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Aperture in C6 & C8 for various configurations

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#1 EdZ

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Posted 23 February 2012 - 05:13 PM

Since recent threads deal with fov of view vignette (light fall off) and aperture vignette (hard loss of aperture due to obstruction), I thought it worthwhile to measure the aperture in the C6 and C8 in various normal configurations.

I’m not measuring light fall off. I’m not reporting effective operating focal length, which I’ve reported elsewhere. Here I’m measuring to find out if I get full aperture or some smaller effective operating aperture.

I used Glenn LeDrew's simple and effective method of measuring aperture. Place a bright light source about a foot away from the eyepieces and measure the light thru the objective. You must refocus to infinity for each individual setup. Since the light beam exiting the objective is a colimated beam, if you wanted to, you could project that beam onto a white wall 5 feet away and it would still show exactly the same diameter as seen when it leaves the objective. I measured at the objective. Simple and accurate. I used three small LED flashlights and dimmed the room.

As you add accessories behind any SCT scope the focal plane needs to move out to accommodate eyepiece field stop position. Since there is a moveable mirror internally, as it moves with relation to the fixed mirror the resultant focal length changes rapidly. Small movements in the mirror result in large changes in the focal length. If the focal plane moves too far behind the back plate, the exit tube may interfere with the light cone, or the primary mirror light cone may not be entirely seen by the secondary, thereby giving a reduced effective aperture. And it has been a contention that the narrow 27mm tube on the C5 and C6 does indeed restrict aperture in numerous configurations. It almost certainly restricts extent of illumination, but here we'll deal only with aperture. You will see that reduction occurs in several instances, but perhaps not nearly as quickly or as severely as has been thought.

Several of the accessories I use for different configurations are of different lengths. Their lengths are listed here:
1.25” standard visual back = 35mm
1.25” TV dielectric diagonal = 75mm
2” diameter extension sleeve = 28mm
2” screw-on Astro Tech SCT diagonal = 130mm
2” GSO rotatable focuser = 100mm with drawtube 10mm out
2” standard refractor diagonal = 113mm
Denkmeier binoviewer with 2 arm powerswitch set to open = 134mm


For the C6, these configurations provide the measured aperture, the last figure on each line. Lengths of the accessories in use and total length behind back plate are in parenthses. Eyepieces used were 16mm SWA and 24mm SWA, both provided same result.

1.25” VB (35) + 1.25” Diag (75) = (total 110) = 150mm aperture

2” extension (28) + 2” SCT Diag (130) = (total 158) = 150mm

GSO focuser (100) + 2” ref diag (113) = (total 213) = 148mm

You can see from these three measures that aperture just started to get reduced with back focus beyond 200mm, . At 213mm of back focus, the C6 loses only 2mm of aperture. But frankly, I wouldn't recommend the SCT focuser on the back of the 6". So heeding that advice, there does not seem to be other normal configurations that result in aperture loss.

One normal way that I could get longer back focus was to try using my Denk binoviewer. There we see the continued reduction of aperture. Using the Denk, back focus gets moved out very long.

1.25” VB (35) + 1.25” Diag (75) + Denk BV (134) = (total 244) = 140mm

2” SCT Diag (130) + Denk BV (134) = (total 264) = 136mm

2” extension (28) + 2” SCT Diag (130) + Denk BV (134) = (total 292) = 132mm

in this last configuration, my C6 and BVs are operating at F= 2270. Since aperture is reduced to 132mm, I'm operating at an effective f/17.

These three additional points give pretty consistent data. Just over 240mm it's down to 140mm aperture. By 260mm backfocus it's reduced to 136mm and at 290mm backfocus aperture is down to 132mm. I'd bet those 4 data reduced aperture points make a fairly consistent graph. It shows aperture doesn't start dropping until you hit slightly over 200mm of back focus and then you would need to be using over 300mm of backfocus to lose an inch of aperture.

What this shows is there does not seem to be any binoviewer configuration that gives full aperture. And frankly, I would not use my BV on the back end in a 1.25" diagonal. So, just about any BV configuration I select would result in a loss of at least 14 to 18mm in aperture.

