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C8 - Actual focal length w/2" SCT style mirror diagonal...

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#1 Dave Bush

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Posted 11 October 2014 - 06:59 PM

Because I'm not going to be able to get out and do this for a while I thought I'd ask.

 

Has anyone bothered to measure the actual focal length of a C8 when using a 2" mirror diagonal that threads directly on the back of the scope (no visual back, extension tube or reducer/corrector)?



#2 doug mc

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Posted 11 October 2014 - 11:49 PM

Hope this helps5095040-C8 Operating Focal Length.jpeg



#3 Dave Bush

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Posted 11 October 2014 - 11:57 PM

One question, the distance from the back port to where?

And is the distance measured from the edge of the the threaded part or the back housing?

Ok that was two questions

#4 hudson_yak

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Posted 12 October 2014 - 09:36 AM

It's measured from the most-rearward surface of the threaded part (what the SCT diagonal flange comes into contact with) to the EP focal plane. The latter depends on the particular EP, it may be near the top of the diagonal EP socket, or could be in or out from that a bit.

 

In the graph, a compact 1.25" diagonal (eg. TV Everbright) is near the 100mm point, a refractor-type 2" diagonal near the 150mm point. An SCT 2" diagonal is about midway between those two.

 

Mike



#5 Eddgie

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Posted 12 October 2014 - 10:50 AM

SCT diagonal has a light path of about 125mm.  

As a good rule of thumb, every millimeter of increased light path adds about 3.1mm to the focal lenght.

 

An increase of 25mm (over the design back focus, where the published focal lenght is to be found, which is 100mm for all of the Celestron SCTs except the C14, which is 150mm) would then increase the focal lenght to (25 x 3.1)  77.5mm.

 

In the chart, the graph shows the focal lenght at 125mm to be right at about 2100mm, so close enough.

Anyway, to answer the OPs question, when using a 2" SCT type diagnoal, focal lenght of the C5, C6, C8, C9.25, and C11 will be about 80mm longer with a 2" SCT diagonal than it would be with a 1.25" diagonal in the standard visual back.

 

It is important to note that in the C11 and C14, the measure is taken from the back of the 3.25"  port.   The 3.25" inch back plate adds about 20mm to this figure, and if you add the 125mm for the SCT diagonal, you see that the C11 is acutally working at about (20 + 125) 145mm of total back focus, and at 145mm of total back focus, the focal lenght is about 2950mm, and the focal ratio os about f/10.6.


Edited by Eddgie, 13 October 2014 - 05:30 PM.


#6 Eddgie

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Posted 12 October 2014 - 11:02 AM

I might add this weird fact.   Assuming that the SCT starts with perfect SA correction, if you use back focus past the nominal back focus (100mm for all but the C14), the SA of the system increases.  Again, between a 1.25" diagonal and a 2" SCT diagonal, the difference is too small to be easily seen.

 

But the more you increase the back focus the more severe the SA gets.  

Special cases like using a 2" diagonal behind a focal reducerand long light path binoviewer can add as much as 1/2 wave of sperhcial aberration.

 

This is not "textbook' stuff that is exempt in the "Real World."  It is easy to see in a star test.

 

If you start with an SCT with perfect SA correction and you stick a lot of back focus on it, you can very quicly induce .2 waves of SA.  So in addition to the focal lenght changes, excessive back focus increases spherical aberration, and in many configurations, can reduce apeture as well.

This is why I have been encouraging people that use SCTs to be careful of controlling the light path.

 

It is also why they should use the factory visual back and 1.25" prism diagonal when they compare their 8" SCT to their 5" APO.     To stich a 2" refractor diagonal on the scope to make the comparison is handicapping the C8 from the outset.     Even if the optics are perfect, this one change will push the contrast transfer to below that of the 5" APO.

Astro-Foren.de has measured this and anyone that wants to challange this claim can see it there....



#7 AhBok

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Posted 12 October 2014 - 12:47 PM

I might add this weird fact.   Assuming that the SCT starts with perfect SA correction, if you use back focus past the nominal back focus (100mm for all but the C14), the SA of the system increases.  Again, between a 1.25" diagonal and a 2" SCT diagonal, the difference is too small to be easily seen.

