Aperture in C6 & C8 for various configurations
Posted 26 February 2012 - 07:14 PM
I chose 3 configurations for the "flashlight" test. Back focus in each case was measured from the edge of the threaded flange to the eyepiece field stop (+/-2mm). The eyepiece used was a 10mm Sirius Plossl. Refocussing to infinity was performed between each change in back focus distance. The disc of light from the OTA was projected onto white paper approximately 300mm (12") from the front of the scope to permit measurement (+/-1mm).
a) 2" adapter tube and 2" mirror diagonal, approx back focus 155mm
b) 2" adapter tube without diagonal, approx back focus 60mm
c) 1.25" SCT mirror diagonal and 1.25" SCT visual back, approx back focus 126mm
The three cases represent the most sensible use of the accessories I have to hand. A 90° erecting prism I own was not included, but would give similar results to the other 1.25" setup.
The results are very interesting on two accounts
Setup - Bck Fcs --------- Measured Aperture ----- CO shadow
A ----- 155 -------------- 117 ------------------- 45
B ----- 60 --------------- 118 ------------------- 45
C ----- 126 -------------- 118 ------------------- 45
(all data in mm)
Setup A, longest backfocus, was measured first and I thought "Aha there's the aperture reduction". However, I was surprised to see that the measurement changed very little, if at all for ANY of the setups.
The measurements lead me to two conclusions:
1) This telescope is really operating at 118mm effective aperture, some 9mm less than the advertised specification
2) It is seemingly tolerant of the additional back focus present when employing 2" accessories.
On point 2 I MUST add further comment. A test suggested by Eddgie some while ago revealed that there is vignetting as one moves to the edge of field with long focal length EPs. This can be detected by observing the ring pattern of a defocussed star, then moving the scope to place this image at different positions in the apparent field. Vignetting can be detected where the concentric ring pattern is disturbed, and this behaviour has been noted in the past with a 35mm EP in the 2" + diagonal setup.
I hope this is interesting,
Posted 26 February 2012 - 08:03 PM
Here's the results
For a range of eyepieces that have field stops from +7/-6 from zero (zero at the shoulder), the scope varied in operating focal length by about 30mm to 40mm for each configuration. When I was able to use both 2" and 1.25" eyepieces, becasue it involved removing the 10mm thick adapter to use the 2" eyepieces, that range varied by about 60mm.
The scope operates longer for eyepieces such as small orthos that have the field stops up inside the eyepiece housing. It operates shorter for eyepieces such as a 32pl that have the field stop well out in front of the shoulder. What this means, for example, when placed in a 1.25" VB and a 1.25" diagonal, a 32mm pl is operating at F=1625 while a 6mm ortho is operating at F=1660.
At each configuration, the average of all the 8 to 10 readings was always within a few mm of the reading taken with the 12.5mm Ultima. That Ultima has the field stop less than 1mm in front of the shoulder.
with a 1.25" visual back (35mm), the scope is operating at F=1400 +/-15mm
with a 1.25" visual back (35mm) and a 1.25" TV dielectric diagonal (75mm), the scope is operating at F=1650 +/-20mm.
With a 2" screw on diagonal (130mm), the scope is operatiing at F=1730 +/- 30mm
With a 2" extension (30mm) and a 2" screw on diagonal (130mm), the scope is operatiing at F=1830 +/- 30mm.
Having modified my rubber knob, I don't need the extension anymore.
with the 2" screw-on diagonal (130mm) and the Denk Binoviewer(134mm) (set to no OCS), the scope is operating at F= 2170 +/- 15mm
It was found previously (see the 1st post) that the scope does not have reduced aperture until configurations result in more than 200mm of back length. So for all of these except the Denk BV, they are operating at full aperture.
The Denk Binoviewer was found to be operating at 136mm.
2” SCT Diag (130) + Denk BV (134) = (total 264back length). This netted F= 136mm thru the Denk.
