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Aperture in C6 & C8 for various configurations

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149 replies to this topic

#26 EdZ

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Posted 24 February 2012 - 12:31 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.


Ahh, that could be it.

#27 Eddgie

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Posted 24 February 2012 - 01:52 PM

Actually, Ken Hutchinson did direct measurments on the C14 and did ray traces for the C9.25 and C11.

The C11 has no effective aperture loss with back focus out to 300mm

The C8, C9 and C14 do though.

He did a Ray Trace on his one C14 and took direct measurement of the focal lenght and they matched the ray trace within 2%.

The C14 has an optimal back focus of 150mm vs the 100mm of the smaller SCTs, and one would think that with the very large baffle, there would not be consequence, but in fact, with minor exceptions, the C11 is basicallly a scaled design.

His chart (which I could have sworn I sent to you) shows that working at 180mm of back focus, the focal lenght of the C14 is only 3970mm, or about f/11.15.

However, the baffling is rather tight, and illumination falloff started at 200mm of back focus which appears to be much like the other SCTs.

At 280mm of back focus, the C14 was only working with about 13.4" of apeture.

Because he was able to get direct phycical measurements from the C9.25, I expect that his ray trace for that scope the C9.25 is accurate as well. According to his ray traces, the C9.25 is a C9.00 at 200mm of back focus, and about a C8.4" at about 280mm of back focus.

Ken's ray trace conformed very well to his physical measurements.

Again, .6 or .7 inch doesn't seem like much, but the view through the Baader was clearly brighter to me. I spend a lot of time doing planetary observing with the C14, and I could tell I was working at reduced aperture. At powers in the 300x region, the view was noticably dimmmed.

With the Baaders, I notice no such dimming.

Ted's Vignetting analysis also seemed correlate quite well to your own measurments for the focal redcucer in place.

The C8, C9.25 and C14 (the C9.25 is actuallly the worst of all the designs he ray traced and I think this validates my "Ratio of secondary/effective aperture" comments I made earlier) all started loosing aperture at about 100mm of back focus. Again, this agrees quite will with your own C8 measurement, so even the C14 is not immune to this despite the fact that it is more than twice the size of the C6!

From this one might surmise that the baffling system was design very specificallly in most of thse scopes to be most perfect for the use of the focal reducer at a back focus of 100mm, and that 150mm 200mm was considered enough for visual use.

The C11 did not loose illumination even with 300mm of back focus. With the focal reducer, it also maintained full aperture to about 165mm of back focus, making the C11 the best SCT for people that want to use crayfords or power switch type binoviewers( though likely not with the focal reducer in place).

We each choose our compromises, but with telscopes being so cheap today, it seems to me that a better approach for someone wanting to get a wide field view would be to ante up the money for a cheap dob.

There was a 6" f/5 reflector on the classified today for $160 shipped! OMG! Why would someone buy a focal reduer and a big diagonal and 2" eyepeice to get a view that is half as large as the 6" f/5 and maybe not as well illuminated at the edge of the field when they could get a great rich field reflector for 160 measely American dollars??

I just don't understand it.
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#28 tonyt

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Posted 24 February 2012 - 05:56 PM

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


Piece of cake - just remove the rubber grip. I use a C6 with a WO sct type 2" diagonal.

Tony

#29 bcuddihee

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Posted 25 February 2012 - 09:04 AM

Eddgie, so I'm thinking about all this and like many others would like to optimize mY c8 for bino-viewing with a 2 inch diagonal. It seems to me that increasing the operating focal length simply means that the light cone widens out a bit as it travels from the secondary through the front edge of the baffle. Is this where the clipping of the light cone occurs? If so couldn't a ray trace determine where the clipping occurs and a piece of the front of the baffle be removed so the baffle interference is eliminated? Just a thought, but if this is a cure for the problem, it seems a straight forward and relatively easy one to do. If anyone else has input into this please chime in.
BC

#30 Eddgie

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Posted 25 February 2012 - 10:16 AM

First, there is a ray trace available for the C8. If you would like a copy, please PM me your email address and I will mail it to you.