 

But the more you increase the back focus the more severe the SA gets.  

Special cases like using a 2" diagonal behind a focal reducerand long light path binoviewer can add as much as 1/2 wave of sperhcial aberration.

 

This is not "textbook' stuff that is exempt in the "Real World."  It is easy to see in a star test.

 

If you start with an SCT with perfect SA correction and you stick a lot of back focus on it, you can very quicly induce .2 waves of SA.  So in addition to the focal lenght changes, excessive back focus increases spherical aberration, and in many configurations, can reduce apeture as well.

This is why I have been encouraging people that use SCTs to be careful of controlling the light path.

 

It is also why they should use the factory visual back and 1.25" prism diagonal when they compare their 8" SCT to their 5" APO.     To stich a 2" refractor diagonal on the scope to make the comparison is handicapping the C8 from the outset.     Even if the optics are perfect, this one change will push the contrast transfer to below that of the 5" APO.

Astro-Foren.de has measured this and anyone that wants to challange this claim can see it there....

 

Eddgie,

 

You got me interested in this some time ago. I used to argue that my C8 with a focal reducer worked poorly at high power using similar quality eyepieces but at the same power with/without the reducer. In fact, it is easy to demonstrate this and most anyone can see the difference. I based this on views using a 2" diagonal with the standard Televue visual back (probably worst case for a C8.) However, your postings made me decide to get the Baader T2 1.25" diagonal with the SCT nosepiece and now my high power views with reducer are very similar to those without the reducer at the same magnification.

 

Now my question is: Do you suppose the reason for the improvement is primarily:

 

a. the elimination of aperture reducing effects of the long image train of the 2" diagonal, or

b. the reduction of SA by keeping the back focus within prescribed limits, or

c. some combination of both. I will tell you that I know how to star test properly (thanks in part to you), and I can see a difference of about 1/4 wave between using the T2 diagonal and the 2" refractor diagonal. I had previously chalked this up to the Baader being a better diagonal than the 2"GSO dielectric, but now I am thinking SA caused by the longer focal length might have been the culprit.

 

Thanks,



#8 Eddgie

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Posted 12 October 2014 - 02:32 PM

Some combination of both.

 

For an SCT with excellent SA correction, every inch of back focus (with no reducer) increases the SA by 1/23rd wave (over-corrects).  This is independent of aperture.   It will do the same thing to a C11 as it does to a C5 or any other f/10 SCT using the same f/2, -f/5 mirror setup.  Every inch of back focus changes the SA by 1/23rd wave. 

 

Now, for a very slighly undercorreted telescope, a bit of extra back focus  could actually improve correction of the system.  But for one starting with perfect SA correction, it will add over-correction.

 

Now for an inch or even two, no one will easily see this in focus.  When binoviewers and focal reducers are invoved though, one can quickly get to a point where 1/4th wave of spherical aberration is present, and I have measured 1/2 of a wave wave of SA in one configuration.   That is a lot of SA .

 

Anyway, to answer your question, my guess is that there are multiple factors.  First, the system is almost always working at much higher powers than many people realize (smaller exit pupil by itself is causing some dimming when SCTs are binoviewed), the system is in most cases working with much more sphercial aberration than they realize (and unless one knows how to use the secondary shadow to estimate SA, they perhaps don't know this), and in many cases, the system is working at reduced apeture and with a larger (by percentage) obstruction. In your case, my bet is that you experienced both some mild apeture reduction and a meningful increase in spherical aberration.

 

Meaningless for small differences such as that between a 1.25" diagonal and a 2" SCT diagonal.  An inch makes to small a difference to see.

 

But when the light path gets long as it does in binoviewers or when using focal reducers, you can indeed do considerable damage to the image.

 

For reference, here is what a lot of SA looks like in a star test.  Second image down shows how the secondary shadow sise differs greatly inside and outside of focus.  This much SA has a very serious effect on contrast.  System with this much SA will be noticably soft as compared to the same scope with no SA.

 

http://www.microsoft...55550#post55550

 

If you want to know for yourself, put your previous configuration on and see if there is much change between the secondary shadow size at 10 waves inside and outside.  If you see more than about 1/4 difference in size, there is enough SA present to lower contrast enough that a skilled observer should be able to see it.