Posted 27 February 2012 - 12:17 AM
Why is it required to know where the entrance pupil lies? As long as one is reasonably careful to set the focus for infinity, and locate the light source sufficiantly distant from the eyepiece, the light transits the system exactly as it does when in use. The emerging bundle from the objective is quite parallel enough to obviate concern if the target screen is any reasonably close distance from it. One could verify parallelism by taking measurements from near and far to see if there is convergence or divergence. Indeed, if not parallel one could adjust focus 'blindly' until the emerging light is collimated. And that's all that's required; parallel light coming out the objective end.
Again, I stress that this test is perfectly valid, and enjoys the simplicity of requiring no optical knowledge, and equipment no more sophisticated than a flashlight and ruler.
Posted 27 February 2012 - 07:29 AM
You could measure the diameter of a disk of paper the same way, by the shadow it casts in a beam of light. It will be harder to do well if the disk is far from the shadow, and it requires the beam to be parallel - plus it requires some perceptual judgement regarding how to define the edge of a blurry shadow.
So - I agree it should give a ballpark value, but it definitely has issues. If you aim a small telescope with a reticle into the front of the main telescope, and focus carefully on the edge of the entrance pupil, and move from right to left on a measuring stage, then you are measuring the entrance pupil itself while it is in focus and you have no requirement that the illumination be perfectly parallel.
On the image side, near the focal point, everything is simpler if the offending stop has no lenses between it and the focal point. In that case, the exit pupil is the stop itself and the vignetting condition is just based on whether or not that stop, as seen from the focal point, restricts the full f/ratio cone of light being delivered, i.e. d/x < F.
This helps visualize the various factors involved in vignetting for an SCT. The e.f.l. changes with primary spacing, and if it gets longer then the the f-number goes up and it is harder for a stop to cause vignetting. The opposite is true with a reducer. Anything that pushes the focal point away from the stop will increase the chance for vignetting.
Posted 27 February 2012 - 10:49 AM
Unless the image of the light formed by the eyepiece is on the large side, and/or the restricting aperture is rather near the focus, the emerging light bundle is actually pretty sharp-edged, and poses no difficulty in measurement.
In the on-axis condition, which is what Ed's tests have been restricted to, the location of the entrance pupil is irrelevant in any event. That's because our concern lies solely with the paraxial rays.
To be sure, entrance pupil location is of potential concern when shifting the light source off axis, but even then for real world testing of this sort it is sufficient to assume it lies at the objective or corrector plate at the front. This will not introduce an error of any significance where it's desired to know relative illumination to within a few or ten percent.
Posted 27 February 2012 - 03:44 PM
I don't have nearly as much data points for the C5 as I developed for the C6. However, quite a few years ago I drift timed about 10 eyepieces using just a 1.25" diagonal for a baseline and then in three different binoviewers. At the time I was testing to see when the clear aperture of the binoviewers completely cut off the field of view in the eyepieces. Just as we have issues not knowing data for SCT scopes, people had and still have issues not knowing that they are not seeing the entire fov in a BV because in some cases the binoviewer acts like a new field stop. That's another topic.
At any rate I went back and found my file witth all that drift timing data. Unfortunately, I only get to use about a dozen data points out of the 50 or so points recorded. For purposes of determining focal length, I couldn't use any data points that were vignetted by the binoviewers. I added those to data from several other scattered files I have. And if I find more, I'll add them to the plot. Anyway, here is a plot for the C5. I do not have aperture data to determine when the C5 starts to reduce.
I've used the C5 in numerous configurations.
with a 1.25" visual back (35) and a 1.25" diagonal (75) it is operating near F=1370.
with an 30mm long extension sleeve and a 2" SCT diagonal (127) it's operating near F= 1530
and with a BV (~110) in a 1.25" visual back (35) and a 1.25" diagonal (75) operating F is near 1700mm.
