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Aperture in C6 & C8 for various configurations

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#126 Eddgie

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Posted 07 February 2015 - 02:54 PM

I of course believe that Frank is mistaken and simple logic would seem to support my belief.

 

Geometric ray traces are very specific in predicting aperture reduction, and in every case where I have measured using a laser, the result has very accuratly correlated to Ken Hutchinsons SCT ray traces. 
.
When I am testing an aperture reduced configuration, and I change the back focus, I see about 2.5mm of aperture change. If I shorten, I get 2.5mm back.. If I lenghthen, I loose another 2.5mm.   Nothing is changing with the test apperatus.

 

The only change is when the mirror moves and the light cone geometry changes with it.   The system behaves exactly like the Geometric ray trace says it should, and no one can convince me that the laser method is in error.  The repeatability of the test and the correlation to the computer generated ray trace date makes it clear in my own mind that the test is very highly reliable for the standard SCT.   I have tested too many scopes in to many configurations to have any doubt at all.  

 

But maybe he can convince you, and that is all that matters here.  Choose to believe who you like.


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#127 Eddgie

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Posted 07 February 2015 - 02:58 PM

And of course the physics also predict the SA change.    People don't believe it, but it is so easy to see.  Meaningless for an inch or two of back focus over optimal, but quite serious at 150mm to 200mm over optimal, lowering the system to below diffraction limited.

 

I have no interest in what people use, and a lot of people say that they use a focal reducer and 2" eyepiece and are happy with it..  I am happy for them.

 

My goal though is simply to help people get the best possible performance out of their SCTs.

 

I see endless amounts of SCT bashing, but a properly configered SCT with decent optics can deleiver very satisfying views.   We all make a choice as to what compromises whe are willing to accept, and when I sit at my EdgeHD 8", I want it to delliever as excellent a performance as I can get given the fact that I want to use binoviewers.

 

And if I need a wider field than I can get from the C8, I change telescopes.


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#128 AhBok

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Posted 07 February 2015 - 04:16 PM

Thanks Eddgie. I am confident I now know what my scope is doing and my results are in line with what you predicted (and the ray trace predicted). I reveved this thread to say thanks to you and a couple of others. I've learned a lot about my gear and have verified to myself that I have squeezed all of the performance out of my C8.
Thanks for sharing your knowledge and experience. I wish you, EdZ and Glenn would co-author a book on practical testing for telescope optics as sort of "how to" companion for books such as Suitor's. Nevertheless, the postings you all have shared are much appreciated.

#129 freestar8n

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Posted 07 February 2015 - 04:40 PM

Here we have a case where the aperture is reduced when a wide angle eyepiece is attached. At least that is how I read it. Unless I am misinterpreting things - the location of the field stop for the various eyepieces in the 2" diagonal was about the same - and changing the type of eyepiece changed the aperture - somehow in correspondence with the apparent field of view.

If that is what is happening then it certainly makes no sense in terms of how the entrance pupil should behave - because the size of the field stop has no role at all. It sounds like the exit pupil wasn't fully illuminated or something - and the reduction is an artifact.

If the field stop is pulled way back to a different location it could result in *something else* acting as the aperture stop - but that isn't what it sounds like here.

Anyway - I have explained how pupils work over and over - and in this thread there was a gradual acceptance that the right way to do things is with careful use of a collimated laser - and with confirmation that the beams are never tapered. That should work ok - but it isn't "the flashlight test" anymore.

Frank

#130 Live_Steam_Mad

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Posted 27 August 2015 - 05:06 PM

And of course the physics also predict the SA change.    People don't believe it, but it is so easy to see.  Meaningless for an inch or two of back focus over optimal, but quite serious at 150mm to 200mm over optimal, lowering the system to below diffraction limited.

 

 

Not sure I would agree with that much change in SA. Wolfgang Rohr found for the test on a C11 SCT (admittedly different OTA), he found that going from 97mm to 157mm BFL made the Strehl go from 0.945 to 0.92 at 588nm (yellow light). There was a graph provided from his friend of increase in SA with back focal length for the C14 but it was way off what Mr. Rohr found when he tested a C11 on the interferometer, but maybe the C14 does behave a lot worse?

 

http://r2.astro-fore...ck-focal-lenght

 

In which case I am sort of glad I got the C11 LOL. I seem to remember a figure quoted somewhere as 1/23 wave PV worsening with every 1 inch of BFL increase?

 

Anyway, for my own C11, which is slightly overcorrected in yellow-ish light (IIRC, I must go and star test it tonight again to check) it seems that going between 1.25" and 2" eyepieces, and even adding a 2x barlow, would not alter the SA to a noticeable amount (I'll try and check that too). When Mars comes back (red light), maybe that will be perfectly corrected for SA LOL... and hot blue stars would be horribly overcorrected... SCT's are nearly always corrected for yellow light and are slightly overcorrected in green light. So mine is a little worse it seems.

 

Regards,

 

Alistair G.


Edited by Live_Steam_Mad, 27 August 2015 - 05:12 PM.


