When used visually, field illumination falloff or vignetting of fairly strong proportion is often easily accepted and sometimes totally escapes visual notice. When imaging or when using image intensifiers though, field illumination falloff or vignetting is far more of a concern, and when heavy focal reduction is used, aperture loss becomes a very real concern.
For aperture loss, the laser projection method is probably the most effective method to use, but for a lot of people getting a calculation of vignetting or illumination falloff can be a challenge.
The good news is that there is a very easy way to estimate the amount of intensity falloff of virtually any kind of telescope configuration using nothing more than a bright star an the knowledge of the size of the field (sensor size, photocatode, or eyepiece field stop).
An obstruction is useful for this, so even if the scope does not have one, adding one made from a cut out piece of cardboard and held over the aperture with a couple of strips of tape will be fine. This is just useful for getting a better estimate of the amount of illumination falloff that is occurring.
The test is easy. Just point the telescope at a bright star and center it in the field of view. Now, defocus the star so that three or four rings are showing (if using an obstruction, maybe six if not, but these numbers are not at all critical).
Now, move the scope so that the unfocused pattern drifts toward the edge of the field and watch the outside edge of the pattern. As you move the scope, you may see that there is a smooth, ark shaped intrusion into the edge of the pattern. When you see this, you are seeing the point where either the off axis rays from the primary are starting to fail to intercept the edges of the secondary (Newtonians) or where a baffle or focuser tube in extending into the edges of the light cone (SCTs are the worse for this, with many only starting with a 6mm to 10mm fully illuminated true field).d
Knowing your sensor size, you should now be able to estimate the size of your fully illuminated field. For example, if using an APS-C size sensor or an image intensifier, if the intrusion occurs when the pattern is half way to from the center tot the edge of the field, this would indicate that the fully illuminated field is only about half the diameter of the chip or photocatode, so the fully illuminated field size would only be about 9mm. If it happened 1/3rd of the way from the center tot he edge, the fully illuminated field would be only 6mm, and so on.
As you continue the drift to the edge of the field, notice that the intrusion now starts to consume more and more of the pattern, and by estimating the amount of the pattern being blanked out, one can estimate the amount of illumination falloff. For example, if by the edge of the field, only half of the area of the pattern remains, then this indicates that the illumination has fallen by 50%.
As long as a subject fits entirely inside the fully illuminated area, then 40% or 50% falloff will not damage the image other than darkening the outside of the field (flats) but for very large objects that extend over most or all of the field, a considerable amount of image brightness is being lost.
Here is an example I would give you (and next time I have a clear night, I hope to take pictures, but I have been plagued by clouds for a month now). Sometimes I use my 6" f/2.8 in a configuration that has a 1.25" eyepiece holder behind the filter wheel. I do this simply because it makes it easy to swap the device with a 1.25" nose to other telescopes. If I do this though, it places the 1.25" filter 45mm in front of the photocatode. In this configuration, the test I just describes shows that the Fresnel pattern starts to loose illumination at only about 3mm from the center of the field, indicating that my fully illuminated field is only about 6mm. Because the vignetting source (the filter) is far ahead of the focal plane, the illumination falloff is not aggressive, but it is constant and by the time the pattern reaches the edge of the field, illumination has fallen to 40%. Now for a galaxy at the center of the field, this is meaningless, but for very faint nebula that stretches outside of the field, this is all to often the difference between seeing it and not seeing it.
Now since my reducer/corrector requires 65mm of spacing, if I want to more fully illuminate the field, I can remove the 1.25" eyepiece holder and attach the NV device directly to the rear of the filter wheel using a T2 adapter. This puts the filter only about 20mm in front of the focal plane. I now have to put a spacer between the reducer and the front of the filter wheel, so this setup does not allow for easy movement of the devices between scopes, but now, I an move the defocused pattern to the edge to the edge of what I estimate to be a 15mm image circle before the intrusion starts, and while the falloff is more aggressive (the amount of pattern lost for each millimeter of movement) by the edge of the circle, I estimate that I still have 80% illumination. This one change produces an extremely large increase in the size of the fully illuminated field, and the amount of vignetting at the edge remains very small. By going to 2" filter I could fully illuminate the entire 18mm circle, but the cost vs the tiny drop in illumination at the edge of the field is substantial, so I am happy to accept this tiny amount of illumination loss as a good compromise in terms of cost for performance. The point here is that I know the behavior of my system so I can make this decision knowing exactly what my money will or won't get me.
The reason I posted this is because I see a great number of cases where people are doing extreme amounts of compression, and I often wonder if they have ever devoted any effort to actually measuring the loss of focal speed over the entire field. Sometimes, it can be quite extreme, making a less aggressive reduction producing almost the same brightness over the average of the field size. For example, a Celestron C6 only starts with a fully illuminated field that is about 5mm in diameter. Compressing this to .33 percent of its original size means that maybe only 1.5mm of the image is fully illuminated, and if the configuration induces aperture loss (easy to do in an SCT without attention to the configuration) the center of the field could be working at well under the "nominal" f/3.3. Of course this number is already false because with transmission loss and secondary shading, the nominal .33 reduction is really only yielding maybe f/3.8 brightness, but that is a different story for a different time.
I have used this method to estimate field illumination for over a decade now. Most calculators do not allow for the modifications induced by focal reduction, so for this case, the method I provided allows for an easy way to make a reasonable estimate of how big the fully illuminated image circle is, and how much illumination loss occurs at the edge of the field.
I hope someone found this post to be useful.
Edited by Eddgie, 26 May 2019 - 09:23 AM.