Astigmatism Question, back to reality.

OK, I think the Markus Ludes/TeleVue question has been beat enough to kill a cat, here's a question for you about a scope at the other end of the scale.

The problem:

I received an Achromatic refractor, 102 mm F6, from a vendor with a forum here. I waited quite sometime for this scope and when shipped it was "as is" because of continued problems with the objectives. New ones are supposed to arrive someday.

Mechanically the scope is quite nice, optically, my example is quite disappointing.

Not even requiring a star test, just viewing at any magnification reveals a great deal of what I take to be astigmatism.

Best focus on a star is not a point or even a point with coma, it is a cross. Viewing the double Castor (about 4 arc seconds), the lengths of the lines are sufficient to connect the two and then some. This would be most apparent at 100X. These lines are nice and sharp but of different colors, one is red, one yellow-white. Using Aberator, it seems this would correspond to about 1 wave of astigmatism.

From what I have gather reading the net, astigmatism in a refractor is caused by either pinched optics or bad glass.

I have tried rotating the elements with respect to each other, I have tried to make sure the elements are free and not overly tightened, the problem remains.

The questions:

1. Am I correct in thinking this is astigmatism...

2. Any thoughts on what I might do other than wait for the next round of objectives??

Note: at this point, this is more of an intellectual curiousity to me, and interesting problem to think about and understand, a learning experience. There is some frustration of course, but I have plenty of other scopes to keep me occupied until this one becomes useable.

Thanks for any thoughts or suggestions.

jon isaacs

Jon, looks like astigmatism. If you haven't already, you could check out if the objective and focuser are aligned with the tube. Achromats this fast can have that much of off-axis astigmatism at ~1 degree off-axis.

I hadn't thought of that the last time I played with the scope, it does seem like a good way to determine which element is bad. My recollection is that it did not rotate when I rotated the front element.

If I try it again, I will pay careful attention rotation and also I will rotate the rear element and see if that changes things.

Thanks

jon

Thanks again to everyone that has offered suggestions. Last night I rotated the inner element without rotating the front element. The astigmatism rotated so I guess the problem is with the rear element.

Anyway, hopefully the new objectives will be here soon..

jon

John

if you see astigmatism in a lens, no matter if achromat or apo or whatever, than mostly it is not in the glas.

there are several possibilities alone or together :

1, most comon problem in low priced achromats: cells are not matched in inside diameter with glas diameter, so lenses can shift sidewards
2, spaces are not using matched thickness.

how to correct such problem without return of the optics:
- you can drill for each lens at the edge of the cell with 120° spacings threads and use plastic screws, they can allow you to push the lens onto perfect axis

good luck
Markus