#131 Live_Steam_Mad

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Posted 27 August 2015 - 05:22 PM

I see endless amounts of SCT bashing, but a properly configered SCT with decent optics can deleiver very satisfying views.   We all make a choice as to what compromises whe are willing to accept, and when I sit at my EdgeHD 8", I want it to delliever as excellent a performance as I can get given the fact that I want to use binoviewers.

 

Indeed, I was just looking at Saturn and got a lovely sharp image with only passive cooling after 30 minutes of my C11 SCT being outside, and the planet was only 13 degrees above the South Western horizon, was only barely clearing my concrete shed's roof, with 1/3 of it's diffraction pattern missing because of obstruction from the shed roof, but I could still easily see the sharp Cassini Division at x175, and cloud banding on the surface, even if there was atmospheric dispersion (blue / red) on it's edges and even though it was boiling in turbulence.

 

Regards,

 

ARG



#132 Procyon

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Posted 12 December 2016 - 01:24 AM

I too would like to pay homage to all who contributed to this great thread. But it seems as if there was some disagreements towards the end on how measurements and calculations were achieved. I now have in my possession an Edge 8 (bigger rear opening than an regular c8) an F7 Edge focal reducer, newer adapters that were not available in 2012, T2 zeiss prism, 2" eps with less than 34-35mm field stops etc. etc. and am trying to put together a system that won't allow for aperture loss.

 

So my question to you Geniuses is straight forward: at what back focus length does an Edge 8 and/or C8 begin to lose aperture and transmission loss? Or even an C6, 9.25, 11 and 14 Edge/non Edge for future considerations for the thread, if possible. For the Edge 8, is the cutoff point 200mm?

Also what confused me a bit is when Ed mentioned that a bare C8 is working at 1800mm FL. Where to start my calculations than to figure out the final FL?

If the answers were already mentioned than my bad for missing them. Thanks again and sorry to bring this thread up after a year, I just find it so interesting. It would be great if the final answers were available to have them as a sticky even. One of the best threads ever concerning SCT's.


Edited by Procyon, 12 December 2016 - 10:06 AM.


#133 GlennLeDrew

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Posted 12 December 2016 - 08:34 AM

Toward the very end Frank did raise valid concerns about a test result which also immediately jumped out at me. And that was where a differing aperture measurement resulted simply by changing the eyepiece focal length. This should not be the case if the exit pupils are fully illuminated.

 

It's simplest to ensure full exit pupil filling with laser light by using a beam expander of sufficient magnification. If such an expander is not to hand, either a small exit pupil is utilized so that the 'native', non-expanded laser beam can fill it, or the laser is simply translated or swept (from a distance, if the latter is done) across the exit pupil so as to ensure the emergent light bundle does become clipped by opposite sides of the entrance pupil/objective aperture.


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#134 Eddgie

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Posted 12 December 2016 - 09:59 AM

Also what confused me a bit is when Ed mentioned that a bare C8 is working at 1800mm fl. Where to start the calculatios than to figure out the final FL?

 

 

I am not going to re-read the thread, but the focal lenght of an SCT varies with the position of the mirrors and the result will be based on where the focal plane was postioned as a result.

 

The standard C8 should be at 2032mm when the focal plane is positioned 100mm behind the flat of the rear port of the SCT.

 

For every 10mm of focal plane change, you increase or decrese the focal lenght by about 31mm.

 

Pay attention because there are a lot of moving parts here.

 

Here is my guess.  The 1800mm figure was probably a figure he got by measuring the focal length with the mirror positioned to place the focal plane at the end of the 1.25" visual back.

 

A Prism diagonal (as provided with the C8 by Celestron (has a light path of about 65mm not including the 1.25" nose and the visual back has a light path of about 30mm.

 

If you took off the 1.25" diagonal and put the eyepiece into a visual back (which you would need to hold the eyepiece to do your projection) then when you re-adjusted the mirrors for the new postion of the focal plane at the end of the1.25" visual back, then you would displace the focal plane about 65mm. 

 

3.1 x 65 eqaul 201mm.    Take this from 2032 and you get a little over 1800mm.

 

So, that is my guess.   For this measurement, Edz probably had the focal plane positioned at the end of a 1.25" visual back and if you remove the 1.25" prism, you take about 65mm from the light path so the result is that you re-space the mirrors and the resulting focal lenght is shortened 31mm for every 10mm of light path removed. 

 

Again, this is just a guess as to how Edz came up with this figure, but it matches my own measurements closely. 

 

 

The focal lenght of the C8 is in fact based on the scope being used as shipped by Ceelstron, using the 1.25" visual back with the supplied 1.25" Prism (even a 1.25" mirror diagonal will increase the focal lenght by about 30mm.  A 2" mirror diagonal used with a visual back that has 30mm of light path will increase the C8 from 2032 to about 2100mm).


Edited by Eddgie, 12 December 2016 - 10:20 AM.

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#135 Eddgie

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Posted 12 December 2016 - 10:12 AM

And I directly measured the aperture loss of my EdgeHD 8" starting at 170mm of back focus, but someone else using an different EdgeHD 8" got a different result with aperture loss not starting even at 200mm.   Clearly it may vary from one sample to the next.

 

I made my measurement multiple times and my result was consistent to within +/- 3mm.  

