How does the field curvature of a C-11 XLT change with/without the Celestron f/6.3 focal reducer? Does the shorter focal length and wider FOV have a worse edge of field performance? What is the story with say a 32mm 1-1/4 inch eyepiece with a field stop of 27mm? Same linear field size but lower magnification must figure in somehow. What's your take on this?
You should try to find a copy of Ray Hutchinson's SCT ray trace. This has graphs that show the spot size for the various standard SCTs.
Ray does allow reproduction for amateur use, so I am posting an example here.
Ray does not have spots for the reducer, but this is what we know about reducers. If the reducer did nothing but reduce the size of the image, the spot size would be reduced by .63 in size because that is what reducers do.
And we can infer from the instructions that Celetron provides that the standard reducer does not make the field flat though it probably does improve it. The key to knowing this is that if you can actually find an original instruction sheet for the f/6.3 reducer, it said that the scope should be focused on a star half way from then center to the edge of the field and this what you do almost always when imaging with a curved focal surface. While the blur of the stars at the center if the field is increase in side, the blur at the outside of he field is decreased in size, so that stars across the field are now going to be more uniform in size rather than have small blurs at the center and giant blurs at the outside.
If the only thing the reducer did was reduce, then the blur would be smaller, but the magnification would be less. If though, you used a higher power eyepiece to make the magnfication the same, if the reducer only reduced, then the blur would be the same angular size in the scope when used at the same magnification. This is in fact the big trick that focal reducers perform for imaging. Even if it does not flatten the field, it reduces the angular size of the blur. Now most modern reducers made for refractors are also field flatteners, but you have to pay attention to how it is advertised. If it is only advertised as a flattener, it won't reduce, and if it is advertised as only a reducer, it may not flatten. Celestron never says their reducer is a flattener and in fact, the instructions hint that it is not a full flattener, though it is possible that it does improve field flatness. My own results with the reducer were such that I thought it was better to simply use 2" eyepieces in the larger scopes. With increased focal speed, the eyepiece aberrations start to come into play, yes??? See, you can' say if the aberration you see is coming form the flattened and reduced telescope, or if it is coming from the eyepiece working with a faster light cone.. There is a lot to consider.
Now maybe Celestron has changed the reducer since that document was published, but that tells me that the reducer flattens the field, but does not make if truly flat.
But I can't answer the question of how flat it is not.
Also, the above chart shows the 8, but as the scopes get larger, the blur diameter gets proportionally smaller but at the same time, for a given eyepiece the magnification gets much higher, so if you are using a C11 with a 31mm Nagler, the bloated star is going to be magnificence by something like 38% more than it would be in the C8. So, the blur is bigger in the C8, but for the same eyepiece, the magnification is higher in the C11, and while the blur is smaller at the focal plane, the extra magnification makes it appear to swell more than it would if it were viewed at the same power.
So, it is pretty complicated, but in the end, in the standard SCT, while the blur diameter gets smaller at the focal plane with aperture, for a given eyepiece, it will be about the same angular size.
Again I am only posting the one for the C8, but if you can find a copy of the Ray's ray trace, it is packed with valuable information like this....
And as can be easily seen, the field curvature of the standard SCT is pretty huge. The field of the EdgeHDs would produce stars that were one third the size at the edge of the field, which is a remarkable improvement. Celestron gives us this figure (three times flatter) in the white paper, so the Edge HD 8" would produce a blur diameter of only 13.3 um at the corner of an APS-C size sensor, while in the standard 8", the blur would be 40 um. That is a huge improvement. See, even the EdgeHD is not truly "Flat." but no telescope really is.
Edited by Eddgie, 24 October 2019 - 11:11 AM.