Found this: https://www.oreilly....04/ch04s15.html
So the mysterious number 57.3 is actually 360/(2*pi).
But according to that, I actually do need to contact each eyepiece manufacturer to ask what the field stop is if it is not listed on their website?
How does that relate to vignetting though? Just that the field stop needs to be smaller than the baffle tube?
>>So the mysterious number 57.3 is actually 360/(2*pi).
More importantly, it converts radians to degrees.
The math behind this uses some geometry and trigonometry and takes advantage of the small angle approximation (that for small angles, the sine of an angle and its measure in radians are approximately the same), which delivers the answer in radians.
>>But according to that, I actually do need to contact each eyepiece manufacturer to ask what the field stop is if it is not listed on their website?
Well, you could just measure the thing yourself (provided that you don't have an eyepiece that essentially starts with a barlow element and has the fieldstop between elements of the eyepiece). Or you can estimate it by computing the field of view with the less accurate AFOV divided by magnification formula, and then work out the implied fieldstop (which assumes an accurate AFOV and zero linear distortion). Don Pensack's spreadsheet will do it for you, I believe. Also Agena Astro's website lists fieldstops for many eyepieces.
>>How does that relate to vignetting though? Just that the field stop needs to be smaller than the baffle tube?
Again, the telescope, not the eyepiece is the source of the vignetting-- or at least the type of vignetting you are worrying about. (Eyepieces may add a small amount of vignetting, but it usually fairly insignificant and has nothing to do with baffle tube diameters).
The telescope produces an image circle. The eyepiece provides a circular window to that image circle. The fieldstop is what creates the edge of this window, and blocks light that the eyepiece is not designed to deal with. If the telescope vignettes before the size of the fieldstop, this vignetting will be present in the field viewed by the eyepiece.
You seem to think that once you hit the diameter of the baffle that the image goes dark almost immediately. That simply isn't the case. You also seem to think that the field is fully and evenly illuminated right up to the point you hit the baffle tube diameter, and that simply is not the case either. Unlike the fieldstop, the baffle tube is not at the focal plane. The falloff it creates is not hard and quick. And the secondary may be a source of vignetting as well.
And I've told you this, or implied it a number of times. For example:
The baffle tube is a restriction but it is out of focus. Falloff will start before you even get to the 17mm point. Telescope designers have to balance full illumination of the field with obstruction size. As a result, most obstructed scopes will show some degree of falloff.
I also told you in the 5SE thread
Yes, a C5 with F/6.3 R/C will vignette with a crop sensor camera, but it's not like it is dark in the corners. You may also want to crop the photos square. Either way, you may still appreciate the extra space from a compositional standpoint. And visually, for night time astronomy, I was surprised how well the combo works. Yes there is detectable falloff at the field edges with a max-field 1.25" eyepiece, but there is useful illumination to the edge. In the day time, I could see how the 5.6mm exit pupil would cause problems for people (your day-time pupil can't accommodate the exit pupil, so it effectively stops down the scope, making the secondary even harder to ignore), but I find the combo usable at night with even my 35mm Ultima. Different people have different sensitivities to the shadow of the secondary, though.
The Ultima has a 29mm field stop. The R/C turns that into a virtual 46mm+ field stop. On a C8 (37mm baffle tube) I don't generally visually notice the falloff at all. And as I note above, I even find the combo useful on the C5, and was surprised how well it works. Human beings don't detect increases in brightness linearly, and our brains are powerful image processors, so we are relatively insensitive to falloff.
Some time ago, I took a photo someone had taken using a full frame camera with a 0.7X reducer on a C8 edge. You need the reducer to make the falloff evident. Then I faked the vignetting in the frame to create an image circle. I also super imposed the fields of a 26mm, 32mm, and 38mm SWA when used with the 0.63X R/C. Here is that image:
the above picture reasonably depicts what I see visually. The 26mm is usable with the R/C. It has a 32.2mm fieldstop, which the R/C turns into a roughly 51mm vitual fieldstop. However, in the 32mm doesn't really give you much more in useable field (things are pretty much dark at its field stop) and the 38mm (an effective 71mm field stop with the R/C) has a significant dark circle around the edges of its field.
And note that on the original full frame image, at 0.7X reduction, the corners are obviously dark, but a little bit of cropping produces a totally useable image.
And here is an approximation of the same SWAs when used at F/10 (no reducer/corrector)
Note the 32mm SWA's image circle (the orange one). That's an eyepiece with a 40mm fieldstop on a telescope with a 37-38mm baffle. And the 38mm has a 45.5mm fieldstop (similar to a 40mm 68 and just slightly smaller than a 41mm 68 or 56mm plossl). Any other eyepiece with approximately the same fieldstop will show approximately this same field with the same amount of vignetting.
Of course, all that is for a C8, but the concept is similar for smaller CATs (just on a smaller scale). I've heard a number of 8" SCT owners talk about the baffle size and say that you shouldn't use an eyepiece with a fieldstop bigger than 37mm, or maybe 40mm. I'd rather have a larger field, even if it means it is a tad dimmer at the edge. The edge also has coma and field curvature. It's already not perfect. But I'd rather have the option of a 1.2-1.3 degree field rather than a 1 degree and the false security that doing so eliminates any and all vignetting. Even a C8 doesn't produce a fully and even illuminated field at the 17mm mark in its image circle, let alone the 46mm mark. I believe its designers were trying to strike a balance between obstruction size and the ability to fully illuminate a 35mm film camera frame (a little over 43mm on the diagonal). I think they did a good job.
Anyway... Yes, if you use an eyepiece with a fieldstop bigger than 17mm on a Apex 90, the image you see will have some vignetting, but the vignetting is intrinsic to the telescope and yes, it is caused (at least in part) by the restriction of the baffle tube. But that doesn't mean that the image produced by isn't useful. And again, even with an eyepiece with a 17mm fieldstop, image circle will not be illuminated absolutely evenly, and will be slightly dimmer at the edges, but that won't be obvious at all. As you get wider, the vignetting may be more noticeable in photographs than in the eyepiece, but again, you may find the compromise acceptable. Or perhaps not. But some of this stuff you just have to try and see...
You seemed to dismiss the C5 on fears of vignetting, but now seem to be considering a scope with and even smaller baffle tube and a similar focal length. I don't get that at all.
Finally, I don't see how that C90 page you linked to confirms your fears at all. Quite the opposite. I saw a bunch of people who use eyepieces like a 32mm plossls, 25mm 60-degee and 24mm 68 degree eyepieces on C90s and are happy with the results. Note that the OP was only unhappy with the 24mm 68 (27mm fieldstop) terrestrially because of its linear distortion, and was perfectly happy with it astronomically.
Edited by jallbery, 07 December 2018 - 12:38 AM.