 

Also note this.  In the EdgeHD white paper, Celstron states this:

When configured for best image, the Celstron EdgeHD 8" should be configured with 133.5mm of back focus.

 

At 133 of back focus, the system is not f/10, it is f/10.45.  The focal length is not 2032mm, it is 2125mm (according to the White Paper).

 

Now,, let's see what happens if I take off the imager and put an eyepiece right into a standard 1.25" visual back attached to the rear port..  This would shorten the light path by about 100mm.

 

Once again, if I take the rule of thumb 31mm of focal lenght change for every 10mm of back focus and I cut off 103mm by sticking the eyepeice directly into the visual back, I shorten the focal ratio by (3.1 x 103) 319mm, so the 2125mm that I would get at 133mm of back focus would change to (2125 - 319) about 1806mm.

 

As I recall, Edz did not say exactly how he got his figure, but that is the way the SCT works, and if I back engineer his measurements I have to conclude that he made his measurment with the system not configured for nominal focal plane, which for the C8 is 100mm behind the port.   Knocking off 70mm will reduce the focal length by 210mm.

 

Bottom line.. The nominal focal length of the C8 is given for the telescope used with the factory supplied 1.25" visual back and the factory supplied 1.25" Prism diagonal (and the same is true for the C9.25 and C11).    If you measure it in any other configuration that moves the focal plane, you change this figure by 3.1mm for every 1mm of focal plane movement. 


Edited by Eddgie, 12 December 2016 - 10:21 AM.

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#136 Procyon

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Posted 12 December 2016 - 11:30 AM

Excellent, thanks. I will just write my current setups here for an example to be used. Perhaps someone has an answer as to how much effective aperture will remain in the end using these configurations. It doesn't have to be 100% precise as Eddgie mentions Scopes can differ. I'm just trying to figure out if some of these setups are causing more than 5-10mm of aperture loss.

 

Edz had said:

(For the C6):

 

The 0.63x reducer changes things. With the reducer you start losing aperture at just over 100mm of backfocus and by 130mm of back focus aperture is down to 140mm. Certainly not nearly as bad as when using the binoviewers, but a 5% loss in a normal 2" diagonal configuration. Not too steep a price to pay, none-the-less, a reduction. Worthy of note, since potentially the shortest configuration is the 1.25" VB and a 1.25" diagonal, there may not be any normal configuration using the reducer that is not aperture reduced.

 

0.63x reducer + 1.25” VB (35) + 1.25” Diag (75) = (total 110mm behind reducer) = 148mm.
0.63x reducer + 2” SCT Diag (130) = (total 130mm behind reducer) = 140mm
0.63x reducer + 2” extension (28) + 2” SCT Diag (130) = (total 158mm behind reducer) = 124mm

 

For the C8, I found that without reducer it is still operating within 1-2mm of full aperture with 240mm of back focus. That's as long or longer than just about any conceivable normal configuration.

In addition, the C8 with a binoviewer at 370mm of back focus is operating at 190mm, a loss of 13mm, 1/2 of an inch.
With a 0.63x reducer in place and my Astro Tech 2" SCT diagonal, aperture thru the reducer measured 190mm. So, a 2" diagonal attached to the 0.63x reducer on the C8 reduces aperture, but only by 13mm.

 

I wouldn't hesitate to use a screw-on 2" SCT diagonal on either the C6 or the C8 as it adds so much more security to the eyepieces you want to use, and in neither case does it reduce aperture. In fact, I'd recommend the use of 2" diagonals on both of these.
And I wouldn't hesitate to use the C8 with that 2" diagonal with the 0.63x reducer as that will cause only a loss of 13mm of aperture, reducing operating aperture to 7.5 inches.

 

Glenn had also mentioned a calculation after some other issues were found that affected some equations:

(For the C6)

 

At 190mm behind the primary baffle exit aperture, the effective aperture is 144mm.

At 310mm, aperture is 128mm.

 

 

The examples Edz mentioned I believe did not factor in the Brightness and Transmission issues that Eddgie mentioned as Edz was calculating only the aperture loss at the time. Eddgie mentions, after I'm sure thousands of experiments, that the ideal maximum back focus a setup should have is 170mm for an Edge 8. Others have said it's 200mm. (For monoviewing and binoviewing, not sure about a Focal Reducer).

 

Having said all that, how does one calculate, or what would be the final aperture length-diamater using these setups as examples? I think it's the Focal Reducer calculations that have confused me.

 

Current examples / setups: 

 

1) 1.25" Monoviewing: 10mm VB + Prism (41mm) + 1.25" Clicklock (32mm) = 83mm Where does that bring my FL and should I add more items (lol) to reach Celestron's recommended 133.5mm Back Focus/light path length? 

 

2) 1.25" Binoviewing: 10mm VB + Prism (41mm) + T2 to Bino Adapter (4mm) + Arcturus Binoviewers (100mm) = 155mm or 183mm if I decide to replace the adapter with the 1.25" Clicklock (32mm) for quicker change.