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

this last configuration with the extension tube would be unusual, as the tube is not at all necessary, but it does show the rapid fall off in aperture as we move further behind the reducer. FWIW, as you move further away from the reducer, the focal length gets shorter, hence the light cone gets wider. So, for several reasons, the reducer should only be considered for use when you can keep the length behind the reducer relatively short, say approx 100-120mm.

Note that when using the reducer, since the reducer completely changes the shape of the light cone, I list the length behind the reducer, not the length behind the back plate.

edit 3-4-11
about 30-40 posts down you will find detailed data on testing the C6 when using my Denk binoviewer with powerswitch set to reducer mode. This turns out to be the most severely reduced/vignetted system.
2” SCT Diag (130) + Denk BV (134) = (total 264) = 136mm
with the reducer I'm operating at 0.60x the F which nets F=1100. However aperture is reduced to 118mm. And field of view is limited to about a 23-24mm field stop.


Perhaps next I'll do the full compliment of configurations for the C8. But a few short notes on the C8 follow.

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.

There are other considerations to be aware of, foremost the operating focal length. It gets very long very fast. I have other posts describing operating focal length. But most people should know by now, in these smaller SCTs, 8" and under, very few if any configurations result in f/10. More likely most people are operating between f/11 and f/12, and with binoviewers more likely over f/15.

But, at least now you know if and when you start to lose aperture, and in some cases, how much is lost..

edz
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#2 thomas68

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Posted 23 February 2012 - 06:17 PM

Thanks Ed, just bought a new C8 in January. It's nice to know these numbers.I bought the two inch diagonal (screw on)XLT, just for added security I guess it was a good choice.

Tom

#3 Eddgie

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Posted 23 February 2012 - 08:47 PM

Wonderful writeup.

A question? You often said a " 2" extension an an SCT diagonal." Did you mean a 2" extension and a refractor diagonal? I don't know what the extension would be required for when using an SCT diagonal.

And this:

What this shows me is there does not seem to be any binoviewer configuration that gives full aperture. And frankly, I would not use my BV on the back end in a 1.25" diagonal. So, just about any BV configuration I select would result in a loss of at least 14 to 18mm in aperture.



As you may have read, I recenty bought a Denkmeier Supersystem, which was based on a 2" diagonal system, and when I did, I felt that the image was being perceptably dimmed.

I went to the Baader Maxbright with T2 diagonal. This system is very short and can keep the entire back focus on the C14 down to about 170mm.

Based on your result, I would think that the Baader Maxbright/T2 would give full aperture in both the C6 and C8.

I know I saw a distinctly brighter image when I changed from the 2" based Supersystem Binoviewer to the Baader Maxbright.

And the T2 Prism is very solid. The Baader Maxbrights are also very light, but the combination seems more than sturdy enough.

I have been recommending the Baader Maxbright/T2 as the best Binoviewer for SCT applications because it retains full aperture.

Again thank you for doing all of the measuring! Very informative post.

Regards.
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#4 Eddgie

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Posted 23 February 2012 - 09:04 PM

Also, just compared this to Ted Hutchinson's SCT vignettting analysis for the C8. For 240mm of back focus, his ray traces indicated that aperture would be 7.5".

His ray trace was perhaps influenced by not being able to get accurate measurements for that scope. He used the design in Telescope Optics.

I think he had measurements for the other scopes.

Oddly though, the ray tracing with respect to the focal reducer seemed to be much more in line with your findings.

Anyway, it is much better to have real world measurements in place of a ray trace. I will have to try this method myself one day!

Again, wonderful writeup. Thank you again for posting!

#5 MRNUTTY

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Posted 23 February 2012 - 09:11 PM

Yes, I really appreciate the thorough treatment of the 2" diagonal use cases.

#6 Eddgie

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Posted 23 February 2012 - 09:50 PM

I also wanted to point out that while 13mm of aperture loss in a C8 does not sound like much, if one does the math, one quickly sees that there is a reduction in brightness of about 15%. I don't think this is horrible, but it is a decrease that most people I think would be able to see.

The way I calculated that was by subtracting the area of the central obstruction (about 5 square inches) and the transmission loss of an XLT system (about 10%). This gives an effective light collection of 40.5 Sq In.

The half inch aperture reduction doesn't sould like much, but now the secondary obstruction is larger in terms of the percentage of collection area, so progressivly more collection area is being lost.