Once again, the eyepieces make a difference. If the eyepiece has the field stop way out in the chrome sleeve, then that eyepiece will be operating at a shorter focal length in the same diagonal as a short UO ortho or any short eyepiece with the field stop behind the shoulder.
There is some variation in the data, because the binoviewers are not all the exact same length. I hadn't thought to measure the focal path length of each binoviewer, but they ranged between 100mm and about 115mm for the Denk Standard (Big Easy version). all of these pieces of equipment are long gone, so I can't check. But you can see the BV data is grouped fairly well up around F=1700. That's three different binoviewers and all BVs without OCS in the path.
Posted 27 February 2012 - 05:28 PM
In the on-axis condition, which is what Ed's tests have been restricted to, the location of the entrance pupil is irrelevant in any event.
I think you are missing my whole point. Everything I'm talking about regards on-axis vignetting, and the location of the entrance pupil affects how blurred the shadow is. I am talking precisely about the assumptions of your test, and I'm not talking about the entrance pupil shifting off-axis. I am talking about the location of the entrance pupil *along* the axis.
I did some tests with an 8" sct and I want to be clear on the technique you are using. Do you use a single LED or an actual flashlight with a reflector around a bulb? Do you hold it with your hand or mounted on a bracket? What kind of eyepiece?
Do you specifically focus the system by eye on infinity and leave it that way, or do you adjust the focus so the beam is coming out parallel and does not appear to change size?
Note that the reported measurements are very precise, +/- 0.5mm out of 150, which is 0.3% - not the 10% you mention.
Thanks for any info-
Posted 27 February 2012 - 07:27 PM
I use a single-LED source of small linear width.
It's held in position on axis, and distant from the exit pupil by at least 10 eyepiece focal lengths.
If I have a choice, I use an eyepiece of middling focal length. This offers the best compromise, whereby the image of the light formed at the focus is both sufficiently small and bright, and focus can be reliably set.
In each instance, focus is set visually for infinity with the eyepiece in place.
The 10% figure I gave was the expected maximum error on the off-axis illumination as compared to that on axis, not the diameter/size of the illuminated beam exiting.
By imaging what amounts to a near point source at the focus, this source serves as a surrogate for a star image produced by the objective. It necessarily follows that the light transits the system in reverse, following the same path as from a point source lying at infinity. If there is a restricting aperture, it effectively becomes the entrance pupil and its shadow restricts the diameter of the collimated beam exiting the objective.
Here's perhaps a better way to visualize things. Light from a star arrives at the objective, necessarily parallel. This light is then refracted/reflected as it transits the system until it is brought to focus as a point image. That restriction which impinges most on the light bundle determines the effective aperture, which is the diameter of the entering, parallel bundle at ANY distance in front of the first optical element encountered. This distance could be anywhere from the surface itself out to beyond the solar system. To the incoming starlight there is no entrance pupil. There is only either a free path to the focal surface or some obstruction.
This is the strength of the flashlight test, if done correctly. It replicates in reverse the passage through the system of light from a source at infinity. Parallel light in = a point image at focus. An effectively point-like light at focus = parallel light back out. The small light source ensures a sufficiently sharp-edged bundle of diameter exactly equaling the effective aperture, which should be reliably measurable from as close as the frontmost optical surface out to some number of feet (closer is better, though).
Not only in the on-axis condition is the concept of the entrance pupil of no consequence. For it's only when ray-tracing for two or more angularly separated sources does the entrance pupil enter into the picture. For any single source imaged at the focus, there is no such thing as an entrance pupil, outside of that restrictor (or restrictors) which limits the size of the entrant light bundle.
And so a sufficiently small light source anywhere on the focal surface well replicates the passage through the system of light from an infinitely distant point source. The size and shape of the emerging bundle exactly enough describes the effective aperture for a source at the corresponding field angle.