 

You can measure the apeture loss (if there was any) directly if you have a bright single LED emitter or better, a green laser.

The SA improvement though was almsot for sure not a function of the prism, but rather a function of the greatly reduced mirror spacing, which greatly improved your spherical aberration correction.

 

The C8 is at its best with a high quality 1.25" prism diagonal.  Not because the prism itself is better, but because the mirror spacing gives optimal SA correction.  If it starts perfect, and you put a 2" diagonal behind it, you lower the contrast.  Only a tiny amount, but this whole game is won or lost by tiny amounts adding up....

 


Edited by Eddgie, 12 October 2014 - 03:10 PM.


#9 Dave Bush

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Posted 12 October 2014 - 03:30 PM

SCT diagonal has a light path of about 125mm.  

As a good rule of thumb, every millimeter of increased light path adds about 3.1mm to the focal lenght.

 

An increase of 25mm (over the design back focus, where the published focal lenght is to be found, which is 100mm for all of the Celestron SCTs except the C14, which is 150mm) would then increase the focal lenght to (25 x 3.1)  77.5mm.

 

In the chart, the graph shows the focal lenght at 125mm to be right at about 2200mm, so close enough.

Anyway, to answer the OPs question, when using a 2" SCT type diagnoal, focal lenght of the C5, C6, C8, C9.25, and C11 will be about 80mm longer with a 2" SCT diagonal than it would be with a 1.25" diagonal in the standard visual back.

 

It is important to note that in the C11 and C14, the measure is taken from the back of the 3.25"  port.   The 3.25" inch back plate adds about 20mm to this figure, and if you add the 125mm for the SCT diagonal, you see that the C11 is acutally working at about (20 + 125) 145mm of total back focus, and at 145mm of total back focus, the focal lenght is about 2950mm, and the focal ratio os about f/10.6.

 

Excellent, so I can safely use 2200mm for my scope at f/10.   If I want it more accurately I'll have to measue (star drift?) for each eyepiece right?

 

Thanks!  Just what I was looking for.



#10 Eddgie

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Posted 12 October 2014 - 03:34 PM

I should say this too.   It is not my place to tell anyone what they should or should not use.  We all make our choices, and it may be indeed desirable to trade off ultimate optical performance for some other benefit such as larger true field.  I get that.   I really do...

 

My goal is simply to help people get the absolute best performance possible out of their SCTs, and a 1.25" visual back and high quality prism diagonal will do more for an excellent C8 than an expensive eyepeice will when that eyepiece is stuffed into a 140mm light path.  1/6th wave of SA improvement will improve contrast much more than an Ortho eyepeice can.  For planetary observing in a C8, my advice is to skip the eyepeiecs and invest in a good 1.25" prism diagonal. used in the standard visual back (or a 2" visual back with a reducer that is no longer than the factory visual back).

 

Likewise, if an SCT is under-corrected, the performance can actually be improved by increaseing back focus.   How much back focus?  Increase the back focus enough so that at 10 waves of defocus (4.4mm inside and outside at f/10) the secondary shadow is exactly 1/3rd of the size of the overall pattern.  If it is bigger on one side of focus than the other, SA is present, and to get the best possible contrast from the scope, the eyepeice needs to be moved in or out enough so that the pattern shows as close to 1/3rd of the pattern size as possibe on either side of focus.  In a system that is undercorrecte by any meaningful amount, going to a 2" SCT diagonal might actually improve things.   

 

But it is rare to find Celestron SCTs with very poor SA.  Most are pretty good, and many are quite excellent.   If there is a difference in the secondary shadow size at 4.4mm of defocus (again, valid for all f/10 SCTs), then it might be possible to improve SA by tweaking the back focus.

 

My advice is not to shortcut this.   Use as close as you can get to 4.4mm, but the most important thing is to have exactly the same amount inside and outside.  Doing the star test without accuratly measuring the distance inside and outside of focus is largely a waste of time.  Only when you use exact amounts of defocus will you get a reliable result.