Next, if the central baffle were made shorter, then the secondary mirror baffle would have to be made longner or off axis light would be able to get into the central baffle and fall directly on the focal plane.

These two baffles work together. If you change one, you have to change the other and if you made the secondary baffle edges longer, then bottom edge of the secondary baffle would now become the part that vignetted the outside edge of the light cone.

There is a way to improve off axis illumination. That would be to use a baffle that is megaphone shaped with the wide end near the eyepiece. This would slighty improve the off axis illumination, but as you can imaging, it would complicate focusing. Either you would have to have a moving secondary (Mewlon does this), or fixed mirrors and focal plane with a focusing tube approach (or something similar).

#31 GlennLeDrew

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Posted 25 February 2012 - 10:57 AM

It would be interesting to determine which contributes more significantly to image dimming, once aperture reduction is in effect. The increase in f/ratio or the reduction in aperture.

For instance, one could first assume that the instrument works always at full aperture, and chart the diminution of exit pupil based solely on the f/ratio derived from the focal length and nominal aperture.

Then do this again using the actual aperture. Compare the difference in the slope, as it were, of image dimming between the two cases.

Naturally, while the aperture is being reduced one must take into account the relative increase in size of the secondary obstruction.

With the data you've compiled, Ed, would you care to take a crack at this?

#32 Eddgie

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Posted 25 February 2012 - 11:06 AM

And this. If you really want to do wide field binoviewing, why not buy a used 6" f/5 reflector and move the primary mirror forward about 5 inches so the system will reach focus with binoviewers.

These scopes can be purchased for as little as $160 used (on just sold on the classified for this price and included shipping!).

Now, you have a system that is better illuminated at the edge than C8 using a lot of back focus, and will give 40x and about 1.6 degrees true field of view using inexpensive 19mm widefield eyepeices!

You could also put the mirror back for rich field viewing in Mono mode. With a 27mm Panoptic, you could get a full 2.3 degree true field with a 5.5mm exit pupil! Of course for this, you would need a 2" focuser.

So, I guess I just don't understand why people would attempt to get a bigger field at the expense of damaging the brigtness and living with very degraded off axis performance when they could just buy a cheap reflector and have a 1.6 degree true field with an exit pupil of 3.9mm

And because the scope could not be used at very low powers, the worst part of the coma would not be visible!

This is just me though. It seems to me that used telescopes are so stinking cheap these days that to have a dedicated or specialzed telescope for wide field biniviewing seems to make perfect sense.

In fact, I have my eyes open for a 6" f/5 scope right now for just this purpose! I really enjoy the Binoviers, but in my 6" APO, I would be limited to a true field that would only be .67 degrees. I can get that out of my C14 with a 41mm Panoptic!!! OMG!

But the 6" reflector could be dedicated to wide field observing where optical quality is not critical, and I could reserve the 6" APO for what it does best, which is the widest most perfect low power observing and high power observing all in one package.

I would think that this approach would be better than a small refractor too, because you get a wider true field than anything but maybe 80mm refractors, but with a far brighter image.

I had considered a 120mm f/8 achromat and cutting the tube, but the extra aperture and wider field of the reflector (at the expense of a little coma, but not much because I would only be using the center of the field) just seems to make much more sense to me.

And a 6" f/5 reflector will mount on anything a C8 will mount on.

I just don't understand the rational for trying to turn a C8 into a wide field scope when there are just so much better options.

And for wide field binoviewing, I don't think you can do better than a 6" f/5 dedicated reflector.

Just sharing the way I think about things...

My bet is that the more people think about this, the more we will be seeing 6" f/5 dedicated binoviewer scopes.

I know tha after looking at all the possibilites and doing the math, it stands out as the very best way to employ binoviers for wide field work. Far better than trying to squeeze out a tiny bit more true field in a C8, and to my way of thinking, even better than a cut down refractor.

Think about it.

#33 EdZ

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Posted 25 February 2012 - 11:33 AM

It would be interesting to determine which contributes more significantly to image dimming, once aperture reduction is in effect. The increase in f/ratio or the reduction in aperture.