The C7.5 would have 44.17 inches of collection area, minus 5" of obstruction, for 39.2 inches of collection. Now, when you account for the 10% transmission, this leaves you with 35.3 square inches (effective) of collection.

By going from a 13mm reduction in aperture to a configuration working at full aperture, you gain 15% brigtness. This is about what I saw when I went from the Denkmeier Supersystem to the Baader Maxbright/T2 diagonal.

Of course in the smaller scopes, this number would be a bit higher, and the bigger the CO is in percentage at full aperture the more brightness will be lost as the aperture reduces further and the CO gets bigger in relation to the shaded/unshaded area.

I agree that no one should worry about 2 or 3mm of effective aperture loss, but as the loss gets bigger the dimming becomes more and more aggressive.

Of course one could always put in a ZOA or Baader Ortho and get all of that brigtness back... :roflmao:.

Tom Hutchinson's work of course did not include transmission loss.
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#7 AhBok

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Posted 23 February 2012 - 10:13 PM

Thanks EdZ. This is one of the most useful posts I've seen.

#8 Eddgie

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Posted 23 February 2012 - 11:03 PM

Here is the brightness loss for a C6 used at 140mm of effective aperutre.


27.43" Sq in of collection at full aperure. 3.75 Sq In of obstruction. With 10% transmission loss the full aperture is giving 21.3 Sq in of effective collection.

Now, in the C6 used with a f/6.3 focal reducer and 2" diagonal, light collection is 23.75 sq. in. of effective aperture. Subtract the constant 3.75 sq in. of obstruction and you effective light collection of 20 sq in, and when you subtract the 10% transmission loss, you have 18 Sq In of effective collection.

By restoring the system to full aperture, you go from 18 Sq In to 21.3 Sq In.

In other words, but going from a C6 with a focal reducer and 2" diagonal to a system working at full apeture, the image will be 17% brighter!

I am only mentioning this because it is easy to underestimate the difference a few millimeters of aperture can make. A millimeter of aperture being lost at the outside of the primary actually adds up very quickly in collection area.

But when the system has a large obstruction, the ratio of the shading from the obstruction increases in proportion to the reduction of effective aperture. When the system is at full aperture, the obstruction is 37% of the effective aperture, but when the effective aperture is reduced, the obstruction becomes more like 40% of the effective aperture.

I can't for the life of me understand how people could not see such a significent difference in brightness.

Especially here on CN where people report seeing as little as 6% difference in transmission on eyepeices! :roflmao:

#9 azure1961p

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Posted 23 February 2012 - 11:22 PM

Eddgie,

Ive appreciated your posts here, thanks for the insight.

Pete

#10 GlennLeDrew

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Posted 24 February 2012 - 01:38 AM

EdZ,
Thanks for doing this! It's refreshing to see some experimental results that go beyond the usual regurgitation of theory and supposition. The next step in this process would be to determine the size of the circle of illumination at the 50%, or more accurately, half-aperture level. A variation on the flashlight test can do this. If you desire details, PM me, as it could be both a tad lengthy and of little general interest to include here.


Eddgie,
When performing the loss due to aperture reduction, it's not necessary to consider transmission loss, such as the 10% figure you've used. ESPECIALLY if only applied to the reduced aperture case, as it makes the loss artificially worse (without having verified your math, based on the wording it seems this is what you may have done.) For this purpose, simply consider the 'before' and 'after' cases as though the transmission efficiency is perfect, as what matters only is the aperture area ratio.
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#11 EdZ

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Posted 24 February 2012 - 06:04 AM

You often said a " 2" extension an an SCT diagonal." Did you mean a 2" extension and a refractor diagonal? I don't know what the extension would be required for when using an SCT diagonal.



the problem with attaching a 2" SCT diagonal to the C6 is that the focus knob interferes with the ability to rotate the diagonal to preferred viewing positions. The base of the diagonal hits the knob and allows rotation only thru a small arc. (Not that I tested the C5, but if I recall correctly, you can't even attach the 2" SCT diagonal to a C5, it hits the focus knob).

The extension is threaded both ends, female to fit the back of the C6 tube and then male to allow the SCT diagonal to screw on. It moves the diagonal out by 28mm and with that extra length the diagonal clears the focus knob. With no interference, the diagonal can be rotated to any viewing position. Unfortunately it adds undesirable focal length.