 

I read all the time how terrible SCTs are, and yet my own experience is that a excellent ones are out there in large numbers, but to keep it performing at its full potential, it is necessary to ensure that SA is not being induced into the system by the configuration being used.    I hate hearing SCT refractor comparisons done by people that have not taken care to ensure that their SCT was configured for optimal function.  And since so many people today are using external focusers and 2" diagonals on scopes designed for best opertation with a 1.25" visual back and 1.25" prism diagonal, I am not surprised that so many of them feel that the SCT is not a good scope. 

 

You can put the wrong tires on a Porsche and hurt it too. 

 

Sorry for the rant.   I have always gotten excellent results from SCTs with good optics.  It surprises me when so many other people lack faith in the design.



#11 Dave Bush

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Posted 12 October 2014 - 03:46 PM

Eddgie,  lots of good info, thank you.  

 

Two more questions for you...

 

1.  So if I've got nominally 2200mm at f/10, by adding the focal reducer to I simply multiply 2200 by 0.63 to get the nominal focal length?

 

2.  Is the difference between using my setup (2" SCT style mirror diagonal) and a high quality 1.25" diagonal with the standard back enough that, in your opinon,

I shoud consider going that route?



#12 stevew

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Posted 12 October 2014 - 06:40 PM

I might add this weird fact.   Assuming that the SCT starts with perfect SA correction, if you use back focus past the nominal back focus (100mm for all but the C14), the SA of the system increases.  Again, between a 1.25" diagonal and a 2" SCT diagonal, the difference is too small to be easily seen.

 

The added S/A by switching my C5 from a 1.25 to a 2 inch diagonal was easily noticed at the eyepiece.

 

Now before you tell me that it's a waste of time to use a 2 inch diagonal on a C5 due to the small baffle tube, I already know that :lol:  

 

I had just put a 2 inch visual back on it to test out a DSLR that I had purchased, and decided to have a look through it with an eyepiece.

That's how I learned that increasing the back focus also increases the s/a.

 

However I think the increased s/a is probably more noticeable in the smaller SCT's than it is in the larger ones.

That's why I don't understand why add on crayford focusers are so popular.

OK maybe to cut down on the mirror shift while doing photography, but I have seen plenty of add on focusers with 2 inch diagonals in them...



#13 stevew

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Posted 12 October 2014 - 06:43 PM

 

 

It is also why they should use the factory visual back and 1.25" prism diagonal when they compare their 8" SCT to their 5" APO.     To stich a 2" refractor diagonal on the scope to make the comparison is handicapping the C8 from the outset.     Even if the optics are perfect, this one change will push the contrast transfer to below that of the 5" APO.

Astro-Foren.de has measured this and anyone that wants to challange this claim can see it there....

Eddie, why a 1.25 prism?

Would there be a difference with a 1.25 mirror diagonal?

Just curious?

 

Steve



#14 AhBok

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Posted 12 October 2014 - 09:15 PM

I'm not Eddgie, but the mirror diagonal has a slightly different length focal path than the prism. I doubt it means much at the eyepiece, but for the sake of a fair comparison using a star test there is a slight difference.


Edited by AhBok, 13 October 2014 - 09:32 PM.


#15 Eddgie

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Posted 13 October 2014 - 09:06 AM

 

I might add this weird fact.   Assuming that the SCT starts with perfect SA correction, if you use back focus past the nominal back focus (100mm for all but the C14), the SA of the system increases.  Again, between a 1.25" diagonal and a 2" SCT diagonal, the difference is too small to be easily seen.

 

 

However I think the increased s/a is probably more noticeable in the smaller SCT's than it is in the larger ones.

That's why I don't understand why add on crayford focusers are so popular.

OK maybe to cut down on the mirror shift while doing photography, but I have seen plenty of add on focusers with 2 inch diagonals in them...

 

 

While some might think that this would be the case, it is is not.   The 1/23rd wave per inch is uniform for any apeture size that uses the same forumla (f/2 primary and -f/5 secondary).    It is not affected in any meaningful way by apeture change.  A C11 will have a 1/23rd wave SA change for every inch of back focus change.

 

Here is a quote from the website Telescope-Optics:

 

"The relation implies that, for nominal primary mirror shift ∆, error induced by extending back focus is independent of aperture. It only depends on secondary magnification and primary's focal ratio. For F1=2, m=5, and K2=0, every mm of reduction in mirror separation (∆=1), or nearly for every inch of focus extension, induces ~1/23 wave P-V wavefront error of over-correction, and as much of under-correction for widening the separation."