 

3) 2" Monoviewing: Same setup as 1.25" monoviewing + an extra 4 mm for 2 adapters + an extra 15mm for a 2" Clicklock (EP's must have a maximum field stop of 34-35mm) = 102mm

 

4) Setup 1 (83mm Light Path)+ F/7 Edge Focal Reducer

 

5) Setup 2 (155mm Light Path) + F/7 Edge Focal Reducer = Mega Aperture Loss?

 

6) Setup 3 (102mm) + F/7 Edge Focal Reducer = Final Aperture? Not worth trying?


Edited by Procyon, 12 December 2016 - 07:48 PM.


#137 Eddgie

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Posted 12 December 2016 - 01:55 PM

These posts have focused only on focal length and aperture reduction.

 

Not mentioned has been the change in spherical aberration that occurs in an SCT when you add back focus.

 

Assuming that the scope is perfectly corrected for SA when the mirrors are spaced for a back focus of 100mm, then every 25mm of additional back focus will increase the SA by 1/23rd of a wave.

 

This is not important for changes of 50mm or even 100mm, but binoviewers are an exception in that unless great care is taken to keep the light path short, the SA can indeed become sufficient enough to degrade the contrast transfer.

 

This is my recommendation for anyone that wants to do high resolution observing with a bibnoviewer in and SCT.

 

Learn to star test.

 

When you have your binoviwer configured, use just enough amplifications (GPC or Barlow) to perfectly offset any SA induced by the binoviewer.

 

I do not know this to be the case, but logic tells me that it is:  Roland Christen says that a GPC sould ALWAYS be used with binoviewers (and for plantery use, I concur).    

Now I do not think it is at all just one of those random chances that Christen ships the Mark V with a mirror diagonal and that he has designed a 2.6x GPC.

 

The question for most people that took the time to think about it would be "Why does he make the 2.6x GPC, when most telescopes will reach focus using a 1.25" diagonal and the 1.7x GPC?

Well, I think the answer to that question is how much the effective light path of the T2 mirror and the Mark V binoviewer results in.

 

If  you look at the Maxbright Owner's Guide, it shows that the effective light path for the Mark V and do the math, what you will find is that using the 2.6x GPC with the Mark V and the T2 mirror diagonal, the effective light path works out very close to 100mm. 

 

In other words, when you use a Maxbright or Mark 5 with the T2 mirror diagonal in an SCT, the light path lengthb places the mirror spacing pretty close to what it would be if you use the factory visual back and 1.25" prism diagonal.

Anyone that thinks this is pure serendipity has not played with path lenght and SCTs.

 

Christen is a brilliant optical designer and since he designed the GPCs and he specifies the mirror in place of the Ziess prism, one has to seriously consider what his rational for these decisions was.

 

For general use, most of this stuff does not matter.

 

If one wants to get the highest possible contrast from their SCT, then one should pay very close attention to these kinds of details.

 

It is not just about aperture and focal length.  Contrast performance of an SCT can also be affected.

 

My advice for years now has been to always Barlow  or GPC up an SCT for planets and always use a Baader GPC for fast refectors if one wants the absolute best performnace possible for high resuliion observing.

 

For everything else, none of this is all that critical and you should just use what makes you happy. 

 

If though you want the best possible contrast from your SCT, use it with a back focus that produced the best possible SA correction and assuming the scope has good optics to start with, then the system that will do this in a standard SCT is the T2 mirror, the 2.6x GPC, and a good binoviewer direct attached to the top of the T2 mirror diagonal.

 

Or... Just use what you like.  Often these small differences will be to small for a non critical observer to be concerned over.   No one should agonize about 1/8th wave of SA change or a couple of millimeters of aperture loss if you re not extremely demanding in your system expectation.   For all but planets, I learned to make a lot of compromises. 


Edited by Eddgie, 12 December 2016 - 02:04 PM.

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#138 Eddgie

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Posted 12 December 2016 - 02:29 PM

And here would be my own prescription for an EdgeHD 8" based on my own ownership, two years of use and having tried to many configurations to list here (dozens).

 

Forget the 10mm connnector and T2 prism.   The reason is simple.. It is a royal pain to reconfigure this kind of system.  The 10mm connector is not at all user friendly and if you want  to go back and forth to 2" eyepeices, it will be an aggravation to do so. Do yourself a favor and dump this aproach unless you intend to only binoview.

 

Get the T2 mirror diagonal.  This diagonal is of the highest possible quality.  Light path lenght is about 50mm. Use the 2" nose on our diagonal. 

 

Use the Televue Short SCT to 2" visual back.  This back is 25mm long and is a compression design and will easily hold the binoviewer in place.

 

Use the binoviewer in direct connect mode (Televue, Maxbright, and Mark V do this out of the box but you can buy adapters for most others to attach directly to the T2 diagonal). 

 

For general binoviewing, just run without the GPC.  The change in system performance will be impossible to see with the difference of 20mm in light path length (vs using the very problematic 10mm SCT to T2 connector).  Even if the system loses a millimeter or two of aperture, you are not going to be able to see this tiny difference.

 

Now when you want to view using a 2" eyepiece, it will be very easy to either remove the Binoviewer and use a 2" eyepiece holder on to of the T2 mirror (good for field stop up to about 36mm or so), or swap in a 2" diagonal with a short nose (many companies now make this kind of diagonal.