This problem does not occur on the C8, so the extension is unnecessary for the C8. Any instances where I will use the extension tube to get measurements on the C8 is simple to get another data point.

Regarding achieving the shortest path, prism diagonals would be a choice to help accomodate a shorter focal path. Is the Baader T2 a prism diagonal? Has anyone measured the light path length thru that diagonal?

I suppose another way to get a shorter back length would be to use a 2" visual back and a standard 2" refractor diagonal. The VB would be nearly the same as the extension tube and a typical refractor diagonal is 20mm shorter than an SCT diagonal.

Wish I still had my C5. I loved that scope.
I'm guessing the results of the C6 would represent the C5 pretty well, since the exit tube is the same and the scope is the same approx f#.

One thing I became acutely aware of after doing this. We sometimes see photos of setups that people use to claim how well specific pieces of equipment seem to perform or as examples of how to achieve extremes. For instance, a C6 with a reducer, a 2" diagonal, a binoviewer and a pair of 24 Pans. Looks beautiful, and appears to represent an outstanding low power wide field arrangement. BUT now I know that such a setup on a C6 could be operating close to, if not less than, 3 inches or aperture, and as Ed points out, that with a 50+mm central obstruction, about the worst possible choice of configuration that someone could come up with. This leads me to exclaim, under no circumstances would I recommend using binoviewers behind a reducer.

edz

#12 Mark9473

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Posted 24 February 2012 - 07:07 AM

Baader T2 diagonals come in four versions:
- a Maxbright mirror diagonal having 53 mm optical length
- a Zeiss prism version having 35mm clear aperture and 41 mm optical length; this is the most used T2 diagonal
- a Baader prism version having 32mm clear aperture and 37 mm optical length
- an Amici prism version having 48 mm optical length

The big advantage is that the T2 prism couples directly to the MaxBright binoviewer without an eyepiece holder on the prism and a nosepiece on the binoviewer.
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#13 MRNUTTY

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Posted 24 February 2012 - 07:51 AM

@EdZ you saved me the cost of a 2 inch diag for my C6.

#14 Jon Isaacs

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Posted 24 February 2012 - 08:23 AM

EdZ,
Thanks for doing this! It's refreshing to see some experimental results that go beyond the usual regurgitation of theory and supposition. The next step in this process would be to determine the size of the circle of illumination at the 50%, or more accurately, half-aperture level. A variation on the flashlight test can do this. If you desire details, PM me, as it could be both a tad lengthy and of little general interest to include here.


Glenn:

I think there are others who would be interested in your technique.

Jon
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#15 EdZ

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Posted 24 February 2012 - 08:52 AM

Baader T2 diagonals come in four versions:
- a Maxbright mirror diagonal having 53 mm optical length


How can you possibly achieve a 53mm light path in a 2" mirror assembly? With absolutely not a single millimeter at either end for metal hosing thickness, , connection to the scope, or room to insert anything without hitting the glass, the length is a bare minimum of 50.8mm?

Or, are you simply quoting the spec measure of the 45° housing, which does not include the scope side or eyepiece side stems, or couplings? If that's the case, then you may not be stating the whole operating light path length.


- a Zeiss prism version having 35mm clear aperture and 41 mm optical length; this is the most used T2 diagonal
- a Baader prism version having 32mm clear aperture and 37 mm optical length



A 2" mirror diagonal has a clear aperture of about 44-45mm. 35mm and 32mm clear apertures don't even sound like 2" diagonals.

Using a 2" diagonal with a clear aperture of only 35mm would reduce your illumnated field by a very strong amount. A 32mm clear aperture 2" diagonal is not much more than the total illumination you would get from a standard 1.25" mirror diagonal. Perhaps these diagonals are acceptable for use with typical 27mm clear aperture 1.25" binoviewers, but these would be a huge compromise to use with wide 2" eyeepieces. The illuminated field would be reduced considerably. But illumination is another topic, one that deserves a lot more attention than I can devot to it here.

edz

#16 Mark9473

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Posted 24 February 2012 - 09:12 AM

It is not a 2" diagonal EdZ, we were discussing T2 diagonals. The scope side nose-piece does not contribute to optical length as it disappears into the focusser drawtube or the clamp you have on the end of your scope. As I said, there is no eyepiece side 'stem' or clamp or whatever; the Maxbright binoviewer attaches directly to the diagonal housing without putting a nosepiece on it. This is where you gain back focus.