 

You can find this qoute in the full context at this link, forth paragraph down: 

 

http://www.telescope...cs.net/SCT2.htm

 

Again, I get why people do all of the things they do.   My goal here is not to tell people how to use their gear.

 

My goal is to help them understand why their C8 may not have compared well to their 5" APO.  If you want to get the best performance possible out of an excellent C8, you have to use it with a light path lenght that results in the best SA correction.  The SCT is starting with a big handicap in the form of a large central obstruction, but if it is an otherwise excellent example, it should outperform a 5" APO on planets.    But if you mix in a 1/6th wave of SA, it does not stand a chance against a good 5" APO.   Maybe that SA is ground into the glass, but because almsot eveyone uses 2" diagonals and long visual backs, they could be inducing SA that is not present when the scope is configured for optimal SA correction.

 

And again, by using the star test, even if the scope is a bit undercorrected, it is possible to find the setting that completly nulls any SA that is present.

 

That is actually a very powerful thing.. You can actually fix SA if it is present in an SCT simply by changing the focus point.

 

It is a good bet though that many people seeing a bit of SA in their systems may be seeing it because they are not set up with optimal mirror spacing.

 

I read how bad SCTs are all the time, but my experince does not really mirror that.  I have had great planetary results using many SCTs (not all were excellent though).   I have been advising for a long time that the best thing that somone can do with an SCT to get the best performance out of it is to keep the light path as short as possible.  While it is true that optical qualiity does vary, and other optical issues are almost always present, SA is perhaps the most damaging optical aberration, and a little SA does more damage than a small central zone for example.  Most SCTs have a combination of small defects, but SA is usually decent, and again, if you now what you are doing, you can tune SA out....

 

But people want a wide field form the scope, and that is a good tradeoff (though to me, you are far better off getting a different kind of telescope if wide field is your goal).  Testing an excellent C8 against a 5" APO though, and calling it a looser without ensureing that it is working at best SA correction is pretty unfair to the design.  Not exactlly "Snow tires on a Ferrari", but more like "Bald tires on a Camry."   Push it hard around corners and it is going to squeal.

 

To respond to your post simply though, the above page will explain why this is a function of mirror speeds and does not change in any meaningful way based on apeture.  f/2 and -f/5 gets you 1/23rd wave SA change for every inch of back focus change.  You do as much damage by putting a 2" diagonal and crayford focuser on the C11 as you do on the C5.  Just the way it works out, becuase both use an f/2 primary and - f/5 seconary.


Edited by Eddgie, 13 October 2014 - 09:12 AM.


#16 Eddgie

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Posted 13 October 2014 - 09:28 AM

 

 

 

It is also why they should use the factory visual back and 1.25" prism diagonal when they compare their 8" SCT to their 5" APO.     To stich a 2" refractor diagonal on the scope to make the comparison is handicapping the C8 from the outset.     Even if the optics are perfect, this one change will push the contrast transfer to below that of the 5" APO.

Astro-Foren.de has measured this and anyone that wants to challange this claim can see it there....

Eddie, why a 1.25 prism?

Would there be a difference with a 1.25 mirror diagonal?

Just curious?

 

Steve

 

The 1.25" prism (which is what used to be shipped with standard Celestron SCTs, yes), results in a mirror spacing that is further than that in a 1.25" mirror because of the lights slower speed thought the prism (no change in speed for a mirror).  This means that the mirrors can be further apart if you use a prism.

 

People think that the Prism was the standard diagonal because it was cheap to make (and make no mistake, they were "cheaply made" but not "Cheap to make".

 

My feeling based on testing many SCTs was that the prism was actually shipped becuase it gave the most optimal mirror spacing (resulting in the best SA correction) and the shortest possible focal lenght for the system.

 

I could be wrong of course, but the math suggests that this is why the prism was the factory equipment.

 

I have to say this too.  These prisms were usually better than people believed them to be.   The housings were as cheap as you could get, but the quality of the prisms was (in every case where I used them for any kind of testing) very good.   The housing gave the impression of poor quality, but I think that was maybe more psychological objective.

