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6se vs Vignetting... my observations.

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#1 BillStar



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Posted 31 July 2010 - 10:08 AM

Ok this is wierd.
When I was setting up for my second night of observing with my new 6se, (at 6 pm) some other "regulars" noticed the eyepieces I was laying out for use. I heard immediate comments of approval, but subsequent grumbles of disdain. Approval over my collection, but disdain about their use in my 6se.

I questioned their concern, and they responded with polite warnings of vignetting with my Panoptic and Naglers, and my 38mm University Superwide. Naturally I became concerned, so I immediately did a test in daylight.

I focussed the scope on my car's license plate using my favorite eyepiece (the 1996-vintage Orion Ultrascopic). The car was parked facing a chain link fence, which was in the field of view.

I then used each of the Naglers, the 27mm Panoptic, and the University 38mm (70 degree A.F.O.V.) to view the scene. I had the others look as well. They were astounded. Here is what we observed:

12mm Nagler T4.
True field = .66 degrees @ 125X.
Slight vignetting... negligable (haze near the very edge of field).

22mm Nagler T4.
True field = 1.21 degrees @ 68X
No vignetting... NONE.

27mm Panoptic.
True field = 1.21 degrees @ 56X
No vignetting... NONE.

38mm University Superwide.
True field = 1.75 degrees @ 40X
Slight vignetting (haze at edge of field).

NOTE: The chain link fence was an excellent daylight reference for edge clarity.

We agreed to re-test during darkness. By 2 a.m. we'd all satisfied ourselves of the following:

12mm Nagler
.66 degrees True field.
No vignetting... NONE

22mm Nagler
1.21 degrees True field.
No vignetting... NONE

27mm Panoptic
1.21 degrees True field.
No vignetting... NONE

38mm University Superwide
1.75 degrees True field
Slight vignetting (hazy star images at edge of field).

NOTE: The 12mm Nagler has a 2-inch/1-1/4 barrel. It may be possible that the 1-1/4 portion causes a slight perception of vignetting.

Okay. So what gives?
I get predicted vignetting with the 38mm (1.75 degrees True field). However, I'm not seeing it at 1.21 degrees True field, and acording to a lot of folks, I should be. In fact the 27mm Panoptic (1.21 degrees) is extremely bright at the edge.

I think this warrants some discussion, as I've been told numerous times by everyone who knows, that I'm vignetting over 1 degree of true field. Or then, maybe I've got a one-of-a-kind 6se. Not likely.

Please feel free to disagree. I'm new behind this telescope.


#2 Tel


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Posted 31 July 2010 - 11:57 AM

My physics is lousy, my knowledge of optics worse, but perhaps someone could explain to me why we are talking so much of baffle size diameter without thought given to the length of the baffle tube.

If, as I might comprehend it, a final cone of light is projected from the secondary mirror through the baffle tube to its focal point, doesn't it follow that if the diameter of the cone near the secondary mirror exceeds that of the baffle plate, then vignetting will ensue, SO doesn't it equally follow that the potential problem must start at the front end of the OTA and therefore is more associated with the point at which the cone meets the baffle tube and hence its length ?

In other words, if the baffle tube is long and the light cone exceeds its diameter then vignetting will occur because part of the light cone is excluded, BUT if the tube is short and the cone has narrowed sufficently by the time it meets it, then vignetting surely cannot occur. :idea:

I'm sure this doesn't make sense and there's a catch in this hypothesis somewhere ! :confused: But at least perhaps it gets the ball rolloing and I get some further education ! :graduate:

Best regards,

#3 Jon Isaacs

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Posted 31 July 2010 - 12:11 PM

This is my thinking/experience:

The issue with vignetting is not the on-axis light cone but the off-axis rays. To avoid vignetting, an off-axis ray of the appropriate angle must travel from the edge of the corrector, bounce off the primary, off the secondary and arrive at the focal plane of the telescope.

Since the rays are converging to the focal plane, the effect of any obstruction in the optical path that is larger than the field stop and indeed somewhat smaller than the field stop will result in some vignetting.

However, the human eye is relatively insensitive to vignetting so it is rather difficult to detect even if you take a bright star from the center of the field and move it towards to the edge. A faint diffuse object might be a better choice.

I am not sure what the rear port/rear baffle diameter is of a C-6, the C-5 was 1 inch, a C-8 is 1.5 inches.

Jon Isaacs

#4 Midnight Dan

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Posted 31 July 2010 - 05:07 PM

Hi Bill:

I went through this same scenario with my 8SE. I was told that i would get vignetting over 1.2° but I could get to 2° before I saw the black rim around the edge.

The issue has to do with the definition of vignetting. All SCTs suffer from off-axis dimming which means the image gets darker as you move away from center. The wider the view, the more you see the effect. In reality, you don't see the effect visually until the field becomes quite large. If you were to take a flat-field photo it would stand out very clearly, and those in the AP field will refer to this as vignetting.

So, if you ask about how large your scope will go before you see vignetting, you'll get the answer of around 1° with the 6SE and 1.2° with the 8SE.

However, if you really want to know the maximum un-blocked field of view, that's a different question entirely. When I measured it with my 8SE and used various EPs, I found that visually I could not detect any vignetting till I got to about 1.5°, and the field was not blocked until I got to 2°. Even between 1.5° and 2° I didn't really find the off-axis dimming objectionable, although it was noticeable (it's easier to see in daytime viewing).

Someone else on this forum (can't remember who) did some similar tests on the 6SE and found that the view became blocked at about 1.5°.

You're reporting 1.75° which may be due to a number of reasons. The most likely is that I suspect you calculated the FOV rather than measured it. With SCTs, since there are 2 mirrors of differing focal lengths moving relative to one another during focusing, the actual focal length of the scope can vary significantly depending on your focal point. This depends on the size of your diagonal, position of the field stop of the EP, etc. So the calculated field could be off quite a bit from the real FOV.

To measure the TFOV is really quite easy with these mounts. Set it up in the daytime with a given eyepiece. Find a distant object and align the scope so that when you use only the left-right buttons, the object traverses the widest part of the field. Now set the object at one side and use the menu system to read the azimuth. Use the left-right buttons to move it to the other side and take an azimuth reading again. Subtract the two and you've got your true field of view.

The only thing you have to be careful about is the effect of backlash. To eliminate this, just be sure you always approach the edge of the field from the same direction. For example, start with your object outside the field to the left. Use the right button until the object is just appearing at the left edge of the field. Take the reading and then continue to use the right button until the object reaches the right edge of the field. Then take your second reading.


#5 mclewis1


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Posted 31 July 2010 - 07:25 PM

The C6 has a 27mm rear opening/baffle tube diameter. The maximum fully illuminated from the numbers on the C6 is just over 1° (depending how it's calculated it can be in the area of 1.1°). Most folks can't detect any drop in a star's intensity until it drops by more than 10%. With a C6 most folks report that they can go out to at least 1.3° before they start to see any drop in intensity (vignetting). Many folks use and enjoy a C6 and eyepiece combinations that provide 1.5-1.7°, it's really up to what each of you can personally put up with.

Testing during the daytime or on a bright night time object (moon) will not show you the onset of vignetting, it takes some pretty severe vignetting to become obviously visible to most folk's naked eyes. It takes a really trained eye to notice the drop of intensity in the stars as they move towards the edge of the fov and into an area of vignetting ... and even a well trained eye can't pick up the first 10% or so.

So basically don't bother with the numbers, use whatever eyepiece you like and provides a comfortable view, but don't suggest that there's no vignetting at 1.2 to 1.3° in a C6, you just can't see it yet ... and if you can't see it who cares, right?

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