Look here, esp. Table 2 on page 7 and Fig. 10 and 11 on page 8:
http://www.baader-pl...on_emglisch.pdf
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#17 Mark9473

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Posted 24 February 2012 - 09:13 AM

You asked "Is the Baader T2 a prism diagonal? Has anyone measured the light path length thru that diagonal?".
I just replied to that question.

#18 Eddgie

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Posted 24 February 2012 - 09:37 AM

The Baader diagonal light paths are only given for the length of the light path through the diagonal housing itself.

Baader lists the lenghth of the light path for each component to allow one to calculate how much light path they will be working with for a given configuration.

In the Baader Maxbright system, the Bino head attaches directly to the prism body.

So, for example, the standard Baader T2 Diagonal (the one with the shortest possible light path) is 38mm, and the bino head is about 110mm. I use it in my C14 and the nose goes into a AP 2" visual back on my C14, which adds about 30mm.

This entire configuration adds about 178mm of back focus so based on your measurements, using the typical adapter, most SCTs could keep working at full aperture even with the binoviewer.

This is why I sold the Demkmeier. The view was noticebly dimmed on both the C14 and the EdgeHD 8". I had calculated this prior to purchasing the Denkmeier, but was seduced by the power switch system. The miniute I tried it though, I realized I had made a mistake. The image was noticebly dimmed, and I could see the illiminatin falloff (I am super sensitive to this... Don't know why).

Going to the Baader Max bright/T2 restored me to full brigtness in both the C14 and the EdgeHD. I saw a detectable improvement on pleantery performance in both scopes. The image was brighter and contrastier (due to the brigntness change I think). I was careful to calculate magnifications so that exit pupil differences were minimized when makeing the comparison between Mono and bino views.

It is rare that I say that this or that change made a difference because I think the tendency on these forums is to say that tiny changes are often possible to detect, and my own experience indicated that this was rarey true.

But this change was for me, glaringly apparent.

This is why I love the Maxbright/T2. The view is astonishingly good. Again, based on your measurements, it would appear that even with a bit longer estension, even the C6 could work at very near full aperture with the Baaders, but care would be needed to keep the system as short as possible.

For the C6, I would find a way to shorten the focuser knob.

Here is a picture of the Baader on the C14.

This is a superb setup, and in my opinion, the best possible configuration for most moving mirror telescopes where effective aperture reducation may take place due to back focus issues, where trimming every millimeter is important.

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#19 Eddgie

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Posted 24 February 2012 - 09:47 AM

Oh, by the way. There is a Ziess and Baader prism. I cannot imagine that there is much to be had by going to the Ziess because the view with the Baader prism is superb. I detect no difference between it and my Televue Dielectrics.

And the housing is very sturdy though the bino head itself is so light that any good 1.25" diagnal should hold it as it if used with light eyepieces and the diagonal housing is metal.

The Ziess prism has a bit longer light path, but it has a bit bigger aperture. The Baader has a 1.25", but the Ziess I think has a bit bigger opening because for the Mark V with its bigger apertures in the bino, it could exploit it.

But for the standard T2, the Baader priam is fine.

It also allows one to exchange the barrel for either 1.25" or 2". I was going to go with a 1.25" but when I considered that the thick ness of the 2" to 1.25" adapter might make it hard to reach focus when used on my 6" APO, I went with the 2" Nose. Good thing too, because I only have about 3 or 4mm of infocus to spare when using these on the 6" APO (though I did not buy them for that reason, it is nice to be able to use them on the AOP If I want).

Again, this is why Baader lists the light paths indivigually. It allows you to calculate with fairly good precision how much back focus you will get in any configuration.

I love the Baader syestm for my SCTs.

Based on your own work, I may even get a 1.25" nose piece so I can use them in my C5!

#20 EdZ

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Posted 24 February 2012 - 10:05 AM

I see. So the shortest lengths given for the housing only are those that can be used to calc backfocus with a binoviewer. Just surfing thru some of that data I found what may be the shortest configuration fo a 54mm T2 assembly if connected directly to an SCT, to use with eyepieces.
10mm for the SCT comnection
54mm for the T2
47 to 60mm depending on which eyepiece extension you choose.
TOTAL = 111mm to 124mm.