 

But the best way to know that the scope is performing optimally is to use a star test (as I described above) and use whatever diagonal or extension gives the best SA correction.    By being configured for "Optimal" performance, I of course mean best performance at the center of the field.  If "Optiamal" to someone else means a 7.5" aperture with a 1.3 degree field of view, than I can't argue that.  Optimal means different things to different people.

 

But if the goal is to try to be a 5" APO, you have to be on your game and not driving your Camry to the fight with bald tires on it.



#17 Eddgie

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Posted 13 October 2014 - 09:40 AM

I have also often been interested to know if the prisms in any way corrected for the spherochromatism that comes from the corrector.

 

Many people don't realize that SCTs have some spherochromatism.  Now funny thing is that a prism diagonal can also have sphero-chromatism.

 

While the amount of spherochromatism present in the SCT is less than it is in may expensive APOs, it is none-the-less, and error that is present.

 

I had wondered at one point if the prism somehow offset this.  

 

I think Roland Chirsten or someone once wrote that for a longer focus refractor, a prism was a better choice than a mirror, but as the refractor got fast, the Spherochromatism induced by the fast light cone made a mirror a better choice.

 

I am not an optical engineer, but I have never seen this subject addressed before, and am not smart enough to work it out for myself.   Rohr at Astro-foren.de tests SCTs all the time in different wavelengths of light, but he never passes the light though a prism.  Iwould be curious to know if the use of a prism in the light path would have improved some of the spectral correction.  

Now the amount is so very low that the SCT is more "APO than" all but the finest APOs money can buy, but anything you do that takes energy from the Airy Disk of a star affects the image.  And all of these little tiny things tend to add up!  And how is the spherochromatism affected by overcorrection? 

 

Very complex.    But also, very small errors.  1/6th wave of SA by comparison though is a serious error.  Just thinking out loud on the subject, but there is nothing ever really written on it.

 

A well corrected C8 though (one that showed perfect SA correction with the factory diagonal) will be at its best when the light path is kept the same.  If it had some LSA though. changing to a 1.25"mirror could improve the SA correction.  If it starts with perfect LSA with the 1.25" prism though, and you put a 2" diagonal and Crayford on it, you have not helped it.   Have you hurt it enough to matter?  Not my question to answer. Don't put it up against your friend's 5" APO though.  You might come away thinking your C8 is a horrible scope that is challanged to beat a 4" APO.    


Edited by Eddgie, 13 October 2014 - 09:43 AM.


#18 Eddgie

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Posted 13 October 2014 - 10:01 AM

Ok, I have spend way to much time on this, but I could not leave this until I offered this final couple of things.

 

Here is a star test for an otherwise perfect C8 with 1/6th wave of lower order spherical aberration (LSA) as generated using Aberrator.

 

Now a lot of people would look at this star test and say "WOW" that is a great C8!

What I see though, in a very clinical way, is 1/6th wave of LSA.

 

And when I plot this C8 against a 5" APO, it can no longer match the 5" APO at the important visual frequencies.

 

Red dashed line is C8 with 1/6th wave LSA.   Green line is perfect 5" APO.

 

One sixth of a wave of LSA is in fact, in this particular comprison, a game changer.   I have been ranting about back focus on SCTs for a long time now (expecially in the Binoviewer forum).   I always see these posts of how this or that small refractor beat this or that C8.  And you know what?  I usually tend to beleive them because I believe that in most cases, the C8 was not optimally configured. But there is no reason why a 5" APO should beat an excellent C8 from a design stanpoint.   Physics says that an excellent C8 should match the contrast of a 5" APO.   My own experience is that this is indeed the case.  It won't stand a chance though if it has been configured in such a way that a meaningful amount of LSA has been induced.. All of this little stuff adds up.

Attached Thumbnails

  • sixth wave sa.jpg
  • 5 inch wins.jpg

Edited by Eddgie, 13 October 2014 - 10:11 AM.


#19 REC

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Posted 13 October 2014 - 10:11 AM

Lot's of interesting information here.  Until Eddgie told me I was operating up to about f/2600 using a refractor type 2" setup with my BV, I had no idea it was that long and also cutting down my aperture by almost an inch!  I since then picked up a Baader T2 prism diagonal to shorten the light path.

