Miss-aligned Diagonals

I have recently experienced three severely miss aligned diagonals. Two were reputed to be quality optics, the third an aftermarket. All were two inch. I bought a SV NH on Amart. I expected great images. Jupiter was a fuzzy ball, the moon all had cute little tails. So off went my NH to SV for collimation. Vic emailed that collimation was in spec and indicated it might be the diagonal. I did a check with diagonal in place and a straight thru. With the latter Jupiter had grown clouds bands and the moons were point images. Now I ask for return rights when I buy a diagonal.
what is your experience?

Bob:

I always most always check my diagonals with a Howie Glatter Laser when I first get them. Most are misaligned to some small degree, sometimes to a greater degree. I have three mirror diagonals that put the beam right down the center, one is an older 2 inch TeleVue Everbright, one is William Optics 2 inch Carbon Fiber dielectric and one is a 1.25 inch William Optics Dielectric. A few thoughts:

- In general a somewhat misaligned diagonal does not affect the view significantly, certainly not the way it does with a Newtonian. The optical lever arm is long when shining the laser down the tube but when you are viewing, the light path is reversed, the eyepiece is close and lever is short. The result is that the center of the field is slightly moved off center and the optical axis is tilted slightly.

- The generic Chinese 1.25 inch mirror diagonals are almost always misaligned, if anyone reading this has one, it is worth checking. Some are rather severe, barely making it through the lens of a ST-80... The misalignment always seems to be the result of the Silicone used to attach the mirror to the back of the diagonal, it's a large blob. Removing the mirror and carefully reattaching it with Silicone seems to be the ticket with these.

- Like John, I also have a Japanese 1.25 inch TV diagonal that is not well aligned...

Jon

bob Stenstrom said:

I have recently experienced three severely miss aligned diagonals. .....
what is your experience?

I’ve checked two Televue and one William Optics diagonals (2-inch) and all three are miss aligned. In the case of the Televue (at least the ones I have that are more than 5 years old) the mirror is an octagonal shape and just dropped into a milled slot. There is a little piece of foam plastic behind it. I don’t see how it is possible for the mirror not to be a little off.

George N

Hi,

After reading Roland's post regarding barlows, in the eyepiece forum, and considering the placement of the diagonal in the optical train relative to the point of focus, I am not sure that any of the testing or alignment techniques I have seen used are actually all that valid.

It would seem to me that a laser, or other alignment tool so close to the point of focus would show enormous alignment errors when projected back through the scope, if even the slightest offset occurred.

I would think that the mirror surface quality, and reflective quality of the diagonal are way more important than trying to get it perfectly aligned, especially if the alignment issue is not due to the mirror, but instead is due to the way the alignment of the diagonal fits in the scope could be more of an issue depending on the focuser and how it holds the diagonal, or how the diagonal holds the eyepiece.

Has anyone ever seen a way to really determine if there are issues with the alignment of a mirror, prism, or diagonal?

Wouldn't a simple star test, on Polaris, followed by rotating the diagonal be a much simpler and accurate way to check everything?

Timm