saturn and Jupiter in evidence, I obviously needed to rethink planetary eyepiece power. After even more research into this matter it was apparent that the first step was to calculate the operating focal length of my SCT with the binoviewer. There are star drift methods whereby one measures the time it takes for a star to traverse the field of view but that seemed complicated as there are too many moving parts like choosing just the right star at the celestial equator for the math to work out. Then I found what was for me an easier, done in the daytime with no moving parts, method to determine effective focal length based on this article written by Professor Edz:

 

https://www.cloudyni...of-an-sct-r2335

 

 

I started with my Televue 85 which has a focal length on 600mm, which I have verified that mine actually is, to get the baseline measurement. I propped up a measuring tape against a telephone pole down the street extending up from the ground against the pole about 8 feet. I had to use duct tape to attach the measuring tape about 6 feet off the ground so it would not curl and bend down in the slight breeze. My neighbor watched me attach it to the pole next to her mailbox probably wondering what was going on but did not ask. From my yard 100+ yards away the 12.5mm Morpheus showed 79.5 inches of the measuring tape through it at its maximum field of view top to bottom with 48X magnification.

 

Armed with that information I then did the same thing with the 8" Edge SCT. The 12.5mm Morpheus in my usual 2" Baader BBHS prism diagonal showed 22 inches of the measuring tape and in the binoviewer showed 17.125 inches. Using the 48X magnification provided by the Televue 85 and its 79.5 inch starting point I determined that the actual magnification at the Baader diagonal was 173X and at the Maxbright binoviewer 223X. From that I figured that at the diagonal the system operates at focal length 2163mm (f10.8), and at the binoviewer 2788mm (f13.9). There is a magnification factor increase of 1.08X at the diagonal and 1.39X at the binoviewer. Feel free to correct my math but I checked it several times.

 

So anyone newly getting into binoviewing, like me who wants to utilize the binoviewer on an SCT, really needs to do this one time calculation of the extra magnification factor provided by the binoviewer on their own telescope by one method or another. My understanding is that that is not necessary on other types of telescopes perhaps someone can add more information here as it might relate to that.

 

For me, my next course of action is to hopefully be able to exchange the 12.5mm Morphei for 14mm Morphei which should provide 199X, about the original power I had been looking for. I will bet things will look a little smaller but sharper and less, and hopefully not at all, muddy.

 


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#2 Eddgie

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Posted 02 August 2020 - 11:45 AM

Only telescopes with a negative power secondary and mirrors that move for focusing will experience the focal length increase. This means mainly it is MCTs and SCTs that will have this issue.

 

Also, with the EdgeHD 8", it may be useful to measure the aperture.  The reason is that the EdgeHD 8" looses aperture with less flange to focal plane distance than the standard C8 (at least mine did).   If you are running the Baader 15mm SCT to T2 connector to attach the diagonal to your scope, then this configuration probably works at full aperture but if you are using something like a Clicklock visual back, this might be too much.   Maybe my EdgeHD 8" was not representative of all of them (As best I know, it was the first EdgeHD 8" sold in the US) but that scope started to loose back focus at about 180mm of flange to focal plane distance. 

 

But that is not the worst of it.  When you add a lot of flange to focal plane distance, the SCTs can start to develop spherical aberration.  The design of the system is such that in theory, the system will only have perfect spherical aberration correction if the primary mirror spacing is at a specific distance from the corrector.  As you move the mirror in either direction, the spherical aberration will increase.  For every 25mm, the correction will change by 1/23rd of a wave.  If you flange to focal plane distance is kept very short, this usually is not enough to make a difference that you can see, but in theory, the further you push back the spacing, the more error.

 

This is why I recommend that a GPC or the Televue Bino Vue 2x always be used for planetary viewing (in any scope, whether it needs it or not to reach focus) but for the SCT in particular, so as to avoid increasing SA too much.

 

This too. If the system is undercorrected, moving the focal plane further out can actually improve the SA!  The best way to know if your configuration is generating SA is to measure it at infinity focus with and without the binoviewer. 

 

A lot of these things are by themselves so subtle that it might not be easy to see the degradation, but if you throw in a little SA and a little Spherohromatism, then add a little aperture loss, you probably get to the point where a good observer might be able to see the difference.

 

Sorry it happened to "your" scope, but as you can see, configuration does count. 


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#3 AstroBrett

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Posted 02 August 2020 - 12:08 PM

Your calculations presume that the field of view is not changed when using the binoviewer, and you may want to check that presumption. In other words, using your method, if I inserted a field stop to reduce the field of view, I would calculate an increase in the magnification, which clearly does not occur. I might suggest using a reticle eyepiece to measure the distance a known angle subtends on the tape would avoid the potential pitfall of relaying on the edges of the field of view, with the inherent presumption that this does not change.  

 

Good luck !

 

Brett



#4 AstroBruce

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Posted 03 August 2020 - 07:49 PM

Your calculations presume that the field of view is not changed when using the binoviewer, and you may want to check that presumption. In other words, using your method, if I inserted a field stop to reduce the field of view, I would calculate an increase in the magnification, which clearly does not occur. I might suggest using a reticle eyepiece to measure the distance a known angle subtends on the tape would avoid the potential pitfall of relaying on the edges of the field of view, with the inherent presumption that this does not change.  