This is the key. As we're concerned at any one time with only a single image source/point, the entrance pupil is obviated entirely. And so one wishes to form as small an image of the light at focus as possible. I think less than 0.5mm should suffice. A suitable distance to place a light from the eyepiece then must be at least 10 eyepiece focal lengths for a 5mm diameter source, or 20 focal lengths for a 10mm source, etc.
Posted 27 February 2012 - 09:12 PM
Parallel light from a single star emerges out the eyepiece as a parallel pencil of light. If one placed a screen behind the eyepiece, no matter the distance the same-diameter illuminated circle would be seen. So where is the exit pupil? There is none for a single point source. Your eye could be placed almost arbitrarily distant from the eyepiece and that star would be as well seen as long as the eye is properly in line.
Now we consider the light from a second star some angular distance from the first. The pencil of light from it emerging out the eyepiece will be offset from the first pencil, crossing it at some distance behind the eye lens. Now we have an exit pupil, which is the location at which the eye's pupil must be in order to see both stars simultaneously and fully illuminated.
In similar vein, the entrant bundle for any single point source of light accommodated by the system has no entrance pupil outside of the aperture limiter(s). It's only when considering two or more sources together does the entrance pupil as a system stop become meaningful.
Posted 27 February 2012 - 11:24 PM
Likewise, I don't have nearly as much data points for the C8 (Yet) as I developed for the C6. However, over the last two years I did several drift timings to establish the operating parameters for my C8. Because I have farily decent notebooks, I was able to collect all of that here.
As I develop more data, I'll add to the plot, but I wouldn't expect it to change much. Here is a plot for the C8.
I've used the C8 in numerous configurations.
1.25" visual back (35mm)
2" SCT diag (119 to 127mm) depending on use of 1.25" insert or not.
30mm extension (30) and 2" SCT diag (119 to 27)
GSO focuser (100 to 115) + ref diag (100 to 110)
30mm extension (30) and 2" SCT diag (119 to 127) + Denk no OCS(134)
GSO focuser (100 to 115) + ref diag (100 to 110) and Denk no OCS (134)
There are data points for each configuration. You can see that sometimes a single configuration can vary by as much as 25mm, depending on Crayford focuser drawtube and use of 1.25" diagonal insert or 2" eyepieces. Couple that with the fact that eyepieces can have the field stop vary in position by 20mm and that can mean as much as a change in operating focal length of 75-125mm at one single configuration. That sort of variation does not occur with all configurations, for instance the only variation you could get with the use of just the visual back would be only that due to eyepiece field stop. All the same notes related in the C6 plot post are relevant. Eyepieces make a difference, so there is a range at every configuration, dependant on the position of the field stop.
Posted 27 February 2012 - 11:27 PM
Has it even been taken apart?
Is there a way to shorten the central baffle?
If the baffle tube is threaded at the base and held to the mirror back with a nut, perhaps it would be possible to shorten the baffle by loosening the nut on the rear and threading the baffle in a few millimeters.
I really have no idea of how these things are put together, but this would make sense for it to be done this way.
It might only take reducing the baffle length 4 or 5 millimeters to resture the full aperture.
If this were to be the case and it resulted in recovery of the full aperture, perhaps it would be a case where the assmebly line people were not properly trained.
If the Meniscus corrector is full aperture and the mirror is full aperture, then it would seem that the only possible source of the vignetting would be an overly long secondary baffle (unlikely) or a central baffle tube that is sticking out to far.
If you ever decide to research it, please PM me and let me know what you found out. I would be very interested to know if this is a design errror or a fabrication error.
Posted 28 February 2012 - 06:31 AM
So where is the exit pupil? There is none for a single point source.
I really don't know where to begin. The pupils have nothing to do with the object being looked at. The exit pupil is the image of the aperture stop in image space formed by any intervening lenses - period. It has a size and location in image space. For a normal afocal telescope, it is just behind the eyepiece.
If you have an interior aperture near the focus of the telescope that is acting as the aperture stop, then the exit pupil of the overall system will be shifted back slightly - but it will still have a well defined size and shape in image space.