This makes it dead easy to switch back and forth from BV to Mono.   The 10mm connector by comparison is quite tedious to change. 

 

Ignore tha 15mm difference of light path you get between the 10mm connector and the TV short 2" back (and to be 100% honest, even a standard 2" visual back is not going to make a difference even the most discerning user is ever going to see).

 

Get a 2.6x GPC for planets and stick it in the top of the diagonal for this kind of session. 

 

If you are going to use only Binoviewer, then even here, the short back and the T2 mirror are going to still be the optimized system because the light path lenght will still be in the 100mm range with the 2.6x GPC.

 

For most observing, you will never see the difference between using Televue back (or even a standard 2" back) and the spacing will be near perfect when you use the GPC.

 

If the GPC is to much trouble (and it can be a lot of trouble to be quite fair about it) go with a TV Barcon in front of the diagonal.   This will be much easier to configure and will give very close to the right mirror spacing for optimal correction.  (most regular 2" units will give to much magnfication). 

 

Don't stress over 10mm or 20mm of light path difference for general observing. It just does not matter and you will absolutley not be able to detect a change in system performnce  (even the true field difference is difficult to see). 

 

Instead, my own suggestion is to make the scope more user friendly for the different configurations that you see yourself using.   Getting hung up on light path length for general observing at the cost of making the system hard to reconfigure is simply not worth a change in performance that you won't be able to see.


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#139 Procyon

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Posted 12 December 2016 - 02:46 PM

 

When you have your binoviwer configured, use just enough amplifications (GPC or Barlow) to perfectly offset any SA induced by the binoviewer.

 

I do not know this to be the case, but logic tells me that it is:  Roland Christen says that a GPC sould ALWAYS be used with binoviewers (and for plantery use, I concur).    

Now I do not think it is at all just one of those random chances that Christen ships the Mark V with a mirror diagonal and that he has designed a 2.6x GPC.

 

The question for most people that took the time to think about it would be "Why does he make the 2.6x GPC, when most telescopes will reach focus using a 1.25" diagonal and the 1.7x GPC?

Well, I think the answer to that question is how much the effective light path of the T2 mirror and the Mark V binoviewer results in.

 

If  you look at the Maxbright Owner's Guide, it shows that the effective light path for the Mark V and do the math, what you will find is that using the 2.6x GPC with the Mark V and the T2 mirror diagonal, the effective light path works out very close to 100mm. 

 

In other words, when you use a Maxbright or Mark 5 with the T2 mirror diagonal in an SCT, the light path lengthb places the mirror spacing pretty close to what it would be if you use the factory visual back and 1.25" prism diagonal.

Anyone that thinks this is pure serendipity has not played with path lenght and SCTs.

 

Christen is a brilliant optical designer and since he designed the GPCs and he specifies the mirror in place of the Ziess prism, one has to seriously consider what his rational for these decisions was.

 

For general use, most of this stuff does not matter.

 

If one wants to get the highest possible contrast from their SCT, then one should pay very close attention to these kinds of details.

 

It is not just about aperture and focal length.  Contrast performance of an SCT can also be affected.

 

My advice for years now has been to always Barlow  or GPC up an SCT for planets and always use a Baader GPC for fast refectors if one wants the absolute best performnace possible for high resuliion observing.

 

I think I see what you're saying here. And I believe I just experienced this just the other night while observing the moon. My Binos came with a 1.85x GPC and a 3x GPC. When I first tried it with the 1.85x there was a lot more back focus that was needed I believe (than without it). And the view looked a bit less contrasty. Than I inserted the 3x GPC and I needed to readjust the focusing the opposite way. The view looked just as good and possibly better than the standard view without the GPC (on the moon). But way better than with the 1.85x. 


Edited by Procyon, 12 December 2016 - 03:40 PM.


#140 Eddgie

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Posted 12 December 2016 - 02:47 PM

And for someone that is not famaliar with the GPCs, here is why I mentioned to the OP that it can be a pain to use.

 

The Mark V was redesigned from the previous version to accomdate the GPC inside the front of the binoviewer.  For use with the Mark V, the GPCs are the same, but the lenses are reversed in the lens cell.

 

This means that to re-configure the Mark V, one only has to remove it from the T2 diagonal and screw the GPC into the bottom, then re-mount it.

 

For all other binoviewers that I am aware of that attach by T2 thread, you have to completely unscrew the binoviwer to access the GPC.   Now some will say that if you use a quick connector you would be able to demount the binoviewer but the GPC mounts inside the diagonal, and to get at it, you would have to remove the quick connector ring!    This would still be easier than unscrewing most binoveiwers at the T2 thread, but now you have to demount the BV, unscrew the connector ring, mount the GPC, then put it all back together again.  

 

For this reason, it starts to look quite appealing to use a Barcon in front of the diagonal.  The reconfiguration is greatly ficiltated because it is easier to pop the diagonal out and slip in the Barcon (or similar amplifier). 

 

Having a scope this is easy to re-configure should be a much higher priority than worring about 20mm or 30mm of light path.