That's 20 to 6mm less than a standard SCT diagonal, but I'm not sure it it maintains the ability to rotate the diagonal.

If you use the T2 diagonal with a nose that inserts into an SCT, then you need to add the 35mm for the SCT visual back, same as you would if using a refractor diagonal. Optical path would be almost identical to a refractor diag.

The big savings in length comes when you attach their diagonal to their binoviewer. Under any other circumstances, that savings is reduced or there is no savings in length.

edz
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#21 Mark9473

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Posted 24 February 2012 - 10:25 AM

Oh, by the way. There is a Ziess and Baader prism. I cannot imagine that there is much to be had by going to the Ziess because the view with the Baader prism is superb. I detect no difference between it and my Televue Dielectrics.

The Ziess prism has a bit longer light path, but it has a bit bigger aperture. The Baader has a 1.25", but the Ziess I think has a bit bigger opening because for the Mark V with its bigger apertures in the bino, it could exploit it.

But for the standard T2, the Baader priam is fine.

One additional advantage is that the Zeiss prism with its 35 mm clear aperture could be used with *some* 2" eyepieces that don't have too large a fieldstop size. I'm getting the Zeiss T2 prism for my Mewlon just in case one day I'd want to put a Baader 31mm Aspheric 2" eyepiece on it.
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#22 Eddgie

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Posted 24 February 2012 - 10:41 AM

You'r right. I should have done the transmission for the reduced effective aperture scope at full apeture, but it it doesn't change the outcome very much.

I do like to include transmission though, becuase in this example, the telescope is working more or less with the same brightness of a 5" refractor.

Considering that you can buy inexpensive 5" achomats, or even 6" Newtonians relativly little more than the cost of a focal reducer, 2" diagonal, and 2" widefield eyepeice, one could instead just buy a telecope that is better suited to the task of wide field observing to begin with.

#23 EdZ

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Posted 24 February 2012 - 10:50 AM

I just typed a very long post


and network issues just dropped me. Lost it all.

frustration ensues. Sigh...

#24 EdZ

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Posted 24 February 2012 - 11:23 AM

to briefly summarize.

I question the focal path length thru the Baader binoviewer as 110mm. You might want to actually test it. My old small Burgess $200 binoviewer had a path length of 110mm, and it had 21mm prisms. The Baader does not appear to be much smaller than the Denkmeier. The Denk has 26mm prisms. I think the Baader prisms are equal or larger. To test my Denk, I brought out my very long focus tube 90mm refractor. The draw tube is about 6 inches long. First I focused with just an eyepiece and measured the draw length of the tube. Then I focused with the Denk (with no power switch lens in the path). The change in position of the refractor focuser tube is the focal path length inside the Denk. I found that the focal path length thru my Denk is 134mm. But, the Denk specifications states the focal length path is 114mm.

While you may be able to use your shortest configuration binoviewer in the smaller SCTs with no aperture loss, it will still push you to a very very long focal length, perhaps F=1900 in the C6, and over F=1600 on the C5. Sure you could do it, but you might not want too.

I stopped using a configuration on my C6 that netted F=2000 because image quality suffered. I use the C6 at F=1830. I stopped using configurations on my C5 over F=1370, meaning, I returned my C5 to it's native 1.25" diagonal and 1.25" eyepieces only. I've since sold it.

If I put your short Baader into my 2" SCT diagonal on my C8, it would be about 240mm, and therefore would have little to no aperture loss. So the C8 may be a point of diminishing returns for the need to achieve the shortest BV configuration. In the larger scopes it might not be necessary at all.

The beauty of the C8 is that it starts out very short. With nothing but a 1.25" visual back (no diagonal) the C8 is operating at F= 1800. I believe the C9.25 works the same way.

The C8 reaches F=2000 (f/10) when you put in a 1.25" diagonal. The C5 and C6 do not follow this same pattern. With the exact same visual back and 1.25" diagonal, the C5 is operating at F=1370 and the C6 at F=1650 (both f/11).

edz

#25 Mark9473

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Posted 24 February 2012 - 12:27 PM

The 110 mm is confirmed by Baader on the link I provided above.
EdZ, I think you are confusing with the Baader MarkV which does have larger prisms.


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