The entrance pupil, on the other hand, will be dramatically shifted deep within the telescope in object space. This shift places strict demands on the collimation of the beam and the finite size of the light source when using your method because the shadow is being thrown a long distance from the pupil.
I'll do some more tests using your parameters with a 8" sct and post some results and explanatory diagrams of the layout in image space.
Posted 28 February 2012 - 10:45 AM
[EdZ: take note of the following...]
Here's one way to do that, and probably achieve higher brightness at the exiting bundle. Use a green laser and a beam expander. Don't have the latter, you say? But you do! A finder or bino will serve perfectly. Have it focused for infinity, aim your laser into the eyepiece, and coming out the objective is a collimated laser beam whose diameter is larger by an amount equal to the magnification.
The reason for doing this is only to fill the exit pupil with light, for this will then ensure that the objective is fully illuminated. A laser alone will only partially fill the objective if the exit pupil is larger than the beam. (This suggests that on the absence of a beam expander one could install an eyepiece which delivers an exit pupil less than or equal to the beam width.)
The laser/beam expander will present to the telescope a very small source, which is further shrunk by the eyepiece. This should deliver pretty sharp shadows for obstructions rather near to the focus. Note that it does not matter at what distance the laser placed with respect to the beam expander's eyepiece, or at what distance the beam expander's objective lies behind the scope's eyepiece.
If you anticipate some maxim exit pupil on the scopes you will test, the beam expander need not deliver a larger exiting laser beam. If 7mm is the maximum and your laser's beam width is 1mm, a 7X or 8X finder or bino will do.
Posted 28 February 2012 - 05:48 PM
The actual values for a 200mm f/10 'scope with a 25mm eyepiece, where you have vignetting caused by a 20mm stop 250mm away from focus, would have the entrance pupil shifted 14 meters (a long way) into the telescope, with a diameter of 160mm. If the bulb is 5mm, it would become 357mm at a distance of 1.4km. This would cause blurring of 3.5mm all around the shadow. The taper of the beam would be small, but only if everything is set up perfectly. If the focus is slightly off, then any taper of the beam would be projected over 14m, making the size of the shadow unpredictable.
The exit pupil at the eyepiece plays a role because any light that lands outside it will not end up illuminating the disk. With a 25mm at f/10, the exit pupil is only 2.5mm - which means if a bulb is 10" away, only a small fraction of its light goes in that little hole - and that light ends up filling a space about 200mm in diameter, with a reduction of irradiance of (200/2.5)^2 or 6400.
So I took an 8" sct and placed a wire inside the flange on the back of the OTA, so it would show as an obstruction in the shadow. If the shadow is clear it will have a sharp edge. I found that in order to illuminate it well I had to place the bulb fairly near a 20mm eyepiece - and at that point the shadow of the wire was blurred, meaning any shadow would be hard to measure.
In terms of focusing the eyepiece "at infinity" - that is somewhat ill-defined because normally people focus to a comfortable location much closer - perhaps even 20" away. If there is any error in that focus value then the beam will be tapered, and that taper will change the projected shadow size over the 14 meter throw distance.
So these are all my concerns for this measurement, and the reason the recommended way I have seen to measure aperture is with a microscope/telescope looking in. If measurements are made by this method, I would check them with physical measurements of the actual stop diameter and its distance from focus, which is fairly simple geometry that can be measured directly, and will tell you the entrance pupil size and location.
Posted 28 February 2012 - 07:32 PM
After all, the image of the light should be easy to keep under 1/2mm.
As to focusing by eye. This why a not so long focal length eyepiece is desirable. For then the longitudinal error on eyepiece placement should not exceed a couple of millimeters. This degree of error is of little consequence.