For instance, one could first assume that the instrument works always at full aperture, and chart the diminution of exit pupil based solely on the f/ratio derived from the focal length and nominal aperture.

Then do this again using the actual aperture. Compare the difference in the slope, as it were, of image dimming between the two cases.

Naturally, while the aperture is being reduced one must take into account the relative increase in size of the secondary obstruction.

With the data you've compiled, Ed, would you care to take a crack at this?


Well, unfortunately, I can't get wrapped up in testing like I used to, although I do have a plan to test operating F at each of the congigurations I used, since I already have some of them. Aslo I have a few more configurations which will provide more data points.

On another note, I think you might have to question the validity of the ray trace for the C8 in that article, since I've already found several data points that don't agree with data in that article.

Based on what I've already forund for the C8, that it is still operating within 1-2mm of full aperture with 240mm of back focus. And I know the C8 with a binoviewer at 370mm of back focus is operating at 190mm, a loss of 13mm, 1/2 of an inch. So, I removed the GSO focuser from the C8. By doing so, I'll have a 110mm SCT diagonal that with care provides rotation, and my 134mm BV, for a total of 244mm back focus. Now I'll have my 26mm clear aperture binoviewers on my C8 operating at 200mm or better aperture and at a shorter focal length that I had previously.

and even another note, I don't care to buy a f/5 Newt to modify it. But thanks for reminding us that it is (was) available. I had an f/5 6" Newt. I sold it. But I didn't start this thread to discuss my likes or dislikes in scopes, or to develop a wide-field plan. My purpose is to identify aperture loss in the C6 and C8.

Based on the dicussions in another thread related to this topic, I showed tests results which disproves the often heard comment that the narrow exit tube limits the field of view. We know it has an affect on illumination of the field, that that's not how some people refer to the issue. There's probably a lot of relevance and worth to have the operation F, the aperture reduction and the operting field of view for every data point, but it's a lot of work. If it were my day job, I'd do it.

It was an easy fix to remove the rubber from the focus knob shaft, cut off about a half inch and replace it. I've already removed the 30mm extension sleeve so I can use just my screw-on SCT 2" diagonal on my C6. It's a perfect fit. Clears by an 1/8 to 1/4 inch. Oddly enough I never tested the operating focal length for this configuration.

Really, all I wanted to do is show real data for aperture loss, when there is no loss and when it starts and by how much. I will be posting the actual data using the C8 for all the same configurations. And I do plan to develop the data for the operating F for each scope at each configuration. After that's done Glenn, maybe we can discuss what to do with it. But I gotta get there first. And there's that day job. :>)

edz

#34 Jason Martin

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Posted 25 February 2012 - 12:57 PM

If you really want to do wide field binoviewing, why not buy a used 6" f/5 reflector


but then...

I had considered a 120mm f/8 achromat and cutting the tube, but the extra aperture and wider field of the reflector (at the expense of a little coma, but not much because I would only be using the center of the field) just seems to make much more sense to me.


It looks like you are saying a 6" f5 would be great for wide field, but the off axis doesn't matter because you would only be using the center of the field... isn't that a little self defeating?

#35 erraph

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Posted 25 February 2012 - 01:35 PM

Good evening,

I made the same measurement for an orange C8, using a Baader 2" lock,a 2" Televue diagonal and a Baader 2" -> 1.25" adapter. These sum up to about 17cm of backfocus, including the SCT-thread at the back of the scope.

I measured an aperture of 20.0cm +- 2mm, which in the worst case means a loss of 5% of aperture.

Could someone please mirror the pdf-document of Ken Hutchinson measurements? (Not feeling like registering at yahoo just do access the file...)

I actually have a question. While modern C8s have a flat back, the old ones have a "bulge" sticking out on which there is the SCT-thread. Thus do the "old" C8s have per se about an inch more of "forced" backfocus due to the differnt style of the mirror casing?