 

Good luck !

 

Brett

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#5 swsantos

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Posted 03 August 2020 - 08:47 PM

Your calculations presume that the field of view is not changed when using the binoviewer, and you may want to check that presumption. In other words, using your method, if I inserted a field stop to reduce the field of view, I would calculate an increase in the magnification, which clearly does not occur. I might suggest using a reticle eyepiece to measure the distance a known angle subtends on the tape would avoid the potential pitfall of relaying on the edges of the field of view, with the inherent presumption that this does not change.  

 

Good luck !

 

Brett

My understanding is that that field stop issue is a problem if the eyepiece's field stop is larger than the binoviewer's meaning that the binoviewer is not allowing me to see the entire field of view of the eyepiece.

 

The 12.5mm Morpheus's field stop is 16.8mm, much less that that of the Maxbright II's so I think I am OK.


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#6 Eddgie

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Posted 04 August 2020 - 09:36 AM

My understanding is that that field stop issue is a problem if the eyepiece's field stop is larger than the binoviewer's meaning that the binoviewer is not allowing me to see the entire field of view of the eyepiece.

 

The 12.5mm Morpheus's field stop is 16.8mm, much less that that of the Maxbright II's so I think I am OK.

Yes, this is only a problem if the field stop is larger than the rear aperture of the binoviewer and even in that case it depends on how much larger and how far away from the rear aperture the field stop is. 

 

If the field stop is much larger by well displaced, you see vignetting, the strength of which is determined by how much larger the field stop is and how far away it is.

 

If the field stop is very close to the rear aperture (some eyepieces have the field stop near the telescope end of the barrel) then the vignetting will be more severe, and if it is very close to the end of the barrel, the rear aperture of the binoviewer actually becomes the field stop for the system because it is so close to the focal plane.  In this case, the rear aperture sets the true field limit. 

 

I too am not sure what the previous poster was saying but if you used the same eyepiece for both measurements, your result should be reliable.

 

I do have to say though that these figures overall seem quite high. Are you using some other light path lengthing component like a Baader Clicklock or external focuser? Are you using the factory 1.25" visual back?  With the factory 1.25" back we know the system is f/10.4 and if you were using the stock components that is what I would expect to see.

 

This is speculation and only speculation. It is possible that the corrector in the EdgeHD 8" has some mild Barlow component. Now I don't know this to be true, but if it is, it could explain why your numbers are so high. If it has some kind of mild amplification, then just like with a regular Barlow, the further you put it in front of the focal plane, the more magnification it gives.  Your figures are higher than even I would have expected them to be, and this is a possible explanation.  With a standard (non-HD visual back) and binoviewers I would have expected only about 225mm to 250mm of focal length increase.

 

We also know that while the other EdgeHD models kept the same f/2 and -5 power secondary formula for the mirrors, the 8" does not.  Celestron does not say what the figures are in the white paper, but I seem to recall that somewhere it was stated that the 8" version required a slight deviation from the standard f/2 -5x presrscription, so if this were the case, it could be that for a given amount of focal plane displacement, the 8" might pick up a bit more focal length.

 

I don't know the answer here but the focal lenght you are reporting does seem to be higher than the numbers for the standard SCTs would be so I am just curious as to why that would be.  I think your test measurements were well done (infinity would be better, but the distance you used should be enough to get close. 

 

Anyway, the focal lenght increase is absolutely expected, but in this case, the magnitude seems out of step from what I would have expected.  If the light path is as short as possible, then perhaps it is something to do with the design of the EdgeHD itself. 



#7 swsantos

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Posted 04 August 2020 - 11:39 AM

I do have the Baader Clicklock as a visual back, and although I know I could shorten the light path with a different arrangement, I like to be able to swap back and forth between the binoviewer attached to the T2 Zeiss prism diagonal and an Ethos eyepiece attached to a 2" Baader BBHS prism diagonal. The clicklock makes that just about as easy as changing an eyepiece. My understanding for your previous posts and others is that I am not likely losing any aperture by the addition of the clicklock visual back alone given that the rest of the components are optically short. I would not want to not use the clicklock unless there was a very compelling reason because of the binoviewing/cyclops mode quick change it facilitates. Hate unscrewing things outside at night.

 

There is no external focuser, I use the Feathertouch microfocuser.

 

Those magnification measurements I took were with the 1.25 GPC installed in the top of the T2 diagonal because that will be my high power setup with the binoviewer and I was trying to figure out what it would take to get about 200X out of the binoviewer + GPC configuration.

 

Steve



#8 Eddgie

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Posted 04 August 2020 - 01:06 PM

 

 

Those magnification measurements I took were with the 1.25 GPC installed in the top of the T2 diagonal because that will be my high power setup with the binoviewer and I was trying to figure out what it would take to get about 200X out of the binoviewer + GPC configuration.

 

Steve

Ah, I missed the presence of the GPC.  Apologies for that.

 

The GPC will offset the additional light path of the Clicklock and I do recommend using them for planets.

 

Thanks for clarifying and catching my miss on the GPC. 


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