Posted 29 February 2012 - 03:06 AM
On the other hand, directly measuring vignetting is straightforward if you know the focal length and focus location for a given setup. Does the baffle tube vignette? What is its diameter and how far is it from the focal point? If that ratio is less than the current f/ratio, then it vignettes. It only becomes messy if there is a lens between the suspected vignetting stop and the focus point - but for an sct there usually isn't one.
It's also important to realize that you can look in the front of the telescope and directly see the entrance pupil if there is a stop back there that is vignetting. It will appear very small and far away - but the smallness is only due to its distance in object space. It is actually large, presumably a bit less than 200mm, and about 14 meters away. You will see all the possible stops receding into the distance, and any one of them could cause vignetting - but you need to know its actual size and distance in object space to tell which one is really smallest. Usually there are only a few options possible from the physical layout, and you just need to measure their sizes and distances from the focal point.
Posted 29 February 2012 - 06:17 AM
Posted 29 February 2012 - 07:55 AM
Is there a spec sheet somewhere on binoviewers out there on the backfocus distance ?
What are Televue, Mark V or the Denk II with powerswitch ?
I have Denk II with powerswitch that I've used with SCT's
and will have to check it's performance on various SCT's I use it on. If used on a C11, it may still be good but not on anything else: C5, C8, C6, C9.25 or C14, Meade 2045, JSO125, Parks Jovian 4 or how about on ETX or Questar ?.
I think there is a 1.25 inch nose piece that I can use on the Denk II and bypass the powerswitch (which is a huge convenience !) May still not cut it....
Should I use my Televue with a 1.25 diagonal instead or
the Zeiss/Baader Mark IV o Mark V as my other choices.
I'll have to get around to getting a test setup...
Posted 29 February 2012 - 10:20 AM
On my C14, I think I mesured 30mm for the AP visual back, about 105mm for from the face of the Televue 2" diagonal mirror box (don't measure the nose because it overlaps inside the AP visual back), about 18mm for the power swithc box (again, you don't include the Bino nose piece because it overlaps the top of the diagonal) and about 118mm for the bino itself.
I measured about 270mm of back focus in this configuration.
The view with the Denkmeiers Supersystem appeared very dim to me, and Ted Hutchinson's SCT vignetting analysis (a ray trace that he created based on measurements provided by Celstron) showed the system working at about 13.25" of effective aperture. Planets were noticeably dim.
Some of this I took as bino dimming, but having owned a C11, I could have sworn that for a given magnificaion the view didn't really look any brighter than the C11 using monovision!
I went to the Baader Maxbright with the T2 diagonal. In this configuration, I only have about 180mm of back focus, and I could immediatly see a noticeable improvement in image brightness.
To me, planetary observing was noticably improved by keeping my full aperture because the image was about 15% brighter (using similar magnificaitons). For me, a brighter image has always been a big asset for seeing the lowest contrast detail, so I thought the image was much better when I restored my full aperture.
While Ken's Vignetting analysis may not be exact for the C8 (he used some measurements that were not accurate for that scope) he had direct measurements for the C9, C11, and C14, so the ray trace should be closer for those scopes.
All of the SCTs he traced except the C11 were loosing a considerable amount of effective aperture with 270mm of back focus. The C9.25 was the worst case.
If you want a copy of this, PM me your email address and I will send it to you.
Posted 03 March 2012 - 10:32 AM
I tried to take this reading on the ruler directly, but could not see any difference in the light shining thru the ruler.
I then set up a white board some 6 or 8 " in front of the aperture and projected the light onto the white board. At full aperture the circle of light was quite sharp. I set up to read on a reduced aperture configuration and could see that the projected light was a little bit fuzzy in the outer 2-3mm. I could not possibly have seen that directly on the ruler. While just a bit fuzzy, it did not hamper me from getting a decent measure to the outer edge of the projected light.
Posted 03 March 2012 - 11:56 AM
cast by a light source of discrete size.
I'd love to see a result from a beam-expanded laser. Or for simplicity make the exit pupil about 1/2mm and use the laser directly. Again, given that the laser light is collimated, it can be placed right up to the exit pupil.