Raphael
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#36 bcuddihee

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Posted 25 February 2012 - 01:40 PM

First, there is a ray trace available for the C8. If you would like a copy, please PM me your email address and I will mail it to you.

Next, if the central baffle were made shorter, then the secondary mirror baffle would have to be made longner or off axis light would be able to get into the central baffle and fall directly on the focal plane.

These two baffles work together. If you change one, you have to change the other and if you made the secondary baffle edges longer, then bottom edge of the secondary baffle would now become the part that vignetted the outside edge of the light cone.

There is a way to improve off axis illumination. That would be to use a baffle that is megaphone shaped with the wide end near the eyepiece. This would slighty improve the off axis illumination, but as you can imaging, it would complicate focusing. Either you would have to have a moving secondary (Mewlon does this), or fixed mirrors and focal plane with a focusing tube approach (or something similar).

Not sure I agree here...a slightly shorter main baffle tube may let some stray light in but heck in a newt there are no baffle tubes and stray light doesn'seem to be a biggie especially if the tube is flocked.
EdZ..ant thoughts as to the idea of shortening the baffle tube to minimize light cutoff as the fl increases, or am I missing something here?
Bc

#37 Eddgie

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Posted 25 February 2012 - 02:32 PM

I believe that the back focus is measured from the flat at the rear of the SCT port. In other words, if you put a flat card across the rear opening, this would be 0mm.

On the C11 and C14, this would be at the rear of the port with the 3.25" adapter removed.

I am just about positive that this is the position Ken Hutchinson used in his analysis because this is the last point in the telescope itself that could vignette the off axis rays. He also does a different ray trace for the C11 and C14 with the 3.25" adapter on and off, and again, because this is the last point where the telescope itself can vignette the off axis rays when the reducer is used on the C11 and C14.

If this were the case, and this means that your back focus was for example 12mm less than what you measured (if the flange were 12mm of course). So, if you measured 200mm from the rear cell, then if measuring from the flat at the rear of the port, then the back focus would be more like 188mm.

Here is an example where Rohr has measured the effects on strehl for a system with different levels of back focus.

Notice that the scope is a C11 and he measured from the rear face of the 3.25" plate, which would approximate the position of the rear of the baffle tube opening.

Rohr measuring back focus

In reality, I think it would be more accurate had he measured from the flat at the rear of the opening, but this way, he is only 3mm or 4mm off (the thichness of the plate at the rear of the 3.25" plate).

I have seen in the past other references that also mentioned that this was the correct way to measure back focus: from the first possible point where the focal plane can be accessed.

This is because this is as close as you would normally be able to get a full aperture corrector lens.

Accordin to Ken Hutchinson's analysis (and he did not have a C8 for measurement, so he had to work from somone elses measurments), this would give you about 7.85" of aperture and about a 5% illumination loss.


I can email you Ken's paper if you PM me your email address. I tried to attach it here but it is 2m in size.
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#38 hopskipson

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Posted 25 February 2012 - 03:10 PM

Hi all

Very interesting thread even to a newbie like me :bow:. I have a c8 that I want to add a binoviewer. I would like to minimize the loss of apperature. I already have a 2" gso diagonal if I add a Williams Optics or Orion BV'er will I run into this problem? The Baaders look great but a bit out of my $$$ range.

"I just don't understand the rational for trying to turn a C8 into a wide field scope when there are just so much better options."----Not all of us have the room or significant other patience :grin:for any other options. Just my 2 non-cents.

James

#39 EdZ

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Posted 25 February 2012 - 03:19 PM

Hi all

Very interesting thread even to a newbie like me :bow:. I have a c8 that I want to add a binoviewer. I would like to minimize the loss of apperature. I already have a 2" gso diagonal if I add a Williams Optics or Orion BV'er will I run into this problem?
James


No you will not run into any problem. You will be operating at full aperture.
See my last post prior to this.

edz

#40 EdZ

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Posted 25 February 2012 - 03:23 PM

I made the same measurement for an orange C8, using a Baader 2" lock,a 2" Televue diagonal and a Baader 2" -> 1.25" adapter. These sum up to about 17cm of backfocus, including the SCT-thread at the back of the scope.

