Paul Neesam said:
Hi guys, finally got some first light with my new homebuilt DOB, 16-inch f/4 mirror. The views were outstanding, by far the best I've had without question...but I noticed something I had not noticed with my other smaller scopes I've used, when moving around the surface of the moon to view across the moon surface the curvature of the moon required me to refocus. I could refocus without issue but is this normal, is this a good sign showing the optics are good or does it show an issue? I was not using a paracor corrector and was using a 2-inch FMC 30mm, I did Laser Collimation.
Thanks
Paul
Paul: I am not quite sure what you mean when you say that moving across the surface of the moon required you to refocus. But here's a few thoughts that hopefully will clarify this for you. Viewing the moon at such a low power is not good test and there are a number of factors that one needs to consider.
The short story is I would not be concerned.
The longer story:
- The moon is at infinity for all practical purposes so the curvature of the should not require refocusing.
- However, the diffraction limited or coma free field of view of an F/4 is very small, about 1.1mm. Translated into an angle for a 16 inch mirror this corresponds to about 2 arcminutes. This means only a small region is sharp so that if the view is at all off-axis you may find refocusing helps sharpen things up.
- Another part of the equation is the eyepiece, you mention 30mm FMC, this eyepiece produces a magnification of about 54x and a 7.5 mm exit pupil and a very bright image. There are a number of issues here. If you haven't used an F/4 scope, particularly an F/4 Newtonian, the number of eyepieces that handle the steep angled light cone are few, the number of 30mm eyepieces that do it effectively is very limited, the 31mm Nagler is probably at the top of the list with a few rather expensive stragglers providing reasonable but somewhat less pleasing views.
So in an F/4 used without a coma corrector and with a basic eyepiece, one expects to see quite a variety of off-axis aberrations, it would likely be some combination of coma (from the mirror), astigmatism and field curvature from the eyepiece, it is likely that viewing off-axis (and remember that the diffraction limited fov is only 2 arcminutes) may be sharper with some refocusing to minimize the blur size.
-The final part of the equation is your eye. That 30mm FMC eyepiece is producing a 7.5 mm exit pupil. That exceeds the diameter of the typical dark adapted eye.
More importantly, the brightness of the moon will cause your eye to constrict, if you ware not using a filter, then it will be down around 2mm, day time diameter. Under these circumstances you would be seeing a dark spot in the center of the view from the shadow of the secondary, if you are looking straight on. Assuming a 20 percent secondary, the shadow would be 20 percent of 7.5mm in diamter or about 1.5mm.. With the eye open to only 2mm, this shadow is typically quite obvious.
With a moon filter, your eye would be more open but IMHO, it is unlikely that it would be fully open.
In either case, you have a rather complicated situation. You have a large exit pupil with a hole/shadow in the center. Your eye cannot take in the entire exit pupil at the same time but only some part of it. This means you are not looking at the entire mirror but rather only a small part of it. If your eye is constricted to 2mm, then you are only looking at 2mm/7.5mm x16 inches of the mirror or a bit over 4 inches.
As you move your eye around, to look at different parts of the moon, it is mostly likely that you are looking at different parts of the mirror, particularly because you are probably avoiding the central shadow. This in itself may require refocusing.
- So when you consider all the factors here, you are not likely to be looking at the center part of the mirror, the eyepiece has inherent aberrations that may require refocusing, the comatic blur in combination with all this may show a cleaner view with refocusing, etc, etc I am not surprised that some refocusing is required.
The test of the optics is not at low powers but at high powers where the actual diffraction patterns can be resolved. And in a 16 inch scope, that takes some pretty darn good seeing...
So bottom line: I wouldn't worry...
Just enjoy it
Jon