 

Even if the observer is going to BV exclusively, I would still recommend the TV short (or similar) back and 2" nose on the T2 mirror because when it come time to put the scope away, break down is greatly simplified (unless one is using quick connectors).

 

The Mark V is horribly expensive but it does eliminate some of these issues, but at a price that even I found very difficult to justify. 

 

My point though is think about the overall usablity of the system and what it would take to re-configure before committing to highly specialzed gear like the 10mm SCT to T2.   I had one and I used it, but I was using the Mark V (easy break down) and I was only binoviewing.   But before I went 100% binoviewer, I used the TV short visual back and this was far superior in terms of general utility and ease of configuration and the change in system performance was so small that it was impossible to see.


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#141 Eddgie

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Posted 12 December 2016 - 02:59 PM

Well, I am sorry for all the words, but that is the point.

 

I remember on the Binoviewer forum some years ago that people were skeptical of my own observations.

 

One day, one member posted that he had indeed decided to play with various configurations and despite having been somewhat  skeptical in previous posts, the more he did limiting magnitude and contrast, the more he came to realize that these things can matter. 

 

The point I have made to the OP several times now though is that minor infractions only matter a tiny bit.  For most visual use, these will go completely unnoticed even if the observer tries to see the difference.

 

A discerning observer though, should have no difficulty seeing the difference between a poorly configured SCT and a perfectly configured one.  

 

I am guilty of being quite compulsive about a lot of things, but in time, I came to realize that my compulsiveness was based more on my own demand to feel like I am experiencing perfection than the actual observation result indicated I was achieving.

 

In time, I came to accept that for general observing it was often more in my head than it was at the eyepiece.

 

I can see the dimming 3/4th inch of aperture loss but I can't see the dimming of 1/10th inch.   

I can see the contrast loss of 1/4th wave, but I can't see the contrast loss of 1/6th wave.

 

I learned that sometimes a tradeoff like 20mm or 30mm of back focus really does to little damage to see and should be ignored if the goal of increasing the easy of use, utility, or other aspect of the system.

 

For planets though, I remain compulsive. 


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#142 Procyon

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Posted 12 December 2016 - 07:55 PM

And here would be my own prescription for an EdgeHD 8" based on my own ownership, two years of use and having tried to many configurations to list here (dozens).

 

Forget the 10mm connnector and T2 prism.   The reason is simple.. It is a royal pain to reconfigure this kind of system.  The 10mm connector is not at all user friendly and if you want  to go back and forth to 2" eyepeices, it will be an aggravation to do so. Do yourself a favor and dump this aproach unless you intend to only binoview.

 

Get the T2 mirror diagonal.  This diagonal is of the highest possible quality.  Light path lenght is about 50mm. Use the 2" nose on our diagonal. 

 

Use the Televue Short SCT to 2" visual back.  This back is 25mm long and is a compression design and will easily hold the binoviewer in place.

 

Instead, my own suggestion is to make the scope more user friendly for the different configurations that you see yourself using.   Getting hung up on light path length for general observing at the cost of making the system hard to reconfigure is simply not worth a change in performance that you won't be able to see.

Hey Eddgie, those were great explanations, thanks. What you say in the end to make the scope and setup more user friendly is always what I wanted to do. But at the same time I always wanted to add to the mix the shortest light path as I possibly can. I mean why not, it's all about adding numbers together. A nice mixture of both is what I believed in doing from the very beginning because it made the best sense. 

 

I believe that the 10mm + T2 Zeiss Prism + 1.25" clicklock + binoviewer or single 1.25" eyepiece works really great for me. No screwing around, (with screws), and provides such a quick change setup between binos & eyepieces that it makes it such a pleasure to observe in both modes, even in -10 Celsius weather. I understand that the adapter that would allow me to directly connect the binoviewer to the T2 diagonal is probably not worth replacing the 1.25" clicklock because it will make switching from binomode to mono less user friendly. I ordered it to experiment and to see for myself if replacing the clicklock with it, thus saving 30mm of light path, will make a difference with a certain setup but I'm sure i'll stick back the clicklock for better ease of use.

 

Concerning the T2 Zeiss Prism vs T2 Mirror diagonal and 2" eyepieces, my logic told me this: (tell me if it makes sense)... If I keep the 34mm aperture T2 Zeiss Prism and add a 2" Clicklock which I just ordered (with the 1mm adapter that connects the prism and 2" clicklock) and use an eyepiece with a maximum field stop of 34-35mm, I would than not have the need to unscrew the fussy 10mm T2 to SCT VB and insert a 2" diagonal.

 

The only drawback would be that I would not be able to use a great eyepiece like the 31mm King Nagler T5 with its 42mm field stop and that is why I ordered it's little brothers, the 26mm Nagler T5 with a 35mm field stop and the 22mm Nagler T4 with its 31.1mm field stop. It's not that I'm trying to cheap out and save money by not buying a 2" diagonal, I just like versatily, it's always user friendly. The only comprimise I see here is not being able to use the 31mm for a .21 degree of tfov increase compared to the 26mm (1.2 to 0.99) and a .3 difference if using a focal reducer (1.7 to 1.3). 