Posted 03 March 2012 - 01:16 PM
Since I've been testing both aperture reduction and operating focal length in a C6 for the past two weeks, and there was just a question about the effective power and field using a Denk with powerswitch in a C6, I thought I'd try this arrangement.
I've already measured the aperture reduction of the Denks in normal (open powerswitch) mode. And having seen the poor effects of using my binoviewer with a Celestron reducer, which is located at almost the same position, I suspected it would get similar results in Denk reducer (starsweeper) mode.
Denk in startsweeper mode in the C6 operates at a focal reducer of 0.60x. Starting focal length is F=1830. Effective focal length thru the Denk in 0.60x Denk reducer mode is F=1100. Power in my 20mm eyepiece was 55x.
Similar to Denk in open powerswitch mode, when in reducer mode aperture was measured to an extent of 122mm. A major difference however was that In the flashlight aperture test that aperture showed very strong vignette at the edges, a quite obvious darkening of the field edge in the projected circle of light, all the way down to an aperture of 107mm. However, in my daylight measures, that outer darkening could not be seen at all.
So effective aperture by flashlight was 122mm, but vignetted, and operating focal length was F=1100mm. Power with a 20mm eyepiece was 55x and with a 64° eyepiece I could see a bit less than 1.2°.
I put in a 24mm eyepiece and measured exit pupil. Having tested focal length, I know it's F=1100. With a 24mm eyepiece, I get an exit pupil of 2.65 to 2.70mm. That eyepieece is operating at 1100/24 = 45.8x. With that exit pupil range, it gives a resultant aperture of 45.8x2.65=121mm to 45.8x2.7=123.6mm.
In the flashlight test of the Denk reducer, I got an extent of 122mm, but with a strong fuzzy area all the way down to 107mm. But the actual bright exit pupil test shows a midpoint of 122mm. I'd say based on that exit pupil test the outer edge of the fuzzy circle of light seems to be the right diameter.
To continue with the vignette thru the view, we should expect that stepping up to a 27mm field stop, the 24SWA, the widest field of view using Denk reducer mode, with any 1.25" eyepiece is about 1.45°. However, that's not what we get. The 24SWA is strongly vignetted and the maximum field of view in Denk reducer mode even using the 24mm SWA with it's 27.25mm field stop, happens to be only 1.25° with a poor undefined edge.
So just as was shown that the Celestron 0.63x reducer cut the effective aperture by a large amount, we see the Denk BV in reducer mode also cuts the aperture by a very large amount, in fact even more.
Posted 03 March 2012 - 02:58 PM
A major difference however was that In the flashlight aperture test that aperture showed very strong vignette at the edges, a quite obvious darkening of the field edge in the projected circle of light, all the way down to an aperture of 107mm. However, in my daylight measures, that outer darkening could not be seen at all.
This gets to the crux of what I have been talking about. This flashlight test is based on measuring the diameter of something based on its shadow, and if the entrance pupil is far away and the light source isn't a point, and has a tapered beam, then the shadow will be blurred and have an unknown size.
What you describe as vignetting isn't vignetting at all - it's just an artifact of the way you are measuring aperture, as depicted in my diagram above. There is no "vignetting" of the entrance pupil. It is what it is - and the goal here is to measure it accurately.
Similarly, on the viewing side, if the aperture is greatly reduced by a stop somewhere, you would never notice it as vignetting per se - but as dimming of the central part of the field due to the reduced size of the entrance pupil. The vignetting might even be reduced by the presence of the stop, in terms of percentage illumination loss at the edge.
So this gets to my point about the flashlight test. It should be pretty accurate if the stop is near the front of the telescope, but if the stop is near the focus, then the shadow becomes very ill-defined and not a good way to measure the size of the entrance pupil. That is shown in the apparent blurring and "vignetting" of the shadow you are trying to measure.