I measured an aperture of 20.0cm +- 2mm, which in the worst case means a loss of 5% of aperture.



Did you mean to say
I measured an aperture of 20.0cm +- 2mm, which in the worst case means a loss of 0.5% of aperture.

edz

#41 EdZ

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Posted 25 February 2012 - 03:28 PM

I have seen in the past other references that also mentioned that this was the correct way to measure back focus: from the first possible point where the focal plane can be accessed.

from the flat at the rear of the SCT port. In other words, if you put a flat card across the rear opening, this would be 0mm.



By adding each individual piece length that makes up the configuration behind the scope, that's describes exactly what I did.

edz

#42 erraph

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Posted 25 February 2012 - 03:59 PM

Good evening edz!

No, I meant 5%, as 198^2/203^2 makes 0.95 :)

#43 EdZ

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Posted 25 February 2012 - 05:13 PM

Good evening edz!

No, I meant 5%, as 198^2/203^2 makes 0.95 :)


I see
so then you didn't mean to say
"in the worst case means a loss of 5% of aperture"

perhaps you meant to say
a loss of 5% of area of aperture or light gathering.

Thanks for the clarification
I tripped over the reference to aperture

edz

#44 Ed Holland

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Posted 26 February 2012 - 01:39 AM

This is a great thread, it clarifies understanding, corrects some received wisdom and provides some concrete figures - Thanks everyone.

One potential addition to the data here would be some info for users of the Orion/Skywatcher/Synta 5" Maksutov design that is popular. I hope to find time to perform the effective aperture test on my telescope, and think this might be of value to other readers.

I have 3 setup combinations to hand, based on the accessories to hand:

1) Adapter threaded ring and 1.25" SCT diagonal from a C8

2) 2" adapter tube and Orion 2" mirror diagonal for refractors.

3) As supplied visual back and 1.25" Lumicon erecting prism. This has restricted aperture compared to a 1.25" mirror & results in abrupt vignetting with eyepieces of longer focal length (35mm Parks Gold Series).

Let me work on this tomorrow. I'll also try and evaluate the backfocus distances for each configuration. Up until now, configuration 2 has been used most frequently. It is noteable, however, from the effect on magnification that it does push the f ratio up. Now I have a good 1.25" diagonal available it makes sense to try it.

I could start another thread with the data if that is preferred, or add it here if that is considered on topic?

Cheers,

Ed

#45 EdZ

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Posted 26 February 2012 - 07:16 AM

That certainly would be on topic. You should decide whether a specific thread titled accordingly will direct readers to more easily find data for that scope. It's a toss-up, keep subject matter in one place, or keep scope models in one place.

There's already discussion in this thread on several other sizes, so adding you 5" Mak would be in keeping with that trend. If I had my C5, I think I'd add it here.

edz

#46 freestar8n

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Posted 26 February 2012 - 07:18 AM

I'm not sure this is such an accurate way to measure aperture because if there is something near the focus that is acting as a stop, the entrance pupil will be shifted far back from the corrector, making the measurement of the diameter very sensitive to any divergence of the beam. I think that if you identify the structure that is blocking light you can directly calculate the entrance pupil diameter more accurately. This also makes the measurement independent of the eyepiece or anything beyond the stop itself, but it does require you to identify what is causing the restriction, e.g. the 27mm tube or something. You should be able to tell this just by looking through the front of the 'scope with the eyepiece removed.

You can measure the effective focal length very accurately with a ccd and a plate solve. Otherwise you can use star timing with an eyepiece as long as you know its focal length and apparent field accurately.

If you measure the effective focal length, f, and you know both the focal point and its distance, x, from the structure acting as an aperture stop, then the entrance pupil diameter will be f/x*d, where d is the diameter of the stop - since it is simply re-imaging the stop and magnifying it to the size it appears in object space. If that value is greater than the corrector diameter then the corrector is defining the entrance pupil and you don't have on-axis vignetting. If that value is less than the corrector diameter, then *it* is the entrance pupil diameter, exactly. When you use a reducer, f is decreased which makes the apparent size of the stop smaller - thereby increasing its chance of restricting the aperture.

