Hello folks,
Browsing the telescopes at TS again. I have a 71mm F5.9 and 115EDT F7 that I use for visual. NOT for imaging, since I use newtonians on an EQ mount for that.
I was thinking what if I get a get a "fast" and flat field refractor around 90-100mm to fill in the gap and be a more portable setup. The 115EDT is really long and unwieldy, and since it is front-heavy, the eyepiece end sticks out a lot and changes height for different altitudes. Right now I am using a Stellarvue alt-az mount and Amici diagonal, and very happy to just push the scope around with correct-image.
The advantage of this type of scope is that it is physically shorter, plus I can just drop in a camera to do EAA / plate solving without worrying about field flatteners and lens spacings. In this instance, I am referring to the 4+ lens scopes with the flattener embedded further up the tube, so that I can insert diagonals and barlows and etc and just set the focus. NOT referring to the kind where the flattener is added retroactively and must be a certain distance from the eyepiece or camera.
My questions are if there is any drawback to getting this type of scope.
1) The resolution should be determined by the aperture anyway, so what's the difference between F7 and a particular eyepiece vs F5.6 and a slightly shorter length eyepiece?
2) Will the flat field "help" some of the eyepieces be more in-focus near the edges? Or are most eyepieces already designed with field curvature in mind? For example, I have a Baader 8-24x zoom and Paradigm 15mm and both have a little bit of field curvature.
Thanks
First Radius of Curvature (RC) in a doublet or triplet is totally a function of focal length. It does not matter what aperture. If you are going with a doublet or triplet, the longer the focal length, the flatter the field. The RC is about 1/3rd of the focal length, so a doublet or triplet with 1000mm focal length will have an RC of about -330mm. By comparison, a reflector has an RC that is about the same as the focal length, so a reflector with a focal length of 1000mm will have an RC of about 1000m.
As mentioned, you can also add a field flattener, but these are typically designed to work with imagers and a specific spacing is required for that flattening to be most effective. If you use a diagonal behind it, don't expect the same pinpoint stars you would get with a camera sensor.
Telescopes like the TV 101 will have much flatter fields but of course this comes at a cost. To get that field to be flat, the TV 101 essentially has a built in field flattener that is optimized for the focal plane to be much more distant than is typical with field flatteners designed for imaging.
The easiest way to deal with field curvature is to use longer focal length, narrower field eyepieces of the same apparent field. The longer the focal length, the less visual accommodation is needed to keep the field looking sharp. For example, rather than a 31mm Nagler, a 41mm Panoptic has lower power but still offers a wide field of view (in this case, even wider than the 31mm Nagler) will show stars sharper at the edge of the field.
A 27mm Panoptic has a 30.5mm field stop vs the 27.4 field stop of the 20mm Nagler and once again, due to the shorter focal length, even though it has a bigger true field, the 27mm Pan will have less power and stars will appear sharper at the edge.
In these cases, it is not that the eyepiece is better, it is that anywhere at the outside of the field, the defocused blur is smaller and your eye has an easier time accommodating any defocus.
Young observers can accommodate 2 diopters of defocus but seniors will struggle to accommodate .5 diopters of focus. Depending on your age, the use of a longer focal lenght eyepiece with about the same size true field will generally produce sharper stars at the edge.
As powers go up, you can use wider field eyepieces but at the lower power end of the spectrum, where you are seeing well away from the center of the field.
A flat field eyepiece is not always the best choice. They are best with telescopes with low field curvature, such as Newtonians, because in those scopes, the field is much less curved than with refractors.
Here is a graphic that explains why field curvature in an eyepiece can actually be good! Scroll down about half way on the page and you will see how eyepieces with field curvature can actually make a telescope that has field curvature produce a field that is sharp all the way across.
https://www.telescop... toward the eye.
So, in a telescope with a curved field, the best eyepiece would be one that has a curve that is as close to the curve of the telescope's field as possible.