 

If that works out and there is no vignetting I'll be VERY happy. And if it doesn't, oh well, I think it was worth experementing and than I'll gladly buy a bigger diagonal to use a 31mm Nagler as well.

 

Also, say I want to switch from a 26mm 2" Nagler to a binoview setup, I will insert an Antares 2" to 1.25" adapter inside the 2" clicklock (I hope this works) and than insert a binoviewer or 1.25" ep. If this works and the clarity and brightness of the view is not affected, it will make for a very versatile 1.25", 2" & bino setup I find. Does that make sense?

 

I have a feeling it should work. Right now, to go even further, I'm trying to configure these setups with an F7 Edge Focal Reducer which should be here tomorrow. This is where I was confused with how Edz made those calculations because he's done them with a C6. 

 

Edz provided us with these calculations for when using an F/6.7 Focal Reducer with a C6:

 

0.63x reducer + 1.25” VB (35) + 1.25” Diag (75) = (total 110mm behind reducer) = 148mm. 
0.63x reducer + 2” SCT Diag (130) = (total 130mm behind reducer) = 140mm
0.63x reducer + 2” extension (28) + 2” SCT Diag (130) = (total 158mm behind reducer) = 124mm

 

110mm of back focus + the reducer made you lose 2mm of aperture out of the C6's 150mm 6" mirror.

130mm made you lose 10mm and 158mm of back focus made you lose 26mm or a full inch +1cm.

 

I would love to know these calculations for when using a C8 or Edge 8. 

 

If I use a focal reducer with a 2" setup consisting of 102mm light path, I think I should be ok. 

 

But where will that bring my 8" aperture if I use the F7 Focal Reducer with a 1.25" binoviewing setup consisting of 155mm backfocus? (or 183mm of backfocus if I decide to leave the 1.25" cliclock on). I guess I'll find out with the first clear night sky after everything arrives. I'm just curious to see how the double cluster would look through binos (w/16mm Naglers, 24mm Panoptics or 25mm TV Plossls) and a focal reducer. :) Please don't tell me to just get a dob lol, there must be a way to do this with an SCT!

 

155mm lightpath + F7 Focal Reducer what will be the result? Would adding a 3x GPC help? I guess I'll try that too.

 

If the 2" setup doesn't work I'll bite the bullet and get a bigger diagonal. But if it works and the bino+focal reducer experiment fails, than I'll just stay as is and use the focal reducer with 1.25 or 2" ep setups. That is when I will maybe look to find a quicker setup for when switching from focal reducer to a bino setup.


Edited by Procyon, 13 December 2016 - 10:19 AM.


#143 Eddgie

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Posted 13 December 2016 - 09:16 AM

Well, my entire point above was that in the end, the utility you expect from the system (ease of reconfiguration) should be as important to you as the difference 20mm of light path will make.   

 

It sounds like you have thought through your own wants and needs and have come up with a configuration that will make sense for you and that is by far and away the most important thing.

 

I am doubtful you will get full aperture if you use any kind of focal reducer coupled with any kind of binoviewer.   I have found a system on an SCT that used a binoviwer with a reducer and worked at full aperture.   

The problem is nothing to do with what kind of reducer you use.  It is the cutoff of the light cone by the primary baffle that results from the back focus.

 

Now, I did in fact get one reduced configuration to work with on minimum of aperture loss.

 

On my C14, I used an Astro-Physics 3.25" to 2" back.  On to this, I put a 1.25" standard T2 Prism.

 

Russ and Denkmeier made a custom dovetail that allowed me to mount my Binotron/power switch directly to the top of the prism using a quick connect ring.

 

Now one must remember that the C14 was designed to work with more back focus than the other Celstron SCTs. 

 

This configuration resulted in an aperture of 13.9 inches in low power mode.     This was the very best I have ever done using any kind of reducer on any SCT.

 

Every other reducer/binoviwer on every other SCT in every other configuration produced aperture loss between .7 and 1.5 inches.

 

On a C5, a standard configuration with a binoviewr in low power mode reduced the aperture from 127mm to 91mm. 

 

It is very hard to use a reducer and a binoviewer and keep full aperture.  I have not myself found a way to do it.

 

Again, my point though is to use what you like.   The benefit of having a wider true field is a powerful one and if you lost half an inch of aperture to get a wider field, there is no great sin in that. 

 

Russ and I had many discussions about this and in the end I came to believe that he was correct, and that using a power switch in an SCT in low power mode, even at the cost of some aperture reduction, did indeed provide the highly desirable benefit of producing a much wider true field.

 

If I were ever to get another SCT for binoviewing (unlikely) I would go to the Denk integrated diagonal and a BV with a power switch.  I learned that the gain it true field was far more important than working at full aperture. 


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#144 Eddgie

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Posted 13 December 2016 - 09:35 AM

Here is the best reduced configuration I ever tried on an SCT.

 

Here is the dovetail connector I got from Russ installed on the Binotron, which allowed it to be attached to a T2 diagonal using a quick connector:

 

Baader adapter.jpg

 

 

 

This is the Binotron coupled to the top of a T2 Mirror (for use in my 6" Apo) and on the C14, this gave 13.9" in low power mode.