A standard way to measure entrance pupil diameter is by facing a microscope/telescope into the front of the lens system and focusing on the image of the stop as formed by the preceding elements of the lens system. You then translate from one side of the stop to the other and measure the separation. This only depends on the stop itself and the lenses preceding it - which is all that determines the entrance pupil location and diameter.

In some cases x and d are both fixed and you can modify the setup to change the effective focal length. In that case you can calculate the maximum focal reduction that starts to give on-axis vignetting.

Frank

#47 Derwin Skotch

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Posted 26 February 2012 - 02:11 PM

Thanks for starting this thread, I have long wondered exactly how much aperture reduction was taking place and now have some idea, and ideas on how to measure it.

BTW, I know it's off topic here, but I have read in the binoviewer forum (and now here) how great it would be to shorten a low f ratio scope to reach focus. A simple ray trace will show that a shortened 6" f 5 scope coupled to a bino with 25mm entrance aperture and an optical path length of 120mm will have a fully illuminated field diameter of only 1.2mm. Is this really better than what can be accomplished with a SCT?

#48 GlennLeDrew

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Posted 26 February 2012 - 02:56 PM

Frank,
For on-axis measurement of effective aperture, the flashlight test is quite accurate. Here's why.

The eyepiece forms a very small image of the light at the common focal surface (when the scope is focused at infinity--important) and on axis. This image sends light back through the system 'in reverse', and any restrictions reveal themselves as a reduced-in-diameter collimated bundle exiting the objective. Because the light source is on axis, no near-to-focus restrictions have any impact whatsoever.

To appreciate this, shine a light into the eyepiece from about a foot away and on axis. Look into the objective to see how tiny an image of the light is formed by the eyepiece.

It's worthwhile to point out that the smaller in size the light the better, for it will provide a closer to point-like image, which makes for a sharper edge on the emerging bundle and hence improved accuracy.

If the light is aimed into the eyepiece from off axis, one can explore the off-axis vignetting. For example, if a 2", 30mm f.l., 80* AFoV eyepiece with 46mm field stop is inserted, one could aim the light from as far as 40* off axis, and the light source at the focal surface will be located some 23mm off axis. Now the exiting beam will be tilted 1/2* or more off axis, and it will be non-circular due to asymmetric clipping. The surface area compared to that of the on-axis, circular bundle, gives us directly the illumination at the focal surface for that off-axis distance. And if the light is aimed into that same eyepiece 20* off axis, the corresponding off-axis distance is about 11.5mm, assuming a linear relationship between linear off-axis distance and angular offset as seen in the eyepiece.

This is the basis of the method I alluded to earlier, and which Jon suggested I elucidate. I would be inclined to try its variant, which bypasses the eyepiece and instead places a tiny light source directly at the focal surface and allows off-axis displacement to arbitrarily large distance beyond the 46mm circle of an eyepiece field stop.

#49 GeneT

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Posted 26 February 2012 - 03:41 PM

Excellent report! Thanks for issuing it.

#50 freestar8n

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Posted 26 February 2012 - 04:07 PM

I understand the assumptions being made, and the key one is that the beam is "collimated" when the system, including the eyepiece, is "focused at infinity." My point is to emphasize that the location of the entrance pupil being measured has no correspondence with the "front of the telescope." The actual entrance pupil could be 10 meters behind the front of the telescope, which means that the beam could easily be off from the true diameter it has at the actual pupil.

I think the method is basically ok for a ballpark value, but it does rely heavily on the assumption that the beam is perfectly collimated and easily measured even far from the entrance pupil. If you can find it recommended in an optics text I'd be interested to see it. There are many tricks for finding things like nodal points, and the method I described using a microscope focused on the entrance pupil is the main technique I'm aware of to measure aperture.

My other point is that there is no need to measure it optically when you can make a few physical measurements and know the value exactly.

Frank


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