 

Also one nice thing about the Baader diagonal, it comes with a fine tune focusing ring so you don't have to move the mirror in and out to focus.

 

Ed, quick question if you will:  I also picked up a 10mm SCT adapter to screw directly into the T2 prism and eliminates the need fro the factory standard visual back, is this the best way to go to insure maximum contrast and performance in my 8" SCT?

 

Thanks,

 

Bob



#20 Eddgie

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Posted 13 October 2014 - 10:29 AM

Lot's of interesting information here.  Until Eddgie told me I was operating up to about f/2600 using a refractor type 2" setup with my BV, I had no idea it was that long and also cutting down my aperture by almost an inch!  I since then picked up a Baader T2 prism diagonal to shorten the light path.

 

Also one nice thing about the Baader diagonal, it comes with a fine tune focusing ring so you don't have to move the mirror in and out to focus.

 

Ed, quick question if you will:  I also picked up a 10mm SCT adapter to screw directly into the T2 prism and eliminates the need fro the factory standard visual back, is this the best way to go to insure maximum contrast and performance in my 8" SCT?

 

Thanks,

 

Bob

 

For binoviewers, yes.  For mono-viewing,  it will not be enough difference from a standard 1.25" diagonal to make a difference, but if your scope has perfect SA correction with the standard 1.25" prism, going to the 10mm would very slighty undercorrect the scope.

 

Not enough to matter though.

 

The only time you are going to see a really meningful difference is when you can change the light path by 3" to 4".  This is why I tend to devote a great deal of effort to the topic on the binoviewer forum. 


Edited by Eddgie, 13 October 2014 - 11:13 AM.


#21 Terra Nova

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Posted 13 October 2014 - 10:37 AM

I think this discussion is excellent enough, and the information therein, both timely and timeless enough that it should be pinned at the top of this forum. Thanks particularly to Eddgie for providing us with an in depth explanation as to the what and why of this important question.



#22 Dave Bush

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Posted 13 October 2014 - 11:45 AM

This thread got side tracked and one important question (at least to me) seems to have been missed.

 

If I've got nominally 2200mm at f/10, by adding the focal reducer do I simply multiply 2200 by 0.63 to get the nominal focal length?

 

Thanks.



#23 Eddgie

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Posted 13 October 2014 - 05:20 PM

Your question was answered in the second and forth post I believe, though not as the result of a direct measurement, but how to estimate it, and these figures are very accurate.

 

While I have not measured the focal length using a 2" SCT diagonal, I have measured for a variety of configurations, and theC8 with an SCT diagonal will be about 2100mm.

 

Sorry, did not meant to hijack your thread.  I thought your question had already been answered.


Edited by Eddgie, 13 October 2014 - 05:31 PM.


#24 Dave Bush

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Posted 13 October 2014 - 05:37 PM

No need to apologize.  My question about the focal reducer wasn't answered unless somehow I missed it.

 

Let me reset....

 

My diagonal is a GSO Dielectric.  It's 2" and screws directly onto the back of the scope.  It's an SCT style diagonal.

 

In your first reply you said...

 

"SCT diagonal has a light path of about 125mm."  and then...  "In the chart, the graph shows the focal lenght at 125mm to be right at about 2200mm, so close enough."

 

That's just what I was looking for.  But thats for when the diagonal is attached directly to the scope back.

 

My other question is, if I am using 2200mm as my focal length, if I then attached the f/6.3 reducer/corrector wouldn't the resulting focal length be 0.63 x 2200mm or 1386mm?   Or does adding the reducer/corrector, since it pushes back the location of the eyepiece, mean it can't be so simply calculated?



#25 hudson_yak

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Posted 13 October 2014 - 07:34 PM

It's the second of your guesses. While the focal length of the basic scope is increasing, the additional spacing from reducer to focal plane is increasing the reduction factor of the reducer. The nominal .63 factor occurs with the 100mm spacing of a 1.25" diagonal. The SCT vignetting report shows an overall focal length of 1150 for 125mm spacing, for a reduction factor of .54 assuming 2125 focal length for 125mm spacing with no reducer. For 150mm spacing the factor is 1050/2200 or .48.

 

Mike




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