 

Binotron on T2 Mirror 2.jpg

 

For the 13.9" configuration in the C14, the T2 Prism was used.  The T2 Mirror reduced aperture to 13.7".

 

 

Using a 2" diagonal with a normal visual back and normal 2" eyepeice holder, the aperture reduction in low power mode was 1.6".

 

The moral of this story is that light path length matters and one should make reasonable efforts to keep it short, but that sometimes there is a goal that necessitates some compromise, and often this compromise is worth it to achieve that goal.  Getting a wide true field out of a binoviewer is the kind of goal that makes the compromise of some aperture loss a reasonable one.    The only question then is how much aperture loss one can accept.  

 

If I were ever to binoview an SCT again, I would use the Binotron with power switch with a short light path diagonal setup as shown simply because I came to value the power switch and the low power mode far more than I cared about some aperture loss in low power mode.

Russ was right.   It is a powerful convenience and worth half an inch of aperture in low power mode.

 

Do what works best for your observing needs and compromise with BV is unavoidable.


Edited by Eddgie, 13 December 2016 - 09:44 AM.

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#145 Sarkikos

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Posted 13 December 2016 - 10:36 AM

Why the Baader T2 Mirror Diagonal in the 6" APO instead of the Baader T2 Prism Diagonal?  

 

The Baader T2 Prism Diagonal provides the shortest light path.  (I have the Zeiss version.)   I have heard that a mirror diagonal might give a better image in refractors below about f/6.

 

For me, though, concerns about the best way to binoview SCTs are moot.  I only binoview my 10" Dob, and only for planet/lunar.  Now that makes things much simpler.

 

Mike


Edited by Sarkikos, 13 December 2016 - 10:48 AM.


#146 GlennLeDrew

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Posted 13 December 2016 - 05:28 PM

A prism diagonal of the same physical dimensions presents a shorter optical path length than does a mirror, due to the refraction in glass moving the focus rearward by a distance equal to about 1/3 the glass thickness.

 

At not small exit pupils, the spherical (and chromatic) aberration introduced by a prism are likely not resolvable, even for fairly fast objectives. And so in the quest for wider fields with minimal aperture reduction a prism diagonal can be a sound choice. And at f/10 SCT speeds, a prism is fine at highest powers.


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#147 Sarkikos

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Posted 14 December 2016 - 12:05 PM

If abberrations potentially induced by the prism diagonal are not resolvable at "not small exit pupils" (i.e., wide exit pupils and low power),  prism diagonals produce miminal aperture reduction for wider fields, and for slower instruments and higher powers prism diagonals are fine, then when would it be better for an observer to use a mirror diagonal?  What advantage would a mirror diagonal ever have, except maybe lighter weight?

 

Why use the mirror diagonal in a 6" APO rather than a prism diagonal?

 

Mike



#148 Procyon

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Posted 14 December 2016 - 12:30 PM

Hey Mike, I think Eddgie just mentioned the Mirror diagonal because it was possibly a better solution to me for when switching eyepieces to binos, kind of a better user friendly solution to what I use. But I see Glenn says the Prism produces some aberrations in low power wide exit pupils? 


Edited by Procyon, 14 December 2016 - 12:39 PM.


#149 coutleef

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Posted 14 December 2016 - 12:36 PM

from what i understand
prism diagonals do not perform as well as mirrors for fast refractors

it all depends on the speed of the refractor

Edited by coutleef, 14 December 2016 - 12:36 PM.


#150 GlennLeDrew

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Posted 14 December 2016 - 06:50 PM

In convergent light (as for a light cone) a plane parallel glass plate (which a prism diagonal effectively is) introduces spherical aberration and spherochromatism. The steeper the light cone, and the thicker the plate, the worse these aberrations. But in all cases there is some threshold in exit pupil diameter above which the aberrations are irresolvable.

 

Refractors are no more or less afflicted than any other design, except insofar as existing color aberrations might be altered (or even corrected for if designed for use with a prism.)

 

Note how thin plates such as filters pose no real problem (if decently figured) even with quite fast systems. This illustrated the dependence on glass block thickness. And so a 2" prism will enforce a somewhat larger exit pupil threshold than will a 1.25" prism for its induced aberrations to be not resolved.

 

Mirror diagonals are preferred due to their absence of those aberrations of refraction, as well as their having just a single surface instead of three--or four, in a roof prism.

 

For decades, even to this very day, perhaps, f/10 SCTs have shipped with 1.25" prism diagonals. At such system speed a well figured and coated prism holds its own against a mirror. Even a 2" prism is likely fine at small exit pupils regarding the induced SA.

 

Note also that the requirements on aberration control are more relaxed for the observation of dimmer fare like so many DSOs. Beyond the solar system, pretty much just double stars and densely packed star clusters (where the stars are seen as reasonably 'bright') impose more stringent constraints. No nebula or galaxy possesses sufficient surface brightness to permit full visual resolving power. And so for the DSO 'fuzzies' aberrations can be of larger magnitude. (Try focusing on a galaxy in a starless field, then swing to a star to see how close you got. ;) )


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